Multi-Row Roller Bearing Mounting and Maintenance RKB BEARING KNOWLEDGE COLLECTIONS

Transcription

1 RKB BEARING KNOWLEDGE COLLECTIONS Multi-Row Roller Bearing Mounting and Maintenance

2 CONTENTS 1 GENERAL INFORMATION Multi-Row Tapered Roller Bearings Multi-Row Cylindrical Roller Bearings 8 2 MOUNTING OF MULTI-ROW ROLLER BEARINGS Preliminary Operations Chock Preparation 11 Shaft Preparation Mounting of Four-Row Tapered Roller Bearings Bearing Parts Preparation 12 Mounting of Unsealed TQO Bearing with Pressed Steel Cage, Variant I (large size bearings) 14 Mounting of Unsealed TQO Bearing with Pressed Steel Cage, Variant II (small and medium size bearings) 17 Mounting of Unsealed TQO Bearing with Pin-Type Cage 18 Mounting of Sealed TQO Bearing 19 Dismounting of Four-Row Tapered Roller Bearings Mounting of Four-Row Cylindrical Roller Bearings Bearing Parts Preparation 22 Mounting of the Bearing Inner Ring onto the Roll Neck 22 Mounting of the Bearing Outer Rings into the Chock 25 Mounting of the Chock onto the Roll Neck 25 Dismounting of Four-Row Cylindrical Roller Bearings 26 2

3 3 LUBRICATION OF MULTI-ROW ROLLER BEARINGS Grease Lubrication Oil Lubrication 31 4 MAINTENANCE OF MULTI-ROW ROLLER BEARINGS Typical Maintenance Activities Checking and Readjustment of a TQO Bearing Axial Internal Clearance (Bench End Play BEP) Storage Need for a Well-Balanced Bearing Maintenance Program 42 DISCLAIMER 43 3

4 1 GENERAL INFORMATION Multi-row tapered and cylindrical roller bearing assemblies are mostly used on roll necks of rolling mill stands where two major constraints recommend their use. On the one hand, we have to consider that rolls are subjected to very high radial loads and axial loads of different degrees of magnitude. Nevertheless, compared to the radial loads, the axial loads are relatively small but they have to be absorbed in either direction. To carry these loads, radial or combined (radial and axial), roll neck bearings must be made of materials having high hardness, resistance to rolling fatigue and wear, and good dimensional stability in order to provide high performance under very difficult operating conditions (high operating temperature, water or roll coolant mixed with dirt, etc.). On the other hand, the available mounting space is drastically restricted, especially in radial direction. Consequently, a bearing of low sectional height but with a very good load carrying capacity, like multi-row tapered or cylindrical roller bearings are, seems to be the best choice for roll necks. The rolling mill stands are very particular applications which, in addition to the already mentioned requirements, also request the bearing change to be made on the occasion of the roll barrel regrinding. For this reason, the frequency of roll change should be also taken into consideration when selecting the roll neck bearings. When using bearings whose inner rings are tightly fitted on the roll neck, this task is made more difficult in case of multi-row cylindrical roller bearings (where the chock together with the outer ring and the roller-and-cage assembly can be withdrawn, leaving the interference-fitted inner ring on the roll neck) and of four-row tapered roller bearings (that generally are clearance-fitted on the cylindrical roll neck and thus the chocks can be easily removed). Whatever combination of loads and work conditions, the customer will be able to find the correct RKB multi-row roller bearing to perfectly suit his specific needs in terms of maximum load carrying capacity within the minimum space. This fact will decisively contribute to lower the cost of the customer application. In addition, upon request, the RKB Bearing Industries Group can design and manufacture any type of multi-row roller bearing regardless their dimensions. Today s needs for higher performance bearings have encouraged RKB to manufacture products with significantly enhanced characteristics in terms of top performance at lower costs. Since fatigue strength, wear and shock resistance under heavy rolling loads are requirements for roll neck bearings, premium quality carburizing grade bearing steels are normally used in the manufacture of RKB multi-row roller bearings. Starting from technology and engineering and ending with systems management, production and marketing, all of the resources of the RKB Bearing Industries Group are focused on continuous product improvement and service enhancement to add value to customer applications. Skill and experience in handling, mounting and dismounting multi-row roller bearings are crucial to guarantee a long bearing service life. For this reason, RKB recommends reading thoroughly the instructions provided in this publication and using the appropriate tools during the mounting/dismounting process. In case of need, do not hesitate to contact RKB Technical Team for assistance. 1.1 Multi-Row Tapered Roller Bearings It is well known that tapered roller bearings are separable and this feature commonly eases their mounting and dismounting. Nevertheless, even if they are also separable, the special internal geometry of the four-row tapered roller bearings does not permit their installation by following the same mounting sequence as in the case of multi-row cylindrical roller bearings (fit the inner rings onto the roll neck, fit the outer rings into the chock and afterwards push the chock onto the roll neck). For this reason, the complete bearing has to be obligatorily first mounted into the chock and, only afterwards, the chock together with the bearing can be pushed onto the roll neck. This situation contradicts the general rule according to which the bearing inner ring should be installed with an interference fit on the journal. This means that the bearing cones must have either a clearance fit on roll neck (although the risk of inner ring creep increases significantly) or an interference fit, obtained through their tapered bores in conjunction with the tapered roll neck. The relative rotation between the bearing bore and the roll neck can result in strong heating and severe wear. However, wear can be minimized by abundantly lubricating the mating surfaces of the cones and the roll neck. For this purpose, inner ring bores are frequently provided with helical grooves that play a double role: create an important deposit for supplementary grease (in this way improving the lubrication of the roll neck) and collect and embed the abraded hard particles. Radial grooves, designed to have the same purpose as heliacal groves, may also be provided in the abutting cone and spacer faces. This is the case of slow to moderate mill speeds, where, in order to accommodate roll installation and removal, four-row tapered roller bearings are generally mounted with a clearance fit on the work roll necks. Note that since the acting loads are lower than in other situations, bearing wear could be considered moderate, rolls wear out faster than bearings and consequently, when changing the rolls, the bearings can be also changed (before bearing wear becomes critical). However, four-row tapered roller bearings with a cylindrical bore cannot be used for all rolling mill applications. When mill speed exceeds m/min and/or in the case of heavy loads and severe environmental conditions, 4

5 the cones of back-up roll bearings must be mounted with interference fits to avoid cone creep. In this case, mill stand design generally encompasses four-row tapered roller bearings with tapered bore cones fitted on tapered roll necks. It is also important to mention that this type of assembly can facilitate and speed-up bearing mounting and dismounting. In most rolling mill applications, work roll bearing arrangements are lubricated with grease. Improving the lubrication quality and, especially, the lubricant cleanliness inside the bearing eventually results in the achievement of a longer bearing life. In order to retain the grease inside the bearing, preserve its cleanliness, cut maintenance costs and, last but not least, protect the environment, bearing designers and users try to reduce grease consumption. In order to achieve these objectives, RKB has developed four-row tapered roller bearings with integrated seals which have the same main dimensions as the unsealed ones. The grease consumption with a sealed bearing is largely decreased and operating and maintenance costs are heavily reduced. Since the sealed tapered roller bearings obviously keep the lubricant cleanliness at a high level for long time, they generally have a longer life than those unsealed. However, the basic dynamic load rating of this type of bearing is lower than that of unsealed bearings of similar dimensions, due to the reduction of the available space for the rollers. Sealed bearings are mostly used when work rolls need to be changed often because of their worn barrels, as well as in the case of back-up rolls in mills provided with oil-air or oil-mist lubrication. Note that in both hot and cold strip mills, where large quantities of fluids (water or dirty roll coolant) can penetrate and compromise the bearings supporting the work rollers, special measures for bearing sealing should be taken. Large tapered roller bearings (as well as large cylindrical roller bearings) can be provided with pin-type cages and pierced (through-bored) rollers. Due to the large inertial forces (as a result of the great accelerations and decelerations of rolls) this cage design is recommended especially for reversible stands. All of the multi-row tapered roller bearings manufactured by RKB are supplied assembled with the required axial internal clearance (Bench End Play BEP). The correct assembly of the bearing components allows to obtain the correct value of the BEP and assumes that the lubrication holes of the cup spacers are evenly spaced between the adjacent single-row and double-row cup. Usually the appropriate BEP range is obtained by adjusting the width of the spacers between cones and cups, respectively, but in some designs (bearings without spacers) the axial internal clearance is already adjusted in RKB manufacturing plants. Note that when the bearing is fitted on the shaft and in the housing with an interference fit, the axial internal clearance is reduced and the remaining axial internal clearance after mounting is called Mounted End Play MEP. The Technical Fiches provided by RKB supply all the information necessary to calculate the radial internal clearance (RIC) of the bearing using the given values of BEP and e (Fig. 1): Given: Calculation: RIC min = RIC max = Radial internal clearance: RIC = BEP e 1.5 Example BEP = 0.350/0.400 mm e = = mm = mm RIC = 0.091/0.104 mm Fig. 1 Radial internal clearance calculation In most rolling mill applications, the radial load always acts on the bearing cups along the same direction and, therefore, only a small part of the cups is actually loaded. For this reason, the bearing may face premature failure. To avoid this issue, and consequently to fully exploit bearing potential and extend its life, and according to the 5

6 maintenance program, it is necessary to turn the bearing cups so that the working area of the cups will be regularly changed (for more information, see chapter 4). In order to ease this operation, the side faces of the cups are divided into four zones marked with special references (Fig. 2) made at intervals of 90. Fig. 2 Special reference that marks a loading zone of an RKB TQOS bearing cup The RKB Group produces various types of multi-row tapered roller bearings composed of several parts assembled together in a well-defined sequence that ensures the correct functioning of its products (Fig. 3). Fig. 3 RKB four-row tapered roller bearing types The RKB TQO configuration (Fig. 4 a) is a four-row taper roller bearing composed of two double-row cones, one double-row cup, two single-row cups, four pressed steel cages, four rows of rollers, one cone spacer (also available with lubrication grooves) and two cup spacers (with annular groove and lubrication holes). The paired roller rows are in face-to-face configuration. The width of cup and cone spacers is factory adjusted which means that each spacer of each bearing is face ground after accurate measurement of the distance between adjacent cups and cones in order to obtain the appropriate initial BEP. So the bearing parts cannot be interchanged and are individually marked for proper assembly. Lubrication grooves and oil holes are provided in the cup spacers and in the double cup. Furthermore, lubricant slots in the cone side faces and cone spacer let the lubricant go through the bearing to the roll neck. The RKB TQO/G configuration (Fig. 4 b) is a four-row taper roller bearing composed of two double-row cones (with helical groove inside the bore), one double-row cup, two single-row cups, four pressed steel cages, four rows of rollers, one cone spacer (also available with lubrication grooves) and two cup spacers (with annular groove and lubrication holes). The paired roller rows are in face-to-face configuration. This bearing is always mounted on the shaft with a loose fit and the helical grooves provided inside the bearing bore improve the lubrication of the roll neck, collect the abraded particles and thus the wear between mating parts is considerably reduced. The RKB TQO/EG configuration (Fig. 4 c) is a four-row taper roller bearing composed of two double-row cones (with helical groove inside the bore), two central single-row cups, two single-row cups, four pressed steel cages and four rows of rollers. This type of configuration differs from the other types of TQO by the absence of spacers. The BEP is obtained by adjusting the width of the cones and of the cups, respectively. The paired roller rows are in face-to-face configuration. The RKB TQO pierced rollers configuration (Fig. 4 d) is a four-row taper roller bearing composed of two double-row cones (with helical groove inside the bore), one double-row cup, two single-row cups, four rows of pierced rollers, 6

7 one cone spacer and two cup spacers (with annular groove and lubrication holes). The rollers are enclosed between two steel rings and guided with pins passing through the center of the roller. The pins are threaded in one side and welded on the other. This configuration is particularly suitable for large size bearings and permits to increase the number of rollers compared to the configuration with pressed steel cages, thus improving the values of basic dynamic and static load ratings (Cr and C0r). The paired roller rows are in face-to-face configuration. a) b) c) d) e) f) g) h) i) Fig. 4 RKB four-row tapered roller bearings: a) TQO; b) TQO/G; c) TQO/EG; d) TQO with pierced rollers and pin-type cage; e) 2xTDI set; f) TQOS/AVS1; g) TQOS/AVS2(/1); h) TQI; i) TQIT The RKB Set 2xTDI configuration (Fig. 4 e) consists of two paired double-row tapered roller bearings (TDI), separated by two different spacers, one between the two internal cups and one between the two cones. To easily mount the bearing on the roll neck, RKB designs and produces special cone spacers with a tapered profile that assures their self-centering on the cone shoulder (Fig. 5). 7

8 Fig. 5 Self centering cone spacer on cone shoulder (set 2xTDI) The RKB TQOS/AVS1 configuration (Fig. 4 f) consists of a four-row taper roller bearing composed of two double-row cones (with helical grooves inside the bore and sealed with an AVS Anti-Vortex System cone seal set), one double-row cup, two one-side extended cups, four pressed steel cages, four rows of rollers, two cup spacers (with annular groove and lubrication holes), two lateral seals, and two O-type seal rings. The paired roller rows are in faceto-face configuration. The RKB TQOS/AVS2 and TQOS/AVS2/1 configurations (Fig. 4 g) consist of a four-row taper roller bearing composed of two double-row cones (with helical groove inside the bore and separated by an AVS cone spacer seal set), one double-row cup, two single-row cups, four pressed steel cages, four rows of rollers, two cup spacers (with annular groove and lubrication holes available only in the TQOS/AVS2 design), two lateral seals, two separable flanges and two O-type seal rings. The paired roller rows are in face-to-face configuration. The use of sealed bearings simplifies mill stand design, reduces grease consumption and routine maintenance requirements, and consequently bearings attain longer service life. The seals, fitted on both sides of the bearing set, are produced in several designs depending on bearing size and application needs. The most common ones are the integrated lateral unitized seals (European version), the integrated lateral narrow seals (compact seal concept) and the loose flange lateral unitized seals (Japanese version). So the basic design with the garter seal, made of reinforced fluoroelastomer (FKM) and steel spring, is located in the integral external cups (TQOS/AVS1). Alternatively, the separate seal carrier flanges are matched to the cups and a chock type seal runs on extended surface of inner rings (TQOS/AVS2 and TQOS/AVS2/1). Furthermore, RKB seals have been redesigned in new type to increase seal durability and efficiency: while maintaining an overall narrow profile, these seals optimize the available space by utilizing the area directly adjacent to or underneath the cage bore, with the result of the usage of longer rollers with consequent increased bearing load capacity similar to open type version. RKB sealed four-row taper roller bearings are usually provided also with O-rings in the outboard cup outside diameter to seal contaminants from the bearing outer diameter and/or with a cone seal set designed to accommodate relative motion between inner rings and to prevent build-up of negative pressure (AVS technology for longer bearing life and less lubricant deterioration). The RKB TQI configuration (Fig. 4 h) consists of a four-row taper roller bearing composed of one double-row cone, two single-row cones, two cone spacers, four pressed steel cages, four rows of rollers, two double-row cups and two loose flange rings. The paired roller rows are in back-to-back configuration. This arrangement is recommended for all those applications where high stiffness and resistance to overturning moments are required. The RKB TQIT configuration (Fig. 4 i) consists of a tapered bore four-row taper roller bearing composed of one double-row cone, two single-row cones, four pressed steel cages, four rows of rollers, two double-row cups (available with annular groove and lubrication holes) and one cup spacer (available with annular groove and lubrication holes). The paired roller rows are in back-to-back configuration. The main difference between TQIT and TQI lies in the bore shape, which is tapered in TQIT and cylindrical in TQI. Finally, note that the mounting sequence needs to be carefully and exactly followed especially in the case of TQO and TQOS bearings where the BEP is adjusted by adjusting the width of the spacers. 1.2 Multi-Row Cylindrical Roller Bearings Within a given axial and especially small radial mounting space multi-row cylindrical roller bearings offer the largest load carrying capacity. This is the main reason for which this bearing type is suitable and widely used for the highest radial loads and for a wide range of speeds. Thus it is no surprise that these bearings are the ideal products for rolling mills at the back-up roll position where radial loads can be extremely high, especially as they can tolerate moderate to high mill speeds. Different types of multi-row cylindrical roller bearings are used to support the roll necks of back-up rolls but the specific type of product naturally depends on the rolling stand design. When choosing a certain bearing, the stand designer as well as the user should consider that the use of multi-row cylindrical roller bearings in metal rolling mills exposes them to very heavy stresses, and sometimes to impact loads. The multi-row cylindrical roller bearings can accommodate only radial loads. So, if these bearings are used, the axial loads (if exist) have to be accommodated by a separate bearing. Therefore, they are usually mounted together with deep groove or angular contact ball bearings or with tapered roller bearings which take up the axial 8

9 loads. RKB four-row cylindrical bearings are designed with well-balanced cross-sections to provide the highest possible radial load carrying capacity within the bearing envelope. The roller and raceway profiles are designed and manufactured to allow optimized stress distribution while minimizing the effect of roller-edge loading. Moreover, the rollers and outer raceway sections are carefully matched to ensure equal load distribution from row to row. a) b) c) d) e) f) Fig. 6 Main RKB four-row cylindrical roller bearings: a) A2D; b) C2C; c) D2C; d) F2C/EVO; e) G2B; f) Q2AC/EVO Table 1 Main RKB multi-row cylindrical roller bearings RKB design Rings Rollers and cages a No. of inner rings No. of outer rings No. of loose flange rings No. of intermediate rings No. of wider inner rings with concentric shoulder No. of double pronged cages No. of window type cages Split rollers Pierced rollers (applicable) A2D (Fig. 6 a) C2C (Fig. 6 b) D2C (Fig. 6 c) F2C/EVO (Fig. 6 d) Yes G2B (Fig. 6 e) Yes b Q2AC/EVO (Fig. 6 f) Yes a machined brass cages b alternate long/short rollers The multi-row cylindrical roller bearings are generally mounted with an interference fit on the roll neck. However, when these bearings need to be mounted with a loose fit on the roll neck, the RKB Group usually supplies them with C2 radial internal clearance and a helical groove inside the bore and/or lubrication grooves in the side faces of the bearing rings for an efficient lubrication of the mating parts (e.g. suffix AC). The four-row cylindrical roller bearings are 9

10 of separable design, which considerably simplifies mounting and dismounting, inspection, and maintenance. Note that either all bearing components can be mounted separately, or the rings (with integral flanges) and the rollercage assemblies can be mounted independently of the inner rings. The RKB Bearing Industries Group typically provides the inner rings with additional grind stock. To achieve a particularly good running accuracy, this allows the mill operator to optimize the roll precision by grinding the inner ring raceway after its mounting on the roll neck. Inner rings can also be provided in a finished state without the need for an additional grinding after mounting on the roll neck. Note that both inner rings (designation L) and outer ring assemblies (designation R) are interchangeable with similar parts of other bearings, this being helpful for quick roll replacement. Moreover, RKB multi-row cylindrical roller bearings allow direct interchange of bearing parts with main premium competitor products, but we recommend doing this only in case of emergency. To match the applications in the proper way, the RKB Group produces multi-row cylindrical roller bearings in several designs (Table 1) within given geometric series. The various designs differ basically in the number of inner and outer rings, in the number of loose or integral flanges on the outer ring, in the cage type, in the number of rollers in the cage pocket, etc. Large multi-row cylindrical roller bearings can be provided with through-bored rollers and pin-type cages. This cage design is necessary for reversing stands because of the great acceleration and deceleration forces. Fig. 7 RKB full complement NNUV bearing with six rows of rollers (available also with four or eight rows of rollers) It is well known that a high radial load carrying capacity is reachable if bearing incorporates a large number of rollers but this fact is limited by the presence of the cage. A solution could be the full complement bearings which are cageless bearings. RKB full complement multi-row cylindrical roller bearings with four, six (Fig. 7) or eight rows of rollers have low cross section, high radial load carrying capacity and remarkable stiffness. The low cross section allows for relatively large roll neck diameters in comparison with the roll diameter. Note that this bearing type is not suitable for high mill speeds. The RKB Bearing Industries Group has a flexible manufacturing program of different types (also special) and sizes of multi-row cylindrical roller bearings. They have cylindrical bore and some sizes are also available with tapered bore as required by the application or by maintenance request. Furthermore, RKB can produce four-row cylindrical roller bearings sealed at both sides by radial rubbing seals or with wider inner ring with concentric shoulder. As in the case of multi-row tapered roller bearings, in order to avoid the loading of the same zones of the outer rings, and consequently to fully exploit the bearing potential and extend its life, and according to the maintenance program, it is necessary to turn the outer rings so that the working area of the rings will be regularly changed (for more information, see chapter 4). 2 MOUNTING OF MULTI-ROW ROLLER BEARINGS The performance, load carrying capacity and service life of a bearing depend not only on its quality but also on correct mounting. Therefore, the mounting should be done only by experienced personnel. The RKB Technical Team is at customer s disposal for initial mounting, briefing the fitters and for any other eventuality. Cleanliness during 10

11 mounting is a prerequisite of major importance for the correct performance of the bearings and to help them not fail prematurely. 2.1 Preliminary Operations Chock Preparation Clean the bore and remove scratches, burrs or sharp edges with fine sandpaper. Blow the lubrication and drainage holes and ventilation ducts with compressed air and clean them with solvent. Remove any remaining metallic particles from all holes with a magnetic rod. Check bore form and dimensional accuracy in 4 parallel planes (perpendicular to the bore axis); in each plane, bore diameter should be measured in 4 positions, offset by 45 each (Fig. 8); Coat the bore internal surface with a thin layer of oil or grease. This light lubricant layer will help ease the assembly and will reduce fretting or corrosion between cups and chock bore in service. Fig. 8 Checking of the chock bore dimensional accuracy Shaft Preparation Remove scratches, burrs, or sharp edges with fine grit sandpaper. Check the journal form and dimensional accuracy in 3 parallel planes (perpendicular to the shaft axis); in each plane, bore diameter must be measured in 4 positions, offset by 45 each (Fig. 9). Coat the journal external surface with a thin layer of oil or grease. Fig. 9 Checking of the shaft dimensional accuracy 2.2 Mounting of Four-Row Tapered Roller Bearings As mentioned before, the four-row tapered roller bearings are separable, like the cylindrical roller bearings, but, unlike them, they cannot be mounted by following the same mounting sequence (fit the inner rings onto the roll 11

12 neck, fit the outer rings into the chock and push the chock onto the roll neck). In consequence, they follow their own particular mounting procedure. Four-row tapered roller bearings are precision mechanical components and should therefore be handled with appropriate care when mounted and dismounted. It is important to use the proper tools and to follow the instructions provided to avoid bearing damage. When mounting four-row tapered roller bearings, the individual components of the bearing must be mounted in the correct sequence. Parts belonging together are identified by letter markings. Besides, extreme care has to be taken so that the parts of one bearing are not mixed with those of another one when several bearings are mounted at the same time Bearing Parts Preparation The bearing should not be removed from its original packing until the chocks and rolls and all accessories are ready for mounting. The anti-corrosion oil does not have to be washed out as it does not react with any of the commonly used rolling bearing oils and greases. If oil is used as the lubricant, coat all bearing components with a thin layer of this oil. If grease is used as a lubricant, prepare in advance the necessary amount of grease (see paragraph 3.1) for lubricating the entire bearing (Fig. 10 a), coat cap raceways (Fig. 10 b) and seal lips, if need be (Fig. 10 c), and fill in each double-row cone with half quantity of the remaining grease (Fig. 10 d). a) b) c) d) Fig. 10 Greasing an RKB TQO bearing Fig. 11 Greasing the inner ring extensions and the annular grove for seals of an RKB TQOS bearing 12

13 If the bearing is sealed, coat the cone extensions for the seals with grease and fill in the annular grooves for the seals (located in the middle of the bearing) with grease (Fig. 11). Seals of the RKB sealed bearings are commonly made of fluoroelastomer (FKM). This is a special purpose fluorocarbon-based synthetic rubber characterized by very good wear and thermo-chemical resistance, which lead to superior performance, especially in high temperature applications in different environments. Seals made of FKM have exceptional properties even under harsh environmental conditions and can withstand operating temperatures up to 200 C (390 F). Although this compound is very stable and harmless up to these temperatures, if exposed to temperatures above 300 C (570 F), it gives off hazardous and dangerous fumes which can become extremely harmful if inhaled, come into contact with eyes or, in isolated cases, with human skin. This can occur, for example, if fire or a welding torch are used when dismounting the bearing. Moreover, even if the seals have been cooled down, they still represent a danger and under no circumstances should be touched with bare hands. When dismounting a bearing that has been extremely heated, follow the safety rules depicted in Fig. 12. WARNING! Safety rules for handling bearings with seals made of fluoroelastomer (FKM) Use protective goggles, gloves and suitable industrial breathing sets. Put the seal residual fragments remained after dismounting in a safe plastic or glass container accordingly marked to indicate the dangerous material. Immediately consult a doctor in the following cases: o o o when fumes are accidentally inhaled; when fumes come in contact with eyes. First flush the eyes with water! when seal debris touches the skin. First wash the skin with water and soap! RKB takes no responsibility for any injury caused by the improper handling or use of seals made of fluoroelastomer. Fig. 12 Safety rules for handling bearings with seals made of fluoroelastomer (FKM) a) b) Fig. 13 Mounting sequence of an RKB TQO bearing 13

14 The four-row tapered roller bearing of the TQO type (or 2xTDI set type) is still the preferred solution throughout the metals industry, due to its superior radial and axial load carrying capacity, mounting simplicity, and easy load zone control. For this reason, in the following, the mounting procedures for the TQO/TQOS bearing types are presented in detail. For the bearing correct mounting, all of its parts have to be assembled and mounted following the letters (from E to A) marked on cups, cones and spacers (Fig. 13 a). The general mounting sequence of a TQO bearing is presented in Fig. 13 b and the complete mounting procedures described below depend on bearing size (large or small and medium size bearings) and design (bearings with pressed cage, bearings with pin-type cage, sealed bearings). Note that in the case of rolls mounted on such bearings, one chock is located in the housing with keeper plates (locating bearing), while the opposite one can float (non-locating bearing) Mounting of Unsealed TQO Bearing with Pressed Steel Cage, Variant I (large size bearings) STEP 1 (Fig. 14 a) Fasten the roll-side cover onto the chock (if need be, lay the O-ring in position). In case, mount the labyrinth on the chock cover. If it is more convenient, mount it just before Step 9. Place the chock on level supports with bore axis in vertical position. Lift the single-row cup DE with a special lifting tool and lower it with side E down into the chock. If the bearing is mounted for the first time, place the cup marked zone in the load direction and mark the position on the top of the chock. In all other cases, take into account the bearing recommended rotation inside the chock. Make sure that adjacent parts abut each other completely and check the seating of the cup against shoulder with a feeler gauge. STEP 2 (Fig. 14 b) STEP 3 (Fig. 14 c) STEP 4 (Fig. 14 d) STEP 5 (Fig. 14 e) STEP 6 (Fig. 14 f) STEP 7 (Fig. 14 g) STEP 8 (Fig. 14 h) STEP 9 (Fig. 14 i) Mount the cup spacer DD (provided with lubrication holes) over the cup DE. Take care not to nick the bore surface or raise burrs on the spacer. Lower the double-row cone CE with face E down. While cone handling, pay attention that it is not placed on (or hung from) the cage but only on (or from) the inner ring, otherwise cage damage could be irreversible. Gently rotate the cone to allow roller settlement. Add cone spacer CC (provided with equally spaced lubrication slots to allow lubricant to reach shaft surface). Place the double-row cup BD with face D down. Mount the cup spacer BB (provided with lubrication holes) on the cup BD. Take care not to nick the bore surface or raise burrs on the spacer. Lower the double-row cone AC with face C down. While cone handling, pay attention that it is not placed on (or hung from) the cage but only on (or from) the inner ring, otherwise cage damage could be irreversible. Gently rotate the cone to allow roller settlement. Add the single-row cup AB to the stack with face B down. Make sure that all cups and cup spacers are placed so that the load zone markings are aligned in a row. Mount the chock cover without gasket. Tighten evenly four equally spaced cap screws, until the cover is uniformly seated against the side face of the chock and cups and cup spacers abut each other. 14

15 a) b) c) d) e) f) g) h) i) j) Fig. 14 Mounting sequence of an unsealed RKB TQO bearing with pressed steel cage. Variant I (large size bearings) 15

16 It is highly recommended to turn the chock into a horizontal position and, using a clamping device similar to that used in sealed bearing mounting (Fig. 19 a), gradually and evenly tighten the cones and cone spacer. Use a feeler gauge to check that these parts abut each other. The clamping fixture must touch only the cone faces (not the cages that could be easily damaged in this way). Measure the width G of the gap and determine the final gasket thickness S (Fig. 15). Remove the cover again. Insert the gasket of required thickness. Replace the cover and firmly and crosswise tighten the screws. If a compressible gasket is used check the gap and tighten or untighten all cap screws until the initial value G of gap width is reached. Remove the clamping tool. Metallic shims: S = G G0 G0 = mm Elastic cork or other compressible material: S = ksg G ksg = Caution: due to overall cup width variation, the gasket thickness defined for a certain bearing assembly should not be used for another bearing assembly (even if it is of the same type and dimensions)! Fig. 15 Final gasket thickness (all dimensions in mm) STEP 10 (Fig. 14 j) STEP 11 (Fig. 16) Push the labyrinth ring (heated in an oil bath to C) onto the roll neck. While the labyrinth ring cools down, it must be pressed tightly against the roll body face. Align the chock (with the assembled bearing) with the shaft and push it onto the roll neck with side E forward until side E of the double-row cone abuts the labyrinth ring. Mount seal, spacer (A), nut (B), and split clamping ring (C) into position Tighten the nut until it abuts the clamping ring and the spacer abuts the bearing cone. Loosen the nut in order to leave an axial clearance of about mm to enable free cone rotation and prevent wear to the cone faces in case they creep on the roll neck. This axial clearance also allows minor float to take place through the loose cone fit. Fig. 16 Axial clearance of RKB TQO bearing cones on roll neck 16

17 2.2.3 Mounting of Unsealed TQO Bearing with Pressed Steel Cage, Variant II (small and medium size bearings) STEP 1 (Fig. 17 a) STEP 2 (Fig. 17 b) Fasten the roll-side cover onto the chock (if need be, lay the O-ring in position). In case, mount the labyrinth on the chock cover. If it is more convenient, it is also possible to mount it just before Step 3. Place the chock on level supports with bore axis in vertical position. Put on wooden supports the single-row cup DE with side E down and place on top of it the cup spacer DD (provided with lubrication holes) so that the load zone markings are aligned in a row. Lift the cup cup spacer assembly with the lifting tool and lower it with side E down into the chock. If the bearing is mounted for the first time, place the cup marked zone in the load direction and mark the position on the top of the chock. In all other cases, take into account the recommended bearing rotation inside the chock. Make sure that adjacent parts abut each other completely and check the seating of the cup against shoulder with a feeler gauge. Place on wooden supports the double-row cone CE with face E down. While cone handling, pay attention that it is not placed on (or hung from) the cage but only on (or from) the inner ring. Complete the whole bearing set by putting the other parts on top in the following order: cone spacer CC (provided with equally spaced lubrication slots to allow lubricant to reach the shaft surface); double-row cup BD with face D down; cup spacer BB (provided with lubrication holes); double-row cone AC with face C down. Gently rotate the cone to allow roller settlement; single-row cup AB with face B down. Make sure that all cups and cup spacers are placed so that the load zone markings are aligned in a row. Lift the stack with the lifting tool and lower it into the chock. Take care not to nick the bore surface or raise burrs on spacers or cups. Make sure that all the load zone markings are aligned in a row. Check with a feeler gauge if adjacent parts abut each other completely. STEP 3 (Fig. 14 i) a) b) Fig. 17 Mounting sequence of an unsealed RKB TQO bearing with pressed steel cage. Variant II (small and medium size bearings) Mount the chock cover without gasket. Tighten evenly four equally spaced cap screws, until the cover is uniformly seated against the side face of the chock and cups and cup spacers abut each other. It is highly recommended to turn the chock into a horizontal position and, using a clamping device similar to that used in sealed bearing mounting (Fig. 19 a), gradually and evenly tighten the cones and cone spacer. Use a feeler gauge to check that these parts abut each other. The clamping fixture must touch only the cone faces (not the cages that could be easily damaged in this way). Measure the width G of the gap and determine the final gasket thickness S (Fig. 15). Remove the cover again. 17

18 STEP 4 (Fig. 14 j) STEP 5 (Fig. 16) Insert the gasket of required thickness. Replace the cover and firmly and crosswise tighten the screws. If a compressible gasket is used check the gap and tighten or untighten all cap screws until the initial value G of gap width is reached. Remove the clamping tool. Push the labyrinth ring (heated in an oil bath to C) onto the roll neck. While the labyrinth ring cools down, it must be pressed tightly against the roll body face. Align the chock (with the assembled bearing) with the shaft and push it onto the roll neck with side E forward until side E of the double-row cone abuts the labyrinth ring. Mount seal, spacer (A), nut (B), and split clamping ring (C) into position. Tighten the nut until it abuts the clamping ring and the spacer abuts the bearing cone. Loosen the nut in order to leave an axial clearance of about mm to enable free cone rotation and prevent wear to the cone faces in case they creep on the roll neck. This axial clearance also allows minor float to take place through the loose cone fit. Mounting of Unsealed TQO Bearing with Pin-Type Cage STEP 1 (Fig. 18 a) Fasten the roll-side cover onto the chock (if need be, lay the O-ring in position). In case, mount the labyrinth on the chock cover. If it is more convenient, it is also possible to mount it just before Step 4. Place the chock on level supports with bore axis in vertical position. Put on wooden supports the single-row cup DE with side E down and place on top of it the cup spacer DD (provided with lubrication holes) so that the load zone markings are aligned in a row. Lift the cup cup spacer assembly with the lifting tool and lower it with side E down into the chock. If the bearing is mounted for the first time, place the cup marked zone in the load direction and mark the position on the top of the chock. In all other cases, take into account the recommended bearing rotation inside the chock. Make sure that adjacent parts abut each other completely and check the seating of the cup against shoulder with a feeler gauge. STEP 2 (Fig. 18 b) a) b) c) Fig. 18 Mounting sequence of an unsealed RKB TQO bearing with pin-type cage (with pierced rollers) Place on wooden supports the double-row cone CE with face E down. While cone handling, pay attention that it is not placed on (or hung from) the cage but only on (or from) the inner ring. Complete the stack by putting the following parts on top in the correct order: cone spacer CC (provided with equally spaced lubrication slots to allow lubricant to reach the shaft surface); double-row cup BD with face D down; 18

19 STEP 3 (Fig. 18 c) STEP 4 (Fig. 14 i) cup spacer BB (provided with lubrication holes). Make sure that all the load zone markings are aligned in a row. Check with feeler gauge if adjacent parts abut each other completely. Fasten the eye bolts into the provided holes. Using appropriate cables lift the assembly and lower it with side E down into the chock. Take care not to nick the bore surface or raise burrs on spacers or cup. Place on wooden supports the double-row cone AC with face C down. While cone handling, pay attention that it is not placed on (or hung from) the cage but on (or from) the inner ring. Complete the stack by putting the single-row cup AB (with face B down) on top of it. Fasten the eye bolts into the provided holes. Using appropriate cables lift the assembly and lower it with side C down into the chock. Check with a feeler gauge if adjacent parts abut each other completely. Make sure that all cups and cup spacers are placed so that the load zone markings are aligned in a row. Take care not to nick the bore surface or raise burrs on cup. Mount the chock cover without gasket. Tighten evenly four equally spaced cap screws, until the cover is uniformly seated against the side face of the chock and cups and cup spacers abut each other. It is highly recommended to turn the chock into a horizontal position and, using a clamping device similar to that used in sealed bearing mounting (Fig. 19 a), gradually and evenly tighten the cones and cone spacer. Use a feeler gauge to check that these parts abut each other. The clamping fixture must touch only the cone faces (not the cages that could be easily damaged in this way). Measure the width G of the gap and determine the final gasket thickness S (Fig. 15). Remove the cover again. Insert the gasket of required thickness. Replace the cover and firmly and crosswise tighten the screws. If a compressible gasket is used check the gap and tighten or untighten all cap screws until the initial value of gap width G is reached. Remove the clamping tool. STEP 5 (Fig. 14 j) STEP 6 (Fig. 16) Push the labyrinth ring (heated in an oil bath to C) onto the roll neck. While the labyrinth ring cools down, it must be pressed tightly against the roll body face. Align the chock (with the assembled bearing) with the shaft and push it onto the roll neck with side E forward until side E of the double-row cone abuts the labyrinth ring. Mount seal, spacer (A), nut (B), and split clamping ring (C) into position. Tighten the nut until it abuts the clamping ring and the spacer abuts the bearing cone. Loosen the nut in order to leave an axial clearance of about mm to enable free cone rotation and prevent wear to the cone faces in case they creep on the roll neck. This axial clearance also allows minor float to take place through the loose cone fit. Mounting of Sealed TQO Bearing STEP 1 (Fig. 19 a, Fig. 20 a) Fasten the roll-side cover onto the chock (if need be, lay the O-ring in position). Place the chock on level supports with bore axis in vertical position. On the special clamping-lifting device complete the whole bearing set by putting the parts in the following correct order: if the seal is mounted inside the single-row cup DE (e.g. TQOS/AVS1) start with it and place it on the extended arms of the clamping-lifting device. If the seal is mounted inside a seal carrier (e.g. TQOS/AVS2 or TQOS/AVS2/1) place it first with the seal concavity upward on the extended device arms and then put into position the single-row cup DE with side E down; cup spacer DD (provided with lubrication holes); double-row cone CE with face E down. While cone handling, pay attention that it is not placed on (or hung from) the cage but on (or from) the inner ring; 19

20 cone spacer CC (provided with equally spaced lubrication slots to allow lubricant to reach the shaft surface); double-row cup BD with face D down; cup spacer BB (provided with lubrication holes); double-row cone AC with face C down. Gently rotate the cone to allow roller settlement. If the seal is mounted inside the single-row cup AB (e.g. TQOS/AVS1) place the cup with the face B down on the stack. If the seal is mounted inside a seal carrier (e.g. TQOS/AVS2 or TQOS/AVS2/1), place the single-row cup AB with the face B down and add the seal carrier on top of the stack with the seal concavity downward. Make sure that all parts are placed so that the load zone markings are aligned in a row. Retract the upper arms of the clamping device so that they will abut the face A of the double-row cone AC. Be careful not to touch the seal and fasten the whole stack. Check with a feeler gauge if adjacent parts abut each other (especially the cones and cone spacer). STEP 2 (Fig. 20 b) a) b) Fig. 19 Mounting sequence of a sealed TQO bearing Lift the assembly with the clamping device and lower it with side E down into the chock. Take care not to nick the bore surface or raise burrs on spacers or cups. If the bearing is mounted for the first time, place the cup marked zone in the load direction and mark the position on the top of the chock. In all other cases, take into account the recommended bearing rotation inside the chock. Check with a feeler gauge the seating of the double-row cup DE or seal carrier against chock shoulder. Mount the chock cover without gasket. Tighten evenly four equally spaced cap screws, until the cover is uniformly seated against the side face of the chock and cups and cup spacers abut each other. Tilt the chock into a horizontal position and release the clamping device enough to retract its lower arms so that they will clamp the face E of the double-row cone CE, and gradually and evenly tight again the stack. Be careful not to touch the seal. Use a feeler gauge to check that cones and con spacer abut each other. Measure the width G of the gap and determine the final gasket thickness S (Fig. 15). Remove the cover again. Insert the gasket of required thickness. Replace the cover and firmly and crosswise tighten the screws. If a compressible gasket is used check the gap and tighten or untighten all cap screws until the initial value G of gap width is reached. Remove the clamping tool. STEP 3 (Fig. 19 b) Push the labyrinth ring (heated in an oil bath to C) onto the roll neck. While the ring cools down, it must be pressed tightly against the roll body face. Align the chock (with the assembled bearing) with the shaft and push it onto the roll neck with side E forward until side E of the double-row cone abuts the labyrinth ring. Mount seal, spacer (A), nut (B), and split clamping ring (C) into position. 20

21 a) b) c) d) Fig. 20 Mounting of a sealed RKB TQO bearing STEP 4 (Fig. 16) Tighten the nut until it abuts the clamping ring and the spacer abuts the bearing cone. Loosen the nut in order to leave an axial clearance of about mm to enable free cone rotation and prevent wear to the cone faces in case they creep on the roll neck. This axial clearance also allows minor float to take place through the loose cone fit. Caution: in the case of rolls mounted on TQI bearings, the cups are clamped (zero axial clearance Fig. 21) on the operating side (locating bearing). On the drive side (non-locating bearing), the cups are permitted to float axially in the chock bore due to clearances between the cup and the cover faces (Fig. 22). The drive side not only floats through the cups of the bearing, but also through the chocks in the mill frame window. This arrangement permits free expansion and contraction of the roll caused by variations in roll temperatures. Fig. 21 Zero axial clearance of the RKB TQI bearing cups inside the chock bore of the roll operating side (locating bearing) 21

22 2.2.6 Fig. 22 Axial clearance of the RKB TQI bearing cups inside the chock bore of the roll drive side (non-locating bearing) Dismounting of Four-Row Tapered Roller Bearings If the bearing has to be dismounted for inspection and maintenance, dismounting will be performed in the reverse order of mounting. It is important to use appropriate tools and follow the above instructions and recommendations to avoid bearing damage. 2.3 Mounting of Four-Row Cylindrical Roller Bearings Four-row cylindrical roller bearings differ by their design (Table 1 and Fig. 6). Customers can order either the complete bearing or the single components (outer ring assembly or inner ring).rkb four-row cylindrical roller bearings are precision mechanical components and should therefore be handled with appropriate care when mounting and dismounting. In particular, the individual components of the bearing must be mounted in the correct sequence, by means of appropriate tools and according to the instructions provided to avoid bearing damage Bearing Parts Preparation The bearing should not be removed from its original packing until the chocks and rolls and all accessories are ready for mounting. The anti-corrosion oil does not have to be washed out as it does not react with any of the commonly used rolling bearing oils and greases. If oil is used as the lubricant, coat all bearing components with a thin layer of this oil. If grease is used as the lubricant, prepare in advance the necessary amount of grease for lubricating the entire bearing and fill each double-row outer ring with half quantity of the grease STEP 1 Mounting of the Bearing Inner Ring onto the Roll Neck STEP 2 (Fig. 23) STEP 3 Push the labyrinth ring or backing ring (heated in an oil bath to C, depending on the interference) onto the roll neck. While the labyrinth ring cools down, it must be pressed tightly against the roll body face so that it abuts the roll body without a gap. Heat each inner ring with cylindrical bore (which is fitted with interference on the roll neck) to C in an oil bath (Fig. 23 a) or using an induction coil (Fig. 23 b). When the inner ring is heated in an oil bath, use a thermostat to avoid excessively high heating temperature. After the rings are taken out of the oil bath, wipe off the oil in the bores and on the faces of the rings. If the roll neck end is not shaped to guide the rings (stepped sections), we highly recommend using a mounting sleeve (Fig. 24). 22

23 a) b) Fig. 23 Heating of an RKB bearing inner ring by means of: a) oil bath; b) induction coil Fig. 24 Mounting sleeve (to ease the mounting of inner rings) If inner rings are heated up in an oil bath, they can be pushed on the roll neck either manually (small or medium size bearings) or using mounting grippers (large bearings, Fig. 23 a). The grippers must always carry the ring with its axis in a horizontal position. If inner rings are heated up using an induction coil, push manually or using an appropriate lifting device the coil together with the inner bearing rings on the roll neck (Fig. 25). Fig. 25 Mounting of an RKB bearing inner ring on the roll neck using an induction coil Caution: after mounting using an induction coil the bearing rings and roll necks always remain magnetized. Consequently, they must be demagnetized after mounting. This can be done also by means of the induction coil 23

24 itself. The induction coil is pulled over the mounted part with the current switched on and then slowly removed to a distance of 1 2 meters from the parts. After cooling down, the rolling bearing inner rings should abut the labyrinth ring without a gap. Smaller inner rings can be mounted without a gap between them by pushing a mounting sleeve against the ring face while the rings cool down. RKB recommends that after mounting onto the roll neck the inner ring raceway diameter (also known as the F dimension) should be measured (Fig. 26) in the same condition as shaft diameter measurement (Fig. 9). Fig. 26 Measuring the diameter (F dimension) of the inner ring raceway of an RKB four-row cylindrical roller bearing a) b) c) d) Fig. 27 Mounting of an RKB bearing outer ring into the chock 24

25 Mounting of the Bearing Outer Rings into the Chock Since the outer rings of cylindrical roller bearings are loose fitted in the chocks, manually insert the smaller outer rings into the chock. In case of large size bearings, use the threaded holes for eye bolts, which are usually provided, to insert them into the chock more easily by means of cables (Fig. 27 a) or special lifting and clamping device (Fig. 27 b). In case of very large bearing outer rings mount them with their axis in a horizontal position. Therefore the rings can be inserted into the chock on a beam suspended in ropes or cables (Fig. 27 c, d). If the bearing is mounted for the first time, place the cup marked zone in the load direction and mark the position on the top of the chock. In all other cases, take into account the recommended bearing rotation inside the chock. The desired load zones of the outer rings should be carefully positioned in the same direction. Make sure that adjacent parts abut each other completely and check the seating of the outer ring against shoulder or loose flange with a feeler gauge. Mount, if need be, the thrust bearing. Mounting of the Chock onto the Roll Neck Check if the labyrinth ring and the inner rings have been fitted onto the roll neck. Carry in horizontal position the pre-assembled chock (complete with outer rings and the thrust bearing) to the roll neck by means of a crane. Align the chock as closely as possible with the roll neck so that the chock can be easily pushed onto the roll neck. Push the pre-assembled chock onto the roll neck (Fig. 28). Fig. 28 Mounting of the chock onto the roll neck If bearing is of double pronged cage type pay particular attention to the rollers: they do not have to hit the inner ring (previously mounted on the roll neck) and tilt (Fig. 29 a). If this happens, the bearing installation may be compromised. The pre-assembled chock installation is much easier and can be automatized, thus considerably reducing the time needed to replace the rolls or/and the bearings, when bearings with the innovative RKB window-type cage are used (Fig. 29 b). Moreover, this special RKB GX type configuration alternating long and short rollers allows to get a better load distribution and to reduce the edge contact pressure (typical of standard designs). Another important feature is the higher angle and length of the chamfer on the inner ring raceway, specific for each type of geometry. When pushing the pre-assembled chock onto the neck roll, pay attention not to produce score marks on the rollers and inner rings. Mount the axial clamping and sealing system (the parts holding the thrust bearing and the four-row cylindrical bearing inner rings on the roll) onto the shaft and make sure that it is properly secured after its mounting and adjustment. 25

26 a) b) Fig. 29 Window-type cage innovation: a) mounting difficulties of a standard four-row cylindrical roller bearing; b) RKB innovative solution: window-type cage (RKB GX design) Dismounting of Four-Row Cylindrical Roller Bearings Dismounting of four-row cylindrical roller bearings from the roll neck encompasses two separate steps: withdrawal of the chock from the inner rings and extraction of the inner rings from the roll neck. After removing the axial clamping and sealing system (the parts holding the thrust bearing and the four-row cylindrical bearing inner rings on the roll) from the roll neck, the chock can also be withdrawn from the roll neck as a complete unit. When only the rolls are replaced, the chock is mounted onto the new roll neck only after the inner rings have been installed on it. Dismounting of the different bearing components for inspection is effected in the reverse order of mounting, using the same equipment. Withdrawal of the inner rings mounted with an interference fit from the roll neck requires special equipment and RKB recommends using the same induction coil used during mounting. To release the interference fit of cylindrical roller bearing inner rings, they must be quickly heated to C. While heating, the roll neck should rise its temperature as little as possible, because the goal is only to obtain sufficient clearance between the inner ring and the roll neck to easily extract the inner ring. In some cases, especially with large bearings and only if roll design permits, the inner rings could be extracted hydraulically. But in this case often the fitting surfaces are damaged by fretting corrosion or even worse are cold welded, which may lead to great difficulties in the extraction. In some exceptional cases and with extraordinary precautions the inner rings can also be heated with a circumferential gas burner in order to be dismounted, but the RKB Group does not recommend this procedure and does not offer any warranty that the bearing will work properly. Caution: when the inner rings are fitted with clearance on the roll neck, the labyrinth ring abutting the roll shoulder is also fitted with clearance. The assembly design permits the labyrinth ring to be removed together with the bearing rings and thus the components of the bearing arrangement to be held together as a unit. 26

27 3 LUBRICATION OF MULTI-ROW ROLLER BEARINGS It is well known that up to 40% of premature bearing failures are due to incorrect lubrication. Even high quality bearings can exhibit best performance only when they are lubricated correctly. Rolling bearings must be lubricated to prevent the harmful metal-to-metal contact between the rolling elements, raceways and cage and to protect the bearing from corrosion and wear as well. In addition, bearing lubricant has to ensure dissipation of heat, elimination of contaminants, flushing away of wear debris, lubrication of the seal lips and filling of labyrinth seal gaps. It is worth mentioning here that the function of lubricant in seals is different from that in bearing itself and therefore it is better to lubricate bearings and seals separately, choosing the best lubricant for each, even if, unfortunately, in many cases this solution is rarely chosen and implemented (too complicated design, high costs, possibility of making mistakes by substituting one lubricant with another, etc.). The general principles of hydrodynamic lubrication can be applied, within certain limits, to explain lubrication of rolling bearing elements. However, in the case of rolling bearings, the contact area is extremely small and, consequently, the load pressures may reach high values, in the range of 1 3 GPa. Under such extremely high pressures, an elastic deformation of the contacting surfaces occurs, the load-bearing area increases and the oil is totally squeezed from between mating surfaces. Consequently, the oil pressure increases by several orders of magnitude and, therefore, a thin viscoelastic lubricant film, which becomes capable of supporting the external load, is maintained between the surfaces. This special lubrication regime is known as elastohydrodynamic lubrication in which the friction and film thickness between the two bodies in relative motion are determined by the geometry and the elastic properties of the bodies, by the relative speed, and by the viscosity of the lubricant at the actual pressure, temperature and rate of shear. where: Condition to be met to use grease lubrication (only indicative): P C P C lim_grease P equivalent dynamic load, kn; C basic dynamic load rating, kn; P C lim_grease load ratio limit for grease lubrication; F adjusted speed index, mm min -1 : Kb Dm D d n P = 1 C lim_grease F F F = K b D m n bearing type factor (2 for tapered roller bearings, 3 for cylindrical roller bearings); mean bearing diameter, mm: bearing outer diameter, mm; bearing bore diameter, mm; bearing speed, rpm. D m = D + d 2 Fig. 30 Procedure to determine if grease lubrication is suitable for an application In the particular case of rolling bearings, the thickness and load carrying capacity of the lubricant film mainly depend on the viscosity and other oil properties and on the bearing internal geometry, speed, and size. When the bearing starts to run, the contact between mating surfaces of rolling elements and raceways is mostly metal-tometal, because the oil film thickness is small, in relation to surface roughness, hence a large number of metallic asperities come into contact. As speed increases, the lubricant film thickness also increases and fewer asperities make contact. In order to obtain the desirable lubricant full film, bearing geometry, application conditions (speeds 27

28 and loads), and lubricant properties have to combine to form an oil layer so thick that even the highest peaks do not penetrate the film and do not come into contact with each other. If this situation is achieved during operation, the subsurface stress will be the only responsible for the bearing fatigue failure and a long bearing life is expected. In this context the role of the temperature cannot be excluded. The most appropriate working temperature for a rolling bearing is the temperature that requires the minimum lubricant quantity necessary for an optimal lubrication. Among other important factors (especially those connected to operating and environmental conditions), bearing loads and speeds are of crucial importance when selecting the appropriate type of lubricant for a certain application. As a basic orientation in using grease lubrication, users can follow the procedure depicted in Fig. 30. If the mentioned conditions are not fulfilled, oil lubrication can be chosen. Nowadays, many companies produce a large number of special rolling bearing oils and greases with extremely different compositions and properties, making therefore the selection of an appropriate oil or grease for a specific application very difficult. The RKB Bearing Industries Group advises his clients not to change the lubricant that the designer has chosen for the application (rolling mill in this case). However, if lubricant changing is compulsory or desirable, we recommend customers ask their lubricant supplier for exact data. In addition, we ask our customers to pay attention to the fact that some lubricants (especially greases) are not compatible and therefore cannot be mixed. However, if it is necessary to mix lubricants, RKB recommends consulting the companies producing different lubricants for compatibility. 3.1 Grease Lubrication Grease is formed of a combination of mineral or synthetic oil and up to 30% (or even more) of an appropriate thickener resulting in a semi-solid lubricant. Metal (sodium, calcium, lithium, aluminum or complexes of metals) soaps are generally used as thickening agents, but modern greases could contain non-metallic thickeners also (bentonite, clay, polyurea, silica gel, PTFE, FEP, etc.). The type and quantity of the thickener used and the viscosity of the base oil dramatically influence grease consistency or stiffness. A grease of a given consistency may be obtained in several ways by varying oil viscosity and thickener percentage. For this reason, greases with equal stiffness do not necessarily have the same performance. When choosing a grease type for a certain application, grease consistency, temperature range at which grease efficiently operates, and rust-inhibiting properties are the first main factors to be taken into account. Grease consistency (Table 2) is a grease stiffness measure and is classified according to the scale of the National Lubricating Grease Institute (NLGI), based on the penetration depth of a standard cone sank into the grease, at a temperature of 25 C, for five seconds. The depth of penetration is measured on a scale of 10-1 mm and the softer the grease (allowing deeper penetration into the grease), the higher the penetration NLGI number is. The test method is in accordance with ISO 2137:2007. Table 2 Classification of greases by NLGI consistency number NLGI number ASTM worked penetration (10-1 mm) Appearance at room temperature very fluid fluid semi-fluid very soft soft medium hard hard very hard extremely hard The temperature range over which a certain grease can be used depends largely on the type of base oil and thickener used, as well as on additives. In Table 3 temperature range and other characteristics for greases with mineral oil base are presented. The data contained in this table provides average values and only some basic data for initial orientation. For more information, customers are asked to address to grease manufacturers to obtain precise details regarding individual grease. Most of the greases listed are available in several consistency (penetration) classes. When grease is with synthetic oil base the lower and especially upper limits could be extended. Sodium (Na) based greases are water-soluble, meaning that they absorb water (emulsify with it) and form a rustinhibiting emulsion without altering their lubricating properties. These greases efficiently protect bearings against rust 28

29 ensuring that water cannot infiltrate into the bearing, but quite often they can become very soft over time and, if water enters, they may be washed away from the housing. Calcium (Ca) based greases are frequently stabilized with 1 3% water and, if temperature increases, water may evaporate in such an extent that the grease separates into mineral oil and soap. Since lithium and calcium base greases are insoluble in water, they do not provide anticorrosion protection. Such greases should under no circumstances be used unless they contain a rust-inhibitor. Table 3 Temperature range for grease with mineral base oil Thickener type Temperature range ( C) Water resistance good fair Load carrying capacity Sodium (Na) a poor good Sodium (Na) complex b fair fair Lithium (Li) excellent fair Lithium (Li) complex good good Calcium (Ca) c excellent fair Calcium (Ca) complex d good good Aluminum (Al) good fair a emulsifies with water b for higher temperatures c good sealing against water d for higher temperatures and high speeds Greases containing EP (extreme pressure) additives are used for extremely loaded rolling bearings as rolling-mill bearings are because these additives increase the load-carrying capacity of the lubricant film. Note that it is recommended to lubricate medium and large sized roller bearings with such EP additivated greases. Calcium and lithium base greases containing EP additives (mainly lead compounds) have very good rust inhibiting properties, adhere well to the bearing contact surfaces, are insoluble in water and for these reasons they are suitable for applications, such as rolling mills, where water can penetrate the bearing housing. Where operating conditions permit and condition given in Fig. 30 is fulfilled (please pay attention to the limit cases that require a closer study), grease could be the lubricant of choice for roll neck bearings due to its inherent advantages: Lower mounting costs compared to oil lubrication. Reduced maintenance requirements and no need for including piping or pumping equipment. No need for constant level indicators. Easier to retain in housing (compared to oil). Cleaner usage (no barbotaging as with oil). Easier and cheaper to seal (than for oil). Better sealing properties against contaminants. Disadvantages of grease include its inability to remove heat or wash away wear products, the possibility of accumulating dirt or other abrasive contaminants, and a potential incompatibility problem if thickeners of different types are mixed. If grease lubrication is suitable for a certain application (see Fig. 30), note that there are several important factors to consider in choosing the appropriate grease: Bearing load and speed. Operating conditions. Environmental conditions. Load and speed range (Fig. 31) is of major importance when selecting the appropriate grease for a specific rolling mill application. For rolling mill bearings operating in normal load and speed range, greases with high-viscosity base mineral and/or synthetic oil (bearing grease K according to DIN 51825) can be used. In the high load range operating conditions greases with higher viscosity base mineral and/or synthetic oil and with EP additives (bearing grease KP according to DIN 51825) will be the best choice. Greases for high speed bearings with EP additives (same as KP class) have to be used for bearings operating in high speed range. For rolling mill applications involving high speeds and/or high loads, the operating temperature often increases, requiring temperature-resistant grease or adequate cooling measures. Consequently, the speed, load, and temperature limits of the lubricating greases must be obtained from the grease manufacturer. 29

30 Also the operating and environmental conditions influence grease selection. For non-horizontal rolls, since the grease may leak out from the bearing, a stiff grease (NLGI grade 3 or sometimes even 2), highly adherent and resistant to working, should be chosen. Stiff grease is also suitable in applications where bearing contamination by foreign matters is very likely. In wet environments or where roll mill is sprayed vigorously by water, roll neck bearings are exposed to corrosion due to condensation or water splashing. Obviously, the greases used for such applications must have anti-corrosive properties and the bearing has to be protected by seals. High load range P/C > 0.15 F < P/C < 1.15 Normal load and speed range F < High speed range F > P/C < 0.15 Fig. 31 Load and speed ranges for grease lubrication The replenishment or renewal intervals of the grease are other criteria for grease selection. For example if there are long lubricating ducts or if the application requires a large amount of grease, greases of consistency NLGI grade 2 (or even 1, if advisable) must be selected so that they can be pumped easily. An important issue in successful grease lubrication of roll neck bearings lies in determining the appropriate amount of grease to be used. The right amount and distribution of grease within the bearing and its housing promise very long service lives. As a general rule, one can consider that the necessary grease quantity to lubricate a bearing should represent 1/2 to 2/3 of bearing free volume, according to the bearing speed (the higher the speed, the lower the amount of grease to be used). The equation used to find the free volume of a bearing is presented in Fig. 32. To determine the corresponding weight of grease, an approximate density of 0.9 kg/dm 3 can be used. where: D d Bearing free volume: V = π (D 2 d 2 ) T m ρ outer diameter, mm bore diameter, mm T bearing width, mm m bearing mass, kg ρ average steel density (about 7.8 kg/dm 3 ) D = mm d = mm T = mm m = 3117 kg ρ = 7.8 kg/dm 3 Example: TQO Bearing free volume: V = π ( ) = dm3 7.8 Fig. 32 Free volume of a bearing in dm 3 (1 dm 3 = 1 liter) Usually, the free spaces in the chocks (on both sides of the bearing) are large enough to allow grease to escape from the bearing and, for this reason, if the bearing runs at high speeds, these cavities should not be filled with grease. In the case of low-speed bearings lubricant friction due to working is negligible and, therefore, their housings should be filled with grease close to full capacity. Note that an excessive amount of grease would cause an undesirable churning of the grease resulting in a decrease of grease lubricity and, therefore, an overheating of the bearing. In Fig. 33 a summary of the main guidelines for the use of grease in rolling mill applications is presented. 30

31 In the case of four-row tapered roller bearings supporting horizontal rolls, grease should be supplied at two points (Fig. 34). If the roll body is reground without removing the chocks, the lubricating system must allow bearing regreasing through the necks. When sealed four-row tapered roller bearings are used, we recommend providing drain holes on each side of the bearing so that the bearing seals have minimum exposure to water and other wet materials. Grease: Mineral or synthetic base oil with viscosity of mm 2 /s (cst) at 40 C; Base oil viscosity index of at least VI = 80; Lithium or calcium soap; Extreme pressure additives (EP); Consistency class (NLGI grade): (1), 2, 3. Grease quantity: 1/2 2/3 of bearing free volume, according to bearing speed (the higher the speed, the lower the amount of grease to be used). Caution: avoid an excessive amount of grease that may cause overheating of the bearing! Fig. 33 Main guidelines for the use of grease in rolling mill applications Fig. 34 Grease lubrication system 3.2 Oil Lubrication Mineral oils are basically hydrocarbons obtained from crude oil, but include different quantities of hydrocarbon derivatives containing elements such as nitrogen, oxygen and sulfur. Despite of the expansive development of synthetic oils, solid lubricants and wear resistant polymers, mineral oils are probably the most commonly used lubricants in many major industries, because of their advantages compared to grease: Relative low costs. Availability. Ability to remove heat and wash away wear debris and contaminants. Capability of being easily pumped, circulated, filtered, cleaned, heated, cooled, and atomized. Ease of replenishing (particularly when the relubrication interval for grease is very short) and renewing. 31

32 Recyclability and versatility. Suitability for many critical applications (extreme loads, speeds or/and high temperatures). Most of the above mentioned features are valid and even more efficient for synthetic oils. Despite their incontestable positive features, mineral oils also have several serious and sometimes major shortcomings, such as: Difficulty to seal or retain them in bearings and housings. Decisive importance of oil level (or oil flow in high-speed bearings) proper control. Rapid oxidation and significant viscosity drop at high temperatures. Spontaneous combustion or even explosion (oxidizing agents are needed). Solidification at low temperatures (below freezing point). All these drawbacks of mineral oils and the strong demand for high performance lubricants led to the development of synthetic lubricants that have some important advantages, such as: Capacity to work at high temperatures (without decomposing) as well as at very low temperatures (without freezing). Less strong decrease of viscosity with increasing temperature (higher viscosity index values). Reduced toxicity. Low vapor pressure. Resistance to aging (deterioration). Reduced fire hazard. The most important characteristic of oil is its viscosity. Among a large number of viscosity types, in the lubricant industry the most used is the kinematic viscosity (ν) hereinafter called simply viscosity and its usual unit of measure is mm 2 /s (also known as cst). Since viscosity varies with temperature it is a common practice to indicate the viscosity of oil at a certain reference temperature. Most often this reference temperature is 40 C and the oil viscosity at this temperature is designated by ν 40. The viscosity of oils decreases as temperature increases, but some oils feature a favorable viscosity-temperature behavior, meaning that their viscosity varies less with temperature than the viscosity of other oils. An arbitrary but much used measure for the change of viscosity with temperature is the (kinematic) viscosity index (VI), whose scale was set up by the Society of Automotive Engineers (SAE). Although this scale was originally ranged from 0 to 100, today s mineral oils can attain values beyond 100 and synthetic oils can reach even values over 400. As one can see in Fig. 35, two different oils having the same viscosity at 40 C, but with different viscosity indexes, will reach different values of viscosity at the working temperature (tw). Thus, the higher values of VI represent lower degrees of oil viscosity change with temperature. Fig. 35 Oils having different VIs experience different variations of viscosity with temperature Lubricating oils are commonly classified by their kinematic viscosity index. They are generally grouped into high, medium and low, as in Table 4. The advantages and disadvantages of certain mineral or synthetic oils to lubricate specific applications must be carefully considered when selecting a lubricant and designing a lubrication system. Oil lubrication is suitable when high speeds or operating temperatures make difficult or even impossible the use of grease (Fig. 30) and when it is important to quickly evacuate the heat from the bearing and housing. For high-speed operation, since the heat generated in rolling bearings increases with oil viscosity, it is necessary to select low-viscosity oil (otherwise the bearing temperature would increase too much). On the contrary, for very slow 32

33 speeds, viscous oils are used to ensure a sufficiently strong oil film. Anyway, excessive oil viscosity may cause skidding of rolling elements and exaggerated lubricant friction which can result in severe overheating and irretrievable raceway damage. On the other hand, insufficient oil viscosity may result in metal contact and possible premature failure. Table 4 Classification of lubricating oils by viscosity index Group Viscosity Index Low viscosity index (LVI) Below 35 Medium viscosity index (MVI) High viscosity index (HVI) Very high viscosity index (VHVI) Over 110 Selection of proper oil means selection of proper oil viscosity and this is based on expected operating temperature, speed, and bearing geometry. According to the bearing speed (n) and size (Dpw pitch diameter which can be easily approximated by the bearing mean diameter, Dm), the oil must have a certain (minimum) viscosity at the operating temperature in order to form the elastohydrodynamic lubricant film able to withstand the bearing loads. According to ISO 281:2007 this reference (rated) kinematic viscosity ν1, required to obtain adequate lubrication condition, can be determined using the diagram in Fig. 36 a. Bearings should be generally lubricated with oil whose operating viscosity ν (at working temperature) at least equals the rated viscosity ν1. With this value and with the estimated working temperature and using the diagram in Fig. 36 b, it is possible to determine the minimum necessary viscosity of oil at reference temperature (40 C) and the appropriate ISO viscosity grade of the chosen oil. a) b) Fig. 36 Choosing the appropriate oil: a) reference kinematic viscosity (at working temperature) depending on bearing size and speed (according to ISO 281:2007); b) viscosity-temperature diagram for ISO grade mineral oils In Fig. 37 an example of oil selection for the bearings of a rolling mill application is presented. Note that the diagram in Fig. 36 b is valid only for mineral oils with a viscosity index of about 95. For different oils please ask oil manufacturer or RKB Technical Team for correct calculation. The same procedure applies to determine the base oil viscosity of greases. A large part of bearing lubricating oils is represented by the additivated mineral oils. They comprise special compounds called additives that by chemical and/or physical action contribute to the improvement of the lubricant properties and transmit better performance characteristics to the lubricants. The major bearing oil additive families are: Anti-oxidants (AO) or oxidation inhibitors: additives which retard the appearance of oxidation products and therefore considerably extend lubricating oil life. 33

34 Corrosion inhibitors or rust inhibitors: chemical compounds that, when added to oil, diminish metal corrosion rate, preventing rust formation on metallic part surfaces during inactive periods. Formation of a coating and passivation layer that covers the metallic surfaces and does not allow the access of corrosive substances (moisture and atmospheric oxygen) to them represents the background of the corrosion inhibiting mechanism. Oil composition, amount of water, and many other factors contribute to the corrosion inhibiting efficiency. Given Bearing TQO : Dm = mm Speed: n = 50 rpm From Fig. 36 a: Reference kinematic viscosity of oil (at working temperature): ν 1 = 60 mm 2 /s Given Working temperature of bearing: tw = 75 C From Fig. 36 b: Minimum requisite kinematic viscosity of oil at 40 C (ISO grade): ν 40 = 320 mm 2 /s Fig. 37 Example of oil (base oil of grease) selection Anti-wear (AW) additives: complex substances that reduce wear in the mixed friction/lubrication regime. They form a film to surround metallic parts and, if the loads are light, they succeed in separating the surfaces and avoiding their metal-to-metal contact. Under extreme pressure conditions, the AW additives become ineffective, therefore appropriate EP additives are necessary. Extreme pressure (EP) additives: similarly to AW additives, they adhere to metallic surfaces, preventing them from touching even at high or very high pressure and therefore decrease the wear and the probability of part seizure. When a thin oil film between two surfaces of small area endeavors very high pressure for a very short period of time, called relaxation time, its behavior becomes rheological (viscoelastic behavior, almost as a solid body). This fact helps keep apart the two surfaces and avoid the metal-to-metal contact. A major role of EP additives is to increase the relaxation time from picosecond to nanosecond range. EP oils perform well over a large range of temperatures, speeds and bearing sizes and prevent damage of the parts during starting and stopping of the application. Detergents: additives used to clean and neutralize oil impurities (which otherwise would accumulate in deposits) by emulsifying the oxidation products, they maintain them in suspension, and prevent them from attaching to lubricated surfaces. Even if many detergents have the same effect as several other additives, it is totally unrecompensed to use them as universal additives. Friction modifiers: additives (mostly solid particles, such as graphite, molybdenum disulfide, wolfram disulfide, etc.) added to lubricants with the purpose to reduce the friction between the lubricated part surfaces. Dispersants: additives which keep fine contaminant particles in suspension (to avoid their coagulation) until they are retained by filters (if case be) or removed together with the oil when replaced. Water is also held in suspension as a stable emulsion. Metal deactivators or metal deactivating agents (MDA): additives which contribute to oil stabilization by deactivating metal ions appeared in lubricants as a result of the oxidative processes with the metallic surfaces and therefore inhibit the action of metallic (especially copper) particles as catalysts in the oxidation processes. They also retard the formation of gummy residues. Defoamers or anti-foaming agents: chemical additives, generally insoluble in foam, that prevent the production of air bubbles and foam in the oil which could lead to lubricity loss of lubricant, pitting, and even corrosion when oil embedded air comes into contact with metallic surfaces. Having low viscosity, they rapidly spread on foamy surfaces where they cause air bubbles rupture and surface foam breakdown. Viscosity index improvers: additives which reduce the decreasing rate of the oil viscosity with increasing temperature. At high temperatures, they increase the viscosity, and at low temperatures they improve the oil fluidity. 34

35 Emulsifiers: additives which help form and stabilize an emulsion (mixture of insoluble substances), usually mineral oils with water. Pour point depressants: additives which improve the fluidity of oil at lower temperatures (lower the pour point). Seal conditioners: additives which cause gaskets and seals swell and therefore prevent oil leakage. Thixotropic additives: compounds which improve the grease property to soften when mechanically stressed and to return to its initial consistency when left to rest. Special additivated preserving oils are also thixotropic. As for oils used for lubrication of roll neck bearings, the special and unfavorable conditions in which they often operate have to be considered: low-speed and highly loaded large size bearings, hardly axially loaded roller bearings, etc. In such conditions it is more likely that the non-additivated oil film is inappropriate, but if additives (especially EP additives) are used, they form a separating film between mating parts (rolling elements and raceways, cage and guiding lips, respectively) in order to prevent wear and premature fatigue breakdown. EP additives must be even more used, as the bearings are in situations where they are subjected to combined loads so that P/C > 0.15 or/and the operating viscosity ν is lower than the viscosity ν1. In addition, oil additives should provide, if necessary, oxidation stability, anticorrosive protection, foam reduction and fine distribution of insoluble contaminants in suspension. For applications where bearings are subjected to great thermal stressing, high-temperature oils with superior non-deterioration properties must be used and, where temperatures vary within a large range, oils with VI improvers are appropriate. For extremely high temperatures, synthetic oils (polyglycols or polyalphaolefins) are preferred because they are very resistant to deterioration (aging). The correct answer to which is the most adequate oil to a specific roll neck bearing application? should come either from experience or reliable tests. Note that there are some other oil properties such as flash point, pour point, neutralization number (NZ), saponification number (VZ), carbon residue, etc. that could be very important in many specific rolling mill applications. Fig. 38 Circulating oil lubrication system Another important issue in roll neck bearing oil lubrication is the method of lubrication adopted in a certain rolling mill application. Nowadays several methods of oil lubrication are widely used: Oil bath lubrication seems to be the least used method in roll neck bearing lubrication due to the fact that oil has to be changed frequently (or very expensive and resistant to aging synthetic oils have to be used). This happens because the limited available spaces in the chocks determine that only a small quantity of oil is provided, which deteriorates fast, taking into account the heavy mechanical and thermal stress acting on it. If however this method is used, it is worth mentioning here that, as with grease, an excessive quantity of oil can cause churning and considerable heat generation. Bearing friction torque increases also with the quantity of oil and this could represent considerable power loss. Therefore, 35

36 it is essential for the maximum oil level not to be higher than the center of the lowest rolling element. On the other hand, when the quantity of oil is too low, many metal-to-metal contacts occur, resulting in rapid temperature rise and in possible bearing seizure. Circulating oil lubrication (Fig. 38) is the most used oil lubrication method for the usual speed range of the roll neck bearings in rolling mill applications. Since in these applications energy loss due to frictional torque inside the bearing, combined with external heating of the roll neck and an insufficient heat dissipation are often present, this lubrication system permits not only a good lubrication but also cooling of the bearings and carrying away of contaminants and water. In a circulatory system the outlet must be larger than the inlet to prevent the possibility of an excessive amount of oil in the bearing. Oil injection lubrication is suitable where a circulating oil lubrication system is not efficient enough to cool both the bearing and the roll neck or where rolls have extremely high speeds. In this system the lubricant is injected through lateral nozzles into the bearing and, similarly to the circulating oil lubrication system, some supplementary features (oil reservoirs, pumps, inlets, outlets, oil coolers, etc.) are necessary, which implies extra costs, at all negligible. a) b) Fig. 39 Throwaway oil lubrication systems (oil mist or oil-air) Throwaway oil lubrication (Fig. 39) is often used for high-speed rolling mill bearings. There are two widely used types of such lubrication systems: oil mist lubrication and oil-air lubrication. With oil mist lubrication a mist of oil and air (atomized oil in compressed air) is transported through pipes to the bearings. Condensing nipples immediately before each bearing position cause the oil to be supplied to the bearing in droplet form. The small quantities of oil can be accurately regulated, and consequently the lubrication friction is negligible. Even if there are several advantages of using compressed air as a means of transportation for atomized oil (e.g. the quality of the sealing is increased by the chock overpressurization produced by the compressed air and by the air escaping at the seals), it should be known that the escaping air still contains atomized oil which could have a negative impact on the environment. In the case of oil-air lubrication the non-atomized oil is intermittently fed in a precise quantity to the lubricating pipe system and transported through it, by the compressed air, to the bearings. Since oil is not atomized, high-viscosity oils with EP additives can be used. As with oil mist lubrication, the effectiveness of the sealing is increased, but this system is not as harmful to the environment as oil mist lubrication system. For both oil mist and oil-air lubrication systems, an oil bath is necessary. The small quantity of oil transported by the compressed air, after it lubricates the contact surfaces, reaches the oil bath and contributes, in this way also, to the bearing lubrication. Unlike other oil lubrication systems, throwaway oil lubrication systems ensure bearing proper lubrication even during start-up or other temporary transitive working regimes. With a horizontal shaft, as a general rule, the bath oil level must ensure that the bottom rolling element is half immersed in the oil and for this reason the chock is provided with oil drain holes so as to fulfill this requirement. 36

37 4 MAINTENANCE OF MULTI-ROW ROLLER BEARINGS The life of multi-row roller bearings, which often endure extreme working conditions in terms of speeds, loads and temperatures, depends to a great extent on maintenance and periodic inspections given by the mill operators during rolling mill working cycles. In this case, bearing maintenance targets two important aspects: lubricant quality and bearing wear level. 4.1 Typical Maintenance Activities Lubricating properties of oils or greases deteriorate due to contaminant infiltrations, ageing and mechanical working and, even in the case of most performing sealing solutions, all lubricants become inevitably contaminated in service and must, therefore, be replenished, renewed (grease) or changed (oil) periodically. From the rolling mill productivity point of view it is vital that these maintenance actions are performed on the occasion of roll changes. Regarding grease lubrication, (re)lubrication intervals are defined as the minimum grease service life (determined by laboratory tests and based on 10% failure probability, similar to the calculation of bearing rating life) that closely depends on grease type, bearing type, size, speed, and temperature, and load type and magnitude. In the case of unfavorable operating and environmental conditions (high temperatures, extremely large loads, etc. typical for roll neck bearings), the lubrication intervals become significantly shorter. Moreover, water, moisture, mill scale, and other contaminants penetrating the bearing might cause a severe reduction of relubrication intervals, if the sealing is inappropriate. Consequently, on the occasion of roll changes, the condition of grease and seals must be carefully examined and based on these observations appropriate values for relubrication intervals can be obtained. In the case of oil lubrication, it is recommended to determine the oil change intervals based on several oil analyses. First, the oil has to be analyzed at about two months from the introduction of fresh oil into the lubrication system. Based on the results of the initial analysis, after a period of time (preferably on the occasion of roll change), the second oil analysis is conducted in order to determine oil properties (viscosity, neutralization number NZ, saponification number VZ, etc.), and its content of water and solid foreign particles. Note that a constant presence of even a small amount of water in the oil may lead to an important decrease of oil change intervals. In order to find the condition of the bearing from the wear point of view, and reveal developing trouble areas before they become serious, the bearing has to be removed from the roll neck and periodically inspected. It will be dismounted firstly at hours operation to perform a complete bearing cleaning as well as a seal examination. If the bearing is in a good state and seals prove their efficiency, next inspection can be postponed at about 2000 hours operation. After each removal of the chock from the roll neck, the appropriate maintenance activities can be as follows: Registering bearing position in the mill and cup/outer ring position in the chock: recording in a bearing fiche the chock number, roll number, stand number and its location in the mill, cup/outer ring position in the chock (load zones that have been used), tonnage/hours operation from the bearing initial mounting until the recording moment, measurement data regarding bearing, neck and chock and other inspection details may be very useful for the maintenance team. Bearing removal from the chock: to remove the bearing from the chock and handle it during inspection, specific lifting and handling devices must be used. Dismounting of the bearing has to be done in reverse order of operations performed for mounting (see chapter 2). Bearing cleaning: small bearings (or a small number of bearings) could be cleaned with kerosene or solvents, and large bearings (or a large number of bearings) may be cleaned in tanks filled with hot neutral oil with the viscosity of 22 mm 2 /s at 40 C. Tanks must be provided with cleaning agent heating and re-circulating systems. Industrial detergents should be avoided. After cleaning in the hot oil tank, bearings could be washed and rinsed with hot alkaline water solution. Ultrasounds can be used also for bearing cleaning. Cleaning operation should be thoroughly performed and as it must eliminate any contaminant (water, scale, foreign solid particles, old lubricants, etc.) which can produce further possible bearing wear. After bearing cleaning (rinsing), especially if their inspection is postponed, bearing components should be coated with a thin layer of light oil to protect them against rust. Examination of the bearing for wear and minor in-house repairs: cleaned bearing should be visually inspected with great care to identify areas that require minor in-house repairs. The roller inspection implies cage free rotation and roller individual turning. Raceways should be inspected also. Pay attention to the condition of cups/outer rings because they provide information about troubles in the mill. The presence of rust stains on the parts means that water entered the chock most likely through faulty or damaged seals, while the contamination of the lubricant with hard foreign particles, scale or even dirt is indicated by more or less severe bruising. All small spalls and pits encountered on roller and raceway surfaces should be repaired by loose metal grinding and sharp edge smoothing using only 37

38 grinding and polishing tools, in order to stop spall spreading and avoid loose metal from reaching the bearing contact surfaces (and producing important damages there) when bearing is remounted. Unfortunately large rusted or corroded areas cannot be completely removed, but by using sandpaper with grit specification of 240 or 320, as much as possible damaged surfaces have to be polished. In this manner, when the bearing is put back into service, rust contamination is prevented. Note that bearing outer rings/cups are stationary parts and practically, by proper selection of the load zone (quadrant), a lot of wear problems are thereby avoided. Careful determination and readjustment of the bearing axial internal clearance (Bench End Play BEP) of four-row tapered roller bearings: please refer to section 4.2. Chock inspection and repair: repair damages of bore surface by performing similar operations as in case of bearing raceway repairs. Measure the bore diameter (see 2.1.1) and register the results in the chock fiche. Inspection of the lubricant condition: check grease/oil quantity and condition, and, when appropriate, renew or replenish with fresh grease, or fill up or change the oil. Bearing assembly into the chock: before assembling the bearing into the chock, check the bearing fiche to be sure to use the right load zone (quadrant). The RKB Group recommends inspection and rotation of the cups/outer rings to change the loaded zones every hours operation (first rotation 180, second rotation 90, and third rotation 180 again). Follow the instructions presented in chapter 2. Inspection of the roll neck: clean and repair neck surface, measure the journal diameter (see 2.1.2) and record the results in the related shaft fiche. Installation of the chock onto the roll neck: follow the instructions presented in chapter 2. Caution: with sealed four-row tapered roller bearings, the RKB Group recommends performing additional inspections after hours operation: Check grease quantity and condition, and, if it is in a small amount or contaminated or even discolored, fill the bearing and labyrinths with grease in a proper amount. Carefully inspect bearing seals and change them if damaged. Finally, it is advisable to always have three and a half sets of bearings for every stand, which is a really costeffective solution. One of the sets is mounted on the rolls in the operating position, the second one is mounted on the rolls removed for barrel regrinding and the third set has to be anytime accessible for use. Another half set must be in stock for any undesired, but potential bearing failure. In the case of cylindrical roller bearings with tightly fitted inner rings, it is indicated to have supplementary inner rings for mounting on the roll neck. Periodic mounting and dismounting operations for the inner rings are not needed if the rolls are replaced at regular intervals. 4.2 Checking and Readjustment of a TQO Bearing Axial Internal Clearance (Bench End Play BEP) The TQO bearing wear can be evaluated by determining, at least once a year, the actual axial internal clearance of the bearing, operation that includes several special measurement activities. RKB is fully available to help or instruct customer maintenance personnel in the bearing measurement procedure. If the value of the actual axial internal clearance (actual BEP) is at least twice the initial BEP (provided with the related RKB bearing Technical Fiche), then the actual BEP needs to be readjusted. Caution: since bearing parts tend to wear unevenly, the end play in each set of rollers may exhibit some differences. Generally, in favorable operating conditions, there should not be much difference between the sets. However, if some unusual operating conditions severely load a certain zone (often one end) of the bearing or damaged seals at one side of the chock allow localized contamination of the lubricant, then the differences between the actual bench end plays of different roller sets could be important. For the actual bench end play determination, the bearing, without the spacers (two cup spacers and one cone spacer), should be put on a flat, solid surface with the lower single-row cup DE (with face E down) placed on wooden (counterbored, if possible) supports (Fig. 40) so that there is some cage clearance and bearing free rotation is permitted. Caution: whenever the bearing is stacked up, the proper stacking sequence must be followed in order to have the correct setting clearance. After the bearing to be measured is stacked up, it is necessary to load it, to properly seat all of its parts as shown in Fig. 40. The slab used as a weight should be placed on the single-row cup AB on face A, centered on cup outside diameter and counterbored so that the cage is clear and free to rotate. This weight is significant, especially for the 38

39 bearings with extensive service, because their components are likely to be out-of-round and have to be seated accordingly. The load applied (slab weight) should be equal to at least half of the bearing weight. Fig. 40 Stacking and loading of an RKB TQO bearing in order to determine its actual axial internal clearance (BEP) Caution: for large bearings a lifting system to place the slab on top of the bearing is required and, for safety reasons, the lifting chains or ropes (with some slack) must be kept in place all the time it takes to measure. After the bearing is loaded, in order to seat the rollers properly, all the parts must be rotated separately and, if necessary to help seating (but with the scope to protect the bearing as well), a thin layer of light oil can be used. In the case of bearings which have operated for an extended period, in order to completely seat the parts, it is possible that a significant rotation of the bearing may be required. The verification of the correct roller seating of all four sets of rollers is accomplished by trying to insert a 0.05 mm feeler gauge between the roller large end and the cone rib at four different locations (offset at 90 each). When all parts are properly seated, the gaps between the cups and the gap between the cones are measured each at four places 90 apart around the bearing (Fig. 40). For each gap, the average value of the four width measurements is used in the following calculation of the BEP as the width value of the respective gap (Table 5). The actual respective cup and cone spacer width values are then measured.. In order to verify the parallelism of the spacer faces, the above measurements should be performed at four places offset by 90, and the average value of the four readings (for each spacer) is then obtained and used as the width value of the respective spacer (Table 5). Table 5 Measurements needed for the determination of the bearing actual axial internal clearance (BEP) Dimension Width SBB of the gap corresponding to the spacer BB Width SCC of the gap corresponding to the spacer CC Width SDD of the gap corresponding to the spacer DD Width of the spacer BB Width of the spacer CC Width of the spacer DD Average of 4 measurements VmSBB VmSCC VmSDD VmBB VmCC VmDD To obtain more precise measurement results, it is highly recommended to arrange the bearing parts in reverse order in the stack (single-row cup AB placed with face A down on the wooden supports and the slab seated on face E of the single-row cup DE) and carry out a second measurement. Adjust the values in the second column of Table 5 by retaining the average value of eight (instead of four) measurements. After these measurements are done, the difference between a certain spacer width and the same spacer gap measurement (all average values) is the end play in the two rows of rollers adjacent to that spacer (Fig. 41). 39

40 Caution: if at least one of the three values of the actual bench end plays for different sets of rollers is at least twice the initial BEP (given in the related RKB Technical Fiche), then the axial internal clearance of the bearing needs to be readjusted. Actual BEP = Spacer Width (VmBB, VmCC, or VmDD) Spacer Gap (VmSBB, VmSCC, or VmSDD) Fig. 41 Actual axial internal clearance (BEP) of an RKB TQO bearing The bearing axial internal clearance can be adjusted to the required amount by regrinding the faces of every spacer so that each of them is wider than the measured space by the amount of end play desired in the bearing. Consequently, first of all it is necessary to target the value of the desired bench end paly in the bearing, considering that, due to the wear, it is impossible to obtain the same value of the axial internal clearance as that of the corresponding brand new bearing. RKB recommends that the target axial internal clearance is 1.3 times the original axial clearance stated in the related Technical Fiche. Having this new value of the bench end play, the spacer widths can be calculated (Table 6) and the spacers reground to the calculated values. As an example, the determination of the actual axial internal clearance (BEP) of the RKB TQO A2AHA1ZBBT4B bearing is presented in Table 7. Since the actual axial internal clearance of the bearing presented in Table 7 exceeds twice the original BEP (as in the related RKB Technical Fiche) spacers should be reground. As an example, calculation of these adjustments is presented in Table 8. Table 6 Readjustment of the axial internal clearance (BEP) of RKB TQO bearings Dimension Initial axial clearance (BEP) as in RKB Technical Fiche New axial clearance (when actual axial clearance becomes at least 2 BEPTF) Equation BEPTF BEPNEW = 1.3 BEPTF Width of the reground spacers: Spacer BB Spacer CC Spacer DD BBNEW = VmSBB + BEPNEW CCNEW = VmSCC + BEPNEW DDNEW = VmSDD + BEPNEW Table 7 Example of determination of an RKB bearing actual axial internal clearance (BEP) a Dimension Measurements Designation Average a b c d (a+b+c+d)/4 Width of the spacer BB VmBB = Width SBB of the gap corresponding to the spacer BB VmSBB = Actual BEP of the roller sets adjacent to the spacer BB BEPBB = VmBB VmSBB = = Width of the spacer CC VmCC = Width SCC of the gap corresponding to the spacer CC VmSCC = Actual BEP of the roller sets adjacent to the spacer CC BEPCC = VmCC VmSCC = = Width of the spacer DD VmDD = Width SDD of the gap corresponding to the spacer DD VmSDD = Actual BEP of the roller sets adjacent to the spacer DD BEPDD = VmDD VmSDD = = a all dimensions are in mm 40

41 Table 8 Example of readjustment of an RKB bearing axial internal clearance (BEP) a Dimension Designation or equation Value Initial BEP (as in RKB Technical Fiche) BEPTF New (target) BEP BEPNEW = 1.3 BEPTF = = Width b SBB of the gap corresponding to the spacer BB VmSBB = Width b SCC of the gap corresponding to the spacer CC VmSCC = Width b SDD of the gap corresponding to the spacer DD VmSDD = Width of the reground spacer BB BBNEW = VmSBB + BEPNEW = Width of the reground spacer CC CCNEW = VmSCC + BEPNEW = Width of the reground spacer DD DDNEW = VmSDD + BEPNEW = a all dimensions are in mm b average value 4.3 Storage Multi-row roller bearings should be stored in their original cases (Fig. 42) in warehouses (Fig. 43) where the temperature is maintained at ºC and the relative air humidity does not exceed 60%. Bearings should not be stored directly on the ground, but on shelves, palettes or other wooden supports that are at least 20 cm above the ground. Moreover, no vibrations should be allowed in the storage areas, because the bearings daily subjected to even minor vibrations may be damaged during the storage period. The warehouses are supposed to be used only for bearings storage and never for chemical and corrosive materials. Fig. 42 RKB specific wooden cases used for large bearings Fig. 43 Detail of one of the storage facilities of RKB in Balerna (Switzerland) 41