FKL Bearings in Vibrating Screens

Transcription

1 FKL Bearings in Vibrating Screens

2 Bearings in Vibrating Screens: Vibrating screens are used for grading (i.e. separation of solid materials by grain size) of materials and other vibratory machinery such as road rollers and saw frames are among the machines subjected to the most severe stresses. The rolling bearings in the exciter units of these machines must support not only high loads and high speeds but also accelerations and centrifugal forces. Many of these applications involve adverse environmental conditions such as contamination and moisture. Design of Equipment and Process: The function of a vibrating screen is to sort particulate material made up of a distribution of sizes into specific size groupings. Vibrating screens generally handle particles of size between 5 mm and 250 mm. They are found in a variety of industries and in a range of processes. Its Main Elements: A vibrating screen consists of three main elements: Deck : The machine shakes the material over a screen or grating. Material smaller than the apertures of the screen fall through it and can be transferred elsewhere, while larger particles continue to move along the screen surface. The surface of the screen is called Deck and the structure to which the deck is attached is called Screen Box. A vibrating screen can consist of several decks, whereby the material may be selected into several size groupings, Vibration inducer : Generates the vibratory motion needed to move the unsorted material across the deck. Frame : Structure on which the vibratory screen is mounted. Basic elements of a typical vibrating screen mechanism : Frame mr 2 Centre tube mr 2 Fig: 1 Benefits of Screen Duty Bearing: Increased uptime Improved reliability Resistance to contamination Higher productivity levels Reduced noise and vibration levels Excellent wear resistance 2

3 Types of Vibrating Screens: There are three main types of screens: Free motion or two-bearing screens Circular motion or four-bearing screens Linear motion screens Bearing performance requirements : Vibrating screens place unique demands on the bearings used in them. Therefore, the bearings designed for such applications have to be able to withstand extreme operating conditions such as high speed, loads, accelerations and temperatures. The important features of a Vibrating Screen Application are as listed below: 1. Rotating Outer Ring Load : In most of the screens, it is the outer ring that rotates and not the inner ring. Hence the outer ring must be fitted in the housing with interference fit. 2. Heavy Load : The relatively high load of this application demands a high capacity bearing usually a Spherical Roller Bearing. 3. Misalignment : Most screens use a single long shaft supported by two bearings. In such machines, shaft bending and consequently, misalignment is likely to occur. Thus, Spherical Roller Bearings are usually preferred for use in these applications. (See Fig-1) 4. Axial Movement of Shaft due to Thermal Expansion : Most of the Vibrating Screens run hot due to heavy load and speed, as well as the friction in the Sealing arrangement. At times, heat is also transmitted from the process material, which itself is hot. This build up of heat causes the shaft to expand with respect to the housing, when hot, and to contract, when cold. Spherical Roller Bearings are, therefore, fitted on the shaft with a loose fit between the inner ring and the shaft in order to facilitate such movements. 5. High Speed : Typical operating speeds of a Vibrating Screen vary between 800 to 1200 rpm. 6. High Acceleration : Due to reciprocating motion of the vibratory application, bearings are subjected to high g or acceleration. Normal cages cannot withstand such accelerations. Hence enhanced cage design needs to be used in such applications. 7. High Temperature : The combination of heavy load, high speed and the Seal friction lead to a very high level of heat generation. The bearings, therefore, must be capable of withstanding high temperatures for a considerable period of time, without any loss of performance. 8.Contaminated Operating Environment : The environment, in which the vibratory screens are required to operate, is highly contaminated most of the time. While external seals are used in order to minimise ingress of abrasive foreign particles, infiltration by some amount of such particles is practically unavoidable. The bearings, therefore, should be capable of tolerating presence of a certain amount of such particles. Bearing Design : Spherical Roller Bearings of 223 series have been specially designed by FKL in order to fulfil the required operational requirements of a vibrating screen application. Inner Ring : Absence of the flanges minimises the sliding surfaces within the bearing, thus reducing the friction and allowing the bearing to run cooler at higher speeds. Outer Ring : The outside diameter has the central lubrication groove with three holes feature (W33) as standard. Guide Ring : The design incorporates a floating guide ring situated between the two rows of rollers, so as to ensure that the rollers travel along the correct paths on the raceways.. In bearings sizes , the guide ring is centred on inner ring, while in sizes , the guide ring is centred on outer ring. 3

4 Cage : Due to reciprocating motion of the vibrating application, Screen Duty Bearings are subjected to high g or accelerations. Normal cages cannot withstand such accelerations. Hence, strong cages, with advanced design, are needed. FKL Screen Duty Bearings incorporate a single piece machined solid brass Cage, centred on Outer Ring Raceway, as a standard feature. Internal clearance : The standard bearing clearance is optimized at C4 class. Should any other clearance class be required, it should be clearly specified while ordering. Tapered bore : Spherical Roller Screen Duty bearings are also available with a tapered bore with taper 1:12, identified by the suffix K. Suitable adapter sleeves of series H23 and withdrawal sleeves of series AHX 23 series can be selected from the FKL General Catalogue. Tolerances : Bearing housings should be made of steel or spheroidal graphite cast iron (e.g., GGG40). The thickness of the housing material in the radial direction should be at least 40% of the bearing width, in order to eliminate the risk of it's deformation. Since the outer rings of the eccentric bearings fitted in a vibrating screen are subjected to rotating outer ring load, these must be fitted with an interference fit in the housing so as to avoid creeping of the bearing ring with respect to the housing and the consequent temperature rise and wear. For bearings in the range , it is recommended to be ISO grade N6 IT4 / 2, whereas for sizes and above, it is recommended to be ISO grade P6 IT4 / 2. The inner ring is supposed to have a loose fit on the shaft so as to allow the shaft to expand freely during operation, due to temperature rise and consequent thermal expansion. For bearings in the range , it is recommended to be ISO grade g6 IT5 / 2, whereas for sizes and above, it is recommended to be ISO grade f6 IT4 / 2. Surface roughness on bearing seatings housing bore and on the shaft should be maintained as 1.6 microns R max. a Sealing arrangement : Vibrating screens are required to work in very dusty surroundings. Since contamination by hard particulate matters is a serious cause of bearing failure, the sealing arrangement of the screen must be designed to be highly effective so that foreign matters do not penetrate the seals and reduce the life of the bearings. It is generally recommended that a combination of a vertical labyrinth seal and a positive contact seal such as a V ring seal be used. The gaps in the labyrinth seal are to be dimensioned according to the figure below: Bearing bore d mm to to to to Labyrinth gap dimensions mm 0,6 0,7 0,8 0,9 2,5 3,0 3,5 4,0 Fig 2: Labyrinth dimensions 4

5 It is to be noted that labyrinth seals are only effective so long as the passages are purged regularly with clean grease. Maintenance procedures are to be drawn up accordingly. In applications, where a more effective sealing arrangement is desirable, the V ring seal may be replaced by a garter lip seal as shown in figure. For grease lubricated applications, the lip should be facing outwards so as to prevent ingress of contamination. Fig 3: Labyrinth with garter lip seal for grease lubrication In oil lubricated applications, the seal orientation should be reversed in order to retain the oil, and an additional seal should be used so as to prevent ingress of contamination, as shown in the fig.below. Lubrication : Fig 4:Labyrinth with garter lip seal for oil lubrication Grease lubrication : In most of vibrating screen applications, grease lubrication is usually used, since this method enables a relatively simple design of the arrangement as a whole, and of seals in particular. Usually, a grease having Lithium base and mineral oil, having a consistency of 2 or 3 on the NLGI scale is used. The base oil viscosity is required to be in the range of cst at 40 C. The grease should contain rust inhibiting and extreme pressure (EP) additives. Under heavy duty conditions, a supplementary addition of 1 3% MoS Molybdenum disulphide) is considered beneficial. In view of the 2 ( vibration, the mechanical stability of the grease has to be good in order to prevent excessive bleeding of oil (softening or oil separation from the soap base). When hot material is being screened, a high temperature grease e.g., Na-synthetic or Bentonite grease should be used. Amounts of grease to be used and re-lubrication intervals are given in the table below. Installation of an automatic 5

6 Table 1: Recommendation for grease lubrication for Spherical Roller Bearings series 223 Bearing No. Grease charge Relubrication Interval on mounting g on relubrication 900 r/min r/min r/min working hours 2000 r/min VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C

7 Oil lubrication : Oil lubrication has to be used when the speed or temperature of operation renders grease lubrication impossible. Oil bath is the simplest method of oil lubrication. Oil circulation, preferably combined with a cooling system, is sometimes used particularly for larger screens in order to control the bearing temperature. Oil mist lubrication is used in machines having smaller bearings running at high speeds. In oil mist lubrication systems, the pressurised oil mist acts as a protection against ingress of contamination. 0 For oil lubrication, an oxidation resistant mineral oil having a minimum viscosity of cst at 40 C is recommended for use. The oil should have an EP additive along with a rust inhibitor and anti-foam additives. The oil level in an oil bath should be up to the centre of the bottom most roller in stand-still condition. Oil quantity recommendations for Spherical Roller Bearings are given in the table Table 2: Recommendation for oil lubrication for Spherical Roller Bearings series 223 Bearing No. Distance from shaft axis to oil level mm Flow rate per bearing with circulating oil Lt/min VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C

8 Table 3: Recommendation for oil lubrication for Spherical Roller Bearings series 223 Bearing No. Distance from shaft axis to oil level Flow rate per bearing with circulating oil Lt/min VIB M C VIB M C VIB M C VIB M C VIB M C Mounting and dismounting of bearings : VIB M C VIB M C VIB M C The level and condition of oil must be checked from time to time. Normally, the oil should be changed after six months of operation. Mounting and Dismounting of Bearings: Prior to assembly, all dimensions of the mating components of the bearings such as housing bore and shaft diameters conform to the stipulated dimensional tolerances. Accuracy of forms such as cylindricity, out of roundness and taper - should be measured by taking readings at three points around the periphery and at both ends of the contact surface ( see fig ) Fig 5 :Simple method to check accuracy of forms a b It is advisable to ensure that dismounting will be as easy as possible, before the mounting job is undertaken. Therefore, it is preferable if holes for dismounting bolts are drilled and tapped in the housing flange right at the design stage, so that removal by using jacking bolts may be facilitated. (See the figs) below: 8

9 Fig : 6 Designation Specification: Bearing Number VIB M C4 Prefix: 223 Dimension Series Code 08 Bore Diameter Code Suffix: M Bearing with Brass Cage VIB Vibratory C4 Internal Clearance Table 4: Bearing Tables Bearing No. Boundary dimensions (mm) Load Ratings (KN) Dynamic Static Mass (kg) d D B C r C or VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C

10 Bearing Tables : Bearing No. Boundary dimensions (mm) Load Ratings (KN) Dynamic Static Mass (kg) d D B C r C or VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C VIB M C Bearings with larger sizes can be offered on request. 2.We can also provide taper bore on request. 3.Vibrating screen bearings with pressed steel cages can be supplied on request. For further details, please contact your nearest FKL Office. 10

11 Industrial Applications of vibrating screens : Crusher Machinery Cement Machinery Mining and Construction Equipment Pulp and Paper Processing Equipment Marine and Offshore Machinery Metal Industry Equipment Railway Axle Boxes 11

12 Slovakia Kolkata Ahmedabad FKL, s.r.o. Nádražná ul. 2535/33/D Skalica, Slovakia india@fkl.sk Corporate & Registered Office E 3/7, Gillander House, 3rd Floor, 8, Netaji Subhash Road Kolkata , West Bengal (India) Tel: /32/33 Mobile: E mail: marketing@fklindia.com Indian Manufacturing Plant Paiki No. 143, Near Divya Bhaskar Press, Sarkhej Bavla Highway, Chacharwadi Vasna, Changodar, Matoda, Ahmedabad E mail: ahmedabad@fklindia.com E mail: marketing@fklindia.com