NTN TECHNICAL REVIEW No.7426 New Product High Load Capacity Cylindrical Roller Bearings Takuya OZU NTN has developed high load capacity cylindrical roller bearings appropriate for gearbox of wind turbines and other industrial machinery. By using "Keystones" instead of the conventional cage, the new bearings have increased the load capacity without reducing speed performance. As a result, the new bearings have 1.5 times or more the rating life of current caged bearings. 1. Introduction The gearbox on wind turbines is located high off the ground and is therefore difficult to access for service. Therefore, the bearings in the gearbox must provide long life with a high degree of reliability. Even though the size of wind energy farms has been growing in recent years, demand for smaller, lighter nacelles have been on the rise in order to mitigate loads on towers. Given this, compact bearings for gearbox applications that boast greater load carrying capacity are greatly needed. In response to these needs, NTN has developed a unique high load capacity cylindrical roller bearing product (Fig.1) that incorporates keystones in place of a cage. As a result, our new cylindrical roller bearing product boasts a greater load bearing capacity without loss in running performance. The rated life of the new product is more than 1.5 times longer than that of conventional caged cylindrical roller bearings. 2. Structure and Features In the new NTN design, resin-made keystones are situated between the rollers to prevent roller-to-roller contact. Since these keystones are independent of each other, tensile stress otherwise resulting from roller-to-roller contact and separation does not affect them. As a result, the size and number of the rollers can be increased because the keystones can be thinner (Fig. 2). In Table 1, the performance of the new design is compared with that of a conventional design. Longer life The size and the number of the rollers are optimized to enhance the load carrying capacity of the bearing. As a result, a rated life that is more than 1.5 times as long as that of a conventional caged bearing design. Higher speed Roller-to-roller contact occurring on full complement roller bearings is avoided by use of a unique keystone design. Through optimization of the shape and roller guiding scheme with the keystones, limiting speed and smearing resistance equivalent to a caged design have been achieved. Industrial Engineering Dept. Industrial Sales headquarters -9-
High Load Capacity Cylindrical Roller Bearings Keystone (resin) The size and number of rollers cannot be increased because the mechanical strength of the cage needs to be maintained. New design Cage Full complement roller Fig. 1 High Load Capacity Cylindrical Roller Bearings A roller comes into contact with an adjacent roller that is rotating in the opposite direction, leading to greater frictional resistance. Fig. 2 Comparison of structure with conventional CRB Handling ease Even when the inner ring is removed, the rollers on the keystone design do not disassemble. Thanks to this feature, the inner ring and outer ring subassembly can be handled independently, leading to easy bearing installation on machines. Table 1 Comparison of performance with conventional CRB Type Bearing size (mm) Dynamic rating life (kn) Static rating life (kn) Rating life ratio Allowable running speed (oil) min -1 Conventional design with cage Full complement roller 153218( equivalent to NJ233E) 133 116 141 174 16 187 1.6 1 1.9 2 2 1 3. Evaluation Test (1) The following tests were performed by assuming that each test bearing was used on the output shaft of a gearbox for a wind turbine. 3.1 Test bearings In general, the non-locating bearing on the output shaft is often an NU or NJ cylindrical roller bearing type, in the case the test bearings used were NJ2324E. The new design tested is a prototype identical to a conventional design, but that has an increased number of rollers compared to the conventional design. In Table 2, the technical data for the conventional design (with a cage) is compared to that of the new design. Conventional design (machined cage) Table 2 Test bearings Type (rolling element separators) Appearance 122686 3862 13 Bearing size (mm) Roller size (mm) Number of rollers 15-91-
NTN TECHNICAL REVIEW No.7426 3.2 Temperature increase comparison test The running performance of the new design was compared with that of the conventional design in terms of temperature increase. As shown in Fig. 3, the temperature increase pattern with the keystone design closely matches that of the conventional design, and the new bearing was capable of trouble-free operation up to 35 min -1 (dn/dmn = 42,/67,). 3.3 Acceleration/deceleration test To evaluate resistance to the smearing that can occur in non-load operation, the keystone bearing was subjected to an acceleration/deceleration test under a set of low-load conditions. As can be seen from the test results plotted in Fig. 4, the temperature rise on the outer ring was limited to a stable level of approximately 15 C. (Additionally smearing did not occur during the test.) 3.4 Oil-resistance test with separator pieces To determine oil resistance resin test pieces made of the same material as that used for the keystones were immersed in the lubricating oils that are used in wind turbine gearboxes. The tensile stress of the test pieces was taken and plotted in Fig. 5. As can be seen from the data, the test pieces immersed in each of the two lubricating oils did not exhibit significant losses in mechanical strength. Loading condition Radial load Fr= 98 kn Rotation speed Up to 35 min -1 Lubricating oil Turbine VG56 Lubricating system Circulating lubrication Temperature rise on outer ring C 1 9 8 7 6 5 4 3 2 1 Conventional design 1 2 3 4 Fig. 3 Temperature rise Rotation speed min -1 Loading conditionradial load Load of housing weight (21 kg) only Rotation speed 18 min -1 Lubricating oil Turbine VG56 Lubricating system Circulating lubrication Run duration 24 h Temperature rise on outer ring C 8 6 4 2 min -1 18 5 1 15 2 25 7s Run duration h Run pattern 5s 12s 4s Fig. 4 Accelerated test result -92-
High Load Capacity Cylindrical Roller Bearings Test piecejis 7113 No. 1 dumbell test piece Oil used for immersion Synthetic oil equivalent to VG32 from Company A Synthetic oil equivalent to VG32 from Company B Immersion temperature1 C Immersion duration2 hrs. Tensile strength MPa 8 6 4 2 A B 5 1 15 2 Immersion duration h Fig. 5 Oil resistance of separator 4. Evaluation Test (2) Additional tests were performed with an N-type cylindrical roller bearing (a type that has a doubleribbed inner ring). This type of bearing is often used in planetary gear applications. On certain planetary gears, the bore surface of the gear may be used as a raceway surface for the outer ring. With this type of arrangement, an N-type cylindrical roller bearing that lacks an outer ring is often used (Fig. 6). Bearings for planetary gearing are usually used with their outer rings allowed to rotate. However, for the these tests, the inner ring was allowed to rotate. 4.1 Test bearings In Table 3, the technical data for the conventional design (full complement roller bearing) is compared with that for the new keystone design. Fig. 6 Example of using N type Planetary gear Table 3 Test bearings Conventional design (full complement roller bearing) Type (complete with rolling element separator assembly) Appearance 11.619.276.6 29.56 17 Bearing size mm Roller size mm Number of rollers 16-93-
NTN TECHNICAL REVIEW No.7426 4.2 Temperature rise comparison test To evaluate the running performance of the keystone design, the temperature increase characteristics were compared to those of the conventional design. As shown in Fig. 7, the temperature increase of the keystone design was limited compared to the conventional design. There was an approximately 1 C maximum difference in temperature increase between the new and conventional designs. In addition, after undergoing the test. The conventional design exhibited smearing, something not apparent in the keystone design (Fig. 8). Loading condition Radial load Fr= 69 kn Rotation speed Up to 2 min -1 Lubricating oil Turbine VG56 Lubricating system Oil bath Temperature rise on outer ring C 7 6 5 4 3 2 1 Conventional design 5 1 15 2 25 Fig. 7 Temperature rise Aborted when abnormal vibration occurred. Rotation speed min -1 Fig. 8 Bearing condition after test -94-
High Load Capacity Cylindrical Roller Bearings 5. Conclusion A unique high-load capacity cylindrical roller bearing design incorporating keystones has been described, and the results from evaluation tests with this new design have been discussed. The author believes that this new bearing design, which features the advantages not only of bearings with cages but also those of full complement roller bearings, will contribute significantly to longer bearing life and improved reliability for transmission systems. The new high-load capacity cylindrical roller bearing design can be applied to systems other than gearboxes, so NTN will market this design as a high load capacity bearing product line. Photo of author Takuya OZU Industrial Engineering Dept. Industrial Sales headquarters -95-