Doug Lucas and Biswanath Nandi, The Timken Company
August 21, 2013 | 0 Comments
Performance, cost and efficiency issues associated with bearings commonly used in today’s wind turbine gearbox high-speed output and intermediate shafts can be addressed with a bearing type for wind turbine gearboxes called the locator bearing, which offers an alternative solution.
The locator bearing combines a single-row tapered roller bearing (TRB) that supports prevailing radial and thrust loads across raceways during the normal positive torque conditions, with a carefully designed secondary rib ring to support thrust loads in reverse torque conditions such as braking or motoring. This single-row bearing type can act as a substitute for different bearing combinations.
Gearbox output shafts are sometimes equipped with a non-locating cylindrical roller bearing (CRB) and an axially locating four-point-contact-ball (FPCBB), see Figure 1. The CRB supports only radial load while the FPCBB supports the entire thrust load in the system.
Figure 1 – Current bearing arrangement
In Figure 2 another bearing arrangement is displayed applying two single-row (similar or dissimilar series) TRBs at the locating position. Special attention is paid to selection of the proper raceway angles to insure that both bearings rows maintain sufficient size load zones during the full spectrum of operating conditions.
Figure 2 – Bearing arrangement with two tapered roller bearings
Inadequacies in Existing Arrangement
Designed with point contact, the FPCBB is designed primarily for supporting radial load and to a certain degree for moderate thrust loads. This bearing design is not ideally suited to support the constant and fluctuating gear thrust loads that are prevalent during normal operation. Under pure axial loading, the balls contact the race at an angle and rotate across that contact angle, but at the same time around the centerline of the shaft. This causes micro slipping between the balls and raceways introducing surface initiated damage modes such as the peeling shown in Figure 3.
Figure 3 – Peeling on balls
The outer ring of the ball bearing must be loosely fitted in the housing to prevent any radial load from transmitting into this bearing. This requires keyways to prevent rotation of the outer ring: unlike the locator bearing, which is tightly clamped between housing shoulder and end cap.
The arrangement shown in Figure 2 is a good alternative to four-point-contact-ball combinations, although adequate distribution of load and traction forces between the bearing rows remain a design concern.
Locator Bearing Design
Figure 4 shows a typical locator bearing design. In Figure 5, a locator bearing is fixed at the right end of the shaft to support radial and thrust loading from the gear, while a CRB is floating at the opposite end. The bearing resembles a single-row tapered roller bearing having an additional rib ring attached to the outer race.
Figure 4 – Typical configuration of locator bearing
Figure 5 – Bearing arrangement with locator bearing
Figure 6 illustrates how the locator bearing supports thrust load in both the directions. The major thrust (thick arrow line) is due to predominant helical gear thrust that is present during positive torque conditions for more than 99% of the duty cycle. Support of the loads is across the raceways. The tapered roller bearing accommodates the combined loading with pure rolling motion without any of the micro slip that takes place with the ball bearing.
Figure 6 – Load sharing