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
However, occasional faults in the grid or generator, and resonance during braking procedures will force reverse thrust for short intervals. In other cases the generator serves as a motor to drive the turbine rotor at low speed for maintenance, and in some cases of low wind, just at the cut-in speed. All of these conditions require the locator bearing to support axial load in minor direction (thin arrow line, Figure 6). This minor thrust load is transmitted through roller large and small ends, the inner race rib, and the outer race rib ring.
The primary features and benefits of the locator bearing include:
1) Reduced axial space compared with two-row arrangements.
2) Easy adaptability to existing designs.
3) Improved gear contact positioning due to the centering characteristic of the locator bearing operating with 360º load zone.
4) Reduced system cost.
5) Reduced stress and increased bearing life because of the 360º load zone and all the rollers are in contact as illustrated in Figure 7, throughout the full range of positive torque during normal power generation.
Figure 7 – 360° load zone
6) Optimized race/roller contact angle based on the ratio of radial (Fr) and axial load (Fa). Careful attention is given to the bearing design so that the thrust induced by the radial reaction on the locator is always less than the gear thrust. Since the ratio of the bearing radial reaction and the gear thrust will be constant for the duty cycle, the gear thrust will always be sufficient to keep the bearing seated.
7) Smaller outer flange is available on either end of the outer race to prevent possibility of reversing orientation during assembly (Figure 8).
Figure 8 – Bearing with locating outer flange
8) Internal clearance is precisely set at the bearing manufacturer. Hence, manual bearing setting during assembly is completely eliminated.
9) The range of axial endplay should be closely controlled to insure proper running clearance throughout the spectrum of operating load and temperature conditions. Since the fixed locator bearing is used in conjunction with a floating CRB, the need for manual adjustment is eliminated.
10) According to AGMA 6006, allowable maximum contact stress at equivalent load should not exceed 1300 MPa at the output shaft and 1650 MPa at intermediate shaft bearings. Contact stress in a locator bearing supporting both radial and axial load is less than this and much less than that of the CRB that supports only radial load with reduced load zone (see Figure 9).
Figure 9 – Comparison of contact stress
Locator Bearing Configuration
Standard locator bearings are designed with an above centerline cage that retains the rollers with the inner race. Alternatively it can also be provided with a below centerline cage that retains the rollers with the outer race. Such a configuration can be supplied with two separable rib rings (see Figure 10). Preferred construction is largely dependant on the gearbox assembly procedure, mainly when assembling heavy intermediate shafting in a vertical orientation in the housing.
Figure 10 – Locator with two detachable ribs
High-speed shafts are often subject to service inspection and change out. So it is an advantage that the entire output shaft assembly with locator bearing attached can be slid out the housing during inspection.
Debris Resistant Bearings
Wind turbine gearboxes have a history of bearings and gears experiencing early damage and fatigue. Because the output shafting experiences many more revolutions, the bearings on this shaft are even more vulnerable to contamination effects. To improve the reliability, the locator bearing can be supplied with debris resistant bearing materials and ES300 coatings on rollers. Figure 11 shows one such set of experimental results with statistical significance that bearing life can be improved by a factor of 4.5 when the ES300 coating is applied.
Figure 11 – Fatigue life test result: ES300 technology
Although the locator bearing is best suited to high-speed low thrust environment such as gearbox high speed and high-speed intermediate shafts, it has some limitations that restrict its use in other applications. This bearing type cannot tolerate internal bearing preload under any condition. The axial capacity of the bearing in minor direction across the ribs is limited compared to the generous axial capacity in major direction across the raceways. However the minor axial thrust capacity is very similar to the axial capacity of NJ style CRB’s, which have found wide usage in wind turbine gearboxes. It’s interesting to note that bi-directional bearings like the locator have been applied in many automotive and aerospace applications exposing them to thrust loads in the minor direction with very good success. These operating regimes of load and speed are well established. So the primary challenge to wind turbine gearbox designers is to be able to identify their entire duty cycle so that the locator performance can be evaluated under all known circumstances. In the end, application testing of the locator in the gear box should be conducted to verify is performance.
While the four-point-contact-ball bearing and cylindrical roller bearing combination have found wide spread usage at the high speed output shaft positions in wind turbine gear boxes, output shaft bearing reliability still remains a weaker part of the system design. Fxed two-row tapered roller bearings are now finding favor. The locator is one more alternative to be considered for the output shaft fixed location. Narrower than the wider two-row alternatives, this bearing style can offer designers an opportunity to reduce the length and weight of the gearbox. Because one instead of two rows are applied, efficiency is improved by approximately 30-50%. Since the locator runs constantly with a 360º load zone in all positive torque conditions, shaft movement is minimized and gear contact is no longer a function of the bearing clearance. Concerns for smearing races in unseated conditions are eliminated. Although there exists very substantial experience with using bi-directional bearings like the locator bearing in many different applications, it is important that field testing is performed to validate usage, since there are many transient conditions in a wind turbine gearbox that remain unaccounted for at the design stage.
Doug Lucas is chief engineer for wind applications at The Timken Company in Canton, Ohio, U.S.
Biswanath Nandi is a former group leader of new product development and design at Timken Engineering and Research India in Bangalore.
Images courtesy The Timken Company