Technical Paper MAIN SHAFT SUPPORT FOR WIND TURBINE WITH A FIXED AND FLOATING BEARING CONFIGURATION Tapered Double Inner Row Bearing Vs. Spherical Roller Bearing On The Fixed Position Laurentiu Ionescu, Thierry Pontius www.bearing.sg
Introduction Today s usage of a single double-row radial spherical roller bearing at a fixed position on a wind turbine main shaft is not a preferred bearing solution, and should be avoided in future designs. While there isn t an official maximum limit, a conventional ratio of permissible thrustto-radial loading deemed acceptable for two-row spherical roller bearings is between 0.15 and 0.20. That means the axial load should only be 15% to 20% of the radial reaction of the two-row bearing. In some applications, this ratio may be stretched to 0.30 or 0.35, but issues related to unseating effects, abnormal load distribution between rows, roller skewing, retainer distress, excessive heat generation and roller smearing may increase. Since this ratio at a fixed position of a wind turbine main shaft is often in the vicinity of 0.60 resulting in only one of the two rows supporting both the radial and thrust loading the bearing cannot operate as it was originally designed. Therefore, other fixed bearing solutions need to be considered to improve performance and reliability. This paper reviews the benefits of applying a combination of double-row tapered roller bearings (TDI) and cylindrical roller bearings (CRB) on the main shaft of wind turbines. Analysis shows that using a preloaded doublerow tapered roller bearing improves the fixation of the main shaft, thereby reducing the opportunity for false brinelling on the bearing raceways and input components of the gearbox. The internal clearance can be optimized with preload to help ensure excellent system stability even in the most extreme wind conditions, and the raceway profiles can be optimized in order to operate properly even with a highly misaligned shaft. Main Shaft Bearing Comparison Some wind turbine main shafts are equipped with two spherical roller bearings (SRB). A fixed SRB carries the radial and axial loads from the rotor while the floating SRB carries only the radial load. Both bearings are mounted with a radial clearance in the mounted configuration. The mounted clearance plays a major role in developing the bearing spring stiffness in both radial and axial directions. Therefore, the radial translation of the main shaft and the axial movement are affected by the initial clearances and the fitting practices. Minimizing radial translation is beneficial to both bearing and system performance. An improved bearing arrangement for supporting wind turbine main shafts is using a preloaded TDI bearing for the fixed position and a CRB for the floating position. Analytical Comparison During Operation of the Main Shaft Fig. 1. Main shaft bearings Fig. 2 shows a load schematic imposed by the rotor blades on a typical wind turbine. www.bearing.sg
Fig. 3. Main shaft model Fig. 2. Loads and working system of axis During operation, loads arrive in the form of a radial load from the mass; a side load from the wind; and rotor thrust and over turning moments from the blades in two planes. The cycle used for this comparative study is an equivalent one resulting from a binned load cycle with over 500 variable loads Fx, Fy, Fz, My and Mz at constant rotational speed of 16 rpm. Condition Fx Fz My Mz Time number (N) (N) (Nm) (Nm) (%) 1 250 000-500 000 600 000 450 000 27.50 2 250 000-500 000 600 000-750 000 22.50 3 250 000-500 000 0 450 000 8.25 4 250 000-500 000 0-750 000 6.75 5 250 000-500 000-600 000 450 000 19.25 6 250 000-500 000-600 000-750 000 15.75 Table 1. The equivalent cycle Main Shaft Model Fig. 3 shows the main shaft model used for the bearing system analysis. The loads Fx, Fz and the moments My, Mz are acting in the center of the hub at a distance of 2000 mm from the fixed bearing. Side loads from wind Fy are small compared to Fz and are ignored in our example. The distance between each bearing position is 1000 mm. The gear box weight (200000 N assumed in our example) is acting at the end of the shaft at a distance of 2500 mm from the fixed bearing position. Fig. 3 illustrates a solution above the center line composed of a TDI and CRB arrangement. Below the center line, the solution illustrates an SRB and SRB combination. The bearings are lubricated with grease that has an ISO VG320 viscosity. The operating temperature is assumed at 40ºC. Position Fixed Floating Type Principal Dimensions [MM] Bore O.D. Width TDI 750 1040 320 9090 0.500 axial preload SRB 750 1090 335 10200 0.300 radial clearance CRB 710 1030 236 8060 0.200 Radial clearance SRB 710 1030 236 7020 0.100 radial clearance Table. 2. Data of bearings for comparison study Radial Rating [Hn] Table 2 summarizes all the data about the bearings selected for the application. Comparison Study Results Condition Table 3 Adjustable Life L10A fixed position TDI SRB cr Adjusted life l10a (hours) Average mounted setting [mm] Floating position CRB SRB 1 224000 107000 15000000 2800000 2 6500000 8700000 >1E+8 16520000 3 347000 120000 31500000 4800000 4 10900000 18000000 >1E+8 >1E+8 5 225000 108000 15100000 2850000 6 653000 8700000 >1E+8 16500000 weighted 420000 197000 27300000 4800000 www.bearing.sg -
Adjusted life represents the calculated bearing life, taking into consideration all of the application environmental factors such as operating temperature, misalignment, load zone, lubrication and stress level. Its equation is: where L 10 = catalog life [hours] a i = environmental factors The results show that the adjusted life for each bearing, and for the entire system, is higher for the solution TDI and CRB combination that for the SRB combination. Even though the TDI s rating is smaller than the rating for the SRB, the performance of the TDI bearing is better due to its ability to be preloaded. Preload helps improve the distribution of the loads between both rows and improves the load zones. The TDI bearing is adjusted with 0.500 preload and adds greater rigidity to the system compared with an SRB that is set to operate full-time with internal radial clearance. The TDI bearing provides improved load distribution between two rows, decreasing the radial and axial deflection on each row. Radial Displacement Fig. 5 shows the radial displacement of node 1 (rotor side) on the main shaft for each condition from the equivalent cycle. Shaft Deflection Fig. 4 shows the plots for the radial main shaft deflection versus the location of the deflection point for condition 1. Fig. 5. Main shaft node 1 radial displacement Notice that for the same loads, radial displacement of the shaft at node 1 is 25% to 30% less for the TDI and CRB configuration. Axial Displacement Fig. 4. Main shaft deflection The results show that a main shaft mounted with a preloaded TDI deflects less than it would if it were mounted with an SRB. This is the case for two reasons: Wind turbine applications involve both the main shaft radial deflection and main shaft axial displacement. In fact, the axial displacement of the main shaft transfers directly into the input shaft of the gear box. Unless careful attention is paid to the specification of the SRB clearance, the control of the axial clearance and the location of the planetary, axial displacement can have a negative impact on the performance of the planet carrier bearings mounted in the gear box.
Reducing the axial movement in the static condition reduces the opportunity for false brinelling. Fig. 6. Main shaft axial displacement The axial displacement of the main shaft depends largely on the system rigidity and the fixed bearing s internal clearance. By using a preloaded TDI bearing in the fixed position, the axial displacement of the right end of the main shaft is almost four times less than when compared with using an SRB. Reducing axial shaft movement reduces the risk of thrusting the gear box input shaft, which is very important. Bearing Load Zone Fig. 7. Load zone of bearings For the TDI bearing, the load sharing is improved between the bearing rows. For the SRB, the entire load is supported by only one row most of the time. In a configuration with fixed and floating bearings, the fixed bearing (TDI or SRB) is loaded radially and axially while the floating bearing (CRB or SRB) is only loaded radially. Because the axial force is acting from the rotor side to the gear box side, only the row adjacent to the gear box will support the entire axial force from the wind, regardless of whether a TDI or SRB is used. See Fig. 7. row adjacent to rotor Fig. 8. Load zone of TDI bearing row adjacent to gear box For the TDI bearing, the load is shared between both rows because: The TDI bearing is preloaded, which eliminates all initial clearance without load.
The preload increases the radial and axial stiffness under operating conditions, thereby reducing deflections during operation. Optimized bearing load zones in wind turbine applications running at low speed have real benefits. At low speeds, the rollers will loose traction and rolling velocity after they exit the load zone, causing them to be pushed by the cage. When the rollers re-enter the load zone and regain traction and rolling velocity under load, skidding and smearing damage can occur. The dynamics of the skidding and smearing will cause adhesive wear at that point on the race and the rollers. It also will negatively impact the ability to build lube film in the load zone. In addition, skidding and smearing introduces highly tensile shear forces beneath the surface of the race/rollers. Especially for through hardened products, this tensile force in combination with tensile hoop stresses, residual heat treat stresses, and rolling contact stress can lead to the formation of premature, axially-oriented cracks. Even when there is enough lube film or extreme pressure additive to prevent metal-to-metal contact, roller sliding induces these same high tensile stresses in the contact. The issues of speed, sliding, skidding, and smearing are potentially worse in greased applications. That s because the grease is more viscous and will resist the rolling motion out of the load zone. The SRB s performance is lower primarily because there is only one row loaded most of the time. This occurs especially in larger diameter SRBs in which the internal radial clearance translates into a fairly large axial clearance in the sphere. Under the external axial loads, substantial movement from the left to the right of the inner ring relative to the outer ring occurs until the right row of rollers is in contact with the outer ring raceway. In this condition, the entire internal axial clearance of the SRB will be contained in the left row because a gap between the rollers and outer ring raceway will leave the left row unloaded most of the time. The unloaded row causes multiple concerns, including: The row capacity of the SRB is underutilized and L10 is reduced. Because loading one row causes thrust loading in the system and possible contact between the roller ends and toe flanges, we would expect extra heat generation, which might complicate lubrication, fit and life. The unloaded rollers can be pushed by the cage at all times, with little or no rolling motion. The result is constant skidding and sliding along with all the issues mentioned previously. False brinelling in static conditions. row adjacent to rotor row adjacent to gear box Raceway Stresses Stress values are related to the load zone values. It is important to notice that when there is an improved load zone in the system, and when the bearing load is shared between many rollers and both bearing rows, the stress values are lower. Fig. 10 shows the mean stresses. Fig. 9. Load zone of SRB bearing.
The TDI bearing setting is preloaded to provide excellent static and dynamic stiffness. The TDI and CRB configuration also offers these additional benefits: Reduction in main shaft axial movement. Maximized global stiffness of the system. Maximized load zones and bearing life L10a due to optimum preload. Preloaded bearing operates with true rolling motion to minimize roller skidding. Reduced axial main shaft movement lowers the risk for raceway fatigue and pitting. Fig. 10. Race stress comparison between spherical and tapered roller bearing models The stress values for the TDI bearings are well-balanced for all the conditions from the equivalent cycle. However, the SRB has greater stresses than the TDI bearing. For the SRB row adjacent to the rotor, there are almost no stresses under some operating conditions because the row is unloaded. Conclusions Reduced axial main shaft movement in the static conditions lowers the opportunity for false brinelling. Reduced axial main shaft movement lowers the risk of thrusting the gearbox input shaft. Applications operating with high misalignment benefit from optimized raceway profiles in the TDI. Acknowledgments We gratefully thank Gerald Fox for his guidance and contribution in creating this paper. For the wind turbine main shaft support configuration consisting of a fixed and floating position, a TDI and CRB combination offers significant advantages over configurations using two SRBs for the following reasons: The TDI bearing offers excellent radial and thrust capacity. The preloaded TDI bearing improves the load zone to balance load sharing and improve L10.
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