Overview
The current use of a double row spherical roller bearing at the fixed end of the wind turbine main shaft position is not a priority and should be avoided in future designs. Although there is no established maximum limit value, it is generally believed that the maximum ratio of axial and radial forces tolerated for a double row spherical roller bearing is between 0.15 and 0.20. That is to say, the axial force can only be 15% to 20% of the radial force. In some applications, this ratio can reach 0.3 or even 0.35, but there may be a list of unstressed forces, two columns of uneven stress, roller tilt, cage deformation, excessive heat generation, and roller spotting. Since the force ratio of the fixed end bearing of the wind turbine main shaft is generally about 0.6, this will result in only one row bearing the radial force and the axial force, that is, the bearing cannot be operated in the original design, so other considerations must be considered. Fixed-end bearing solutions to improve performance and reliability.
This paper mainly describes the advantages of using a double row tapered roller bearing at one end of the wind turbine main shaft position and a cylindrical roller bearing solution at the other end. Analysis shows that the use of pre-tightened double row tapered roller bearings can increase the rigidity of the spindle, thereby reducing the possibility of false indentations in the bearing raceways and gearbox input components. The pre-tightened and optimized internal clearance ensures excellent system stability even under the most extreme wind conditions, and the shape of the raceway is optimized for normal use even under very large eccentric conditions.
Comparison of spindle bearings
Some wind turbines use two spherical roller bearings at the spindle position. Fixed-end spherical roller bearings are subjected to axial and radial forces, and floating-end spherical roller bearings are only subjected to radial forces. Both bearings have a certain radial clearance during installation. The mounting clearance has an important influence on the axial and radial rigidity of the bearing. Therefore, the radial offset of the main shaft and the axial movement of the bearing are affected by the initial clearance and the shaft hole fit. Reducing the radial offset of the bearing is beneficial for improving the performance of the bearing and system. The use of a pre-tightened double row tapered roller bearing at the fixed end and the use of a cylindrical roller bearing at the floating end is a better form of bearing arrangement for the fan spindle.
Analysis and comparison during spindle operation
Figure 1 shows the force diagram of a typical wind turbine.
Figure 1 Load distribution
In operation, one radial force is mainly from gravity, and one horizontal force is mainly from wind. The axial force and the overturning moment of the rotor are derived from the blades. The working condition used for analysis and comparison is an equivalent load condition synthesized by more than 500 different stress conditions at a constant speed of 16 rpm.
Spindle model
Figure 2 shows the spindle model for bearing system analysis. The forces Fx, Fz and the torques My, Mz act on the center of the housing 2000 mm from the fixed end bearing. The horizontal force Fy from the wind is very small compared to Fz and is therefore ignored in our analysis. The span between the bearings is 1000 mm and the gearbox weight (200000 N assumed in our analysis) acts on the shaft end 2500 mm from the fixed end bearing.
Figure 2 shows the arrangement of a double row tapered roller bearing and a cylindrical roller bearing on the center line. Under the center line, two spherical roller bearings are given.
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