Failure analysis of asteroid gearbox

In the early stage of new product development, there must be sufficient simulation analysis and calculation and a large number of experimental verification, and wind power asteroid gearbox is no exception. According to the theoretical mechanics analysis, there are several forces evenly distributed among several planetary gears. These forces provide pure torque through planetary bearing and pin axial planet carrier, and the resultant force is zero. Therefore, in theory, there is no axial force on the planetary gear, and the pin shaft is not subject to any axial force. The pin shaft is usually connected with the planet carrier by a certain amount of interference according to the empirical value. In the early stage, due to the small batch of application of planetary transmission in the industrial field, the planetary carrier was a left-right split structure, with nodular iron castings on the left and steel castings on the right (such as 42CrMo). With the progress of casting technology and the increase of batch, the design was improved into an integrated casting form (refer to figure 1).

It’s hard to get a perfect explanation of where the axial force that forces the pin out to the left comes from. Therefore, the reason has to be found from the beginning, after careful analysis, the finite element simulation found suspicious phenomenon. Finite element analysis focuses on the bending fatigue of gears, the maximum stress and strain at the fillet of planet carrier. The relative strain of left and right spokes of planet carrier only concerns the control of the maximum strain, and it is difficult to pay attention to the influence of strain on the interference of pin shaft, because it is just a hypothesis for simulation calculation – assuming that the interference is large enough and there is no displacement. The function of the early version of the software is also limited, and the influence of the repeated positive and negative force on the creep of the pin can not be simulated.

It can be seen from Figure 2 that the left side of the planet carrier is relatively thin under force, and the left side forms a cantilever beam structure for the right side, so when the middle of the pin shaft is under force, the left side has a torsion effect relative to the right side. When the sun gear is in reverse rotation, the force of the pin shaft on the planet carrier is also changed from left to right repeatedly, and the left end of the pin shaft is equivalent to the bending moment in the left and right directions repeatedly. Even if the interference amount of the left and right planet carrier spokes is the same as that of the pin shaft, but the right is thick and the left is thin, the interference force formed is different. You can imagine that the left and right ends of the pin shaft (steel part) are held by a small hand and a big hand respectively. When the force is applied, there must be greater torsion on the left side (Fig. 2). When the force retreats or reverses, the pin shaft wants to return to its original state or swing to the other side. In the process, because the interference at the right end is tighter, it is difficult for the pin shaft to move to the right, but it can only move to the left side relatively. Repeat this, the pin shaft moves slowly to the left. If the planetary carrier adopts the left-right split structure, this phenomenon will progress faster and more obvious, because the interference force produced by the right steel part is much greater than that of the left casting. This phenomenon is similar to taking out a stick that is inserted into the soil in life. The usual way is to push and pull the stick from side to side, and the stick will slowly withdraw upward.