There are many applications of precision angular contact ball bearings in aerospace. There are oil lubrication and solid lubrication, oil immersed solid cage for oil lubrication and self-lubricating material solid cage for solid lubrication. Spacecraft need a certain working life to have economic benefits, and bearings are often the limiting factor. Aerospace bearings are all under light load, and most of them are cage failure. This paper makes some qualitative analysis and Discussion on the design and application of cage.
1.Analysis and test
There are two important dimensions of cage, one is the gap between cage and ferrule guide surface, the other is the gap between pocket and steel ball. The figure shows the definition of two kinds of gaps (in terms of radius). It is now discussed in three aspects.
Figure 1 definition of guide clearance and pocket clearance
(1) The pocket clearance should not be too small, otherwise it will restrict the movement of the steel ball and cause greater friction between the steel ball and the cage.
(2) Our practical experience is that the ratio of guide clearance to pocket clearance of micro bearings must be less than.
If both clearances are appropriate, the guide plays a leading role. The cage rests on the guide surface of the inner ring and guides the steel balls one by one. Ideally, the steel balls are uniformly distributed. Once it is unevenly distributed, the bow guide will certainly make it slide and reset, so the analysis and discussion take the uniform distribution as the starting point, and frequent sliding is undesirable.
Figure 2 normal internal guidance
If the pocket clearance is smaller than the guide clearance, the guide surface does not work, and the probability of two steel balls clamping the cage is large. This clamping force can be large, resulting in three effects. The bearing friction torque increases, the torque fluctuation is serious, the cage is damaged, ranging from wear to breakage, and the steel ball slides on the channel, causing howling Wear and fatigue.
Figure 3 the steel ball clamps the cage, and the cage is worn in both directions
If the pocket clearance is too large, the peak and peak values of friction moment are also large.
The table shows the friction torque of a solid lubricated bearing when the bore is too small. After redesigning the pocket to make it conform to the relationship between the guide clearance and the pocket clearance, the average torque and the maximum torque are significantly improved. This is the original data of a bearing. We have made dozens of bearings, all of which have the same effect, and there is no failure in long-term operation, which cannot be done before the pocket repair.
(3) If the inner and outer circles are skewed, the trajectory of each steel ball is elliptical. When the inner ring rotates at a uniform speed, the angular velocity of the steel ball on the outer channel is different, which also produces the phenomenon that two steel balls clamp the cage. The elliptical "perigee" steel ball "leads" the pocket of the cage is relatively normal, and the "apogee" steel ball hinders the cage. This phenomenon is inevitable when the skew reaches a certain degree. This can be seen from the fact that the wear of the pocket is always front and rear. So skew is the most fatal defect. To ensure that there is no skew, it depends on the bearing assembly process, which must be guaranteed by the installation design.
In Figure 5, the apogee distance of the ellipse caused by skew is L1, and the perigee distance is L2 (different from the oblique section ellipse of the cylindrical surface).
Fig. 4 inner and outer ring skew
Figure 5 ellipse caused by skew
2. Concluding remarks
In the past, the broken cage was mostly attributed to the insufficient strength of cage material. I hope to improve the strength. Now it seems that there are two reasons for cage breakage. First, the geometric dimension of the cage is not designed properly. Second, the inner and outer rings are skewed during assembly. "Constant pressure preloading" involves the uniformity of the spring. "Positioning preloading" is generally achieved by grinding the end face of the shell. At present, although various measurement methods are used to detect the assembly error, it is difficult to measure accurately, so the bearing life is very fragmented.
The influence of cage pocket clearance and guide clearance on bearing performance has been proved by practice. The inner and outer circles are being tested. The above analysis hopes to cause discussion and correction.
More about KYOCM Angular Contact Ball Bearing:
Angular contact ball bearings have inner and outer ring raceways that are displaced relative to each other in the direction of the bearing axis. This means that these bearings are designed to accommodate combined loads, i.e. simultaneously acting radial and axial loads.
The axial load carrying capacity of angular contact ball bearings increases as the contact angle increases. The contact angle is defined as the angle between the line joining the points of contact of the ball and the raceways in the radial plane, along which the combined load is transmitted from one raceway to another, and a line perpendicular to the bearing axis.
In KYOCM, angular contact ball bearings include Single-row Angular Contact Ball Bearings, Double-row Angular Contact Ball Bearings and Three Point And Four Point Angular Contact Ball Bearings.