Potential Causes for the Observed Instability Effects  Though a final conclusion on the root cause for the observed cage instability effect could not be drawn and  the influencing effects could not be quantified or prov en by an analytical model so far, the observations  made on the H/W supports at least the following assumptions:  The cage instability is mainly caused by the dynamic forces resulting from a non-circular shape of the  Phenolic resin cages (potentially also influenced by local density differences in the material).  A good oil film between the balls and th e cage helps to avoid cage instability.   A weaker (more flexible) cage yields better dynamic stability than a more rigid gage does. In case instability occurs, the rigid cage is expected to produce higher transient forces between balls and cage  pockets than a weaker (more flexible) cage does.    Taking out three balls of the bearing in a way as described above contributed to cage stability. This effect  is potentially caused by a reduction of the lateral cage run-out or by reduction of dynamic cage forces  produced by transient non-linear ball to ball and ball to race contacts. However the observed effect cannot be completely explained and proven without having established a very detailed dynamic bearing model  that would have to be correlated with H/W test results.   Technical Conclusions and Lessons Learnt   Do not trust information gained from “similarity” of bearings. Bearing behavior is defined by a variety of  parameters (size, material, cage dimens ion and tolerances, cage guidance, ball to cage clearance, ball  dimensions, pre-load, mo unting environment, environmental conditions, lu brication, life, operation velocity,  operation profile, etc.) and no application is identical to another one. Check carefully these conditions  before believing in “similarity”.  Thin ring bearings of the given size (diameter in the 100-mm range) and cage dimensions, operated at  elevated speed (above the 500-rpm range) and at the given boundary lubrication cond itions seem to be  sensitive to cage instability effects, especially if the cage is not perfectly shaped.   Improper (non-circular) cage shape can be expected especially if Phenolic Resin cages are used, since  here the potentially in-homogenous local material characteristics tend to cause un-symmetric internal stress levels after manufacturing and at exposure to te mperature extremes. Also local density variations in  the material might contribute to resulting eccentric forces produced at high-speed operation.  In applications requiring bearings of elevated rotation speed and at comparably large dimension, the use  of Phenolic Resin cages is not recommended by the above reasons. Substitutes shall be considered at the  beginning of the design process (e.g. Vespel, PEEK or similar).  Though the effect was not completely understood, it was found that residual dynamic effects due to cage  un-symmetry were damped out and mitigated after reducing the number of bearing balls from 39 to 36 by  leaving every 13 th cage pocket free. Using only bearing cages with an odd number of balls (39)  unsymmetrical spaced, did not cause any improvement compared to an even ball number of 42.  In order to reduce the project risk, advantage should be taken from a simple mechanism design principle  allowing easy dismantling of the mechanism in case of bearing issues. This implies that e.g. bearing press fits should be avoided if possible in order to support an easy and safe bearing integration and dismantling  process.  Programmatic Conclusions and Lessons Learned  The observation was made that bearing suppliers produce very good standard bearing quality in view of  dimensions of races and balls. Therefore a preferred option to mitigate technical and programmatic risks is  to procure standard high quality bearings from experienced bearing suppliers, if no specifically designed  129