As a consequence the identified measure was also introduced into the FM mechanism however here the  observed noise issue was still observed in a reduced form. This led to the conclusion that the positive  effect obviously caused by the reduction of balls in the EQM mechanism was compensated by another effect present in the FM bearings.  The remaining suspicion was, that the cage configuration as used in the EQM bearings was different from  the FM cage configuration. This theory was fed by the fact that EQM and FM cages were manufactured  from different batches of phenolic resin.  Since the existence of the cage instability issue was obviously also related to the environmental conditions  e.g. to the mechanism storage time, time of operation, hot/cold operation and humidity, the theory was  supported, that the poor geometric stability of the Phenolic Resin cages was one of the potential root causes for the noise issue.   Phenolic Resin is known to be quite sensitive to manufacturing processes, storage conditions and  production batch and the resulting quality of a bearing cage is even depending on the location of the used  material within the manufactured piece of raw material.   A non-circular shape of the bearing cage might have caused a “Hula Hoop” effect at the given rotation  velocity resulting in abnormal forces between cage a nd balls with subsequent noise production.   Though at supplier premises the actual cage dimensions were checked on a 3D measurement machine,  the confidence in these measurements was quite low, since the weak thin walled cages were expected to deflect under the contact load of the mechanical measurement device.  The FM1 mechanism disassembled and the bearings were completely stripped on Astrium Site. The  measuring gauge already available from the EQM bearing in spection process wa s used to perform proper  measurement of circularity and diameter of the cages in steps of 1/100 mm on all FM bearing cages.  It was found that the cages used in the FM me chanism were out of tolerance by up to 0.3 mm in diameter,  compared to the EQM cages that were of good circularity.  As a consequence the selection of cages for the FM mechanism was taking place in order to identify the  cages of maximum diameter conformity. These selected cages were then locally overworked by hand in  order to generate a good circular shape.   After vacuum re-impregnation, the re-worked cages were integrated into the bearings again and the  subsequent mechanism test at ambient conditions yielded prefect results. The noise issue on the FM1 was completely eliminated at the operational temperature extremes. Only at low survival temperature,  which was -8 deg below worst-case operation temperature, a slight noise production was still observed.  This was probably caused by cage dimensions ch anging with temperature from the circular shape at  ambient conditions to any non-circular asymmetric shape at low temperature extremes in combination with  the higher viscosity of the lubric ant under this condition.    This finding supported the theory of the high influence of non-perfectly shaped cages at the given bearing  size and rotation speed.  Since the thermal deflection over temperature is reproducible, the noise effect disappeared again  completely after heating up the bearing from the su rvival temperature to the lo w operational temperature  and from there up to ambient.   By using the identified 36-ball configuration supported by selection and optimisation of the available spare  cages for good circularity, noise free performance over the whole required temperature range could be  realized also on the both FM mechanisms.   128