Stress Control in Ball Beaings – MACRO

SUMMARY

The manufacture of ball bearings is done in several steps to achieve bearing rings with very high geometric accuracy. During the various stages - turning, heat treatment, hard turning, grinding - the parts undergo metallurgical transformations giving rise to the appearance of residual stresses in the material which in turn produce geometrical distortions during manufacture. These have a strong impact on the manufacture of bearings of small sections and large dimensions. Indeed, in the case of annular bearing rings, these distortions require to multiply the finishing operations in order to obtain the desired dimensional and geometric tolerances. It may also happen that the distortions observed are too large to be removed by one or more finishing steps, which causes a significant loss of material.
The residual stresses and the distortions they imply are a subject of major academic and industrial interest because of their important impact, not only on geometric tolerances, but also on the performances and life time of mechanical parts. As a result, several existing structures involving both laboratory and industry researchers are currently addressing the issue of stresses - essentially from the point of view of their experimental characterization and the analysis of raw data - to better understand their genesis and ultimately control them. Also, there are currently different methods to determine the residual stresses (destructive or not), all of which require taking into account strong hypotheses having an impact on the result, and it is therefore still difficult today to have a clear idea of the accuracy of measurement methods and their area of validity.
The improvement of the quality and the productivity of the manufacturing processes of ball bearings of low section developed by ADR thus implies a better control of these stresses throughout the process. Such a control will bring a gain of competitiveness in the manufacture of ball bearings which constitutes a strategic activity of the company. ADR will then be able to respond to new, larger and more competitive markets in high-technology sectors such as optronics, SATCOM constellations, navigation of defense equipment and high-precision robotics.
To do this, we propose to create the joint laboratory MACRO or STREBB (Stress Control in Ball Bearings) between the Laboratory of Process and Materials Sciences (LSPM - CNRS) and the ADR company. This LabCom aims to contribute to the improvement of the measurement and the prediction of residual stresses generated at different stages of a fabrication process of complex geometry pieces, in order to allow in the short term the optimization of the fabrication processes and in the medium term the introduction of the developed measurement and calculation methodologies to the manufacturing processes. We aim at limiting the number of finishing operations to maintain the imposed geometric tolerances, and then at extending the research to a better understanding of the links between residual stresses and service life. This project uses experimental characterization tools, but also numerical tools for predicting distortions after different operations. It will include the characterization of the metallurgical, mechanical and geometrical state of the bearing rings at various stages of a conventional manufacturing process, and then the identification of the critical points for the optimization of the process and the design of the parts.