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Three Innovations Advance the Science of Shot Peening
Controlled Shot Peening has long been recognized as a very effective process to prolong the fatigue life of all types of gears. Three recent innovations have extended the state of the art: 1) by a process that provides a smooth surface after peening to increase the resistance to surface fatigue; 2) by a coupon system that makes possible actual X-Ray diffraction curves to monitor production shot peening; and 3) by a computer program that permits a reasonable prediction of residual stress curves. C.A.S.E.sm is an acronym for Chemically Assisted Surface Engineering. Frequently, when the bending fatigue life of very high cycle gears has been increased by shot peening the tooth roots, the next mode of failure is surface fatigue or pitting of the contact faces. There has been a need, particularly for automotive and aircraft transmissions, to extend the surface fatigue life beyond that provided by shot peening alone and smoothing the surface after peening has been found to be very effective. The C.A.S.E. process cuts the relative finishing time, after shot peening, down to just minutes for typical automotive sun gears and provides a surface which, in addition to the compressive stresses from the shot peening, can have an Rsk value of as little as -1.1. Very significant increases in surface contact fatigue (pitting) life have been observed. MILAMsm system. By the use of the new MILAMsm coupons, X-Ray Diffraction (XRD) profiles can now be furnished in a matter of days and at a fraction of the usual prices. The MILAM coupons are dimensionally similar to Almen strips but are thicker and made of the same material and hardness as the part itself. The coupons are used in the Almen fixture so that they reproduce the effect of shot peening in the most critical locations on the part. The convenient new system opens up the use of XRD as a production tool for the evaluation of shot peening quality and is adaptable to Statistical Process Control (SPC) without the need to destroy the part. Peenstresssm is a computer program, developed in conjunction with ENSAM, a French engineering university, that permits the optimization of shot peening parameters during the process development phase. The program library now contains nearly 100 metals and alloys in various heat treatments and will accept inputs for shot size, shot type and Almen intensity. Peenstress will then calculate, and reproduce on screen, residual stress distribution.
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