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Superfinishing and its Effect on Micropitting

By: Martin McCormick (REM Surface Engineering), Marco Bonazzoli (Best Finishing) and Matt Bell (REM Research Group)

Technical Papers and White Papers

Micropitting is a prominent failure mechanism for both highly stressed case carburized gears and bearings that operate consistently in low speed, low load environments where boundary conditions predominate.

Lubricant regime is controlled by lubricant film thickness and roughness. Studies on gears investigating these two critical factors conclude that the surface roughness is more important than the elastohydrodynamic lubrication film thickness.

Superfinishing is an imprecise term usually applicable to technologies which reduce the surface roughness below that of a standard ground condition. Broadly two different approaches are taken; one seeks to maintain the periodic profile and reduce the peak and valley heights. The second seeks to remove the
periodicity and create an isotropic surface profile.

Isotropic surfaces are formed through a variety of planarizing techniques that sequentially reduce the asperity height, starting with the peak asperities.

Under mixed lubrication the peak asperities are able to contact during motion, creating friction and surface damage. Removal of these asperities enables the contact to remain in the full elastohydrodynamic lubricant regime more consistently.

Chemically accelerated vibratory finishing is one such subset of isotropic superfinishing. This technique is applicable to a range of metallic parts such as gears and bearings and is able to reduce the Ra <0.1µm and Rz <1µm. It has been shown to have significant positive impact on case carburized gears producing a range of benefits from noise reduction, increased efficiency, and improved lubrication.

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