Explain the significance of 'Surface Finish' (RA) on the fatigue life of a spiral retaining ring.

Spiral retaining rings are coiled from flat wire, which inherently has a smooth, rolled surface finish on the top and bottom. However, the 'edges' of the wire can have micro-slitting marks or burrs. These surface imperfections act as stress concentrators (notches). Under cyclic axial loads, fatigue cracks initiate at these edge defects. Specifying a 'vibratory deburr' or 'tumble' finish is standard to smooth these edges. For high-cycle applications, a surface finish of 16-32 micro-inches RA is targeted. Additionally, a smooth finish reduces the friction during installation and removal, preventing 'galling' of the shaft or bore surface.