14.12 Surface roughness

The initial (as manufactured) surface roughness Ra of the working profiles of gear teeth depends primarily on the manufacturing method. The surface roughness to be used as input data for Scoring should be the surface roughness (micro-in rms) of the gear tooth profiles after they are run-in. The degree of improvement in surface roughness depends on the surface hardness of the gear teeth, the initial as-manufactured surface roughness and the operating conditions of load, speed and lubrication regime. The surtace roughness of slow speed, low hardness gears with an initial surface roughness of 80 micro-in rms might have up to a 4:1 improvement by running-in to 20 micro-in rms. Medium-hard, medium-speed gears commonly have 2:1 improvements by running-in from say 60 micro-in rms to 30 micro-in rms, while the surfaces of high-speed carburized gears may improve from 25 micro-in rms to 17 micro-in rms by running-in.

Users should obtain data for the surface roughness after run-in from tests on their particular gears. In lieu of this data, the following table gives typical values of surface roughness before and after run-in:




Surface Roughing (micro-in rms)
Gear Tooth
Manufacturing MethodAs ManufacturedAfter run-in



Milling 64 - 125 32 - 64
Shaping 32 - 125 25 - 50
Hobbing 30 - 80 20 - 45
Lapping 20 - 100 20 - 40
Shaving 10 - 40 10 - 25
Grinding 10 - 40 10 - 25
Honing 6 - 20 5 - 15