A comparison of surface roughness measurement methods for gear tooth working surfaces

Surface roughness is a critical parameter for gears operating under a variety of conditions. It directly influences friction and contact temperature, and therefore has an impact on various failure modes such as macropitting, micropitting and scuffing. Typically, gear tooth surface roughness is measured using a stylus profilometer, which yields a two dimensional cross section of the surface from which roughness parameters are taken. Stylus profilometry can produce inconsistent results if measurements are not executed correctly. Variables such as measurement parameters, stylus tip radius, and repeatability of stylus orientation relative to the gear tooth can all impact measurement results. This paper examines measurements from one “shop floor” and one “metrology lab” profilometer, both using two different stylus tip radii on the same gear teeth. Measurements from ground, shot peened and superfinished surfaces are compared. Although stylus profilometry is convenient, a limited amount of information regarding the surface topography of the tooth is retained. Tooth replicas subsequently evaluated with optical interferometry offer an alternative means to measure surface roughness, and allow for retention of a much more complete representation of the tooth surface for future evaluation. The three dimensional surface profile generated by optical interferometry can also highlight features that would be difficult to evaluate using stylus profilometry. This paper compares roughness measurements made using optical interferometry of gear teeth with optical interferometry of tooth replicas. Two different replication techniques are evaluated. The same teeth measured using stylus profilometry are used, thus the interferometry results are directly compared to the profilometry measurements. Lastly, when tooth replicas are taken and measured with optical interferometry, the reference frame of the gear from which the replica is taken is not immediately apparent. A method for correlating tooth replica coordinates to roll angle is also presented, which is shown to be useful for plotting roughness trends at points of interest over the active profile of the tooth.