 Gear pitting area fraction

The gear pitting area fraction measures how much of a stated tooth-flank inspection region is classified as affected by gear surface pitting:Both areas must use the same geometric convention. A projected photograph, an unwrapped flank map and an actual curved surface need not assign the same areas. The gear pitting inspection mask specifies which part of the flank is included, so that an apparent change of damage is not merely a change of inspected region.

For an original image-counting example, let the inspected region contain $N$ equal-area pixels and let $n$ be classified as pitted. If their common scale is $s$ length units per pixel,Thus $n=2400$ out of $N=120000$ gives $\varphi =0.02$, or $2\%$. The common scale cancels from the ratio; uncertainty in the classification boundary, perspective distortion or the inspection mask does not. Image segmentation should therefore be checked against the actual surface, with consistent illumination and a recorded threshold or classification rule.

Combining regions requires area weighting. Suppose one region has $0.6\mathrm{m}{\mathrm{m}}^2$ of pits within $30\mathrm{m}{\mathrm{m}}^2$ and another has $1.2\mathrm{m}{\mathrm{m}}^2$ within $120\mathrm{m}{\mathrm{m}}^2$. Their fractions are $2\%$ and $1\%$, but the combined fraction is $1.8/150=1.2\%$, rather than the unweighted average $1.5\%$.

The ratio measures extent under its chosen convention, not pit depth, lost volume or a universal remaining life. A 2009 NASA endurance study paired repeated gear inspection with vibration and oil-debris monitoring. Its test rig used accelerometers, shaft timing signals and an inductive debris sensor, giving observations of different aspects of the evolving damage.^[ref] A surface fraction can complement these measurements, but it does not make their signals interchangeable.