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spline drive gear teeth wear
Gear tooth is a cantilever beam
gear tooth  pitting
Gear Alignment and The Contact Zone

The picture to the left shows that the pitting is on the right side of the teeth, indicating that the highest contact pressures were to the edge rather than the center. Poor load distribution along the face width means that very little load is on one side and and a larger load on the other side. High contact pressures can result in tooth bending stresses higher than the designed limits, with tooth breakage and surface contact fatigue.

The geer teeth tells you when the rotational axis of the gears are not parallel. If the shafts are mounted in bearings that have positional errors; or the shafts, bearings, or housings deflect under load, then the gears will no longer be parallel to one another.

Seldom do gear teeth contact evenly. Gear teeth will only be parallel on the drafting table. In service they are only parallel by accident --and then only through a small load range. The contact between two mating gear teeth is influenced by:
1. by the elastic characteristics of the housing in which they are contained,
2. by the elastic characteristics of bearings by which they are supported,
3. by the elastic characteristics of the shafts upon which they are mounted,
4. by the elastic characteristics of the gears themselves,
5. by the accumulated dimensional errors in all the supporting parts as will as the errors in cutting of the gears,
6. by the necessary and accidental clearances in the supporting parts,
7. by the amount and nature of the warpage in heat treatment.

When inspecting gears for alignment problems, it is helpful to have knowledge of the wear patterns on previous gears of the same equipment. Is the alignment pattern unusually different from past gears? Obviously, gross misalignement is always cause for concern and further investigation, but some misalignment is normal. Let experience be the judge.
Inspecting Gears
Seldom do maintenance manuals stipulate the rejection criteria for gears. If this is the case you can use standard industry practice, experience, and informed opinion.

The criteria that I established for small piston aircraft engine gearing is the following:

1. If the gear runs smoothly and the tooth face shows even polishing without wear or pitting, then the gear is doing its job.

2.  Inspect steel gear teeth using magnetic particle inspection (Magnaflux). Pay particular attention to the root fillet. Any cracking is cause for rejection. Rejection wording can be something like "fatigue crack at fillet root".

3. Pitting that penetrates the case hardening, such as the pitting shown to the lower left is cause for rejection. Rejection wording can be something like "fatigue pitting at tooth flank"

4. Scuffing or wear that distorts the tooth's profile is cause for rejection. Rejection wording can be something like "scuffing damage has distorted tooth profile"
Uneven contact and pitting
A gear tooth is a stubby cantilever beam. Gear teeth usually break at the base of the tooth on the tensile (loaded) side (red arrow). Inspect the tooth root for cracks. Typically in small aircraft engine gears, magnetic particle inspection and 10x visual inspection is used.
spur gear contact
The metal cycles between compression and tension as the gear tooth both rolls and slides across. Inspect tooth flank for surface fatigue pitting. The flank is simply the surface that contacts the mating gear.

Corrosion will eventually occur on the gears, and it will cause small surface pits. These surface pits will eventually cause spalls and ultimately to cracks.
bendix distributor gear with broken teeth
Broken gear teeth are often signs of a problem somewhere else in the gear train, and not the tooth itself.  In the example shown in the picture, the usual suspect is excessive clearance between the gear support shaft and the bushing. Typically, the excessive clearance is because the bushing has worn. Replacing the broken gear will fix the damage but won't fix the problem. The bushing (part of the distributor block) also needs to be replaced.
Bendix Magneto Distributor Gear
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