Gear Failures
Arthur Griebel
Gears are one of the most common and fundamental forms of mechanical power transmission, and gear design and manufacture are highly refined. But because gears are so common, and because they are always contained deep within the host equipment, gear failures are both frequent and expensive.
Gear failure analysis is more detail oriented than many other types of failure analyses because the failure usually cascades through multiple teeth and sometimes through multiple gears. As an example, teeth that break off by fatigue cracking can be entrained in the mesh and cause overload failure of other teeth. Anybody that has opened a failed gear box and removed handfuls of fractured teeth and metallic debris knows how extensive the damage can be.
During the failure analysis, the analyst must collect enough evidence to reconstruct the sequence of failure at least to the extent of identifying causal and consequential fractures. The fracture mode and crack initiation sites of all broken teeth must be documented. The intact teeth must be examined for evidence of surface wear, sliding contact, tooth tip interference, and other features that might indicate the condition of use of the gear. This is critical because correctly identifying fatigue cracking but failing to recognize the fatigue was caused by tooth tip interference will obscure the corrective action.
All of the preliminary inspection must be completed before the gear is cut up for the metallurgical inspection. The roots are inspected for cracks, the pitch line is inspected for frosting or spalling and evidence of sliding contact, the wear pattern is observed, etc. We attempt to make sense of the observations and interpret them for the customer. For example, gear teeth with little or no wear show that the gear was not heavily loaded. If it failed by overload, an unusual event is likely the cause, not the normal conditions of use.