Professor of Machine Design
Dept. of Mechanical Engineering
Politecnico di Milano
Institute for Machine Elements
Gear Research Centre (FZG)
Edited by Lawrence Kren
Metal gears fail for numerous reasons, some, in part, independent from the gears themselves. Assessing gear damage can be a challenge, especially in industrial equipment. Unlike lab tests designed to isolate a particular failure mode, field failures may combine several modes. Rarely do engineers have all the data needed for a complete diagnosis. And damage that happens after the first failure can alter the final appearance of damaged gears, further complicating the diagnosis.
The good news is there are only five common failure modes: bending fatigue, pitting, micropitting, scuffing, and wear.
Bending fatigue failure is the result of cyclic bending stress at the tooth root. Stress comes from a variable-lever-arm load that moves along the tooth profile during mesh. The damage process follows three stages: crack nucleation, crack propagation, and final unstable fracture. The critical section or crack nucleation site is often at the tooth-root fillet. Here, stresses, boosted by notch effect, reach a maximum.
After nucleation, cracks typically follow a path across the tooth-root thickness, though they can take different paths depending on gear shape and stiffness. For example, gears with a lightweight, thin-rimmed body could crack along the gear rim.
The fracture surface typically has two distinguishable parts; a fatigue-crack growth area and a final unstable fracture area. Socalled "beach marks" may appear when intermittent gear operation frequently interrupts the crackpropagation process. Beach marks may also show up on idler gears, where both flanks of each tooth see a reversed (alternating) stress cycle.
Pitting or macropitting is surface damage from cyclic contact stress transmitted through a lubrication film that is in or near the elastohydrodynamic regime. Pitting is one of the most common causes of gear failure. It also affects antifriction bearings, cams, and other machine components in which surfaces undergo rolling/sliding contact under heavy load.
Damage is often local to the region of negative sliding in the dedendum between the tooth root and pitch line. When mating gears are of the same material and heat treatment, expect pitting first on the gear with fewer teeth because it sees a greater number of load cycles.
Pitting starts with the nucleation of subsurface or surfacebreaking cracks, then propagates under repeated contact loading. Eventually a crack grows large enough to become unstable and reach the tooth surface. There, a small volume of material separates, leaving a pit about 100- m deep. A large pitting-damaged area can modify the tooth profile and trigger vibrations and audible noise. This type of failure happens in both through-hardened and surface-hardened gears, though the latter often exhibit what is termed micropitting.
Micropitting is a relatively recent phenomenon that has become more prevalent owing to an increased use of surfacehardened gears (case carburized, nitrided) made of better-quality, cleaner steel. Modern lubricants with sophisticated additive packages that let gears work in more extreme conditions may indirectly contribute to micropitting.
Micropitting is the formation of small craters on the tooth surface, often in the region of negative sliding below the pitch line. Micropits resemble macropits except they are roughly a factor of ten smaller or about 5 to 10- m deep (0.2 to 0.4 in.) when they first appear.
These craters nucleate from surface short cracks and progressively remove surface material, similar to what happens with abrasive wear. For this reason, engineers sometimes (erroneously) label micropitting as a kind of abrasive wear. But micropits actually are the result of rolling/sliding contact fatigue of the tooth surface and subsurface layer. Fatigue comes from repeated normal and tangential loads in a boundary or mixed-lubrication regime. The ratio of oil-film thickness to mean-surface roughness, is considered a key predictor of this kind of damage.
Micropits have light-scattering properties that impart to the affected area a frosted, light-gray appearance. That is why micropitting is also termed frosting or gray straining. Micropitting changes the tooth profile, mainly in the tooth-flank areas that see negative sliding, though severe cases can involve the entire flank. This altering of the tooth profile and meshing can raise transmission error, dynamic loads, as well as vibration and noise levels. Further, micropitting and the surface cracks that develop because of it often serve as candidate locations for macropitting. Micropitting may also promote bending fatigue failures in tooth flanks.
Scuffing, also termed "scoring" (incorrect according to gear standards), is a severe type of adhesive wear which instantly damages tooth surfaces that are in relative motion. In fact, a single overload can lead to catastrophic failure.
Scuffing welds together unprotected surfaces in metal-tometal contact. Metal particles detach and transfer from one or both meshing teeth. During successive rotations, these particles can scratch teeth flanks in the sliding direction. This type of damage generally happens in areas of high contact pressure and sliding velocity, far from the pitch surface. Conditions there are less favorable to form a protective lubricant layer that would prevent direct metal-tometal contact. This protective layer could be a thick oil film (relative to surface roughness) or an adsorbed or chemically deposited layer established by lubricant additives.
In any case, lubricant and lubricating conditions, not material strength, are responsible for scuffing damage. Scuffing often happens to new gears when tooth surfaces are not yet well run-in. Experiments show that a newly manufactured surface is able to carry only 20% of the load of a well run-in surface. The risk of scuffing goes up as lubricant degrades over time or becomes contaminated with metal particles or water. It is sometimes difficult to distinguish between surface scratches from instantaneous scuffing or those from wear.
Wear is a continuous, abrasive process of material removal from matinggear teeth that happens with or without abrasive particles in the oil. For example, hard asperities on gear flanks can remove material from mating flanks. Removal of the hardened layer from surface-hardened gears accelerates wear. Extremely worn spur gears have pointed teeth and a reduced profile contact ratio. Continual wear of tooth roots weakens the gear until it breaks.
Wear typically happens under boundary or mixedlubricating conditions lacking a thick supporting oil film that would otherwise separate tooth surfaces. Mild antiwear additives that help protect surfaces with adsorbed or reacted layers under critical lubricating conditions lessen wear.