Wear is a well-known criterion of failure for gears. Wear is a result of metallic contact between the tooth flanks. But when a lubricant is involved, the wear-generating mechanism can be quite different. If the pitch line velocity is 0.5 m/s or higher, the lubrication film still has a dominant effect, the metallic contact is only partial, and the gears are running in the mixed-film or full EHL regime. At very low pitch line velocities (less than 0.5–1.0 m/s), the boundary lubrication prevails, and the metallic contact is dominant. The wear behavior in this case is completely different from wear occurring in mixed-film regime. Wear occurring at very slow speed is called in U.S. literature ‘slow-speed wear’ or ‘adhesive wear’ [1]. In German literature also, the term ‘cold wear’ [2] (in contrast to scuffing, which is sometimes called ‘hot wear’) is used. In this paper, slow-speed wear will be discussed, defined as the following: wear occurring either on lubricated gears at very low pitch line velocity or on dry-running gears.

Wear on gears is not an intensely researched topic, so little literature can be found. When compared with the exhaustive investigations carried out on other phenomena such as macropitting, scuffing, micropitting, or tooth flank breaking, it seems that slow-speed wear has literally been left out! But despite this, there are applications in heavy gear applications where wear is a criterion that cannot be ignored.

Worldwide, until 2014, only one standard existed, AGMA 925 [3], which describes wear and proposes a method to evaluate the risk of wear. As will be discussed further on, the wear described by AGMA 925 is the wear occurring in the mixed-film regime; wear at very slow speed is not covered by AGMA 925.

In a quite different area of application, wear is a very important topic—for dry running plastic gears. Over the past few years, the authors have worked closely with a number of manufacturers of plastic gears and the University of Erlangen (Germany) to investigate the problems of gear wear in detail. A calculation method could be developed that can be used to predict where and when local wear will occur on a tooth flank. Parts of these findings have also just been published in the final version of the German standard VDI 2736 [4].

The basic mechanics of slow-speed wear of metallic gears is the same as for dry-running plastic gears. However, the wear coefficients to be applied in each case are very different, and the influence of the lubricant (in particular, the effect of the lubricant additives) is crucial. In 1980, at the FZG in Munich, Plewe [5] published investigations of the adhesive wear behavior of lubricated metallic gears (pitch line velocities 0.007–1.0 m/s). The wear coefficients were determined for additive-free oil. However, a "factor for the influence of lubrication" is required before Plewe's data can be used for a modern gear lubricant, and up to now, very little is known about these factors.

If the wear coefficient is known, the distribution of wear can be defined over the tooth contact area in the contact analysis. If the step-by-step change in the tooth flank (due to wear) is then also taken into account, a realistic prediction of the progression of wear and its effects on noise and vibrations can be made.