Surface treatments for timing gears
With the dominance of the four-stroke engine in both the automotive passenger car market and most types of motorsport, there are plenty of opportunities for suppliers of timing gears to improve either on standard equipment or to optimise these components in bespoke engines.
With timing gears, what we want often want to do is improve durability while decreasing inertia and component mass. A number of surface treatments can help us achieve these goals.
Carburising, also commonly referred to as case-hardening, is a technique that not only improves wear resistance but imparts high levels of compressive residual stresses into the surface of the component. This gives the gear a much greater fatigue strength, as it effectively lowers the maximum tensile service stresses at the component surface. This helps not only in terms of preventing tooth fatigue breakages, it also provides a degree of protection against rolling and sliding contact fatigue that can lead to pitting damage on the tooth flanks. This process cannot be applied to existing gears; the steel used must be of a certain composition suitable for carburising, and it is likely that the teeth will need to be ground following carburising in order to remove the effects of distortion that often occurs when this process is used.
Nitriding is another process that imparts compressive residual stresses to the surface of a component, and it also needs to be done on a special composition of steel. It is used for gears, but less commonly so than carburising.
Shot peening is a mechanical process that aims to put the surface of the gear into a state of residual compressive stress. The usual method of shot peening is to propel a high-velocity stream of hard media toward the component surface; there are other methods of shot peening, such as ultrasonic, but these are very much in a minority.
Shot peening is often combined with carburising in order to give a high degree of compressive stress – the resulting fatigue resistance offers an improvement over either of the processes in isolation. It can of course also be used in isolation, and on existing components with success.
Multi-stage peening, using different peening media and intensities, is often specified for the timing gears in race engines, and this allows an optimum compressive stress profile, with the maximum level of compressive stress at the surface. After initial peening, the maximum level of stress can sometimes be found below the surface: there is often an advantage to have the maximum compressive stress at the surface, especially where a component is loaded in bending.
There is a well documented increase in fatigue strength owing to improvements in surface finish, so we can expect to find gains in durability by using treatments that decrease the surface roughness of the gear teeth. Polishing or superfinishing processes, combined with carburising and shot peening, have been shown to be very effective.
A number of companies find an advantage in making timing gears from through-hardening steels, and dispensing with carburising or nitriding, often using both peening and polishing to acquire a useful degree of compressive residual stress. There is a focus article in Race Engine Technology, issue 74 (November 2013), which deals with carburising, nitriding, peening and polishing in more detail.
Written by Wayne Ward