Gear materials

Tuesday, September 30, 2014

Tags :  transmission

The materials selected for use in transmission systems, particularly the gears, play an important role in the reliability and efficiency of racing transmissions. In this month’s article we will take an initial look at those materials.

Materials for gears in general can be divided in two categories: metallic and non-metallic. Unsurprisingly, gears in racing transmissions fall into the first group. Although there is ongoing research into the potential uses of composite materials for gear construction, their application is as yet not truly feasible. The requirements for transmission gear materials are high surface hardness, good core toughness, fatigue strength for tooth bending, rolling contact fatigue resistance, and high density to resist pitting and sub-surface spalling during heavy use. The most common metals used here are wrought surface-hardening and through-hardening carbon steels.

The needs of motorsport and mass-production transmission manufacturers are broadly similar, but the very high loadings and quest for the smallest, lightest gears possible (not to mention greater freedom from cost and complexity constraints) means that the materials used for motorsport gear construction are subtly different from those found in the average high-performance roadcar.  

Surface-hardening steels are hardened to a relatively thin case depth and the various types include carburising, nitriding, and carbonitriding steels. Surface-hardening steels include plain carbon and alloy steels with a carbon content generally not exceeding 0.25% C.

Through-hardening steels can be comparatively shallow hardening or deep hardening, depending on their chemical composition and method of hardening. They include plain carbon and alloy steels with a carbon content ranging from 0.30 to about 0.55%. The mechanical properties can be tailored by varying the quantities of the allying elements and the heat treatment process: for example, the bending and surface hardness can be improved. Another example of such tailoring would be the addition of molybdenum to help reduce rolling contact friction. 

The highest grade of carburising steels tend to be found in motorsport gear applications, although such materials were often initially developed for other industrial sectors such as aerospace. Taking the gears used in a helicopter’s transmission as an example, these need to be exceptionally strong and resistant to fatigue, with the consequences of failure being far more serious than in a racecar. Also, the budgets available for aerospace development dwarf even the biggest racing operations and thus industries such as defence, aerospace and even OEM vehicle manufacturers have the resources to develop new high-performance materials that race teams do not. There are of course exceptions, and several manufacturers of racing gearboxes have commissioned bespoke steel blends to suit their specific applications.

One area in particular where steels used for motorsport and other high-performance applications differ from more run-of-the-mill materials is their ‘cleanliness’. The number and type of inclusions in steel billets destined for use as gears can have a major impact on ultimate fatigue resistance and strength, so the very best steels are smelted in a vacuum, where there is minimal opportunity for contamination of the metal. It is not unusual for very ‘clean’ steels to be vacuum melted up to three times, giving them a very consistent molecular structure but making them anything but cheap!

This is only a brief overview of the materials and requirements that need to be accounted for when specifying gears for use in racing transmissions, but in future articles we will revisit the subject to cover specific areas in greater detail.

Written by Lawrence Butcher

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