Gear materials and manufacture
In previous RET-Monitors we have looked at the construction of racing gearbox casings; however, as regards performance and durability, the materials and manufacturing techniques used for their internals are even more important.
Although often housed in very different packages, the gears used in transmissions destined for racing use are essentially the same whether they are for a Formula One car or a WRC racer. Generally, racing transmission manufacturers produce gear sets from high-quality steel billets, and it would be fair to say that many manufacturers use very similar materials. This is because there is a limited number of steel producers worldwide who can refine high-alloy steels to the level required for producing reliable components. Some companies, particularly those involved in the construction of Formula One transmissions, have their own in-house metallurgy formulae, often developed in conjunction with mainstream steel suppliers.
There are notable differences in the manufacturing process used to produce gears intended for mass production and those designed specifically for the demands of competition use. Traditionally, high-volume gears tend to be made from forged blanks, whereas competition components are usually CNC-machined from billets. By using billets, the manufacturer can better ensure that material consistency is maintained from one batch of gears to another.
For all but the most severe applications, cutting will usually be done using specially designed and precisely ground tooling, in order to achieve the very close tolerances needed on complex tooth profiles. Lower quality gears are generally produced using off-the-shelf tools to save on costs, and these do not provide as fine a surface finish, which compromises tooth strength. Also, any misalignment due to poor machining has an impact on the performance and wear characteristics of dog engagement gears. Mass-production transmissions generally feature synchro engagement systems, which are much more forgiving in terms of tolerances, due to the slipping of the brass cones used in the synchro mechanism, making it practical to use less precise manufacturing processes.
Such levels of accuracy in manufacturing are vital because tooth design and profile is critical to the power-holding capability of a gear set. This requires that a number of factors be taken into account, including (but not limited to) gear ratio, tooth count and load paths through the gear shafts. For this reason, in the most highly stressed racing transmissions applications, even traditional machine cutting is not sufficiently accurate to produce gears to the tolerance required. Instead, these gears are ground to shape, a process involving the use of shaped grinding wheels that form a ‘negative’ of the gear tooth profile. Throughout the grinding process, the profile of the grinding bit will be constantly monitored to ensure it remains within tolerances.
Traditionally machined gears must be heat-treated after cutting, which can introduce measurable distortion into the piece, negating the precision of the tooth profiles. Grinding, however, takes place after heat treatment and thus gives a finished part with much closer tolerances. A second benefit of grinding is that it allows for gears and shafts to be made as a single component, which is not always practical due to tool path constraints using traditional milling methods. Normally, a cut gear will be welded to its shaft, introducing another potential source of distortion and reducing its structural integrity. With a ground gear this is not an issue, and again gives a better fitting and stronger component.
The final stage of the gear production process will often involve surface treatment of the gears. In some applications this will involve the use of DLC (diamond-like carbon) or similar coatings, but it is more common is for gears simply to be ‘superfinished’ using a number of proprietary abrasive or chemical processes. Depending on the process, such finishing techniques can improve by several percentage points factors such as component durability or result in reduced friction and thus increased efficiency.
Written by Lawrence Butcher