There’s more to gearboxes than simply teeth

There is still a lot of reliance on the experience and flair of the engineer and fitter,’ states John MacIntosh, design engineer at Hewland Engineering.

Hewland manufactures and supplies gearboxes for most low and medium power motor racing classes, and in the higher power categories to a number of works-sponsored rally sport teams, many GT cars, American championship cars and Formula One.

Conventional toothed spur gears are used instead of the helical gears found in standard automotive design, as noise is not a factor, but the power losses and thrust loads of helical gears would be. However, at the extreme speed of operation, the gear tooth involute shape has to be designed to allow for the highly loaded tooth to bend. If no allowance is made, a tooth does not disengage the adjacent tooth on exit, and wear and vibration soon damage the gears.

Vital to successful running of the gearbox is selection of the correct bearing for each of the rotating elements in the gearbox, which is why FAG bearings are specified. The factors involved are often conflicting as saving weight is one parameter, while another is the load function of the bearing, whether it has to take thrust, has a location or support role, is preloaded or even as part of a crown wheel and pinion arrangement.

Generally, the layout of the box is kept simple, comprising three shafts with a bearing at each end. The shafts are hollow and as short as possible, to avoid the need for a bearing to support the centre of the shaft.

Additionally the gearbox should be designed to be able to be stripped and re-assembled as quickly as possible This is not only for repairs and maintenance on track mechanics need to be able to vary the ratios of each gear to match the driver, the track design, the temperature and even the atmospheric pressure.

FAG (UK) Tel: 0800 269371