Ed Evans Talks Technical
Could plastic gearboxes be the next big thing?
“Manual gearboxes will last hundreds of thousands of miles if properly maintained and not abused”
Inside our manual gearboxes the gear changing is done by simple rods sliding gears about on a shaft – there’s nothing high-tech about it. And although the synchromesh assemblies that allow gears to engage without grinding are fairly complicated gadgets, again at detail level their engineering concept is simple. Most of the engineering effort goes into the gears themselves, aiming for durability, smoothness, low vibration, slick tooth engagement and low noise. That’s achieved by using the most appropriate steel alloys, tailoring the hardening process to reduce wear on the teeth, and designing the subtle angles of the tooth profiles to engage and transmit torque efficiently. The result can be a compromise because the best tooth angles for power transmission are not necessarily the best for wear or for minimising noise and vibration. Despite this, manual gearboxes can last hundreds of thousands of miles if they’re not abused and are properly maintained. The other vital component is the oil which provides lubrication between the teeth, helping them slide over each other with minimal frictional resistance, wear, vibration and noise. The oil has to do its job at cold startup temperature and at extreme temperature when the gearbox is hot and under load. Quality oil, topped up and renewed at the right times, also reduces the rate of bearing wear – and it’s the worn bearings that allow radial shaft movement which, in turn, damages the oil seals, leading to leaks.
Gearbox killers include ignoring oil changes, wrong oil specification, harsh or sudden acceleration, not checking or replacing the oil after wading, and labouring the box under load (such as towing or prolonged climbing) in hot climates without an oil cooler.
The next radical development may be plastic gearboxes. In prototypes, the use of structural plastics allows different tooth profiles that transmit drive more efficiently without the compromise of noise associated with steel gears. Plastic gears are lighter, saving emissions and, because the material allows a smaller tooth height, there is less sliding contact between gear teeth, which lowers frictional losses. Plastic gear casings save further weight. But the technology is still under development, and won’t be seen in production vehicles for some years. Until then, look after your old steel gears with sympathetic driving and regular oil changes.