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Gearbox damage caused by coasting
Page 58
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WITH THE increase in permitted road speed to 40 m.p.h., certain heavy goods vehicles are suffering premature gearbox damage, which can arise from frequent or prolonged coasting with the gearbox in neutral. This unwelcome return of the habit of coasting has come about mainly because the maximum governed road speed of many eight-wheeled vehicles barely exceeds 30 m.p.h., and cannot be altered due to rear-axle ratios arranged for trailer operation at 20 m.p.h. A simple answer would be a chartge of ratio to. give a higher speed, but with double drive in the majority of cases the cost would be prohibitive. Therefore, many rigid vehicles will have to operate at this now slow pace until retirement, with the temptation to coast ever present. Apart from the dangers of running out of brakes during coasting, much of the damage and wear caused could be avoided if the driver realized exactly what happens when coasting on long descents. With the gearbox at correct oil level and at normal temperature the constant-mesh wheels are idling slowly, together with the counter or Iayshaft, and the warm oil is barely flowing round the fixed wheels on the lower shaft. Meanwhile, directly above (in most boxes) the mainshaft is really motoring round, well above intended speeds and, 'worse still, no oil is arriving at the mainshaft rear bearings, or to the speedo drive, and at the front end of the main shaft the spigot race may also become starved. These parts are travelling at high speed, they have flung off what oil they retained during drive and are soon operating dry with an exaggerated rate of wear quickly in evidence, until the gears are again in mesh_ This situation can apply to any splashfed gearbox with the shaft layout arranged as described.
by
Handyman