741The need for double reduction final drives

The gearbox provides the means to adjust and match the engine's speed and torque so that the vehicle's performance responds to the driver's expectations under the varying operating conditions. The gearbox gear reduction ratios are inadequate to supply the drive axle with sufficient torque multiplication and therefore a further permanent gear reduction stage is required at the drive axle to produce the necessary road wheel tractive effect. For light vehicles of 0.5-2.0 tonne, a final drive gear reduction between 3.5:1 and 4.5:1 is generally sufficient to meet all normal driving conditions, but with commercial vehicles carrying considerably heavier payloads a demand for a much larger final drive gear reduction of 4.5-9.0:1 is essential. This cannot be provided by a single stage final drive crownwheel and pinion without the crownwheel being abnormally large. Double reduction axles partially fulfil the needs for heavy goods vehicles operating under normal conditions by providing two stages of gear reduction at the axle.

In all double reduction final drive arrangements the crownwheel and pinion are used to provide one stage of speed step down. At the same time the bevel gearing redirects the drive perpendicular to the input propellor shaft so that the drive then aligns with the axle half shafts.

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