Vehicle Differential

Geared device between the driven wheels to allow varying rotational speeds of the wheels around corners. We see differentials in everything from the Chinese cart that has an arrow that always points south, in steam traction engines through to complex electromechanical differentials in formula one racing cars.

My particular interest in the mechanical differential is for the steam traction engine and hence I’m reading up on the fundamentals of the design so that I can get it right in miniature form.

Miniature Traction Engine

Axle Ratio – Axle ratios in the differential ordinarily vary between 2.5:1 and 4:1. The ratio is determined by engine horsepower, torque output and rpm range, and by the vehicle application. The higher the numerical drive ratio, the higher the engine’s rpms in relation to road speed.

Axle Seat Angle – The angle formed by the axle seat and the centreline of the axle pinion; by convention, the angle is positive if the front end of the axle seat is lower than the back end.

Cage – The rotating metal fram which encloses the differential side gears and pinion gears inside the axle casing.

Case – The steel unit to which the ring gear is attached. The case drives the spider gears and forms an inner bearing surface for the axle and gears.

Gears – The gears that transmit engine power to the driving axles and are arranged so as to permit the rear wheels to turn at different speeds as required when the vehicle is negotiating a turn.

Lock – A mechanism which eliminates the action of the differential so that both wheels can be driven for better adhesion on slippery surfaces.

Pinion – The bevel pinion in the differential.

Side Gear – The bevel gear on either side of the differential into the centre of which the axle shaft fits.

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