Car Parts and Accessories

Changing Gear without a Clutch Pedal

The basis for most automatic gearboxes is a set of gears called a planetary or epicyclic gear train. This consists of a sun wheel, planet gears rotating round it, a planet carrier, and an outer ring gear which is known as an annulus.

The sun wheel is mounted centrally. In the simple epicyclic gear, a pair of planets revolve on spindles supported by the U- shaped planet carrier, which is mounted on a shaft on the same axis as the sun wheel. As the carrier rotates, the planet gears turn on their spindles and orbit the sun wheel, with which they mesh. The planet gears also mesh with teeth on the inside of the annulus, which can rotate around the sun and planet gears, also on the same axis. By holding stationary any one of these parts, the others can be rotated to provide different speed relationships according to the respective sizes of the gears.

If the sun wheel is locked and the planet carrier revolves, the planets will roll around the sun wheel and drive the annulus in the same direction as their carrier, but at a different speed.

If the planets are locked, or if the sun wheel is locked to the annulus, the gear train will revolve as one unit, with the planet shaft, sun-wheel shaft and annulus rotating at the same speed.

If the planet carrier is locked and the sun wheel rotates, the planets will drive the annulus in the opposite direction.

If the annulus is locked and the planet carrier revolves, the planets will roll inside the annulus and drive the sun wheel in the same direction at a different speed.

In order to obtain the required number of gear combinations, an automatic gearbox has two, three or four sets of planets `compounded’ together. Some parts of each are connected together permanently; others are connected temporarily or stopped by a system of brake bands and clutches selected by hydraulic shift valves which are located in the bottom of the gearbox. Oil, under pressure to operate the brake bands and clutches, is provided by a pump which draws its supply from the gearbox lubricating oil. Sometimes two pumps are used, driven from the input and output ends of the gearbox.

The gear selector controls the shift valves directly, unless automatic forward drive is selected. Then the operation of the valves is governed by throttle opening and road speed. With a wide throttle opening, the oil pressure is reduced and the gears tend to stay in a lower gear.

When the car reaches a pre-set speed, a governor allows the pump pressure to take over from the throttle control and a change to a higher gear is made.

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