Car Parts and Accessories

Why a car has a gearbox

Road wheels turn at about 1000 rpm if a car is to cover 110 km in an hour. At this speed the engine may be turning at 4000 rpm. The relationship between the rotation speeds of the engine and wheels is the axle ratio. In this case it would be 4:1. As long as the car is driven at a steady 110 km/h on the level, this gearing is satisfactory, but when the vehicle is confronted by a hill, a lower gear must be selected or the engine will begin to falter and eventually stall as little power is developed at low engine revs. Selecting a lower gear enables the engine to turn faster in relation to the road wheels and also multiplies the torque.

Manual transmissions on front-wheel drive cars generally have all ratios indirect. Power flows from the clutch, along the input shaft, through a pair of engaged gears and back out the same end of the transmission. The differential uses a ring gear driven from the output shaft rather than a pinion gear and drive shaft

Leverage in Gears

A simple lever (top) enables a small force moving through any distance to move a large weight through a smaller distance.

Gears work like a series of levers (bottom). The bigger wheel rotates more slowly but carries a greater turning effort

Helical gears have teeth cut at an angle to the axis of rotation. They are curved and form part of a spiral, like a screw thread, called a helix. Helical gears run smoothly and operate quietly

Spur gears, which have straight teeth cut parallel to the axis of rotation, are noisier than helical gears.

In modern gearboxes, spur gears are used mainly in the reverse gear system

The clutch driven-plate is splined on to the input shaft. Splines around the hole fit into grooves milled along the shaft. A component splined on to a shaft rotates with it and can slide along it unless locked in place.

How gear ratios are determined

The lowest gear in the gearbox must multiply the engine torque sufficiently to start the fully laden car moving up a steep hill.

A small car needs a bottom gear of 3,5:1. Other typical gearbox ratios in a small car with a four-speed gearbox are 2:1 in second, 1,4:1 in third and 1:1 in top. All these are multiplied by the axle ratio,

So that, if the axle ratio is 4:1, the corresponding ratios between the engine speed and the road-wheel speed are 14:1, 8:11, 5,6:1 and 4:1.

The same car with a bigger engine developing more torque would not need such a low bottom gear, so the gearbox ratios might be adjusted to 2,8:1, 1,8:1, 1,3:1 and 1:1. Closer spacing of gear ratios makes gear changing easier and quicker.

Alternatively, a bigger engine might be designed to provide easier performance, so that the driver need not change gear so often. This could be provided by a three- speed gearbox. Bottom gear would still have to be 2,8:1 to retain the same climbing ability, but second gear would be about 1,5:1, making a convenient step between bottom and top gears.

NEUTRAL

All the gear wheels except the three needed for reverse are constantly in mesh. Wheels on the output shaft revolve around it and those on the layshaft are fixed. In neutral, no power is transmitted

FIRST GEAR

When a gear is engaged, the appropriate wheel is locked to the output shaft and power, is transmitted. In first gear, the widest ratio is used to provide the greatest torque output, for low-speed driving

SECOND GEAR

In second gear, a narrower ratio gives a smaller increase in turning effort

THIRD GEAR

Third gear uses a still narrower ratio, and top gear (centre) is obtained by coupling the input shaft directly to the output shaft so that power is transmitted through the gearbox without the help of the meshing wheels

FOURTH GEAR

Direct-top gearboxes are usually fitted to cars which have the engine in front and the driving wheels at the back. The direct drive in top gear provides negligible loss of power through friction hut about 3 per cent is lost in other ratios

REVERSE

In reverse gear a third wheel, the idler, has the effect of reversing the normal direction of the output shaft

Indirect gearing

CARS with the engine and driving wheels at the same end usually have the final drive located between the engine and the gearbox, to save space. Each gear ratio is indirect. Direct drive (1:1) will not occur in any gear.

Cars with five-speed gearboxes will usually have a fourth gear that is direct and a fifth gear that is less than 1:1 making top gear indirect. This system is less complicated than an overdrive fifth gear.

Second gear in an all-indirect gearbox

Only two gear wheels are involved in transmitting power from the input shaft at the top to the output shaft beneath. The upper gear is smaller than the lower gear, giving a speed reduction

Top gear in an all- indirect gearbox

The two gears which transmit the power from the input shaft at the top to the output shaft beneath differ in size from the gears used for second gear, and the output shaft runs faster

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