THE TRANSMISSION SYSTEM

The transmission system carries (transmits) the movement of the crankshaft to the wheels. It has three main parts: the change gears; the clutch; the rear axle gears.

A gear is only a strong steel wheel with teeth. If a small gear with 25 teeth turns twice, it will turn a gear with 50 teeth once. This is a low gear. If a large gear with 50 teeth turns once, it will turn a gear with 25 teeth twice. This is a high gear. Gears can have as many teeth as are needed.

The crankshaft of a car engine can turn as fast as 6,000 times a minute. One turn is a revolution. So that is 6000 revolutions per minute (rpm). This speed is too great to transmit straight to the wheels. It is made less by two things:

1. The change gears.

2. The differential gears in the back axle.

Let us look first at the change gears.

The change gears

In the gearbox there are three shafts: 1. the driving shaft is turned by the crankshaft. It has a fixed gear wheel near its end. 2. The driven shaft leads to the back axle. On this shaft there are two gears. One gear is larger than the other. These gears can move along the shaft in grooves. They can move backwards and forwards. They are moved by the gear lever. 3. The lay shaft. Four gears of different sizes are fixed on this shaft. The largest always meshes with the gear on the driving shaft. So, when the engine is running the lay shaft must be turning. It must also turn more slowly than the driving shaft. Its gear is larger.

The gear box is about half full of oil.

First of bottom gear

First gear: The large gear on the driven shaft meshes with the smaller forward gear on the lay shaft. The engine is running fast but the driven shaft and the wheels turn slowly. This gear is used to start the car moving.

Second gear

Second gear: The smaller gear on the driven shaft mashes with the second largest gear of the same size. The one on the lay shaft is only a little smaller. This gear is used to increase the speed of the car.

Top gear

Top gear: the driving shaft joins straight on to the driven shaft. The car is now moving very fast. There is little difference between the engine speed and the speed of the wheels. If the car has to slow down, the driver will have to change to a lower gear.

NEUTRAL GEAR

Neutral gear. The driving shaft is turning but it is not transmitting any movements to the driven shaft. The gears are not meshing. The engine is running but it is not turning the car wheels. This gear is used when the car stops for a short time, like at traffic lights. It is also used when the engine is first started.

Reverse gear (In this gear the car moves backwards).

The fourth gear on the lay shaft meshes with the larger gear on the driven shaft. It does this through a small third gear. This gear reverses the movement of the driven shaft. This is how it does it:

The driving shaft is turning clockwise (CW). This turns the lay shaft anti-clockwise (ACW). The lay shaft turns the third gear clockwise. The third gear turns the driven shaft anti-clockwise. So it reverses the driven shaft’s movements. The car is driven backwards.

Text 14

THE CLUTCH

The clutch helps when the driver changes gear. Strong springs press it against the flywheel. These springs push against the cover plate. Round the inside of the clutch plate there is rough material. This does not slip easily. So, when the flywheel turns, the clutch plate turns too. This movement goes to the gears. When the clutch pedal is pushed down, the clutch plate is pulled away from the flywheel. There is a space. The crankshaft is turning but the movement is not going to the gears or the wheels. Now the gears can be changed. When the clutch pedal is let out the springs push the clutch plate against the flywheel again. The movement of the crankshaft now goes through the gears to the wheels. Note. Automatic cars do not have a clutch like this. They have no clutch pedal.

REAR AXLE GEARS

1. From the chang e gears the movement is transmitted to the back axle by the propeller shaft. At the back end of this shaft is a fixed gear (the pinion).

2. The back axle has two halves. A strong gear (bevel wheel) is fixed to each half.

3. A strong box covers the bevel wheels. This box turns when the axles turn.

4. To one side of this box is fixed a large gear (the crown wheel). When it turns, it turns the box and the axles.

5. The pinion meshes with the crown wheel. When the pinion turns, it turns the crown wheel and the box and the axles.

Round the inside of the box are fixed four small gears (differential pinions). Each pinion can turn freely. These pinions mesh with the bevel wheels on the axles. The differential pinions do not turn when the car is going straight. They make the bevel wheels and the axle turn at the same speed.

The outside wheel has farther to go when the car is turning. So it must move faster than the inside wheel. So the differential pinions turn when the car is turning. Now the outside wheel can travel faster than the inside wheel. So the differential pinions allow the two wheels to turn at different speeds when the car is turning.

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