Lubrication of bearings

CLUTCH

The word "clutch", as used in connection with automobiles, indicates a device attached to cars having change speed gears of the sliding type, which permits the engine to be connected with, or disconnected from, the transmission, so that the car may, or may not, move while the engine is running.

The clutch is connected and disconnected from the flywheel of the engine by a foot lever. When disconnected from the flywheel of the engine, there is no connection between the engine and rear-axle.

When the clutch is connected with the flywheel of the engine, the power of the engine is transmitted to the rear-axle, if the gears of transmission are not in "neutral" position. If gears are in neutral position, the power of the engine will end at the end of the secondary shaft of transmission.

If the clutch is "in" and the gears are meshed, the course of the power from the crankshaft of the engine will then be through gears, through drive-shaft, driving pinion, large bevel gear on differential, through differential gears to axle-hafts and wheels. The car will move and the clutch will permit the car move faster or slower.

Clutches have two chief parts: one part (flywheel) is attached to the crankshaft of the engine, the other part (disk or cone) is attached to the main shaft of the transmission.

When the two parts are separated, i.e. when the "clutch" is thrown "out" by clutch pedal, they are independent of each other and the engine can run without moving the car. The left-foot pedal on all cars of standard design is the "clutch pedal".

GEARBOX

When a bicyclist wants to race on a level track, he gears up his drive wheel with a larger sprocket, so that one revolution of the crank takes him further Yet if he takes his wheel, with this large sprocket on the pedal shaft, out on the road where there are hills, he must get off and walk, or exert an extra lot of power.

The same principle applies to the automobile. For this reason the automobile is provided with four or five changes of gears. These gears are contained in a gearbox usually placed at the back of the clutch.

The principle, upon which all change-speed gears work, is the fact that when two gear-wheels or spur-gears are meshed together, the larger wheel turns more slowly than the smaller.

In the gear-box there are two shafts – the upper one coming from the engine through the clutch, and the lower one continuing to the back axle.

Each shaft is fitted with four or five gear wheels of different size.

Those on the upper shaft are fixed to the shaft itself, but those on the lower shaft are able to slide on a keyway, to right and left along the shaft. The lower shaft is square so the sleeve of the gearwheels can slide backward and forward, but they cannot revolve independently of the lower shaft. In order to vary the speed of the car, it is only necessary to slide the gear wheels along the lower shaft until the correct two gears come into mesh to form the gearing required.

THE DIFFERENTIAL

It is necessary to provide an automobile with a differential, in order that the rear wheels may revolve at different speeds when the car turns a corner, while at the same time both are being driven by the engine.

The differential is an automatic mechanism which operates according to the resistance of the road against the revolving wheels. When a car turns a corner, it is necessary for the outside wheel to revolve faster, because it has a longer distance to travel than the inside wheel. The outside wheel revolves faster than the inside wheel if the car turns to the right.

The axle shaft of the outside wheel being attached to the wheel, must revolve faster than the axle shaft of the inside wheel when turning to the right, and slower if turning to the left. Therefore, to compensate for this difference in speed of the two wheels, the bevel gears on the ends of the axle shafts mesh with the small bevel gears, called compensating or differential gears.

LUBRICATION OF BEARINGS

The shafts which carry the power from the motors to the machines need some kind of support to keep them steady. We call these supports bearings. There are different types of bearings which we classify according to whether they take the load on the shaft or thrust it along the axis of the shaft. The former type is known as a journal bearing and the latter type as a thrust bearing.

The rotating shaft bears on a stationary bush or tube. We therefore have two metal surfaces in close contact with each other, and sliding over each other often at high speed. This causes friction and the bearing will become heated. So we have to protect the metal surfaces from overheat­ing and damage. First of all, we avoid making the shaft and the bush of the same material. The shaft itself is generally of steel, so we use another metal such as cast-iron or bronze or white metal for the bush, and this will prevent damage to the shaft. But of course it will not prevent over­heating.

We can reduce the danger of overheating by using lubrication – a thin film of oil between the two metallic surfaces to keep them apart. The internal friction of oil is much less than the friction between two solids, and generates less heat. Lubrication also offers another advantage. A film of oil on the metal surfaces will prevent them from corroding by protect­ing them from the air.

The rotation of the shaft carries the film of oil round the inside of the bearing and keeps the shaft from contact with the bearing. We can feed the oil into the bearing in several ways. Sometimes we allow it to drip down under the influence of gravity. More commonly, a pump feeds it in under pressure. In motor-car and other engines, we half cover the bearing in an oil-bath, and oil splashes up into it.

The amount of friction can be reduced even more with ball or roller bearings with some grease as lubrication.