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By unknown | Jul 16, 2008 | COMMENTS [ 0 ]

Wherever you find a rotating shaft it will always be supported on bearings to keep it in position and to minimise the friction at the supports.

So bearings are all around us, from the wheels, pedals and steering head of the humblest bicycle, to the hard drive of the most sophisticated computer, from the mechanism inside the smallest wristwatch to the prop shaft of the heaviest ship.

In a car you will find well over 100 bearings, which will be of two main kinds: plain bearings, like the shell bearings supporting the crankshaft and bushings in which the gudgeon pins swivel; and rolling-element bearings, like the wheel bearings.

We will come back to plain bearings in a future article. Today we focus on rolling-element bearings.

The term "rolling-element" refers to the fact that these bearings carry the load and reduce friction by providing smooth metal balls or rollers and smooth inner and outer surfaces for these elements to roll against.

In bearing terminology the surfaces between which the elements roll are called the inner and outer races. The inner race fits tightly over the rotating shaft, so that it rotates together with the shaft, while the outer race is held firmly in the support housing.

If the rolling elements are balls, we speak of a ball bearing, and if they are rollers (which can be cylindrical, tapered or barrel-shaped) we speak of a roller bearing. Bearings in general have to deal with two kinds of loading - radial and thrust.

Depending on where the bearing is being used, it might have to cope with radial loads only (that is, loads always acting along a radius drawn from the centre of the bearing to the outer race, as happens in the bearings supporting the shafts of pulleys connected by a tensioned drive belt), or it might need to cope with thrust loads only, as is the case with the clutch release bearing and the bearings supporting the seat of a swivelling chair, or it may be called on to carry a combination of radial and thrust loading, as the wheel bearings on a car have to do.

Here the radial load comes from the weight of the car and the thrust load comes from the cornering forces when you go round a bend.

Within the category of ball bearings there are many variations. A particularly versatile type is the single row, deep groove ball bearing, which is robust, relatively cheap to produce and able to cope with thrust loads in both directions, in addition to radial loads, even at high speeds.

It is, however, not tolerant of misalignment between the shaft and housing, as would happen if the shaft is deflected.

If that is foreseen, a shallow-groove bearing, or alternatively a self-aligning ball bearing, containing two rows of (caged) balls and a common concave outer raceway, is preferred.

Roller bearings are better than ball bearings at supporting radial loads for a given size.

The ordinary, cylindrical type is often found inside manual gearboxes where it doesn't have to deal with thrust loads or misaligned shafts.

A variation of the ordinary roller bearing is the needle roller bearing. Its rollers are long, thin cylinders, like needles.

Caged needle roller bearings are seen in manual gearboxes where the driven gears run on the mainshaft. They are perfect for this high-speed, lightly-loaded application. Uncaged needle rollers are used in the four cups of a typical universal joint where the speed is lower but the loads higher.

The invention in 1898 of the tapered roller bearing by Henry Timken, a carriage maker of St Louis, provided a bearing that could cope with both radial and thrust loads.

These bearings, in which both inner and outer raceways are conical sections and the rollers are also tapered, are often used as wheel bearings on vehicles, usually as back-to-back pairs to counter thrust in both directions.

lNext week we'll look at how bearings are manufactured.


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