Bearings are intended to support load as well as to reduce friction between moving parts to a minimum.They can take various forms, such as plain bushes, split shells or roller bearings, and they are usually made separately from the component they support so that they can be replaced without changing the whole component. Although servicing will prolong a bearing’s life, replacement will eventually be necessary.
The simplest type of plain bearing is a ‘bush’ – a tube or cylinder made to accurately measured inside and outside diameters. When the shaft which runs in the bush shows any radial play (up and down or rocking movement) or when the bush itself is scored or pitted, it must be renewed. It is often a push fit in its housing, but it may be fitted with a greater degree of tightness called an interference fit. This means that the bush is actually bigger than the hole into which it fits and is assembled by using a press or a drift, or by heating the housing until it expands enough to allow the bush to be fitted. On cooling, the housing shrinks to grip the bush.
Removing bushes of this type calls for an extractor – usually a special tool supplied by the manufacturer – or a drift. This is a solid piece made to fit into the bush with a shoulder which butts up against the bush, so that when it is tapped with a hammer it will push the bearing out of place. An alternative is carefully to drill the bush until it collapses.
New bushes are fitted in the same way, by either pressing, drifting or ‘shrinking’ them into the housing. Because these processes may distort the bush it is then necessary to ream its inside diameter to give the necessary clearance on the shaft. A reamer is an accurate cutting tool and this is a job which calls for some skill, especially when the bush has to line up with another bearing and a pilot piece is attached to the reamer, the job being known as line reaming.
Plain bushes are often found in small-end bearings, gear boxes and swinging-arm pivots and may have a hole in them which lines up with an oilway in the housing.
Split shell bearings are a variation on the same theme but are much easier to remove and refit as they are literally a plain bush cut in half lengthways. The two halves fit into a split housing which bolts together. The bearing shells usually have a projecting tang which locates in a groove to stop the bearing moving inside the housing. This type of bearing is fitted to crankshafts and gearbox shafts and, because of the high loading associated with these components, it is normally fed with oil under pressure.
Removal is simply a matter of unbolting the housing or bearing cap, lifting it from the shaft and carefully prising the bearing from it.The signs of wear are the same as for bushes and new shell bearings must always be fitted in pairs, making sure the shell fits snugly into the housing, with no dirt between the two. The housing or bearing cap must always be fitted in its original position and the parts should be marked before they are disturbed, so that the same caps can be fitted the same way round.To compensate for wear on the shaft, the journal or bearing surface of the shaft can be ground down and undersize shell bearings fitted to suit it – this is a specialist operation which should be left to a machine shop. New bolts should also be fitted to the bearing caps and tightened down evenly to the torque specified in the manufacturer’s maintenance manual.
Roller bearings, as the name implies, carry balls or rollers between the two moving parts, so there is no sliding motion and friction is kept to a minimum. The simplest type is a series of ball bearings held between two cupped plates, like that used in many steering heads. By undoing a central bolt, the two components can be separated and the balls removed. Any signs of pitting or scoring mean that the bearing should be renewed. When ball bearings are fitted so that they each touch the next one, the bearing is called a crowded race; on steering-head bearings, it is common practice to assemble the bearing in this way and then remove one ball, using lubricating grease to hold the balls in place while the bearing is assembled.
The majority of roller bearings come as ready made units, with a cage which keeps the balls or rollers equally spaced. Usually, these cannot be dismantled, the occasional exception being needle-roller bearings. These are so called because the length of the roller is much greater than its diameter.
Wear signs on roller bearings, apart from radial movement of the shaft, are pitting, rattling when the bearing is shaken, or noise when the bearing has been cleaned in petrol and then spun between the fingers.
The roller part of the bearing may be a ball, a plain roller, a barrel-shaped roller or a tapered roller. They each have different load-bearing and alignment characteristics but the basic construction is the same. In most cases, the bearing will be an interference fit, either in its housing or on the shaft. Removal is a case of tapping the shaft from the housing, in which case the bearing will either come with it or stay in the housing. Then, depending on the type, it will either have to be removed with a press, a puller or a drift or the housing will have to be heated. In some cases, the bearing is only a push fit and is located by a circlip or a dowel.
If the bearing has worked loose in its mounting, it may be possible to fix it by using. With the exception of taper rollers and some special purpose bearings, which have lips, these bearings can only support radial loads and cannot take end-thrust or axial loads. To prevent the shaft moving in this direction or to permit a certain specified amount of end float, a thrust bearing is used. This is usually a shim or washer, available in a variety of thicknesses. The shaft and bearings are assembled without the thrust washer, and the end float measured with feeler gauges or a dial gauge. The required end float is subtracted from this to give the thickness of washer which must be fitted.
To prevent lubricating oil running through the bearing and out of the engine, there is usually an oil seal fitted behind the bearing. This will either be contained in a bolt-up housing, or will be a push fit in the engine casting. Removal is quite simple as the seal can be tapped out using a piece of wood as a drift damage to the seal isn’t important because seals should always be renewed when they have been disturbed. Refitting is a different matter and calls for the correct drift, although in an emergency the old seal can be used as a drift. When the seal is squarely in place, the rubber lip – which faces against the direction of oil flow – should be lubricated and the shaft gently twisted through it so that the lip isn’t damaged.
The principle of the bearing is basically a simple one and the types described here cover the majority of applications to be found on the motor cycle. Careful maintenance and timely replacement can prevent serious mechanical damage to your machine.