An Introduction To Bearings types

An Introduction To Bearings types 

Introduction

Rotating shafts need to be supported. Depending on the application, the support is provided by plain bearings, ball bearings or roller bearings. In addition to providing radial support, the bearings sometimes also need to accommodate axial forces or, end thrust, along a shaft. 

plain bearings
The plain bearings for steel shafts are very often made from phosphor bronze. This has good load carrying properties which can be enhanced by impregnating it with graphite,Nylon is sometimes used for light duty low speed applications 
Figure 1 shows a plane bearing which is designed to give both radial and axial support. Plain bearings are best suited to low rotational speeds and steady loads. It is essential that they are adequately lubricated with a relatively low viscosity oil. This is usually delivered under pressure in a re-circulating oil system. The oil also carries away heat and maintains the bearing at steady temperature. Very often a spiral groove, leading from the oil passage, is cut in the internal bearing surface to ensure even distribution of the oil.

FIG 1 Journal and thrust bearing

Another type of plain bearing that is widely used in internal combustion engines and reciprocating compressors, is the split bearing with steel-backed white metal bearing shells. They are used for the crankshaft main bearings and the connecting rod big-end bearings. Figure 2 shows a connecting rod big-end bearing. The shells are made from mild steel coated with a soft tin-lead alloy which is able to absorb the shock loading of the power strokes.

 FIG 2 A split big-end bearing

As with other plain bearings, adequate lubrication is essential and oil is delivered under pressure through oil passages in the crankshaft. White bearing metal has a very low melting point and over-heating can be disastrous. Failure of the white metal layer is indicated by vibration and an unmistakable knocking sound. Great expense will then be incurred in acquiring and fitting a replacement engine.
Ball and roller bearings
Ball and roller bearings are classed as rolling element bearings.They offer less resistance to motion than plain bearings and are able to operate at higher speeds and carry greater loads. They are of course more expensive but when correctly fitted and lubricated, they give trouble-free service for long periods. Ball bearings may be contained in a cage which runs between an inner and outer ring

or race, as shown in Figure 3. The balls and the rings are made from hardened and toughened steel which have been precision ground to size. The balls are not always contained in a cage however,

and for some low-speed applications they are packed freely in grease between the rings. This is known as a crowded assembly and is to be found in the wheel and steering head bearings of some bicycles.

Because of their low-friction rolling action, grease lubrication is often sufficient for ball bearings and in recent years a range has been produced which are greased and sealed for life. As an example of their speed capability, a ball race with 8 mm diameter balls is capable of speeds up to 32 000 rpm with grease lubrication, and this can be exceeded with a re-circulating oil system.

  FIG 3 Caged ball bearing

Ball bearings make point contact with the rings whereas cylindrical roller bearings make line contact. As might be expected, they are able to carry greater loads than ball bearings of the same size. A selection of caged roller bearing is shown in Figure 4. It will be noted that some of the rings are fitted with flanges. These enable the bearings to carry a limited amount of axial load. Where the inner ring has no flanges or one flange only, it can be removed enabling the two parts of the bearing to be fitted separately. The cages for some ball and roller bearings are made from reinforced plastic material to reduce friction. These perform well provided that a low running temperature is maintained. Pressed steel cages should be used for running temperatures above 100 C.

 FIG 4 Caged roller bearings

For applications where high radial and axial loads are to be carried it is advisable to use tapered roller bearings. These are shown in Figure 5. Single row tapered roller bearings can carry axial loads in one direction only. If axial loads are present in both directions the bearings may be paired back to back as shown. The front wheel bearings in motor vehicles, which have to carry considerable radial loads and axial loads in both directions when cornering, contain tapered roller bearings that are paired in 

a similar way.

 FIG 5 Tapered roller bearings

Needle bearings are roller bearings with long, thin rollers. They are designed for applications where there is limited radial space. Very often the inner race is dispensed with so that the needle rollers run directly on the rotating shaft in a crowded assembly.They are intended to carry light loads at relatively low or intermittent rotational speeds.

Care needs to be taken when assembling bearing races in position. Very often the rings needs to be pressed onto a shaft or into a housing and as with oil seals, a special centralising tool should be used to ensure correct alignment and seating. A possible arrangement on an input or output shaft from a machine shown in Figure 6.

 FIG 6 Bearing and oil seal arrangement

The slideways on machine beds and worktables may be classedas linear bearing surfaces. The hardened cast iron slideways on lathe beds are V-shaped as are the mating surfaces in the tailstock

and saddle assemblies. Tool slides and worktable slides are generally of dovetail section. The contact materials are mostly steel and cast iron. Because the movement on these slides is relatively

slow and intermittent, no special bearing materials are required and regular lubrication by hand is all that is required for smooth operation. Where the movement on a slide is rapid and continuous, as

with the ram of a shaping machine, the slides are often faced with a bearing material such as phosphor–bronze. Continuous lubrication is also required either by a wick feed or by a forced feed from a pump.