61 Rolling contact bearings

Bearings which are designed to support rotating shafts can be divided broadly into two groups; the plain lining bearing, known as the journal bearing, and the rolling contact bearing. The fundamental difference between these bearings is how they provide support to the shaft. With plain sleeve or lining bearings, metal to metal contact is prevented by the generation of a hydrodynamic film of lubricant (oil wedge), which supports the shaft once operating conditions have been established. However, with the rolling contact bearing the load is carried by balls or rollers with actual metal to metal contact over a relatively small area.

With the conventional journal bearing, starting friction is relatively high and with heavy loads the coefficient of friction may be in the order of 0.15. However, with the rolling contact bearing the starting friction is only slightly higher than the operating friction. In both groups of bearings the operating coefficients will be very similar and may range between 0.001 and 0.002. Hydrodynamic journal bearings are subjected to a cyclic projected pressure loading over a relatively large surface area and therefore enjoy very long life spans. For example, engine big-ends and main journal bearings may have a service life of about 160000 kilometres (100 000 miles). Unfortunately, the inherent nature of rolling contact bearing raceway loading is of a number of stress cycles of large magnitude for each revolution of the shaft so that the life of these bearings is limited by the fatigue strength of the bearing material.

Lubrication of plain journal bearings is very important. They require a pressurized supply of consistent viscosity lubricant, whereas rolling contact bearings need only a relatively small amount of lubricant and their carrying capacity is not sensitive to changes in lubricant viscosity. Rolling contact bearings have a larger outside diameter and are shorter in axial length than plain journal bearings.

Noise levels of rolling contact bearings at high speed are generally much higher than for plain journal bearings due mainly to the lack of a hydro-dynamic oil film between the rolling elements and their tracks and the windage effects of the ball or roller cage.

6.1.1 Linear motion of a ball between two flat tracks (Fig. 6.1)

Consider a ball of radius rb placed between an upper and lower track plate (Fig. 6.1). If the upper track plate is moved towards the right so that the ball completes one revolution, then the ball has rolled along the lower track a distance 2nrb and the upper track has moved ahead of the ball a further distance 2^rb. Thus the relative movement, L, between both track plates will be 2^rb + 2^rb, which is twice the distance, l, travelled forward by the centre of the ball. In other words, the ball centre will move forward only half that of the upper to lower relative track movement.

l 2nrb 1

6.1.2 Ball rolling contact bearing friction

When the surfaces of a ball and track contact under load, the profile a-b-c of the ball tends to flatten out and the profile a-e-c of the track becomes concave (Fig. 6.2(a)). Subsequently the pressure between the contact surfaces deforms them into a common elliptical shape a-d-c. At the same time, a bulge will be established around the contact edge of the ball due to the displacement of material.

If the ball is made to roll forward, the material in the front half of the ball will be subjected to increased compressive loading and distortion whilst that on the rear half experiences pressure release (Fig. 6.2(b)). As a result, the stress distribution over the contact area will be constantly varying.

The energy used to compress a perfect elastic material is equal to that released when the load is removed, but for an imperfect elastic material (most materials), some of the energy used in straining the material is absorbed as internal friction (known as elastic hysteresis) and is not released when the load is removed. Therefore, the energy absorbed by the ball and track when subjected to a compressive load, causing the steel to distort, is greater than that released as the ball moves forward. It is this missing

Fig. 6.1 Relationship of rolling element and raceway movement

{a) Load«! ball stationary (b) Loaded ball rolling forward

Fig. 6.2(a and b) Illustration of rolling ball resistance against motion

{a) Load«! ball stationary (b) Loaded ball rolling forward

Fig. 6.2(a and b) Illustration of rolling ball resistance against motion energy which creates a friction force opposing the forward motion of the ball.

Owing to the elastic deformation of the contact surfaces of the ball and track, the contact area will no longer be spherical and the contact profile arc will therefore have a different radius to that of the ball (Fig. 6.2(b)). As a result, the line a-e-c of the undistorted track surface is shorter in length than the rolling arc profile a-d-c. In one revolution the ball will move forward a shorter distance than if the ball contact contour was part of a true sphere. Hence the discrepancy of the theoretical and actual forward movement of the ball is accommodated by slippage between the ball and track interfaces.

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