30.1 Introduction

In this chapter we examine three quite different problems involving friction and wear. The first involves most of the factors that appeared in Chapter 29: it is that of a round shaft or journal rotating in a cylindrical bearing. This type of journal bearing is common in all types of rotating or reciprocating machinery: the crankshaft bearings of an automobile are good examples. The second is quite different: it involves the frictional properties of ice in the design of skis and sledge runners. The third Case Study introduces us to some of the frictional properties of polymers: the selection of rubbers for anti-skid tires.

30.2 Case Study 1: Design of Journal Bearings

In the proper functioning of a well-lubricated journal bearing, the frictional and wear properties of the materials are, surprisingly, irrelevant. This is because the mating surfaces never touch: they are kept apart by a thin pressurized film of oil formed under conditions of hydrodynamic lubrication.Figure 30.1 shows a cross section of a bearing operating hydrodynamically. The load on the journal pushes the shaft to one side of the bearing, so the working clearance is almost all concentrated on one side. Because oil is viscous, the revolving shaft drags oil around with it. The convergence of the oil stream toward the region of nearest approach of the mating surfaces causes an increase in the pressure of the oil film, and this pressure lifts the shaft away from the bearing surface.

Pressures of 10 to 100 atmospheres are common under such conditions. Provided the oil is sufficiently viscous, the film at its thinnest region is still thick enough to cause complete separation of the mating surfaces. Under ideal hydrodynamic conditions there is no asperity contact and no wear. Sliding of the mating surfaces takes place by shear in the liquid oil itself, giving coefficients of friction in the range 0.001 to 0.005.

Hydrodynamic lubrication is all very well when it functions properly. But when starting an engine up, or running slowly under high load, hydrodynamic lubrication is not effective, and we have to fall back on boundary lubrication (see Chapter 29).

Under these conditions some contact and wear of the mating surfaces will occur (this is why car engines last less well when used for short runs rather than long ones). Crankshafts are difficult and expensive to replace when worn, whereas bearings can be designed to be cheap and easy to replace as shown in Figure 30.2. It is thus good practice to concentrate as much of the wear as possible on the bearing—and, as we showed in our section on adhesive wear in the previous chapter, this is done by having a bearing material that is softer than the journal—or a journal that is harder than the bearing material.

Crankshaft journals can be “case-hardened” by special chemical and heat treatments (Chapter 29) to increase the surface hardness. (It is important not to harden the whole shaft because this will make it brittle and it might then break under shock loading.) Or bearing materials can be selected form the standard alloys which have been developed for this purpose (see Table 30.1).

Embeddability

Real bearings contain dirt, such as small hard particles of silica. If the particles are thicker than the oil film, they will generate abrasive wear. But if the bearing surface is soft enough, these particles will be pushed into the surface, and removed from circulation (Figure 30.3).

Conformability

Slight misalignments of bearings can be self-correcting if plastic flow occurs easily in the bearing metal (Figure 30.4). But there is a compromise between load-carrying ability and conformability.

Preventing seizure

If there is a breakdown in the oil supply, frictional heating will rapidly increase the temperature of the bearing, and this would normally lead to pronounced metal-to-metal contact, gross atomic bonding between journal and bearing, and seizure. A soft bearing material of low melting point will be able to shear in response to the applied forces, and may also melt locally. This helps to protect the journal from more severe damage, and also helps to avoid major component breakages which would result from the sudden locking-up of moving parts. Figures 30.5 and 30.6 show a typical example of overheating damage caused to the main crankshaft bearing of a marine diesel engine. The white metal bearing is badly scuffed, smeared, and oxidized, but there was no damage to the crankshaft journal.

When designing a bearing for a particular application, it is useful to think in terms of the pressure-velocity (or “PV”) envelope inside which the bearing can function safely (Figure 30.7). We must ensure that the maximum pressure and sliding speed encountered in service will not make the material yield (see Table 30.1) or suffer heat damage. The PV envelope is not simply a property of the bearing material, but also the nature of the applied loading, the lubrication conditions, and the constructional details of the bearing. Although hydrodynamic lubrication should keep the surfaces apart, some frictional heating is generated even under ideal lubrication conditions. Much more heat can be generated if lubrication conditions revert to boundary lubrication or worse.

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