1—3 piston rings, 4 piston, 5 piston pin, 6 circlip, 7 bush, B bearing, 9 connecting rod, 10 flywheel, 11 retainer, 12 pilot bearing, 13 pulley bolt, 14 crankshaft pulley, 15 oil deflector, 16 wave washer, 17 friction gear, 18 crankshaft gear, 19 crankshaft, 20 thrust bearing. 21 main bearing, 22 main-bearing cap, 23 rear oil seal, 24 gasket, 25 retainer
With main bearings, the upper half of the bearing fits into a machined section of a crankcase web, and the lower half is carried in the bearing cap, which bolts onto the crankcase web (Figure 4.17).
With connecting-rod bearings, the upper half of the bearing is carried in the big end of the connecting rod, and the lower half is carried in the connecting-rod cap.
Apart from reducing friction and wear, crankshaft bearings have a number of different requirements, as noted under the headings that follow.
The bearing must be of sufficient size to carry the loads imposed on it. It must also be made of a suitable material so that it will sustain the loads under various conditions of engine operation.
The bearing material must be hard enough to resist wear. At the same time, it must be relatively soft so that it does not damage or wear the shaft on which it operates. A very hard bearing could seize on its shaft if it was heavily loaded.
Crankshaft bearings are subjected to varying loads and are repeatedly stressed. The bearing material must be able to resist this without failure from fatigue. Where unusual conditions exist, or after a long period of service, fatigue failure can occur.
Fatigue occurs when a piece of metal is repeatedly stressed, causing it to flex or bend. The continual bending tends to harden the metal, which ultimately breaks. This is called a fatigue failure.
This term refers to the ability of a bearing to permit small foreign particles to become embedded in it. This is a way in which the bearing can protect itself.
Precision-insert bearings, with their thin layers of alloy material, have very little embeddability and depend on good filtration and a flow of oil to flush away any dust particles that might reach the bearing. Conformability
This refers to the ability of a bearing to conform to small variations in shaft alignment and journal shape.
Insert bearings have a steel back and arc lined with alloys which include copper, lead, tin, aluminium, antimony and cadmium. There are many combinations. These alloys generally have limited conformability and embeddability, and depend on finely ground journals and a good flow of oil to wash away any foreign particles that might reach the bearing.
Engine bearings are overlay-type bearings. A typical bearing of this type has a thin, flexible steel back, with a 0.05 mm layer of alloy and another layer of 0.02 mm of softer material. Some bearings have an extra overlay of thin plating.
A working clearance is provided between the bearing and the crankshaft journal (Figure 4. 1 9).
with the type of bearing material, but 0.02 mm to 0.08 mm is a typical specification.
Too little clearance will not allow adequate lubrication and will cause, rapid wear, bearing failure and shaft damage on the design of the oil pan and the main bearing. Too much clearance can cause loss of oil and low oil pressure. Also, there will be a lot of oil thrown from the bearings and onto the cylinder walls. The piston rings will find this hard to control, and oil will find its way up past the piston into the combustion chamber where it will burn to form carbon and cause other problems.
Engine bearings must be given adequate lubrication. The lubricant reduces friction, removes heat and reduces noise.
The flow of lubricant to the crankshaft and bearings is shown in Figure 4.20. Oil from the oil pan is circulated by the oil pump at the front of the crankcase. It passes through the oil filter, along the main oil gallery and then down through drillings in the crankcase webs to the main bearings.
The connecting-rod bearings are lubricated from the main bearings through holes drilled in the crankshaft webs. Oil thrown from the connecting-rod bearings helps to lubricate the pistons, the cylinder walls and other engine parts.