Friday, June 3, 2011

Crankshaft bearings

Fig 4.16
Figure 4.16 shows a crankshaft and its related parts, including the crankshaft bearings. There are two sets of these: the main bearings, which support the crankshaft in the crankcase, and the connecting-rod hearings, which are located between the connecting rod and the crank-pin journal.
Figure 4.16 Crankshaft for a four-cylinder diesel engine and its associated parts
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
The crankshaft main bearings and the connecting- rod bearings are of the split-sleeve type, which means that they are in two halves. They cannot be used on their own and must be inserted into a housing of some kind that has been accurately machined to suit them.
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.

One of the main bearings is provided with thrust faces to accept end movement of the crankshaft. Thrust faces are either built into the bearings or provided as separate segments. These are faced with bearing material and located at each side of one of the main bearings (see illustrations in the upcoming posts).
Fig 4.17

Precision-insert bearings
The precision inserts used for crankshaft bearings consist of a steel back to which a thin layer of bearing material is bonded. In some bearings, more than one layer of bearing material is used. Replacement bearings are accurately finished to size and are installed directly into their housings without any fitting or adjustment.
A bearing insert is shown in Figure 4.18, with its web parts identified. An oil hole is located in the centre of the bearing. For a main bearing, the oil hole lines up with an oil drilling in the crankcase web, and for a connecting-rod bearing, the oil hole lines up with a hole in the crank-pin journal.
Fig 4.18
Some main bearings have an annular oil groove as shown, others are plain. The bearing insert is prevented from rotating by a locating tang that fits into a slot in the bearing housing or cap.

·       Crankshaft and connecting rod bearings are usually referred to as inserts, but they are also called slippers or shells.

Bearing requirements
Apart from reducing friction and wear, crankshaft bearings have a number of different requirements, as noted under the headings that follow.
Load-carrying capacity
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.

Fatigue resistance
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.
·           The everyday meaning of fatigue is tiredness and this, simply, also applies to metal fatigue.

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.
This refers to the ability of a bearing to conform to small variations in shaft alignment and journal shape.
If a bearing material has high conformability, it will cold-flow slightly to relieve the load on local areas that are heavily loaded. This is very limited for precision-insert bearings.
Corrosion resistance
Bearing materials must be resistant to corrosion because some of the by-products of combustion can form corrosive substances. These could affect the bearing material.
This is one of the reasons why crankcases are provided with positive ventilation. The clean air being circulated through the crankcase removes corrosive gases.

Bearing materials
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.

Bearing clearances
A working clearance is provided between the bearing and the crankshaft journal (Figure 4. 1 9).
Fig 4.19
This varies
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.

Fig 4.20
Bearing Lubrication
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.

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