Sunday, June 12, 2011

Servicing crankshaft

With the crankshaft in the cylinder block, the clearance of the main bearings can be checked and the condition of the crankshaft journals can be checked visually. However, to actually measure the main-bearing journals, the crankshaft has to be removed from the engine. To enable this to be done, the engine must be removed from the vehicle and dismantled.

Fig 5.18
Measuring crankshaft journals
Measure the journals with an outside micrometer as shown in Figure 5.18. Check for both taper and out-of- round. When checking taper, measure each end of the journal. When checking out-of-round, take measurements at 900. There are four places where measurements have to be taken. These are shown in Figure 5.19.
Typical specifications for a crankshaft and bearings are set out in Table. This shows the type of service data provided in workshop manuals, although specifications will vary for different engines.
Fig 5.19
The crankshaft will have to be replaced or the journals ground if taper or out-of-round exceeds specifications, or if the journals are rough, scratched or damaged in any way. The journals can be ground undersize and suitable undersize bearings fitted.

Crankshaft and bearing specifications
              CRANKSHAFT ITEM
Crankshaft runout at centre
0.06 mm
Main-journal diameter
49.95—50.00 mm
Main-journal out-of-round
0.04 mm
Crankpin diameter
44.95—45.00 mm
Crank-pin taper and out-of-round
0.04 mm
Main-bearing clearance
0.030—0.050 mm
Crankshaft end-play
0.10—0.30 mm
Connecting-rod bearing clearance
0.02—0.06 mm
Connecting-rod big-end side

0.10—0.35 mm
Bearing under-sizes
0.25. 0.50, 0.75 mm
Flywheel run-out
0.20 mm

Figure 5.20
Grinding crankshaft journals
A special crankshaft grinder is used to grind crankshaft journals (Figure 5.20). The grinder is fitted with chucks at each end which support the ends of the crankshaft. During grinding, the shaft is rotated and the grinding wheel of the machine is used to accurately grind each journal. This is done to a specified size to suit undersize bearings.
When the main bearing journals are being ground, the crankshaft is aligned with the machine but, when grinding the crank-pin journals, the crankshaft has to he offset in the grinder. This enables the crank-pin being ground to be rotated about its own axis.

Cleaning crankshafts
When a crankshaft is removed from the engine, it should be cleaned in a suitable solvent, particularly the oil passages. To prevent rusting, the journals should be oiled immediately after they have been cleaned.
A crankshaft that has been ground needs to be cleaned properly to remove abrasive and metal particles that are the result of grinding.

Crankshaft alignment
Figure 5.21
To check for alignment, the crankshaft is mounted between centres or in V-blocks on a large hat surface. A dial gauge on a surface block is set up on the journal of the centre bearing. The crankshaft is rotated slowly to measure the run-out (Figure 5.21). A bent crankshaft will have to be discarded, although a slightly bent shaft can sometimes be ground to provide accurate alignment.

See main bearings service>>>>>>>>>>> 

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