Thursday, June 2, 2011

Balance wieghts (for crankshafts)

Fig 4.12 (a)
In automotive engines, which operate at high speeds, centrifugal force is produced by the rotating crank-pins. In a four-cylinder crankshaft, balance weights are placed on the crankshaft opposite the crank-pins. Centrifugal force also acts on the balance weights, but in an opposite direction to that on the crank-pins. The forces tend to cancel out, so that vibration is reduced.
A four-cylinder in-line engine operating at low speeds would need very little balance, because No. 1 cylinder is balanced by No. 2 cylinder, and No. 3 is balanced by No. 4. However, for a shaft operating at high speeds, separate balance weights are used for each crank-pin. This contains the forces within the parts of the crankshaft in which they are produced.

Fig 4.12 (b)
Crankshaft designs
The crankshafts of six-cylinder engines require more complex balance weights, and V-type engines have balance weights arranged differently again. Figure 4.12(a) shows a crankshaft for a V-6 engine which has its cylinders arranged at an angle of 90˚. Also shown are its camshaft and balance shaft.
Many V-type engines have the connecting rods of opposite cylinders connected side by side to a common journal but, in this engine, each connecting rod has its own journal. The crank-pin journals for opposite cylinders are on the same throw of the crankshaft, but they are offset at an angle of 30° to each other (Figure 4.12(b) This is done so that firing order of the cylinders can be evenly spaced exactly at each 120° of crankshaft rotation.
Some crankshafts are solid, but others have hollow crank-pins. This reduces their mass and so reduces the effects of centrifugal force. This helps with shaft balance.

·       Hollow crank-pins do not affect the strength of the shaft because, in ass for mass, a hollow shaft has greater strength than a solid one.

Fig 4.13
Journal overlap
A rigid crankshaft that will resist twisting and bending needs large journal diameters and thick webs. These two requirements have been combined in a design feature known as journal overlap.
With this design, the crank-pins overlap the main journals as shown in Figure 4.13. This makes the crankshaft more rigid. If necessary, the web thickness can be decreased to reduce the overall length of the shaft while maintaining rigidity. Compare Figure 4.13 with Figure 4.14 which has no overlap.

Fig 4.14
Crankshaft fillets
A radius, or fillet, is formed between the edges of the journal and the webs of the crankshaft. These fillets, although small, are quite important because they provide a gradual change in the thickness of the section (Figure 4.14).
With no fillet, this section of the crankshaft will be subjected to a stress rise, which means that it will carry a greater load (and greater stress) than the section next to it. Stress could cause fatigue and a crack could start at this point.

·        A crankshaft that is badly ground during reconditioning could fail because of badly shaped fillets.

Fig 4.15
Figure 4.15 shows a journal with rolled fillets. This design feature is used to reduce local stress points between the journals and the webs. The crankshaft also has journal overlap, so it would not be subjected to the stress rise of a crankshaft without overlap. 

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