|Fig 4.12 (a)|
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)|
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.
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.
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).