15

30° or 210° from TDC for No 1 cylinder. 10.2.6 Six-cylinder engines

So the maximum value for the primary reciprocating couple becomes

= MRecifRL + 1.5 cos 30° + 0.866 sin 30° = 1.732AfRecip/?L(clockwise)

This is similar in form and position to that for the rotating out-of-balance couple resultant. It is of course the maximum value of the primary pitching couple acting on the whole engine.

It is also possible to derive the primary inertia balance by using the concept of 'reverse cranks' referred to earlier. In this case each crank has a moment of 0.5 MRscipR considered attached at each crankpin. The direct crank arrangement will be identical to that shown for the rotating out-of-balance in Figure 10.13. That for the reverse crank will also be similar but rotating in the reverse direction. It follows that the resultant for the two cranks will be as determined mathematically above. The secondary forces will be in balance but again a secondary pitching couple will be present for the whole engine.

Figure 10.15 uses the principle of the reverse cranks to eliminate the primary pitching couple. Here balance weights are provided on the crankshaft to eliminate all the rotating out-of-balance plus half the pitching moment as the 'direct crank' whilst the 'reverse crank'—the reverse rotating balancer shaft— deals with the other half of the primary pitching couple.

The crankshaft arrangement normally used for an in-line six-cylinder engine is shown in Figure 10.16. It will be seen that this is two three-cylinder shafts placed end to end, but with one half reversed. As shown, the shaft as a whole is in rotating force and couple balance, together with the primary forces and couples. To reduce the magnitude of the two opposing internal couples, and hence the loads on Nos 1, 4 and 7 main bearings, four balance weights are indicated placed angularly at right-angles to Nos 2 and 5 crankpins.

A number of other ways of positioning balance weights are possible with a view to relieving the between throws rotating bearing load. The one indicated is the minimum number of weights required to eliminate or reduce, if smaller weights are used, the magnitudes of the two opposing internal couples. Figure 10.17 shows the arrangement of the secondary forces and couples. These, like the primary ones, are innately balanced for the whole engine.

10.2.7 Vee engines

10.2.7.1 The general case—single throw

Figure 10.18 shows a diagrammatic section through a vee engine having an angle 2a between the two banks normally referred to as the bank angle.

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Figure 10.16 Balance of six-cylinder shaft. Rotating and primary balance

Figure 10.16 Balance of six-cylinder shaft. Rotating and primary balance b

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