This is only one of the many pitfalls in practical balancing, which cause the designer many headaches, and are rarely capable of being dealt with by theoretical calculation. Another example occurs in the case of a rotating body which for practical reasons cannot be made symmetrical in shape, though the moments of mass are calculated and counterweights added where necessary to cancel out and give correct balance as in Fig. 4. When running at high speed, however, the effect of centrifugal forc,e causes the flywheel to distort, and thereby displace the masses to a varying extent, thereby unbalancing them. In case readers think this is an unlikely eventuality, I may say that I once worked on a certain type of flywheel magneto which gave a great deal of trouble through this cause, though dynamic balancing tests gave no indication of the source of error.
Balance weights, whatever their type or purpose, should always be located as close to the plane of the unbalanced mass as possible. Thus, in the case of the crankshaft shown in Fig. 3B, it would be better to attach the counterweights to the crank webs than at the points indicated. The practice of fitting balance weights to external flywheels, therefore, is one that cannot be commended; in the case of an overhung crankshaft, any bias in the flywheel would set up a violent rocking couple. Flywheels should always be at least in static balance, and if of any great width, dynamic balancing is desirable. An exception is made in the case of internal flywheels, as in motor-cycle engines, which are close to the crankpins, and usually form the crank webs.
Balance of Reciprocating Masses
We have seen that an unbalanced rotating mass may be cancelled by an equal and opposite rotating mass; in a similar way, an unbalanced reciprocating mass may be cancelled by an equal and opposite reciprocating mass. It is essential that this axiom should be clearly understood; it is no use attempting to balance completely a reciprocating mass by a rotating counterweight, or vice versa. A reciprocating mass can only be balanced by an equivalent mass moving in the same plane, but in exactly opposite phase. Thus it happens that the most popular type of small engine, having a single piston working on a single crank throw, cannot possibly be perfectly balanced; the best that can be done in practice is to use a rotating counterweight to produce a partial state of balance, which may be more or less satisfactory, but can never eliminate, vibration or abnormal mechanical stresses completely. This rule applies, whether the engine is single or double-acting, and whatever method is employed to convert the reciprocating motion of the piston to rotary motion of the crankshaft. I emphasise this point because I am often asked to prescribe a "perfect" balancing formula for a single-cylinder engine, and some fearfully and wonderfully conceived devices--all of them either futile, or too complex for practical application--have been submitted by designers as a solution to this problem.