Theory And Practice In Motor Balancing

By Uncle Frank

Referring to Fig.1, in the Illustration No.1, we see a ball “W” revolving at the end of a rod. This revolving ball is unbalanced in that no equal weight is located on the opposite side of the shaft center. Therefore, the centrifugal force sets up an outward pull with the consequent eccentricity of rotation. Vibration will result. In Fig.2, we see a flywheel with a counterweight “W” which would act the same as the ball just described in rotation. Such a weight must have a counter weight to balance it. Fig.3 shows two weights, “W” and “W1” mounted on opposite sides of a shaft center. Of the two weights, “W1” is the heavier. Naturally centrifugal force would be in favor of this heavier weight. This sketch may be likened to the flywheel, rod and piston shown in Fig.4 a simple single cylinder motor. In actual practice the piston, rings, pin and upper half of the rod do weigh more than the counterweight “W”. If revolved in the same plane, that is, just let fly around this mass would be considerably out of balance. We must remember, however, that part of the weight is reciprocating and part rotating and the two masses tend to cancel each other, anyway enough to effect fair running balance of the motor. The single-cylinder motor with its lack of power impulses, is the most difficult motor to balance. Only very high r.p.m. singles can be balanced nicely at top speeds.

The Vee-type Twin Engine

In the Vee twin motor we find two sets of reciprocating parts moving at almost right angles to each other. This angularity varies with the degree at which the cylinders are located from each other. While one set of parts is moving up and down vertically the other is more or less in a horizontal (back and forth) motion. Study Figs.2 and 3 in Illustration No.2 for graphic explanation. Thus, these dual reciprocating masses more or less combine to make a radial force (centrifugal force) which can be quite satisfactorily balanced by counterweights placed opposite the crank pin. With the unequal motions of the two sets of reciprocating parts in a Vee twin there is sort of dampening effect which is beneficial to smoothness of operation.

In Fig.1 we see both rods held halfway between the cylinder center lines. Thus, the pistons are not quite at top dead center. We can also see that at no time will both rods be in line with each cylinder center line. At this point the rotating mass is at its highest peak, both rods are practically stationary. The resultant force would, therefore, be in a horizontal plane. But this does not last long, for soon both reciprocating members are in motion, thus breaking up the horizontal vibration. In Fig.6, Illustration No.1, we see a series of arrows emanating from a common center. These arrows approximate the direction of forces set up in a Vee twin, the object being to cancel all vertical and horizontal forces, thus breaking up the vibration harmonics.

Referring again to Illustration No.2 Fig.2, we see the rear piston descending until its rod is at right angles (90°) with the crank throw. At this point the piston speed is greatest. The actual location of this point during flywheel rotation depends, of course, upon length of connecting rod and stroke, or throw of crank. Note that while the rear cylinder is pretty well down in its travel, the front cylinder rod and piston are lagging behind. In Fig.3, we see the same rod when it has reached right angles with the crank throw on the upward stroke. Again note the angle of the front cylinder rod. Lower dead center for both rods is approximated in Fig.4. Here again, the mass weight of the reciprocating parts is almost at zero. And in this particular position, the upper section of the rods is in motion against the rotation of the flywheel revolving weights. Here again, opposite forces tend to cancel out vibration.

By Uncle Frank
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