Fig. 2648.
A standing balance is effected when the shaft being supported horizontally and with as little friction as possible, the pulley will remain at rest in any position in which it can be placed. Thus, in [Fig. 2647] let c c represent the two centres of a lathe adjusted in their distance apart so as to sustain the shaft s with just sufficient force to prevent end movement or play of the shaft, and if the pulley p remains motionless when arrested at any point of rotation it is in standing balance. A common method of balancing is to set the pulley in slow rotation several times in succession, and if the same part of the pulley’s circumference comes to rest in each case at the bottom as at b then it is heaviest and its weight must be reduced, or weight must be added on the diametrically opposite side of the pulley. Another method is to rest the shaft horizontally on a pair of metallic strips as b b in [Fig. 2648], the strips resting on a flat horizontal surface d, the testing being applied as before. Sometimes, however, cylindrical pieces are used in place of the strips or pieces b b.
Fig. 2649.
A pulley that is in balance thus tested, may not, however, be in balance when rotated, or, as already stated, a standing balance may not be a running balance, for the following reasons: In [Fig. 2649] is a pulley that if turned true inside and out would be of correct standing balance, because the weight is equal on each side of the shaft; thus the point a, though farther from the axis than b, would be counterbalanced by c, while b would be counterbalanced by d, but as soon as the pulley was put in rotation there would be more centrifugal force generated at a than at b, and more at c than at d, because, though the weights would be equal, the velocities of a and c would be greatest.
Now, suppose that instead of a continuous wide pulley several pulleys were used, being out of true so as to be practically equal in shape to [Fig. 2649], and it is apparent that the fact of pulley a b being out of balance is not removed by pulley c d being out in an opposite direction, and that each pulley will tend to bend the shaft in the direction of its excessive centrifugal force.
