Fig. 2670.
When the direction of rotation of the driven pulley requires to be reversed from that of the driving pulley, the belt is crossed as in [Fig. 2670]. A crossed belt has a greater transmitting power than one uncrossed (or, as it is termed, than an “open belt”) because it envelops a greater arc of both pulleys’ circumference. This is often of great advantage where the two pulleys are of widely varying diameter, especially if the small pulley requires to transmit much power, and be of very small diameter.
But a crossed belt is open to the objection that the surfaces of the belt rub against each other at the point of crossing, which tends to rapidly wear out the laced joint of the belt. By crossing a vertical belt the lower pulley receives part of the weight of the belt.
Fig. 2671.
Fig. 2672.
When a belt connects two pulleys whose respective planes of revolution are at an angle one to the other, it is necessary that the centre line of the length of the belt shall approach the pulley in the plane of the pulley’s revolution, which is sufficient irrespective of the line of motion of the belt when receding from the pulley. This is shown in [Fig. 2671], which represents what is known as a quarter twist; a, b are two pulleys having their planes of revolution at a right angle, the belt travelling as denoted by the arrows, then the centre line c of the belt being in the plane of rotation of a on the side on which it advances to a, the belt will continue to run upon the same section of a. If the pulley positions be reversed, as in [Fig. 2672], the same rule applies, and the side d in the figure being that which advances upon b must travel to b in the plane of b′s rotation, otherwise the belt would run off the pulley; hence it is obvious that the belt motion must occur in the one direction only.