Suppose an open belt to connect pulleys A and B1, on parallel shafts, Fig. 97.

Draw a tangent XY to the pitch circles of the pulleys at the points L1 and L2 ,where the belt leaves the pulleys. Now rotate the central plane of the pulley B1 ,about X Y as an axis, through any angle C, to position shown by pulley B. The central planes (shaded) of pulleys A and B intersect on the line X Y, called the trace of the planes. The axes are now not parallel, but the belt may be made to run in one direction, for it still obeys the general principle of the guiding of belts; i.e., the center line of the belt, on leaving the driving pulley, is delivered into the central plane of the receiving pulley.

Examining the figure, we find that the center line of the belt moves in direction of arrow from L1 to R, and around pulley B to L2 from L2 on the surface of B to R2, thence on surface of A to L1,the starting point. From the point L1 ,where the belt leaves A, until it reaches R, the center line of the belt is in the central plane of, the receiving pulley B, and the belt twists about this line, presenting a flat side to the face of pulley B at R.

Fig. 97. Diagram of Pulley Drive where Shafts Are not Parallel.

From L2, where the belt leaves B, until it reaches R2, the center line of the belt is continually in the central plane of the receiving pulley A, and a similar twist in the belt takes place. If now we attempt to reverse the direction of motion of the belt, the top of pulley A, moving in the direction of the dotted arrow, would carry point D of the center line of belt to the left-hand edge of A, as indicated by the dotted line DE, where it would drop off. Therefore, this belt drive for shafts not parallel is suitable only for motion in one direction.