It very often happens that a belt must connect two shafts which are on the same level, but which are not parallel. The connection can be made, whatever the angle between the shafts, by the use of two guide-pulleys. If the two main-shaft pulleys are of the same diameter, the belt may be made to run in either direction by putting both guide-pulleys - or mule pulleys, as they are often called - on the same shaft, which will be perpendicular to the plane containing the axes of the main shafts. That is, if the main shafts are horizontal, the shaft for the guide-pulleys will be vertical. If the main pulleys are of different diameters, the guide-pulleys may still be placed on the same vertical shaft, but in this case the belt can run in only one direction. If the belt is to run in either direction, the guide-pulleys are placed on separate shafts, which are usually adjustable in position, so that they may be tipped at the proper angle to receive and deliver the belt.
We shall first take the case where the two main pulleys are of the same size. Fig. 104 shows the arrangement, the upper view being the plan, and the lower view, the elevation. R and S are the two main pulleys, and C and D are the two guide-pulleys. The line X Y is the line of intersection of the planes of the main pulleys. The location of the guide-pulleys is sufficiently clear from the drawing, without further explanation.
Fig. 105 shows the arrangement when the main pulleys are of different diameters. Here the pulleys can turn only in the direction shown by the arrows; for if the direction were reversed, the belt would leave the pulleys. The pulleys in plan appear the same as in Fig. 104. In the elevation, the upper guide-pulley, which receives the belt from S, has its central plane tangent to the pulley S at the point where the belt leaves S, as shown by the line AvBv. The lower guide-pulley, which receives the belt from R, has its center plane tangent to R, as shown by CvDv.If the pulleys were to turn in the opposite direction, the upper guide-pulley would have its plane tangent to R, and the lower one, its plane tangent to S.
Fig. 104. Belt Drive for Same Sized Pulleys, Shafts not Parallel but in Same Plane.