In order to reverse a quarter-twist belt, it will be necessary to introduce one or two guide-pulleys to bring the center line of the belt at all times into the central plane of the receiving pulley. Fig. 101 shows an arrangement where two guide-pulleys are used. Let the driving pulley R and the driven pulley P be located as in the case of the quarter-twist belt of Fig. 99. Suppose that the direction of rotation is but seldom reversed, and that the usual motion of the driver is in the direction of the arrow. The part of the belt which has the greater load should have a direct connection between R and P, leaving the slack side to run over the guide-pulleys, thereby decreasing the stress on the belt and lessening the friction in the journals of the guide-pulleys.
Fig. 99. First Variation from Fig. 98. for Quarter-Twist Belt.
Receiving point A is then connected to B, the point of departure from the back of the driven pulley, instead of over to the front face, as in case of Fig. 99; and the belt twists about this line as an axis through an angle of 90°, so as to present flat sides to both pulleys. The plan view of the center line of the belt is the line of intersection AB of the traces of the central planes of R and P. Evidently the belt will run in either direction along the trace of either plane. From the point of departure D of the driving pulley, the belt must be led to the receiving point E of the driven pulley, by means of guide-pulleys. Draw DE for the plan view of the central plane of the guide-pulleys. This plane is perpendicular to the paper, and its traces or intersections with the central planes of R and P are shown in X Y and DH of the front view.
The guide-pulleys are idlers introduced only to form a path for the belt, and do not in any way affect the velocity ratio. They may, therefore, be made of any convenient size to suit the existing conditions. We may then assume on the plan view, that J and K are points on the axes of the pulleys M and N, and draw the guide-pulleys so that the face of M bisects the front face of P at E, and one face of N bisects the right-hand face of It at D. On the front ! and side views, the location of the axes has been assumed as GG and FF, and the corresponding views of the pulleys are drawn according to the principles of projection.
Fig. 100. Second Variation from Fig. 98 for Quarter-Twist Belt.
Examining the belt in passing from D to E, we find its center line goes from D in the central plane of R, to H in central plane of N, around N and M, which have one common central plane, arriving at C, a point in the trace of the central planes of M and P, and from leaving point C to receiving point E. It will be noted that the center line of belt connecting both leaving and receiving points is always in the central planes of both pulleys, and, therefore, the direction of motion may be reversed at will. If the belt be followed around the pulleys, we find both sides of the belt come, successively, in contact with the pulleys. Therefore, the belt must be given a single twist before uniting the ends together.
Fig. 101. Reversible Quarter-Twist-Two-Guide Pulley.
It is usually desirable to have only one side the working side; but if such were the case in this arrangement of pulleys, it would bring a sharp twist in the belt, between M and N, which might be a greater objection.