Fig. 98, is obtained; and if angle C becomes 180°, the crossed belt of Fig. 94 is the result.
If the thickness of belt is neglected, it will be noted that the central plane of B (Fig. 98), which is represented by the trace XY, is tangent to the surface of pulley A; and similarly on the side view, the trace ST of the central plane of A is tangent to the surface of B. Now follow the center line of belt in the direc-tion of the arrows on the front and side views, starting at point A, where the belt leaves the upper pulley. From A to D, and around to the back of pulley B, the center line is shown in the trace X Y of the front view. From B, where the belt leaves the lower pulley, to c, and around to the front and starting point A, the center line is shown in the trace ST of side view. Therefore the center line of the belt is always delivered into the central plane of the receiving pulley, and it will drive satisfactorily in the direction of the arrows. Reversing the motion will cause the belt to run off the pulleys, as in the previous case.
Fig. 98. Diagram for Quarter-Twist Belt.
The belt should always be put on so that the same side of the belt touches both pulleys when it is possible to do so. In making the drawing of the belt we shall call the side of the belt which touches the pulleys the inside, and the other side the outside. Now, referring first to the left-hand elevation in Fig. 98, where the belt lies around the circumference of the upper pulley, from the point c, where the upward-moving part of the belt strikes the pulley, around to A, where the downward-moving part leaves the pulley, only the edge of the belt is visible, and is represented by drawing an arc of a circle from line oc around to oA, with a radius equal to the radius of the pulley plus the thickness of the belt. The location of c may be found closely enough for all practical purposes, by drawing a line from B tangent to the upper pulley, c being the point of tangency, and oc the radius drawn through c. In drawing the arc of the circle from oc to oA, it is well to let it run by these lines a little way in the pencil drawing, and that part which is not used may be erased after the drawing is inked. The descending part of the belt leaves the pulley at A; and from that point to the place where it strikes the lower pulley, it twists through an angle of 90°, coming out over the front of the lower pulley. The edge of the belt, which we see in its full thickness where it leaves the upper pulley at A, twists toward the left, less and less of it being seen as the belt descends, until at d, where it strikes the other pulley, the two corners of this edge coincide, and from there to the bottom of the pulley we see this edge of the belt as a line. At the same time that the thickness of the belt has been disappearing from view, the outside of the belt has been coming into view, until at dd' we see the full width of the belt, the outside corner being the one which is visible. The inside corner is behind the rest of the belt, and while it may be shown dotted in the drawing, it is usually omitted entirely. From d' to the bottom of the pulley the two right-hand corners coincide. Points d and d' are found by drawing the horizontal dotted line through D (side view), and laying off on this line a distance equal to one-half the width of the belt each side of line XY. From ee' the belt goes around the back side of the pulley to bb' where it begins to be drawn off to the left, bb' being on the center line drawn through the shaft, since, by glancing at the right-hand view, we can see that the upward-moving part of the belt leaves the lower pulley at B, which is on a level with the center of the shaft. After leaving bb', the belt remains behind the pulley for a short distance, and is shown dotted. It is projected at its full width at bb', but as soon as it leaves the pulley it begins to twist toward the right, the left-hand edge gradually coming into view until it is seen at its full thickness where it strikes the pulley at C. At the same time, the inside face of the belt, which is toward the front when the belt leaves the pulley, gradually turns toward the right, and appears narrower until it disappears from view - that is, projected as a line. This completes the front view, and the side view is drawn according to the same principles, as is also the plan view.
To be strictly correct, the sides of the belt should be shown curved at A and B, for the belt is pliable and starts to curl and slip sidewise a short distance above these points. In order not to have an excessive side slip, the angles c B A and D A B should not be more than 25°.
Figs. 99 and 100 show what changes are made in the location of the pulleys and the appearance of the belt by changing the direction of rotation of the shafts. In Fig. 99 the lower shaft turns in the same direction as in Fig. 98, but the upper shaft turns in the opposite direction. In Fig. 100 the upper shaft turns in the same direction as in Fig. 98, and the lower shaft turns in the opposite direction.