368 APPLIED MECHANICS : 
312. Belt Gearing for Non-Parallel Shafts.—Motion may be trans- 
mitted directly from one shaft to another when the axes are not in the — 
same plane by means of a belt and two 
pulleys, one pulley on each shaft, provided 
that the pulleys are so arranged that the 
middle point of the width of the belt where 
it leaves one pulley is in the central plane 
of the other pulley. In other words, the 
centre line of each of the straight portions 
of the belt must be in the central plane of 
the pulley towards which it is travelling. 
An example is shown in Fig. 565, where the FIG. 565. 
axes of the two shafts are at right angles to 
one another, but not in the same plane. The arrangement of the two pulleys 
mentioned above is only pos- 
sible when the motion is in one 
direction ; if the direction of 
the motion be reversed, the 
belt comes off the pulleys. 
When it is not possible 
or convenient to arrange the 
pulleys on non-parallel shafts, 
so as to permit of the one 
driving the other directly, 
one or more guide pulleys 
may be introduced. The 
guide pulleys must be placed 
so that all the straight por- 
tions of the belt comply 
with the condition already 
stated. Fig. 566 shows two ; 
pulleys A and B, whose central planes intersect in the line CD. Any 
convenient points E and F are taken in 
CD, and tangents EH, EK, FL, and FM 
are drawn to A and B. (Guide pulleys N 
and O, touching these tangents as shown, 
and having for their central planes the 
planes HEK and LFM respectively, will 
serve to guide the belt between the pulleys 
A and B. As arranged in Fig. 566, the 
belt may run in either direction. 
Another example of the use of guide 
pulleys is shown in Fig. 567. Hic. 67: 
313. Straining or Jockey Pulleys.—A belt passing round two pulleys 
may be tightened without shortening it by placing 
a third pulley on the slack part of the belt, that 
is, the part which runs from the driving pulley to 
the following pulley, as shown in Fig. 568. This 
third pulley, which is called a straining, tighten- 
ing, or jockey pulley, runs in bearings which 
arc loaded with a weight to press the pulley on 
Fig. 566. 
F 
