We have already discussed that in an open belt drive, both the pulleys rotate in the same direction as shown in Fig.

Let

r1and r2= Radii of the larger and smaller pulleys,

x = Distance between the centres of two pulleys (i.e. O1 O2), and L = Total length of the belt.

Let the belt leaves the larger pulley at E and G and the smaller pulley at F and H as shown in Fig. Through O2, draw O2 M parallel to FE.

From the geometry of the figure, we find that O2 M will be perpendicular to O1 E. Let the angle MO2 O1 = α radians.

We know that the length of the belt,

L = Arc GJE + EF + Arc FKH + HG

= 2 (Arc JE + EF + Arc FK)