It is the ratio between the velocities of the driver and the follower or driven. It may be expressed, mathematically, as discussed below:
Let d1 = Diameter of the driver, d2 = Diameter of the follower,
N1 = Speed of the driver in r.p.m., and N2 = Speed of the follower in r.p.m.
Length of the belt that passes over the driver, in one minute = π d1N1
Similarly, length of the belt that passes over the follower, in one minute = π d2 N2
Since the length of belt that passes over the driver in one minute is equal to the length of belt that passes over the follower in one minute, therefore
π d1 N1 = π d2 N2
When the thickness of the belt (t) is considered, then velocity ratio,
