Velocity Ratio for Open Belt Drive:

Figure

Consider a simple belt drive (that is one driver and one follower) as shown in the figure given below

Let

D₁  = Diameter of driver

N₁  = Speed of driver in R.P.M.

D₂, N₂  = Corresponding values for follower

Length of belt which passes over driver, in 1 minute = P.D₁.N₁

Likewise,

Length of belt,

That passes over follower, in one minute = P.D₂.N₂

As the length of belt, which passes over the driver in one minute is equal to length of belt which passes over the follower in one minute, therefore:

P.D₁.N₁  = P.D₂.N₂

Or,   velocity ratio = N₂/N₁  = D₁/D₂

If the thickness of belt 't' is given to us then

V.R = N₂/N₁  = (D₁ + t)/(D₂ + t)