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Explain how many types of belt drive used for power transmission? Also derive their velocity ratio.

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There are three types of belt drive: 

(1) Open belt drive 

(2) Cross belt drive 

(3) Compound belt drive

(1) Open Belt Drive: 

When the shafts are arranged in parallel and rotating in the same direction, open belt drive is obtained. In the diagram (a), pulley 'A' is called as driver pulley because it is attached with the rotating shaft.

Velocity Ratio (V.R.) for Open Belt Drive

Consider a simple belt drive (i.e., one driver and one follower) as shown in fig (b)

Let D1 = Diameter of the driver 

N1 = Speed of the driver in R.P.M. D2,

 N2 = Corresponding values for the follower 

Length of the belt, that passes over the driver, in one minute = Π.D1.N1 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

Or, velocity ratio = N2/N1 = D1/D2

If thickness of belt 't' is given then

V.R = N2/N1 = (D1 + t)/(D2 + t)

(2) Cross Belt Drive When the shafts are rotating in opposite direction, cross belt drive is obtained. 

In the diagram, pulley 'A' is called as driver pulley because it is attached with the rotating shaft. Velocity ratio is same as for open belt,

V.R. = N2/N1 = D1/D2

If thickness of belt 't' is given then

V.R = N2/N1 = (D1 + t)/(D2 + t)

(3) Compound Belt Drive 

When a number of pulleys are used to transmit power from one shaft to another then a compound belt drive is obtained.

Velocity Ratio for Compound Belt Drive 

Speed of last follower = Product of diameter of driver(odd dia) Speed of first driver Product of diameter of follower(even dia) N4/N1 = (D1.D3)/(D2.D4)

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