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Definition of a belt 

Belt is in the form of a loop. It connects mechanically two shafts for transmitting power smoothly. A belt drive consists of shafts, pulleys and a belt.

Materials Of Belts

(I) Leather

(ii) Rubber

(iii) Fiber/cotton

(iv) Balata

Standard Thicknesses of belts

5, 6.5, 8, 10 and 12 mm are the standard thicknesses.

Standard widths of flat belts

25, 32, 40, 50, 63, 71, 80, 90, 100, 112, 125, 140, 160, 180, 200, 224, 250, 280, 315, 400, 450, 500, 560 and 600 mm

Types Of Flat Belts

  1. Based on Orientation

    1. Horizontal

    2. Vertical

    3. Quarter –turn

    4. Right-angled

    5. Crossed

    6. Reversed drive

  2. Based on Load or duty or power to be transmitted

Sr. No.



Peripheral velocity


Light load

7.5 kW

12 m/s


Medium Load

7.5 to 15 kW



Heavy load

> 15 kW

>24 m/s

 Important terminology for belts

(i) Speed ratio

Neglecting the slip and thickness of the belt.

Speed ratio= diameter of bigger pulley/diameter of smaller pulley

Hence Speed ratio = N1/N2

Where N1 is the motor or engine speed OR higher speed

N2 is the machine speed or lower speed

(ii) Center distance

(iii) Peripheral velocity


The center line of belt as it approaches the pulley must coincide with the central plane of that pulley. Otherwise belt will fly away from the pulley.


There are two types of pulleys used with belts.

(I) Flat pulleys——–Used with flat belts.

(ii) Grooved pulleys—–Used with V-belts. Grooved pulleys are called sheaves.

Belt Drive and its Types

A belt drive consists of two shafts, two pulleys and a belt. One of these shaft is a motor shaft on which electric motor is mounted. On the other shaft the machine is mounted to which power is transmitted by the belt drive.   Belt drive is used to transmit power from the motor to the machine shaft. It can be from the engine to the machine shaft. This can also be from turbine to generator.

Motor speed and Pulleys sizes

Normally speed of the motor is high because high speed motors are more efficient. Therefore,  motor shaft is the driving shaft and the machine shaft is the driven shaft. Thus, in most of the cases the machine has Lesser RPM than that of the motor. Hence, pulley on motor will be smaller pulley and the pulley on the machine shaft will be a larger pulley. Belts do not transmit with 100 % efficiency because of SLIP and stretching of the belt. There are two types of belt drive, namely flat belt drive and V-belt drive. The flat belt drive is of further two types, Open and cross belt drives.

 Open Belt Drive

 Both drive and driven shafts run in the same direction in case of an open belt drive. For smooth power transmission, belt on one side is more tight than the other side. In a horizontal drive, tightened side is always kept in the lower side of two pulleys because the sag of the upper side slightly increases the angle of contact of the belt on the two pulleys. More angle of contact means more power transmission.

Cross Belt Drive

In case of cross belt drive, both drive and driven shafts run in the opposite directions. But the more angle of contact (angle of wrap) increases power transmission. Chances of slip of belt are not there.

Rope Belt Drive

Here a rope is used. But these not very common in industries.

 Advantages of a belt drive

  1. Belt drive  is a simple drive.

  2. It is cheap.

  3. No lubrication required.

  4. Have high efficiency.

  5. Requires less maintenance.

  6. Some misalignment is adjustable without loss in efficiency.

  7. Durable and has long life.

  8. Can be used for parallel and non parallel shafts.

  9. Easy to reduce vibrations and noise.

 Disadvantages of a belt drive

  1. Velocity ratio is not truly constant because of slip and stretching.

  2. Heating occurs due to friction. It disturbs its perfect working at higher temperatures.

  3. There is a speed limit of 35 m/s.

  4. Requires center distance adjustment due to stretching.


(I) Farming

(ii) Water pumps

(iii) Mining

(iv) Saw mills

(v) Electrical generators


(i) Stone crushers

(ii) Machine tools

(iii) Paper industry

(iv) Textile industry

(v) High power mills

(vi) Refrigeration and air conditioning machinery

(vii)  Cars