Belts

Belts

Belt drives are commonly used in transmission of power between two shafts which are at some distance. The belts run over pulleys, the pulley connected with the source of power is called the driver pulley and the other pulley is a called a driven. Sometimes belt drives may have idler pulleys also. The belts may have different cross section. The following image shows the different types of belt drives.

Types of Belt - Based on cross section

  1. Flat Belt

  2. V-Belt or Wedge Belt

  3. Timing or Toothed Belt

  4. Round Belt

Types of Belt Drives

  1. Open Belt Drive

  2. Crossed Belt Drive

  3. Quarter Turn Belt Drive

  4. Belt Drives with Idler Pulley

Open Belt Drive

  1. Shafts are parallel

  2. Rotation of pulleys in same direction

Crossed Belt Drive

  1. Shafts are parallel

  2. Rotation of pulleys in opposite direction

Quarter Turn Belt Drive

  1. Shafts are at right angle

  2. Rotation in one definite direction

Belt Drives with Idler Pulley

  1. Used when open belt drives angle of contact is less

  2. Used when required belt tension is not achieved

Materials used for belts

Since belt drives are friction drives, belts are made from materials which have high co-efficient of friction. Since the belts are made of different materials, the coefficient of friction also varies. Hence co-efficient of friction decides how much power is transferred. Some of the commonly used belt materials are listed below.

  1. Leather belt

  2. Nylon-Core belt

  3. Rubber belt

  4. Cotton and Canvas belt

  5. Balata Belting

  6. Rubber on fabric belt

Advantages of Belt Drives

  1. Belts permit flexibility ranging from high horsepower drives to slow speed and high speed drives.

  2. Belt drives are less expensive than chain drives for low horsepower and low ratio applications.

  3. Belts require no lubrication.

  4. Single belt drives will accept more misalignment than chain drives.

  5. Flat belts are best for extremely high speed drives.

  6. Belt drives cushion shock loads and load fluctuations.

  7. Belts will slip under overload conditions, preventing mechanical damage to shafts, keys, and other machine parts.

Disadvantages of Belt Drives

  1. Belts cannot be used where exact timing or speed is required because slippage does occur (only timing belts can be used).

  2. Belts are easily damaged by oil, grease, abrasives, some chemicals, and heat.

  3. Belts can be noisy; also loose or worn belts can be a major cause of machinery vibration.

Law of Belting

The center line of the belt as it approaches the pulley must lie in a plane perpendicular to the axis of the pulley, or must lie in the plane of pulley, otherwise the belt will run of the pulley.

Velocity ratio or Speed ratio of belt drive ( i )

Velocity ratio/Speed ratio is defined as the ratio of speed of the driving pulley (n) to the speed of driven pulley (N). In other terms its the ratio of diameter of the driven pulley (D) to the diameter of driving pulley (d).

i = \frac{D}{d} = \frac{n}{N}

Tight side and slack side

When belt is running over pulley, the friction creates grip on pulley which creates tension on one side of belt which makes the driven pulley to run, this side is called tight side. The other side does not experience same tension, that side is called slack side.

Slip and Its effect

Slip is a type of loss occurring in belt drives. When the friction force between the pulley and belt is less, the belt moves without rotating the pulley or pulley rotates without pulling the belt.

Angle of contact (

\theta

)

The angle subtended by the belt on the pulley is know as angle of contact or arc of contact. For better performance the angle of contact should be between 90° to 170°. To increase the angle of contact idler pulleys can be used.

Belt creep

As the belt moves from slack side to tight side the tension increases. That is tension is less in slag side and high in tight side. This results in elongation of belt in tension side resulting in less thickness on tension side. This is called creep

Belt Wipping

As the distance between the pulley driven by belt increases, the belt begins to vibrate in the direction perpendicular to the direction of motion of the belt drive. This vibrations of belt drives is called wipping.

Centrifugal tension in belt

When the belt runs round the pulleys, a centrifugal force is produced on the belt. This force tends to lift the belt from the pulley surface, resulting in more tension on belt.

{T_1}

- Tension on tight side

{T_2}

- Tension on slack side

\mu

- Coefficient of friction

\theta

- Angle of contact

\frac{{{T_1}}}{{{T_2}}} = {e^{\mu \theta }}