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
Flat Belt
V-Belt or Wedge Belt
Timing or Toothed Belt
Round Belt
Types of Belt Drives
Open Belt Drive
Crossed Belt Drive
Quarter Turn Belt Drive
Belt Drives with Idler Pulley
Open Belt Drive
Shafts are parallel
Rotation of pulleys in same direction
Crossed Belt Drive
Shafts are parallel
Rotation of pulleys in opposite direction
Quarter Turn Belt Drive
Shafts are at right angle
Rotation in one definite direction
Belt Drives with Idler Pulley
Used when open belt drives angle of contact is less
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.
Leather belt
Nylon-Core belt
Rubber belt
Cotton and Canvas belt
Balata Belting
Rubber on fabric belt
Advantages of Belt Drives
Belts permit flexibility ranging from high horsepower drives to slow speed and high speed drives.
Belt drives are less expensive than chain drives for low horsepower and low ratio applications.
Belts require no lubrication.
Single belt drives will accept more misalignment than chain drives.
Flat belts are best for extremely high speed drives.
Belt drives cushion shock loads and load fluctuations.
Belts will slip under overload conditions, preventing mechanical damage to shafts, keys, and other machine parts.
Disadvantages of Belt Drives
Belts cannot be used where exact timing or speed is required because slippage does occur (only timing belts can be used).
Belts are easily damaged by oil, grease, abrasives, some chemicals, and heat.
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).
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 (
)
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.
- Tension on tight side
- Tension on slack side
- Coefficient of friction
- Angle of contact