Open Belt Drive And Cross Belt Drive:
Mechanical power transmission systems are used to transmits mechanical power (as rotational torque) from a driver shaft (such as prime mover) to a driven shaft (such as a machine unit).
The belt drive is one of the mechanical drives (gear drives, chain drives, and rope drives of others) used to transmit speed, torque, and power from one shaft to another. Initially, a pulse of the required diameter is strictly applied to the shaft.
An endless belt is wrapped around the pulse, maintaining the proper tension to avoid slip. The belt, which partially wraps it and passes over the pulleys, acts as a flexible intermediate linkage between the driver and the driven shaft.
The frictional force between the belt and the pulleys helps to transmit speed and power from one shaft to another. The initial stress applied to the belt, as well as the friction characteristic between the mating surfaces, determines the power transmission capability.
For a fixed speed in a driven shaft, the diameter of the pulleys determines the velocity ratio and rotational speed of the driving shaft.
The flexible belt also helps reduce vibration and thus restricts it from propagating the driver element.
Although it cannot provide a constant velocity ratio due to inherent slip, it is very useful for short as well as long-distance power transmission. Slip, however, can prevent prime movers from overloading.
While the ratio of velocity and rotation speed is maintained by the diameter of the pulse for the driver and driven shaft, the direction of rotation can be changed using either open or crossed belt drives.
It also affects the power transmission capacity due to the change in wrap angle between the belt and the pulleys. This classification applies mostly to flat belts because v-belts are always used in open configurations.
In an arrangement where the endless (joint) belt moves from the top of one winch to the top of another pulley without crossing it, it is called open belt drive.
Here the induced shaft will rotate in the same direction as the driver shaft. In addition, the wrap angle is always below 180 °; thus, the power transmission capacity is also reduced.
On the other hand, in the cross belt drive, the belt moves from the top of one pulley to the bottom of the other pulley and thus crosses itself. Here the driven shaft rotates in the opposite direction to the driver shaft.
The wrap angle also increases, resulting in higher power transmission capability.
However, continuous rubbing of the belt leads to the short life and quick failure of the belt. In addition, it requires relatively long belts.
The various differences between open belt drive and cross belt drive are given below in table format.
Difference Between Open Belt Drive and Cross Belt Drive:
Table difference between open belt drive and cross belt drive:-
|Open Belt Drive||Cross Belt Drive|
|In an open belt drive, the belt proceeds from the top of one pulley to the top of another pulley without crossing.||In cross belt drive, the belt proceeds from the top of one pulley to the bottom of another pulley and thus crosses itself.|
|Here the entire belt remains in the same plane in every rotation.||Here belt bends in two different planes in every rotation.|
|Here driver and driven shafts rotate in the same direction.||Here driver and driven shafts rotate in opposite directions.|
|The angle of wrap or contact angle between belt and pulley is always below 180°.||The angle of wrap is always above 180°.|
|Due to the smaller wrap angle, power transmission capacity is also lower.||Due to the larger wrap angle, power transmission capacity is also higher.|
|The length of the belt in the open belt arrangement is comparatively shorter.||For the same pulley size and center distance, a cross belt drive requires a longer belt.|
|Here belt does not touch itself. So no rubbing takes place. Thus belt exhibits longer life.||Due to crossing between two pulleys, the belt rubs against itself and thus undergoes gradual wear. Thus belt has a shorter life.|
|In between two pulleys, the belt vibrates (whips) when the center distance is more.||The Crossbelt arrangement is free from such vibration.|
Belt Configuration of Cross Belt Drive:
- The flexible element may have a different shape in the belt drive. A flat belt is a composite belt that has a rectangular cross-section, where the width is much larger than the thickness (height).
- A V-belt is an endless belt with a trapezoidal cross-section with equal width and height. A ribbed belt is similar to a flat belt, but the inside circumference has teeth that mesh with the corresponding slots on the pulleys.
- Open and cross belt configurations are related to flat belts only. In the open belt arrangement, the belt moves from the top of one winch to the top of another pulley without crossing.
- In the close belt arrangement, the belt moves from the top of one winch to the bottom of another pulley and thus crosses itself.
Bending of Belt:
- In the open belt configuration, the inside of the belt touches the pulley; however, there is no tilting or flipping as the belt moves from one pulley to the other.
- So the belt remains in the same plane throughout the rotation. In a cross belt drive, the belt moves from the top of one pulley to the bottom of another pulley.
- Since only one inner portion of the belt can wrap or touch the pulley, the belt needs to bend 180 ° between the driver and the driven shaft. There are two such turns for every rotation of the belt.
The Direction of Rotation:
- An important advantage of belt drive over gear drive is its ability to rotate the driver and the driven shafts in the same or opposite direction without using other accessories.
- In a single-phase, the gear drive can only rotate in the opposite direction unless intermediate or passive gears are employed. An open belt drive driver can rotate a shaft driven in the same direction as the shaft; Whereas, a cross belt drive can only rotate in the opposite direction.
- For example, if the drive shaft rotates in a clockwise direction, the driven shaft rotates in a clockwise direction when the pulleys are connected by an open belt; otherwise, the driven shaft will rotate in the counter-clockwise direction when the bridge is connected by a cross Belt.
Contact Angle and Power Transmission Capacity:
- A belt drive is a friction drive, which indicates speed and power is transmitted through friction.
- The power transmission capability in flat belt drives depends on the coefficient of friction between the belt and pulleys, the angle of the wrap, belt velocity, and the mass of the belt per unit length.
- The higher the wrap angle, the more power it can transmit without slip. In an open belt drive, this wrap or contact angle between the belt and the pulley (any) is always below 180 °.
- It will be equal to 180 ° if the conductor and the driven pulley do not have the same diameter (no speed reduction).
- On the other hand, in cross belt drives, the wrap angle is always larger than 180 °, unless both pulleys have the same diameter. Accordingly, a cross belt can transmit higher power than an open belt provided that all other relevant parameters are unchanged.
Length of Belt:
- The length of the belt depends mainly on the diameter of the driver and the pulleys and the distance of their center. There is a trivial effect of wrap angle on belt length.
- The belt length must be maintained properly because a smaller belt can produce excessive load on the bearings, while a larger belt will increase slip (unstable velocity ratio), force fluctuations, and power losses.
- For the same diameter as the driver and driven pulleys and their center distance, the cross belt length is always greater than the length of the open belt.
Belt Rubbing and Service Life:
- In the open belt configuration, the belt does not touch itself, whereas, in the cross belt configuration, the belt rubs against itself.
- Such rubbing generates heat, accelerates wear rate, and subsequently shortens the service life of the belt. Thus cross belts require constant adjustment in tension and quick replacement.
Also, Read: Inversion of Double Slider Crank Mechanism
Whipping of Belt:
- Belt vibration perpendicular to the direction of motion is called a perpendicular belt.
- Belt whipping is a dominating factor in belt open drive, especially when the center distance between the driver and the driven shaft is high.
- Belt whipping, along with slip, limits the power transmission capability in open belt drive. On the other hand, the cross belt drive is inherently free of whipping; however, slip may occur.