Open Belt Drive and Cross Belt Drive in Mechanical Power Transmission Systems

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:-

Belt Configuration of Cross Belt Drive:

1. 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).
2. 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.
3. 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.
4. 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:

1. 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.
2. 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.
3. 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:

1. 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.
2. 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.
3. 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:

1. A belt drive is a friction drive, which indicates speed and power is transmitted through friction.
2. 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.
3. 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 °.
4. It will be equal to 180 ° if the conductor and the driven pulley do not have the same diameter (no speed reduction).
5. 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:

1. 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.
2. 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.
3. 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:

1. In the open belt configuration, the belt does not touch itself, whereas, in the cross belt configuration, the belt rubs against itself.
2. 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.

Whipping of Belt:

1. Belt vibration perpendicular to the direction of motion is called a perpendicular belt.
2. Belt whipping is a dominating factor in belt open drive, especially when the center distance between the driver and the driven shaft is high.
3. 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.

Frequently Asked Questions (FAQs) on Open Belt Drive and Cross Belt Drive

What is a belt drive in mechanical power transmission systems?

A belt drive is a system used to transmit speed, torque, and power from one shaft to another. It consists of a flexible belt that wraps around pulleys connected to the driver and driven shafts.

How does a belt drive work?

The belt drive works by using the frictional force between the belt and the pulleys. The initial tension applied to the belt and the frictional characteristics between the belt and pulley surfaces help transmit power and speed from the driver to the driven shaft.

What are the main types of belt drives?

The main types of belt drives are open belt drives and cross belt drives. These configurations affect the direction of rotation, power transmission capacity, and belt lifespan.

What is an open belt drive?

In an open belt drive, the belt moves from the top of one pulley to the top of another without crossing. This arrangement causes the driven shaft to rotate in the same direction as the driver shaft.

What is a cross belt drive?

In a cross belt drive, the belt moves from the top of one pulley to the bottom of the other, crossing itself. This configuration results in the driven shaft rotating in the opposite direction to the driver shaft.

How does the wrap angle affect power transmission?

The wrap angle is the angle at which the belt contacts the pulley. In an open belt drive, the wrap angle is always below 180°, resulting in lower power transmission capacity. In a cross belt drive, the wrap angle is above 180°, allowing for higher power transmission capacity.

What are the advantages of using an open belt drive?

Open belt drives are simpler in design, have a longer belt life since there is no rubbing between the belt layers, and are less prone to vibrations when the center distance between shafts is short.

What are the disadvantages of using an open belt drive?

Open belt drives have lower power transmission capacity due to the smaller wrap angle and may experience more vibrations (whipping) when the center distance is large.

What are the advantages of using a cross belt drive?

Cross belt drives can transmit higher power due to the larger wrap angle and are free from belt whipping, making them suitable for longer center distances between shafts.

What are the disadvantages of using a cross belt drive?

Cross belt drives have a shorter belt life due to continuous rubbing between belt layers, require longer belts, and need constant adjustment and replacement due to accelerated wear.

How does the direction of rotation differ between open and cross belt drives?

In an open belt drive, the driven shaft rotates in the same direction as the driver shaft. In a cross belt drive, the driven shaft rotates in the opposite direction to the driver shaft.

How does belt length affect power transmission?

The length of the belt affects the load on bearings, the stability of the velocity ratio, force fluctuations, and power losses. A cross belt drive requires a longer belt compared to an open belt drive for the same pulley sizes and center distance.

What is belt whipping and how does it affect belt drives?

Belt whipping is the vibration of the belt perpendicular to its motion. It is more prominent in open belt drives with long center distances and can limit power transmission capability. Cross belt drives are inherently free from whipping.

What types of belts are used in belt drives?

Common types of belts used in belt drives include flat belts, V-belts, and ribbed belts. Flat belts have a rectangular cross-section, V-belts have a trapezoidal cross-section, and ribbed belts have teeth that mesh with pulley slots.

Which type of belt drive is more suitable for long-distance power transmission?

Cross belt drives are more suitable for long-distance power transmission due to their higher power transmission capacity and absence of belt whipping. However, they require more maintenance due to belt rubbing and wear.

Can belt drives be used for constant velocity ratio applications?

Belt drives are not ideal for applications requiring a constant velocity ratio due to inherent slip. However, they are useful for applications where slip can prevent overloading of the prime mover.

What factors determine the power transmission capability of a belt drive?

The power transmission capability of a belt drive depends on the coefficient of friction between the belt and pulleys, the wrap angle, belt velocity, and the mass of the belt per unit length.