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Driven

All you need to know about pulley and V-belt drives

by Mike Sondalini

Pulley and V-belt drives have long been used as a method to deliver power to rotating equipment. They are simple in design and application. To be reliable, they must be specified correctly, installed accurately, monitored watchfully and replaced before they fail. This article covers their selection, use and the good practices needed out in the field to insure reliable operation.

History lesson
The V-belt drive evolved from the flat belt drive used to turn machinery shafts in the early days of industrialization. It allows the same power to be transmitted with less belt tension. The simplicity, ease of adjustment and forgiveness of imperfection make V-belt drives a popular way to provide power transmission between shafts.

V-belt drive design
To transmit power between shafts using a V-belt, it’s necessary to keep the belt in contact with the grooved pulley with enough friction to make the driven shaft turn. The important requirement is to maintain the grip between belt and pulley so it is strong enough to transmit the driving force.

The table below lists the key requirements considered in the design of V-belt and pulley drives. Assess all the factors affecting use of V-belt drives. Formulas and material properties are readily available. For practicality, pulleys and belts are only available in standard sizes and a limited range of materials proven successful in industrial applications.

V-belt, pulley construction
The belt is usually constructed of rubber because of its elastic and frictional properties. Natural fiber, fiberglass or steel cords are imbedded into the belt. The cords keep the belt together under tension. The cross-section of the belt determines the power it can handle.

The pulley is usually made of metal. The smaller sizes are machined from forged steel or pressed-steel welded together. The larger sizes are cast iron or cast steel. Because they rotate with a large part of their mass well away from the center, they must be balanced so as not to create vibration.

Belt and pulley materials also have limitations. Rubber and plastic have temperature limits. Chemicals such as acids, caustics and hydrocarbons (typically fuels, solvents, oils and greases) affect them. They can be easily cut and damaged. Also, oxygen in the atmosphere degrades them over time.

Metal pulleys corrode. Chemicals also attack them. Fabrication errors only show up in service. For example, pressed and spot-welded pulley halves can separate in operation.

Advantages of V-belt drives
V-belt drives have many advantages, provided the application in which they are used is comfortably within the design requirements of the belt and pulley materials.

Some of the advantages include:
• They are an easily available and low-cost method to transmit power between shafts.
• They prevent transmission of vibration from shaft to shaft.
• The mounting of pulleys to shafts is quick and easy, requiring little skill and training.
• Suitable alignment requires pulling a string line or placing a straight edge across the pulley faces and checking the gaps.
• The belt can spin in overload conditions and prevent the equipment from being damaged.
• You can easily measure belt tension by the amount of sideways movement of the belt midway between pulleys.
• Angular and linear alignment tolerance between pulleys is generous and permits slight inaccuracies.
• Adjusting the distance between shafts changes the belt tension.
• Simply adding wider pulleys with more belts will transmit more power. Pulleys with up to five belts are not uncommon.
• It’s easy for operators to see if the belt is loose. You can also listen for the squeal of a slipping belt.
• You can transmit more power by wrapping the belt further around the pulleys with a jockey pulley on the loose side.
• You can machine pulley hubs to suit any size of shaft.
• Replacing belts is normally easy, provided there is comfortable access to the drive.
• Changing pulley sizes can easily alter shaft speeds.

See Figure 1.

Issues when using V-belts
Though commonly used, V-belt drives present common problems in the manufacturing and process industries. This is often due to neglecting or misunderstanding the limits within which V-belt and pulley drives operate successfully. The table at right lists the critical factors that affect belt drive operation.

See Figure 2.

Mike Sondalini runs Lifetime Reliability, an Australian Web site for maintenance pros. To learn more, visit www.lifetime-reliability.com or e-mail info@lifetime-reliability.com.