Belt troubleshooting

May 1, 2002
Many gremlins attack belts. Anticipating problems helps belts drive for years to come

Awell-designed, industrial belt drive is capable of operating for several years when used under normal conditions, and properly maintained. Put it in a difficult environment, however, or simply neglect it, and you may end up with a belt that fails in days, hours, or even minutes. Fortunately, by troubleshooting for common problems and making periodic inspections, premature belt failures can be virtually eliminated.

THREAT: Ambient temperature

The most common cause of hot belts, by far, is a hot environment. Overheated belts threaten the whole motion system — they can become brittle, crack, and ultimately fail, often without warning. Their reduced flexibility also decreases drive efficiency.

Applications with inherently high ambient temperatures benefit from ventilation around the drive to help dissipate this harmful heat. Designing sheaves with spokes or fins creates airflow while rotating, which cools belts; external air sources such as fans and air blowers also get the job done. However, fans should not force hotter air from a heat source across belts. More openings in the drive guard are another option; sometimes simply letting heat escape lowers temperatures to acceptable levels.

THREAT: Contaminants

Belt drive contamination is perhaps the easiest failure mode to identify. Belts can be visually inspected for oil or debris, which sometimes push through to the backside of the belt, making visible holes. Grease and oil on a pulley can also induce V-belt slipping, causing glazed, or shiny belt sidewalls. Another sign of oil contamination is when V-belts fall apart in layers; oils tend to weaken compound bonds and make for soft, spongy, and considerably weaker belts. Pitted or streaked sidewalls, or a tensile break, can indicate a larger foreign object in the pulley; an unusually loud drive might indicate smaller debris. Other types of contamination — harsh weather, high humidity, and sunlight exposure — also decrease drive performance.

When pulleys are contaminated they must be cleaned, cleared of rust, paint, or dirt in grooves, and better shielded. After that, maintaining a clean environment will protect the drive, preventing other foreign objects from lodging between the belt and pulley. Anticipating these types of problems in V-belt and synchronous drive systems can also be useful in designing more user-friendly and serviceable equipment.

THREAT: Poor drive design

Drives must be properly designed and built to reach their intended service life. In addition to determining the best size and number of belts to use, an engineer must consider other drive-design factors. For example, pulleys must be manufactured according to industry-accepted tolerances. Minimum recommended pulley diameters must be noted; for example, on a 3V-section V-belt, pulleys smaller than 2.65 in. will be troublesome. If a V-belt has deep cracks that begin at regular intervals in the undercord material, it might be running on a pulley that is too small.

With an unusually loud drive, improper belt cross-section might be the problem. Because belts don’t ride properly in pulleys that have the inappropriate angle, they should always be sized for proper cross-section prior to installation. In synchronous drive systems, sprockets should also be examined to confirm that they have the correct tooth profile.

Belt guards must be designed for adequate worker safety and drive protection, yet still provide ventilation. Structural members of the drive including framework, motor mounts, and machine pads must be rigid, and must not deflect under load. Drives should be designed for minimal vibration, and for ease of maintenance and inspection.

Defective drive components are rarely the cause of a drive problem. If other possible causes have been eliminated and a part is suspected defective, the belt or pulley supplier should help verify the concern and correct the problem.

THREAT: Improper loading

Severe operating conditions or drives improperly designed for their challenging situations can cause high internal belt temperatures. Examples of these situations include:

• Unavoidable high-shock loads — In certain applications, inherent conditions will always be severe (as in rock crushers and logging sawmill equipment). • Use of motors or engines larger than originally specified • Higher loads than expected — Failure to account for startup loads is one cause. • Use of high-speed motors with a corresponding reduction in pulley size

The Rubber Manufacturers Association (RMA) has developed service factor tables that categorize the severity of a variety of applications to help designers determine how rugged a system should be. By decreasing belt loads by designing drives with more or wider belts, internal belt temperature can be lessened. Decreasing belt bending stress also helps. This can be accomplished with a larger pulley or a different belt cross section; on V-belts, a cogged or molded notch version, which has increased flexibility, can also be substituted.

THREAT: Installation

Improper belt tension is the most common and obvious cause of slip, which generates unwanted heat. The friction that occurs when a V-belt slips in a pulley causes belt sidewalls to become glazed, and decreases gripping ability.

After incorrect tensioning, faulty drive alignment is the second most significant cause of failure in belt drives. Misalignment is often indicated by unusual belt or pulley wear patterns; sidewalls become uneven, and covers become frayed; on synchronous belts, uneven tooth and side wear indicates misalignment. Misalignment also makes it hard to maintain proper tension, which compounds problems. Good installation practices require drive alignment and proper belt tension setting. Both V-belts and synchronous belts perform optimally with a tension appropriate for the load to be carried.

Troubleshooting should include pulley alignment inspection and belt size and cross-section checks. During installation, belts should never be forced over the sheave edges. Using the drive center distance adjustment to slip the belt onto the sheaves — without prying — is a better method. Otherwise, tensile cords in the belt could be damaged or severed, greatly shortening life. Removal of one of the pulleys might be necessary to install the belt; when reinstalling them, correct alignment with the bushings is paramount. The bolts that connect sheaves to shafts need conscientious, alternating tightening for consistent torque levels. Finally, bushing hardware should never be substituted with another size.

THREAT: Everyday wear

A common misconception is that metal sheaves and sprockets never wear out. Not so — many application engineers report that a significant number of belt drive problems can be traced to a problem with a metal component. Replacing worn pulleys is the only solution for the unusual belt noise and premature failure caused by this situation. At the end of their service life synchronous belt drives begin to show wear at the base of each tooth. Cracking will usually begin on the forward side of the tooth, in the direction of belt travel.

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