How to detect belt drive problems

When compared with the constant lubrication problems associated with chain drives or the mechanical problems and high costs connected with gear drives, a properly maintained industrial belt drive is a cost-effective, reliable means of power transmission.

Drive problems can arise, however, due to any number of sources — from environmental factors to improper drive maintenance.

When diagnosing a belt drive problem, Gates Corporation engineers say maintenance personnel should rely on their senses of sight, hearing, smell or touch to detect the source of trouble.

While equipment is operating, for example, they should be on the lookout for anything unusual about the way the belt travels around the drive. Does the drive frame flex under load? Is there the smell of warm rubber, or detectable sounds such as chirping, squealing or grinding? Is there an accumulation of dirt on the drive guard that might interfere with proper ventilation?

When the drive is shut down, can the belt be held after it has been running, or is it too hot? It is possible to tolerate up to 60C (140F), the maximum temperature at which a well designed and properly maintained belt should operate. If the belt is too hot to hold on to, then assume there is something wrong that is causing heat buildup. This should be done carefully; wear gloves to avoid burning your hand.

Also, using gloves, feel the sheave grooves. Are they smooth and free of burrs and debris? Inspect the belt for unusual wear patterns, signs of burning, or cracking on the belt.

Troubleshooting tips

Faulty drive alignment and tension are the most significant causes of failure in belt drives today. Misalignment, improper tension or foreign objects in the drive usually are indicated when belts turn over or come off the drive, or by unusual belt or pulley wear patterns.

Damaged tensile cord sections also can cause belts to turn over. The cause can be foreign material in the grooves, shock loading or belt damage during installation. When none of these seems to be the reason, and the belts seem to whip or vibrate, the natural vibration frequency of the drive is sometimes involved. Placing knock-down bars or idlers into the vibrating spans sometimes helps.

The friction created by a loose V-belt slipping in the sheave causes the belt sidewalls to become glazed or shiny and lose their gripping capacity. Grease and oil on the sheave also can cause this condition.

Deep bottom cracks that appear at regular intervals are caused by the V-belt turning around too-small a sheave, including backside idlers. A V-belt that is falling apart in layers could be a victim of oil, which weakens the compound’s bonds, making the belt soft, spongy and considerably weaker. Pitted or streaked sidewalls, or a tensile break, can indicate a foreign object in the sheave groove. If a V-belt exhibits any of the above symptoms, it is a candidate for replacement.

Synchronous belt drives will begin to wear out in their normal service life at the base of the tooth where it joins the belt. Cracking generally will begin on the forward side of the tooth (in the direction of belt travel).

All other belt failures indicate severe drive conditions, misuse or abuse, which may or may not be design-related.

Stop that noise

An unusually loud drive can result from several probable causes: an incorrect belt, worn pulleys, debris in the pulleys, or improper belt tension. Pulleys must always be examined for the correct belt size prior to installation and, in synchronous drive systems, must be examined for the correct tooth profile for the sprockets.

A common misperception is that metal sheaves and sprockets never wear out. Application engineers from Gates report that 70% of the belt drive problems they investigate can be traced to something wrong with a metal component. Replacing worn pulleys is the only solution for belt noise and wear caused by that situation.

Contamination of the belt drive is perhaps the easiest cause of belt noise to identify. The belt can be viewed for oil contamination or embedded debris.

Pilling or dust accumulation on the belt is also readily visible. When the pulleys are contaminated by debris, they must be cleaned, their shielding improved, and any rust, paint or dirt must be removed from the grooves.

The heat is on

The effect of temperature on belt life is important. An internal temperature increase of about 10C (18F) — or a 20C (36F) rise in ambient drive temperature — may cut belt life in half. Primary causes of heat buildup in belts are ambient conditions, improper tension maintenance resulting in V-belt slippage, and unusually severe operating conditions. Selecting sub-minimal diameter pulleys during the design stage also can be a factor.

The most common cause of overheating by far is extremely high ambient temperature. When ambient temperature is the problem, increasing ventilation around the drive can help dissipate the heat. This can be done by recommending pulleys that have spokes and fins that create airflows that cool the belts, or by using external air sources to cool the belt.

Fans and air blowers also can be designed for the system, as long as the fan is not moving hotter air from a heat source across the belt. Also consider providing better ventilation in the drive guard. Simply letting the heat out sometimes can dramatically lower the temperature.

If ambient conditions have been eliminated as the cause for overheating, belt slippage or severe operating conditions become prime suspects. V-belt slippage may be due to improper tension or the presence of grease or oil in the drive. Improper belt tension is an obvious cause of slip that generates unwanted heat. Less obvious causes are poor drive guard design, inadequate ventilation, misalignment or worn pulleys.

Unusually severe operating conditions or improperly designed drives can cause high internal belt temperatures. Examples of severe operating conditions include:

* Use of larger motors or engines than anticipated in the original design.

* Higher driveN loads than expected.

* Use of higher speed motors with a corresponding reduction in sheave size.

Internal heat problems can be corrected by decreasing each belt’s load by designing the drive with more belts, and by decreasing belt bending stress, which is a major factor in internal belt temperature. These stresses can be reduced by using a larger pulley, changing to a different belt cross section, or using a molded notch belt if the drive currently uses banded belts.

The ability to anticipate these types of problems in V-belt and synchronous drive systems can be useful in maintaining serviceable equipment.

Although any belt will wear out eventually, an effective preventive maintenance program can save time and money. Inspecting and replacing belts and faulty drive components before they fail reduces costly downtime and production delays.

This article was prepared by belt drive experts at Gates Corporation. For additional information, use the number below on a reply card in this issue.

Keys to a successful belt drive maintenance program

Install belts properly. Good installation practices require aligning the drive and setting the proper tension for the belt. Both V-belts and synchronous belts do best with a proper tension for the load to be transmitted.

Maintain a clean environment. Protect the drive from foreign objects or other contaminants that may damage the belt or pulley.

Establish a replacement schedule. Depending on the environment of the belt drive, its service life might be six weeks, six months or even years. Replacement of belts approaching their ultimate service life will help prevent unscheduled downtime.

Leave a paper trail. Good records are important. Sometimes maintenance people will replace a motor with one that has higher horsepower. Th
is can lead to premature failure for both belt and metal drive components designed for the smaller motor.

If belts are too hot to handle with a gloved hand, the drive needs proper tensioning or improved ventilation.