The facts of belt life

Here’s what you can do to prevent some of the most common PT belt failures

by Brock Peterson

It’s a fact of life: Power transmission systems inevitably fail.

In general, the more complex the system — the more components and moving parts — the more likely that a failure within the system will occur sooner.

The component of the power transmission system that this article focuses on is belts. More to the point, this article explains why belts fail and what consumers can do to prevent these failures.

Contributors to failure
There are a myriad of causes for belt failure: improper operating tension, misalignment of sheaves or sprockets, worn sheaves and sprockets, extreme temperatures in the operating environment, chemical contamination, and foreign objects, to name a few. Fortunately, for all of these failures, there is a corresponding solution.

Improperly tensioned belts are one of the most — if not THE most — common reason for premature belt failure. There is a tension at which the belt experiences its optimum service life. Anything above or below this tension translates directly into a decrease in belt life. (Purchasing agents would file this under “not getting all the bang for the buck.”)

The trick then is getting to the proper tension. Most belt manufacturers provide belt-tensioning devices in their product lines. These devices run the gamut in price range and ease of use. There are sonic tension gauges that read the vibration frequency of belts and translate them into actual strand tensions. These tend to be on the higher end of cost and require the product manufacturer to provide some knowledge of the belts. They are, however, very accurate. On the other end of the price spectrum, some manufacturers provide a tension device for select lines that is highly cost-effective and very simple to use while sacrificing only minimal accuracy.

Depending on the number of drives with belts and the number of personnel tensioning the belts, a consumer can find the right tensioning device for his or her needs.

Poor sheave and sprocket alignment is another common ailment that accelerates power transmission belt failure. Misalignment causes the belts to experience sidewall stresses and uneven tensile member shock loads. In the end, the belts either suffer a tensile member failure due to sidewall cracking or roll over inside the sheaves. Misalignment can exist as non-parallel shafts, offset pulleys on the shafts or pulleys that have an angular skew at installation.

Misalignment is detectable with several methods. The first involves the use of a straight edge as a method of checking for four points of contact on a two-pulley drive. Basically, when the straight edge is placed against the faces of the sheaves, a well-aligned drive results in contact on both of the sheaves in two places — thus, four points of contact. The downside of this method is needing a straight edge that is roughly the same length as the center distance of your drive. It also can be difficult to guarantee the “straightness” of a straight edge.

The other, more technologically sound method of ensuring sheave alignment involves using a laser alignment gauge. Several different manufacturers now produce these, and their products’ ease of use is a definite luxury. The combination of laser technology and magnets allows consumers to quickly and easily check their systems for all types of misalignment.

Running belts on worn pulleys or sprockets is yet another way to decrease the uptime of your PT system. When pulleys or sprockets wear, the surface that interacts with the belt is compromised — formerly flat sidewalls take on a convex shape.

Belts running on worn pulleys are far more susceptible to slippage and excessive wear in localized sidewall regions. Neither of these situations is conducive to long belt life. Belts run on worn sheaves are more likely to roll over in the sheaves; if the consumer is running belts on worn sheaves with misalignment present, the belts are nearly guaranteed to turn over.

Sheave condition is perhaps the cheapest and easiest of the maladies to monitor. Sheave gauges are usually available wherever belts are sold. The sheave gauges drop right into the groove of the sheave. If you can see daylight in between the gauge and the sheave sidewall, it’s time to replace your sheave.

Environmental concerns can also present concerns with belt performance. Most general-purpose belts aren’t made to handle the very peaks of the temperature extremes or constant contamination via chemicals.

Belts can operate successfully in temperature ranges from around minus 20 degrees to 140 degrees F. Outside these ranges, the success of the belt depends largely on the compounds put inside the belt by the manufacturer. For drives with ambient temperatures above 140 F, it’s important to note that the belt life decreases exponentially with the increase of temperature.

Drives in colder atmospheres see failures of a more dramatic nature. In cold weather, the belts fail because the compounds reach their glass transition temperature and thus tend to simply shatter when it gets too cold.

The best way to protect drives from these failures is to be aware of the environment where the power transmission needs to take place, and then design accordingly. The same is said for drives in areas of high chemical and oil contamination.

Conclusion
In systems with high shock loads or heavy-duty cycles, it’s usually the best-case scenario to have the belt fail in a power transmission system. Changing a belt is far easier and more cost-effective than changing a bearing or replacing a motor shaft each time a system goes down. But following these simple solutions can turn a frantic emergency scramble and costly downtime into scheduled preventive maintenance work.

This article appeared in the Aug./Sept. 2002 issue of MRO Today magazine. Copyright 2002.

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