Just the facts, m'am

Sept. 1, 2007
Belt drive performance can be compromised in several ways. Most often, trouble arises when misguided assumptions are made about belts, their structure,

Belt drive performance can be compromised in several ways. Most often, trouble arises when misguided assumptions are made about belts, their structure, or their application. But knowing the facts can help you extend belt life and boost their power potential.

Myth: Belt part numbers reflect the length of the belt.

While it is often true that belt length is defined by common part numbering systems (RMA, DIN, ISO, SAE, and so on) nominal V belt lengths vary from manufacturer to manufacturer. In general, specifications do indeed define the groove width in which a given belt section has to run; they define the size of test pulleys to measure the belt, as well as a length range into which a given belt must fall for a given nominal length, and the range of length tolerances for a matching set of belts. In fact, the specifications define just about every belt dimension — but they do not define the exact dimensions of the rubber piece. They only define resultant dimensions of test stands.

Why is this? Manufacturers make belts different ways. For example, some alter the wedge shape, and some make the belts narrower. So, different belts will fit into one pulley, and exhibit the prescribed resultant center distance dimension for a given length; nominal production lengths are repeated by their production methods. But no two companies will make a 46-in.-long belt that measures exactly 46 in. So, to measure a belt accurately, engineers must use a test set of pulleys and place the proper tension on the belt. (Using a tape measure to physically measure a belt won't return accurate belt length.)

One final note: When changing a set of belts, it is good practice to reduce the center distance.

Myth: All V belts are alike.

This is untrue. Go into any auto tire store and ask for a set of P225R15 tires for your car. The tire salesman will have several different options, ranging from $30 a tire to $130 a tire. Even though they carry the same part number, differences in the construction of the tires make for different mileage warranties and performance characteristics. The same goes for V belts. It is what you don't see that affects product cost: Quality of cord and fabric, the way the materials are handled, and so on. In general, a more expensive belt uses higher qualities of materials and a higher production standard.

Myth: We must start production now, so we don't have time to retension a new set of belts.

Install and run a new set of belts without retensioning them, and you might just be changing the same set of belts in a few days or weeks. On the other hand, taking a bit of extra time — sometimes just a few minutes — to readjust and retension belts after a few hours of running will make them last for months, if not years. This is because most standard off-the-shelf belts stretch; it is the nature of polyester cord.

For additional cost, reduced or no-stretch belts are available. Most manufacturers offer these higher-quality products.

But again, standard off-the-shelf belts will undergo 30 to 80% of their total stretch in the first two hours of operation. That is why the initial tensions are around 30% higher than the run-in tensions. You must take out the initial stretch of the cord when running.

So, if you must install belts on a run cycle, try to explain to your production foreman that if the belt replacement is not done properly, you will be back in a few weeks changing the belts again, and causing another production delay.

Myth: When I replace belts, I return the motor to its old position, which I carefully mark on the motor base.

This is a noble approach, but in fact, motor bases are often located where the last mechanic left them after retensioning adjustments. In this way, he or she accommodated for stretch in the belts over their lifetime. But for new belts, if the motor is moved back to this position, the system will over-tension them.

Myth: Timing belts are always better than V belts.

In terms of efficiency, yes — a timing belt is always better than a V belt. But, if you need overload protection, then V belts are more appropriate. Timing belts do not yield under excessive loads; they break. In contrast, V belts slip and stretch, and even allow mechanical elements to turn without damage.

Myth: Timing belts don't stretch.

As compared to a V belt, timing belt stretch is insignificant. However, timing belts do need occasional tension adjustments to run correctly. The cord structure in timing belts is designed not to stretch, because they must maintain dimensional accuracy. However, timing belt cord structure degrades over time, mostly from high operation torque loads. As the residual tension in the belt declines, the belt runs more loosely. Eventually the cord becomes so slack that when subject to a shock load, it fails and the belt breaks cleanly. In fact, if this situation occurs often in a particular system, it indicates that a V belt might be more appropriate.

Myth: Timing belts with Kevlar (aramid) cord are better.

This is an across-the-board misconception. Steel, fiberglass, and aramid cord timing belts are equally effective.

Belt cord need only carry the torque and maintain belt dimensions. Therefore, only a small portion of a cord's maximum tensile strength is utilized when the timing belt is run at a given power rating. Again, with timing belts, there is zero tolerance for cord elongation, because otherwise, belts lose tooth-to-tooth registration. In short, aramid, steel, or glass cords meet different requirements for chemical resistance — but not for stretch.

Myth: All high-torque drive pulleys are the same.

Not at all. Each manufacturer has specific belt tooth profiles rounded to fit best into specific pulleys. Engineers and mechanics must know what tooth profiles they are using on a given drive, and use matching belts.

If in doubt, consult the manufacturer of your pulleys to determine their type, and consult the belt manufacturer to determine whether a given belt fits. Note that if your belts exhibit excessive fabric wear on their inner sides, chances are good that the belt is too tight — or running in the wrong pulleys.

Myth: If the drive jumps teeth, I can increase the timing belt tension to fix the problem.

While this may be effective in the short term, you are causing other issues like high loads on shaft bearings. Increasing the belt tension increases the friction between belt and pulley. This in turn makes the timing belt act more like a friction flat belt. It is true that power transmission is increased, and the belt won't jump, but the loads on the shafts are now higher.

Inspect the pulleys for wear and change them if needed. The cord construction in the belt can absorb higher tensions, but not the belt drive bearings and shafts.

Myth: To replace a belt, I can just cut off the old belt and roll a new one in its place.

A belt installed this way may only last weeks before failure. Why? Rolling a belt over a sheave briefly increases the tension in the belt, often to a level that is too high for the cord structure to maintain integrity. Too, the pushing lever or screwdriver used to pry on the belt almost always damages the belt wrapping and cord structure. In some cases, this method is unavoidable; however, in these situations, plan for a shorter belt life.

For more information, visit motionsystemdesign.com or optibelt-usa.com.

Thinking ahead

Don't wait until the end of the design process before considering the belt drive. Also, don't ignore minimum bending radius when trying to limit machine size and cost. Finally: When specifying timing belt drives, the biggest mistake is to use service factors that apply to friction drives. Because timing belts don't slip, they are typically installed without accommodations for overload and clutching. This requires a much higher service factor to carry the load under severe starts and stops — and to accommodate accumulated machine inertia.

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