Push or pull Polyurethane O-ring belts transfer power to conveyor rollers from line shafts or motorized rollers.

Polyurethane belts, with greater elastic memory than rubber, and higher abrasion resistance, are widely used for power transmission and load conveyance. They can be particularly long lasting: Consider a large postal distribution center, where 40,000 urethane belts keep vast piles of mail traveling towards their destinations. Some polyurethane belts can last more than a decade in such an environment. On the same token, others may last only a few years before going limp. What gives?

In general, the average lifespan for high-quality urethane belts is about four to six years, with a typical range of two to 12. Environment, proper selection, and belt quality largely determine life. Other factors are duty cycle, pulley size, belt length and speed, pulley alignment, ambient temperature and humidity, load characteristics, and level of maintenance.

Construction: What to request

Extended-life belts are extruded from virgin urethane. (Recycled urethane, called regrind, improves extrudability, weldability, and cost, but also reduces belt tension and lifespan.) Special cross-linking of long-chain molecules makes these belts more resilient under high tension, with up to 20% stretch. Strong weld joints also extend life: Nearly all unreinforced extruded belts are butt welded, and special welding processes can make them practicably unbreakable. RF energy-focusing processes can produce welds up to 10 times stronger than heat wands. If weld flashes are ground properly and not gouged, they will not “neck down” and reduce belt tension when stretched.

Urethane belts may also be joined by biased splices and overlap welds, but these are primarily used with belts reinforced with polyester, steel, or Kevlar. Adhesives are not recommended because joints affixed with them lack strength.

Belt sizing

We've just reviewed basic construction considerations, but how does a designer appropriately size a polyurethane belt to an application? The first step is to determine the belt cross section needed using one or both of the following two methods.

Don’t get shocked Urethane usually insulates, but some power transmission belts made of thermoplastic urethane embedded with conductive thermoplastic prevent static electric buildup caused by friction between belting and pulleys on conveyors and other machinery. With volume resistivity of 2 x 109 ohms-cm and a static decay rate less than 0.01 sec, this belt dissipates static better than varieties embedded with carbon and metal.

• Disconnect the belt from the drive pulley. Buy a fish scale and tie a string to its hook. Tape the other end of the string to the driven pulley (or roller) and wrap it around several times. Next, pull on the scale and apply the highest force the motor will apply during operation. Read the maximum working tension off the fish scale.

• If the application transports boxes on a conveyor, select an online cross-section calculator and use the working tension on the roller conveyor calculator to mathematically determine the belt's maximum working tension. Beware that this calculation is for an ideal environment in which a conveyor is moving hard-bottomed boxes. Set the coef- ficient of friction to 0.05 or larger if:

• A system's conveyor is old, in a dirty environment, or is poorly maintained

• A system's conveyor has heavy or long rollers, or moves boxes that have soft bottoms or bottoms that rub against the frame

These applications may need a thicker belt. When in doubt, load prototype rollers with the maximum box weight, and use the first method above.

1 Now consult an online cross-section calculator and select a belt cross section and durometer by finding a number on loading tension tables equal to or greater than maximum working tension.

The Right Track. Tracking sleeves (red band) can prevent flat belts from walking down a roller. Another option is crowning.

2 Determine the minimum pulley diameter (MPD) for the selected cross section from the minimum pulley diameter calculator. If the smallest pulley diameter is substantially smaller than calculated MPD, then the application either requires a larger pulley, or a belt cross section with a smaller MPD.(Note that flat belts have smaller MPDs than other cross sections.) Yet another possibility: Choose a smaller cross section and specify more than one belt. With this design, realize that the sum of belt tension capacities must add up to the maximum working tension. One final note: 83A durometer (hardness) is most common and preferred, as it has the longest flex life. Only use 92A durometer belts, for example, when there's no other option (and multiply its MPD by 1.3.

3 Use a length calculator to determine the belt cut length. The final belt size should be expressed in a certain form — to illustrate, 3/16 in. (belt cross section) × 13.5 in. (cut length) 83A (durometer).

4 Select a tension calculator, and determine the maximum tension needed to install the belt. If this tension is so large that it will bend a shaft, then change the percent stretch in Step

3 and recalculate maximum tension — or use larger shafts on the pulleys.

As with any design process, consult experts if there is any doubt about belt size. Free samples are often offered, which can be installed on existing systems to verify that sizing is indeed correct. Incorrectly sized belts may be non-returnable, or carry hefty restocking charges — especially in uncommon sizes.

Go for grip

One common misconception is that rough texturized surfaces grip pulleys and slip less than smooth belts. In fact, surface contact area is what counts with urethane belting: The more contact, the greater the coefficient of friction. Therefore, smooth belts grip better. However, if forced to slip, they tend to overheat and stretch or abrade.

Special rough belting can help overcome problems associated with slipping. Some urethane belts are made with a hard, texturized surface to provide a lower coefficient of friction (down to 0.4) so that belts can slip a little in pulleys without overheating and abrading. These belts are also suitable for use on slider beds or where accumulation occurs — for example, in printing and postal applications — because the rough surface can catch the edges of paper sheets and push them along.