Extreme temperature

Urethane is a thermoplastic, so its physical properties change as temperature rises. For example, at 120°F the material's life span (as measured by resiliency) declines to about 70% of that at room temperature; at 150°F resiliency drops to about 10%. A high-temperature urethane belt is able to perform well up to 230°F, but is more expensive.

In the cold, urethane belts become more brittle. In fact, belts left to sit overnight in low-temperature environments can even take a set difficult to overcome at startup. This can cause even strong welds to shear apart.

Although urethane manufacturers often claim that regular urethane works to -10°F, such use is generally not recommended. Specialty lowtemperature urethanes work better in cold environments. Otherwise, below 10°F, belts made of Hytrel are most appropriate. These work down to -40°F, so are particularly well suited for ice cream plants, for example. Because Hytrel is not as resilient as urethane, it should not be stretched beyond 7% — so care must be taken not to overstretch it during installation.

Tension considerations

Most bearings withstand loads considerably larger than those exerted by urethane belts. For example, a 3/16-in. HT belt may exert an initial force of about 25 lb, but typical 1.9-in.-diameter conveyor rollers handle a maximum load of 250 lb, which is 10 times larger. Moreover, urethane belt tension quickly declines: Five minutes after installation, tension decreases by 30%. After a week, the tension of a 3/16-in. HT belt levels off at about 11 lb.

Nevertheless, ensure that belt tension does not exceed the rating of the application at hand. Online tension calculators are helpful for this.

Flat belts: Where and how

Besides round O-ring belts, polyurethane is also made into flat belts. The latter are useful for powering rollers. Some flat belts are designed to add extra drive to up to four rollers, to effectively make them all behave as though they are motorized rollers. So, each of these coupled roller collections, sometimes called zones, includes one motorized roller but performs as though it has five. The O-ring belts are loaded and move these as though they are only moving one (or at most, 1.5) slaves. This makes the O-ring belt operation much more efficient, minimizing rotational speed loss of slaves far from the powered roller.

Crowned pulleys rule

Here's the catch: All flat belts have a tendency to move sideways on flat surfaces. So, uncrowned flanged pulleys are not recommended for flat belts because these either rub against the flange and abrade, or stretch and walk over the flanges. Crowns prevent flat belts from walking, to keep belts centered. The pulley must be larger at the center than on the sides by about 0.016 to 0.020 in., with center diameters 0.032 to 0.040 in. greater than the outside pulley-edge diameters.

Tracking sleeves, another option, provide a quick way to add rectangular crowns to a pulley or roller. As a general rule of thumb, for elastic belts, sleeve thickness should be about 2% of belt width and about 20 to 40% as wide as the belt. To illustrate, standard tracking sleeves of 1/32-in. thick × ½-in. wide stretch up to 7.5% — but thicker and wider sleeves should be stretched only 2%, or else they become too difficult to install.

Standard sleeves stay in place by their own tension. For larger sleeves, a drop of super glue prevents them from moving. One caveat: Crowns may not work on belts that frequently reverse direction, because it usually takes about three pulley revolutions before flat belts center themselves on crowns. In reversing applications, flat belts with V guides (a small V belt welded to the bottom of the flat belt) and flat pulleys with centered V grooves may be more suitable.

Flat belts work well with boxes, tubs, and plastic or aluminum pallets. However, they are not recommended for use with wooden pallets because exposed nails or sharp wooden slivers can cut them. The most common size used on motorized roller zones is 4 in. wide. Two parallel belts are typically used on each zone, for 8 in. of surface area to prevent boxes from slipping on inclines up to 11°. For extra heavy boxes, belt thickness can be increased, even to 2/32 in.

Urethane flat belts also make inexpensive gravity roller brakes for declines on roller conveyors. Braking force can be adjusted by varying the number, width, thickness, tension and pattern of the belts, as shown in pictures at the top on this page.

How do they work? The primary source of braking is the nature of rollers themselves. The axis of a roller is never precisely at the center of rotation, so total indicated readout, a measure of radius variability, is always greater than zero. As one roller's high and low points are almost always out of phase with those of its neighbors, linking the rollers together produces a resistance to movement, wherein belt are forced to stretch and relax with each rotation. This produces drag, which slows roller rotation and box speed.

For more information, call (800) 770- 2358 or visit durabelt.com.

Advantages of welded endless belts

Most elastic urethane belts are made from extruded urethane cord, cut to length, and butt welded at the ends to make endless belts. The extrusion process orients longchain urethane molecules in the direction of stretch, which increases cord strength and elastic memory.

Alternatively, elastic belts can be injection molded. Here though, weak points can be created at gates, where molten urethane flows into the mold, and knit lines, where streams of molten urethane meet. Also, molding does not orient the molecules in the direction of stretch, so often, molded belts are less strong. Finally, molded O-ring belts have more urethane on their outer circumference than on their inside — so they maintain an O shape, resist straightening and back bending, and contribute to energy loss. This may seem trivial, but on a line-shaft conveyor it can increase horsepower requirements by 10% or more.