Machinedesign 1405 Smaller Composite Pulleys 0 0

Composite Pulleys

Feb. 1, 2003
The world just wouldn't be the same without composites - also known as plastics. The latest composites are power transmission components, including V-belt pulleys, flat pulleys, and roller chain sprockets. They are lightweight, which translates into quieter, more efficient operation. But whether idling or driving, there are less-known benefits to composite pulleys. Here we go over possible objections to the use of composite pulleys to separate fact from fiction and to diffuse it must be metal syndrome


One common misconception is that belts will slip more frequently on composite pulleys than on metal pulleys. Not so! The wedging action of V-belts is so efficient that slip should never be an issue; if a belt does slip in a pulley, it lacks sufficient tension to transmit the load anyway. Testing with wrapped, raw edge cogged, and link V-belts has shown that composite pulleys grip tight. This makes them suitable for use as high-speed idlers on conveyors, packaging equipment, lawn and garden equipment, floor cleaning machines, and other power transmission applications.

Another common misconception it that belts slip on composite pulleys in wet applications. However, misted and even fully immersed pulleys display no additional belt slip on start-up.

Design incompatibility

Growing composite pulley availability in universal sizes (A or 4L, for example) means more interchangeability with industry-standard AKtype pulleys. Design modifications also widen application possibilities. Composite idler pulley diameters can be machined down below the belt pitch line for material handling applications. Radial bracing ribs provide additional belt support and prevent groove deformation. Flat belt idler pulleys, available for a wide range of standard belt sizes and suitable for use as backside idlers on V-belts, can include crowns that are molded in to help center belts and prevent rubbing against flanges. Finally, round belt composite pulleys used with urethane belts, ropes, and cables can eliminate the binding and wedging problems of steel or cast iron pulleys; deep, uninterrupted grooves provide continuous support.

Versatile mounting adapters can simplify the job of assembly and help to reduce overall cost. Sintered iron and press-fitting at the factory makes the finished belt idler assembly more convenient. Standard types are available for most mounting requirements; flexible tooling can also help make setups quick and economical. Bore, clevis, and shoulder adapters for simplified mounting of pulleys make for easy press-fit installation.

Temperature degradation

The success of plastics in the twentieth century was due largely to the fact that they can be melted at relatively low temperatures - and molded quickly. Unfortunately, what may be good for the processor may be bad for the designer. To explain: more industrially mature thermoset rubbers have a permanent structure once they are cured. Thermoplastics used in pulleys, on the other hand, can be affected by ambient temperatures.

Still, composites can be surprisingly good at withstanding temperature extremes. To replicate actual operating conditions (for example, on restaurant vent fans) testing to 200°F has given good results: axial and radial runouts remain unchanged and within specifications. At the other extreme (even to -73°F) composite pulleys have also proven themselves. Even after liquid nitrogen dips before applied tension (up to the running torque of a 50 hp motor) composite pulleys take punishment and exhibit no fracturing. For full thermal behavior information, the single most useful piece of data is Continuous Use Data for thermoplastics, derived from UL’s Relative Thermal Index.

Mechanical wear

Just because composite pulleys are lightweight doesn’t mean they can’t stand up to wear. Finite element stress analysis shows that they can provide superior performance. Even composite pulleys under continuous cycling have demonstrated that they do not lose mass or exhibit detectable wear.

Composite pulleys also decrease wear related to low dimensional tolerancing. This is because the molding manufacture of composite parts allows for greater precision to ensure part consistency; this translates into excellent concentricity and reduced axial and radial runout. Premium- quality composite plastics ensure the highest level of performance. Virgin materials ensure more consistent strength and repeatability. Glass-reinforced pulleys give high strength with temperature and abrasion resistance.

But how do composite pulleys affect surrounding components? Fiberglass-reinforced material doesn’t necessarily equate to quicker belt wear. Continuous cycling of wrapped and raw-edge cogged belts from different manufacturers has shown that the rate of wear for these belts driven by fiberglass- reinforced pulleys stabilized over the course of a month or so. Tested at a 10% forced slip condition - way beyond what might be encountered for any length of time - belts in one study were subjected to thousands of on-off cycles. The wear rates are low, comparable to those typical for cast iron pulleys. Composite pulleys can be used with both coated and uncoated cable to actually provide a substantial increase in cable life over steel or aluminum pulleys. During the molding of pulleys, sintered metal drive hubs can be inserted for greater strength. Metal hubs lock into the plastic with anti-rotational slots in each end face for excellent torsional hold to handle starting and running loads.

The smaller diameters of smaller pulleys allow the use of a solid design, providing excellent strength. Radial supporting ribs provide additional belt support and eliminate groove deformation. Much larger pulleys present greater challenges; the wedging action of a v-belt introduces a downward force on the pulley rim. With its excellent strengthto- weight ratio, designers utilizing composites are able to find lighter, more easily manufactured spoke design pulleys that can withstand these forces. The rim span between the spokes does not buckle; further, the supporting spoke will not collapse under these forces.

Generally speaking, virgin materials in sprockets offer the most consistent strength and repeatability. Using finite element analysis, designers are able to avoid stress points and offer quality-engineered components to ensure high performance.

Chemical degradation

One well-known benefit of composite pulleys is that they are corrosion resistant; this makes them especially ideal for washdown and corrosive environments. However, polymer bases are not invincible and all can be dissolved by some chemical. It’s usually best to consult the manufacturer for detailed information. Following is a partial list of damaging chemicals; concentration, exposure time, and temperature also affect resistance.

• Acetic Acid
• Ammonium Chloride
• Ammonium Hydroxide
• Antimony Trichloride
• Barium Chloride
• Benzoic Acid
• Boric acid
• Bromine
• Butyric Acid
• Calcium Chloride
• Calcium Hypochlorite
• Calcium Thiocyanate
• Chloroacetic Acid
• Chlorosulfonic Acid
• Chromic Acid o m-Cresol
• Formic Acid
• Glycolic Acid
• Hexafluoroisopropanol
• Hydrochloric Acid
• Nitric Acid
• Perchloric Acid
• Phenol
• Potassium Permanganate
• Potassium thiocyanate
• Stannic Chloride
• Stannic Sulfate
• Sulfuric Acid
• Tetrafluoropropane

Some chemicals are harmless when the polymer is not under load, but add stress components and thwe pulley will crack at stresses far below rated strengths. Prototype testing in the chemical is always suggested.

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