During the past few years, plastic sprockets have become a viable alternative to metal sprockets in power transmission drive applications. The reasons are several: they resist corrosion due to oxidation or chemical attack, reduce excessive belt or chain wear, and avoid static or other electrical charge problems. Moreover, they require no lubrication, which prevents unsanitary conditions caused by toxic lubricants used with metal sprockets.

Other advantages of plastic sprockets include low coefficient of friction, which enables drives to operate more efficiently; light weight and quiet operation; and availability in USDA/FDA approved materials.

Because of these attributes, plastic sprockets are widely accepted in food processing, packaging, bottling, wood processing, and waste water treatment industries.

Material choices

Many types of polymers are used to make sprockets, including UHMW-PE, nylon, acetal, polypropylene, PTFE (Teflon), and polyurethane. Because each type performs best under different conditions, you need to choose a material that matches your drive application. Here are the most important performance characteristics of these materials (summarized in Table 1).

UHMW-PE. Ultra-high-molecularweight polyethylene (UHMW-PE) has a very low coefficient of friction, excellent impact strength, and outstanding abrasion resistance. It also offers good chemical resistance and is USDA/FDA approved. This material is well-suited for moderate-load applications. For example, UHMW-PE sprockets are used with flat wire belting (sprockets engage wire mesh) in the food processing industry, with flat top chain in conveyor applications, with engineering class 720 chain in waste water treatment, with H78 offset sidebar chain in the mill industry, and with steel detachable or combination chain in agricultural applications.

Oil-filled nylon. This type of nylon has a low coefficient of friction, good impact and abrasion resistance, and good chemical resistance. It is a rigid material, which resists creep and distortion of sprocket keyways under high loads. Sprockets made from oil-filled nylon include those used with roller chain and flat top chain.

Glass-filled nylon. This nylon material contains glass fibers, usually added during injection molding, to increase its rigidity for high-load applications. Sprockets made from this material typically have some porosity, which makes them weaker than those that are machined from cast nylon. Roller chain sprockets are often made from glassfilled nylon.

Polypropylene. Compared to other polymers, polypropylene offers a good balance of thermal, chemical, and electrical properties. It also has a low coefficient of friction, plus fair impact and abrasion resistance. Polypropylene sprockets are typically used in chemically harsh environments because they resist attack by most organic chemicals, mineral acids, and gases.

PTFE (Teflon). Because of its high cost, PTFE is seldom used for sprockets. However, it offers a very low coefficient of friction and the ability to operate in a wide temperature range. It has excellent chemical resistance, but only fair impact and abrasion resistance.

Polyurethane. When used for sprockets, polyurethane is usually made in a hard formulation, which gives it the rigidity to reduce creep. It has very good abrasion resistance, but tends to have a high coefficient of friction and poor impact resistance. This material is generally chosen because it is easily injection molded, a process that economically produces parts in large quantities. Polyurethane sprockets are typically found in mill and waste water treatment industries.

Acetal. For close-tolerance applications, acetal is an excellent choice. It machines well and has a low coefficient of friction. It is easily injection molded and is often used to make small roller chain or custom sprockets. Acetal is a hard material that resists creep, but it tends to be brittle and has only fair abrasion resistance.

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