Plastic sprockets are making inroads in several industries because they solve a variety of problems that beset their metal cousins. Here are the most common types and how they’re applied.
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.
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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Designs for high loads
An important criteria in selecting plastic material for sprockets is the expected load or torque. Is the load excessive or fluctuating? How will start-up torque affect the sprocket?
Recently, our company sponsored a test program at the Oregon Graduate Institute in Portland, Oreg., to address these concerns as related to roller chain sprockets. In these tests, a hydraulic brake applied load to the keyway of a roller chain sprocket to determine which of the most commonly used materials have the best failure resistance, Figure 1 (Caution: these test results are limited and have not been verified in the field, where results may differ).
If loads are too high for a conventional plastic sprocket, the sprocket can be modified to increase its load capacity. For example, metallic hubs, Figure 2, are often bolted to large sprockets such as those used with H78 offset sidebar chain in mills or 55 series combination chain in agricultural applications. This spreads the load over a larger area of the plastic material to reduce stress and increase load capacity.
For smaller drive components, such as roller chain sprockets or timing belt pulleys, a hexagonal metal insert, Figure 3, spreads the load over a larger area. In a recent test, sprockets with metal inserts exceeded the load capacity of conventional plastic sprockets by at least three times. These metal inserts are often used in the food processing and packaging industries because they trap fewer food particles than bolt-on hubs.
Modified idler sprockets use a roller bearing in the hub to increase operating speed capability. The bearing also acts somewhat like a metal insert, thereby increasing the sprocket’s load capacity. Using plastic idler sprockets makes sense even when the drive sprocket is metal. They reduce noise, increase chain life (by reducing wear), and require no lubrication.
One of the companies using plastic drive sprockets is DSI in Redmond, Wash. DSI makes conveying equipment for the food industry. Their Portioner machine uses high pressure water jets, controlled by a computer, to cut chicken and meat into desired portions. This machine features plastic components such as sprockets, wear strips, bearings, and rollers, Figure 4. Bruce Hegnauer, production manager, says that they chose UHMW-PE sprockets for their conveyors because nickel plated sprockets did not hold up. “The nickel plating flakes off, which causes corrosion and contamination,” says Mr. Hegnauer. He adds that the UHMW-PE sprockets are cost effective and have good wear characteristics. Furthermore, they meet USDA/FDA requirements for food processing plants.
Another company, Coe Manufacturing Co., Portland, Oreg., makes equipment for use in the lumber industry. Their Infeed Table, Figure 5, uses plastic 81X (same as H78 offset sidebar) idler sprockets. This machine feeds lumber into a multisaw edger, where a computer detects the shape of the piece of wood, and positions it to make the most efficient cut. The machine feeds 20 pieces of lumber per minute through the edger. Denis Jones, project engineer says that they chose plastic to minimize the sprocket weight, which comes into play as the assembly moves up and down continuously. “Having a lightweight sprocket reduces wear and tear on the feeder” says Mr. Jones. Using a low-friction UHMW-PE sprocket also lets the system operate more efficiently. Mr. Jones says that his customers specify plastic sprockets mainly to reduce noise and extend chain life.
Plastic works for non-sprockets too
In a novel application of plastic materials, The York Food Systems, Preston, Wash., uses UHMW-PE blocks to drive a flat wire belt in spiral conveyors that operate in cooking or freezing environments. Each plastic block slides on a stainless steel rail without lubrication, while machined cleats in the top surface engage mating surfaces on the bottom of the conveyor belt. Jim Aikins, development engineer, says that UHMW is used in the spiral conveyor for its low coefficient of friction and long wear life. Also, the lubrication-free material complies with USDA and FDA requirements.
Stan Heister is a product manager, Solidur Pacific Co., Portland, Oreg.