Motion System Design

Plastic bearings and rails smooth conveyor ride

Metal components are not always the answer in conveyor applications. Bearings, guide rails, and other components made of plastics offer a smooth alternative for conveying equipment

Packaging, food processing, and other material handling applications often use plastic components within the conveying equipment. Plastic materials provide benefits unobtainable with conveying components made from other materials. For example, in the food processing industry, equipment is subject to frequent washdowns with harsh chemicals to meet FDA sanitary and cleanliness standards. Components such as plastic bearings, chain guides, and guide rails, Figure 1, can withstand such treatment. In conveying applications, metal components often cause high noise levels. Plastic components, however, run quietly, reduce friction, and meet USDA standards. They also offer flexibility in design. They can be machined to meet custom requirements. And products such as plastic guide rails can be bent around conveyor corners more easily than metal rails.

Bearings and other plastic components in roller, chain, and belt conveyors are usually used in low to medium- duty applications. They can handle loads to 3,000 to 4,000 lb with conveying speeds to 1,000 fpm.

In roller conveyors, the rollers can be made of many materials, including PVC plastic, steel, stainless steel, and aluminum. Plastic roll end bearings are often used on each end of the roller to support the shaft. Lubricantfilled hardwood can also be specified for roll end bearings. Rollers are made in several ways including springloaded types. These rollers come with round or hex shafts which spring-load into the conveyor side frames. For the roller, PVC rigid plastic is considered an economical material with good wear and impact resistant properties. This nonmetalic material is available in grades that meet FDA approval. Speed, load, and environmental conditions determine the type of roller for an application.

Chain conveyors are driven by various means including motorized plastic or metal chain sprockets. A plastic or metal chain travels over the sprockets. To reduce friction, noise, and wear, special wear strips and chain guides are placed over the metal parts of the conveyor. These strips and guides are available as flat bar clip-ons for flat chain conveyors. The metal or plastic chain rides on top of these wear strips.

Sometimes conveyor manufacturers require captured chain guides, Figure 2. The chain is retained in the plastic wear strip profiles, which prevents it from running out of the track.

The type of plastic used in these strips and chain guides is typically ultra-high molecular weight polyethylene (UHMWPE). These plastics function well in wet and chemical environments or where abrasive particles are present.

Belt conveyors often use heavy-duty synthetic belts. These belts are supported by two or more rollers depending on conveyor length. Longer conveyors use several sets of rollers between the side frames to support the belt and prevent it from sagging while supporting the conveyed products.

Round and V-belt guides, made of UHMW-PE plastic, enhance the life of the polyurethane, polyester, and coil-spring belts used on these conveyors.

Screw conveyors are typically found in bulk handling applications. The materials handled are often corrosive, abrasive, and sometimes full of moisture. Hanger bearings, which support the rotating shaft of a conveying auger, tend to be inaccessible for maintenance and lubrication. Therefore, UHMWPE and nylon plastics, or hardwood are the materials chosen for these bearings. The plastics and the hardwood are self-lubricating, have a low coefficient of friction, low wear rate, and resist abrasion. Several plastic compositions are available for high temperature (over 220 F) or special speed and load requirements, such as moving 1,000 lb at 150 fpm.

The auger and the trough of screw conveyors can have plastic edgings and liners too. For the auger, a UHMW-PE plastic edging snaps over its outside edge. This edging, available in various thicknesses, reduces noise and wear on rubbing screws, and can eliminate hangers on shorter screws. Selection depends on the amount of space between the auger flight and trough bottom. A trough liner protects metal components from the harsh materials moved through the conveyor, extending trough life.

Spiral conveyors use a spiral shape to conserve space and allow bakery goods to “cure” gradually when moved from an oven to a freezer. Products sit on top of a wire mesh belt, which moves to the next step or station on the conveyor line. This belting sits on two metal rails, each covered with UHMW-PE, nylon, or PTFE plastic bar snap-ons, Figure 3. These plastics come in FDA approved versions and prevent metal-tometal contact between the wire belt and railings, reducing noise and friction.

Spiral conveyors also use plastic guide rails that snap over the metal vertical uprights. These uprights help to hold the conveyor together, and the plastic guide rails prevent contact between the metal belt and uprights.

Cable conveyors, often used in canning applications, use a cable pulley system. The conveyors are driven by a metal cable that is 3/8-in. diameter. For protection against beverage spills, cleaning liquids, and chemicals, the cable can ride in a plastic pulley of UHMW material. This pulley comes in diameters of 2 to 3 in. No lubrication is needed with these cables.

Tom Johnson is product manager at Pobco Inc., Worcester, Mass.

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