Machine Design

Thermoplastic Bearings Offer Design Flexibility

A new bearing material was recently developed to handle the high chloride concentrations in hot tubs. The large integrated spa-bath nozzle spins rapidly under water pressure to deliver an invigorating hydromassage. These bearings were produced without machining.

Thermoplastic rolling-element bearings give engineers a new perspective in machine design because the injection-molded assemblies combine the benefits of rolling-element bearings with the functionality of integrated molded features.

Thermoplastic bearings are often made from POM (acetal), which is a semicrystalline material with good lubricity and fatigue performance. Injection molding the material forms a thin surface layer of ultrafine crystals. By controlling this surface layer, bearing life can be significantly improved in comparison to machining the ball raceways.

Tooling techniques have been developed to mold products complete with ball raceways. In addition to maximizing bearing life, formed raceways eliminate a requirement for postmold machining. Because machining is not required, it is possible to make the bearing races almost any shape imaginable. For example, pulleys, rollers, precision gears, and fixing clips can all be integrated into the bearing races. This allows design flexibility and reduces system costs.

Nonmetallic bearings can be manufactured with outer diameters up to 150 mm. Depending on bearing size, the bearing tracks are controlled to a tolerance of at least ±0.1 mm. Other molded features can be controlled to similar accuracy levels. In the case of gears, for instance, tolerances are commonly held to ISO Grade 6, although more demanding applications are now moving to Grade 4.

Thermoplastic bearings work well in applications that require high speed and precise motion control. One such application is digital printers and copiers that produce up to 65 pages/min. The machines need precise paper handling to produce high-quality images and avoid paper jams. Thermoplastic bearings really come into their own in corrosive or harsh-chemical environments, such as food-processing or swimming-pool equipment. Acetal is ideal in these applications because its tight crystalline structure gives it good chemical resistance and makes it impervious to water absorption.

This information supplied by Sarnatech BNL USA Inc., Foxboro, Mass.

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