Design with Fluorocarbon Resins

Considered next are self-lubricated plastic materials, namely fluorocarbon resins, fabrics, and coatings. Dupont s Teflon, the thermoplastic PTFE, has a low coefficient of friction, p, and a large temperature range from -450°F to -e5(X)°F. The PlFF types are the most common and are available in sheet rod or film. Perfluoro alkoxy alkane (PFA) resins have comparable properties to PTFE. Another fluotinated ethylene propylene is a copolymer, that is, easier to mold, but it has a higher coefficient of fiiction p, and a temperature limit of 390°F. 

The pure resins are limited to light loads and low draft velocities because of cold-flow problems and low thermal conductivity values (k). Therefore, various fillers are added such as glass, graphite, bronze, and These resins and the filled compounds have a pressure-velocity limit of 8000-10,000 psi fpm. Unfilled PTFE has a static p = 0.05-0.08, and a dynamic p = 0.10-0.13. The p values are larger with fillers ranging from 0.08 up to 0.50. The coefficient of friction, p decreased with pressure under dynamic conditions. 

A wear factor K is used in self-lubricated bearing design. The radial wear t in a bearing is given by 

The running clearance in a bearing will change with heat buildup and thermal expansion. Because of high coefhcient of thermal expansion values for polymers, they have not been used in electric motors because of small running clearance requirements that is. 

The total running clearance is composed of a shaft allowance (tolerance) plus an installation allowance plus thermal expansion a allowance. The shaft allowance is about 0.005 in. on a 1.0 in.-diameter shaft and about 0.003 in. on a 10.0 in. shaft. The installation allowance is about 0.002 in. The thermal expansion allowance is calculated from the bearing temperature which has contributions from 

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