Applications of dry film lubricants for polymer gears

Polymer gears can be run without lubrication, however, high running temperatures, driven by high contact friction, mean that the operating life of these gears, especially in medium to high power transmission applications, tends to be low and limited by wear. This paper describes an attempt to control friction and wear by reducing the running temperatures by using a series of solid lubricant coatings deposited on flanks of the polymer gear teeth. Four potential coatings were selected, viz. molybdenum disulphide (MoS2), graphite flake, boron nitride (hexagonal) and poly-tetra-fluoro-ethylene (PTFE). Each coating was used with both reinforced and unreinforced poly-ether-ether-ketone (PEEK) and unreinforced polyamide (PA). Tests were carried out on coated-coated, coated-steel and coated gears running against uncoated gears. Wear rates (in the form of weight loss) and running temperatures were recorded. Results indicate that PTFE provided the greatest reduction in frictional forces and that failure mechanisms were predominately in delamination of the coatings and abrasive wear.