Lubrication Regimes in High-Performance Polymer Spur Gears

Little has been published on the behaviour of polymer gears operating under lubricated conditions. An experimental and analytical programme was undertaken to classify the regimes of EHL under which polymer spur gears operate. In doing so theoretical film thicknesses were calculated and then used to classify the regime according to Johnson's Map. The effects of lubrication on the operating efficiencies of high-performance polymer gears were interpreted and from these results coefficients of friction were derived. In addition to this the effect of tooth geometry was investigated and the beneficial influence of high-pressure angle tooth geometry is demonstrated. At loads typically associated with polymer gears the operating regime is shown to be mixed film lubrication. When high-pressure angle gears were tested at high loads the operating regime became full film lubrication and relatively little tooth flank damage occurred. 1. Introduction Polymer gears are frequently employed in situations where no external lubrication is permitted, such as in food processing machines and in office equipment, such as printers. Unlubricated (dry) polymer gears are limited in both load and speed due to high frictional forces. This creates high temperatures leading to rapid wear and even melting. Developments in polymer materials (e.g., polymers containing glass fibres and an internal lubricant such as PTFE) and gear tooth geometries have moved polymer gears from motion to power transmitters [1, 2]. In many of these applications an external lubricant such as grease is permitted. Very few examples of oil or water lubricated polymer gear applications exist. External lubricants can reduce the friction, compared to dry running gears and the lubricant can also act as a coolant. Polymer gears operating under external lubrication permit much higher loads and speeds and offer the possibility of new design solutions to a number of applications traditionally reserved for steel gears. The reduction of frictional losses in steel gears has been studied by various authors [3, 4]. However, these results are not directly applicable to polymer gear testing. The low modulus of polymer gears makes them resilient when the teeth come into contact. This deformation results in a change in the curvature of the gear tooth, increasing the contact area during tooth contact. Little has been published on externally lubricated polymer gears, the exception being Song et al. [5] who looked at polymer/steel combinations only and compared oil thicknesses and pressures with steel/steel pairs. They reported