Gear lubricants surface speed

The frictional behaviour of polymers differs somewhat from that of perhaps more familiar materials. The frictional force tends to be proportional not to load (as in the classical case) but to speed. The coefficient of friction is very dependent on the nature of the two surfaces in contact, but is generally low, when suitable pairs are selected. This means that plastics gears can usually be run without external lubricants. Often the static friction coefficient is lower than the dynamic, which helps to explain the absence of slip-stick phenomenon exhibited by some plastics systems in motion this is especially marked with PTFE, which has an exceptionally low coefficient (around 0.02). The non-classical response of plastics materials results from their much lower modulus. Their frictional response is characterized by adhesion and deformation. 

Under ideal conditions gear teeth operate with films which effectively separate the surfaces, and the above EHL film formula may be used. However, under demanding conditions of low speed or high temperature, the film predicted by classical EHL theory may be small compared with the roughness present on the teeth. The gears then operate under conditions described as "partial", "mixed" or "micro" EHL. A conventional measure of the severity of lubrication is provided by the lambda-ratio, which is defined as the mean oil film thickness divided by the combined RMS roughness of the two surfaces, i.e. 

The experiments were conducted on a special test rig (Figure 4). The basic principle of the test rig is that the test-rig shaft is coupled with a flywheel and accelerated by an electric motor to a predetermined speed. Then the shaft is braked over the friction surface against the stationary measuring device. Gear shift is by means of a hydraulic cylinder. The force of the hydraulic cylinder is regulated. Lubrication is by oil circulation with temperature control. The test specimen is cormected with the test-rig shaft and with the sensor. 

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