Technical Committee Lubricants

Influence of various base oils on friction and power loss in gears

The frictional behaviour of synthetic lubricants was systematically investigated in a twin disc test rig and a back-to-back gear test rig. For 22 lubricants (mineral oil, polyalfaolefin (PAO), polyglycol (PG), ester and mixtures) the coefficient of friction was measured in the twin disc test rig, dependant on pressure, oil temperature, rolling and sliding velocity. In the gear test rig pressure and velocity was varied for 9 lubricants.
The mineral oil shows an estimated coefficient of friction of about » 0.045, the synthetic lubricants have a lower coefficient of friction. In the twin disc test rig the lowest coefficient of friction with PG as low as 20% of the coefficient of mineral oil was measured, depending on the mixture of EO: PO. In the gear test rig such low coefficients of friction were not measured, but still values of 60% compared to mineral oil were obtained. Esters show in the twin disc test rig a coefficient of friction in the range of 30% to 100 % of the coefficient of friction of mineral oil, in the gear test rig in the range of 60% to 100%.
From these investigations a calculation method for the coefficient of friction in disc and gear contacts was derived. With these equations the power loss and efficiency of transmissions in practice, lubricated with synthetic gear oils can be evaluated.