Aviation oils provide thin lubricating films between highly stressed bearing, gear and other lubricated contacts. The stresses and shear rates cause rapid rheological changes in the oil which affect the traction (friction) between the surfaces. Thin film dynamic forces are associated with the molecular structure of the oil provided by the oil supplier. Traction force divided by the normal load (i.e. traction coefficient) can be viewed as a fundamental property of aviation oil. Its value is a function of stress, temperature and shear rate. Appropriate measurements are made under dynamic conditions to characterize aviation oil products for engineering design purposes and service performance prediction.
Rationale: Bearings and gears in high speed aeropropulsion turbine engines and transmissions generate an enormous amount of heat between rolling/sliding contacts. A good portion of the heat is associated with the shear of highly stressed thin films between surfaces. The bulk film shear stress or traction is critically needed to predict heat generation and to conduct thermal management analysis. Traction coefficient measurements are required over engine relevant speeds, stresses and temperatures.