Evaluating the Impact of Oil Viscoelasticity on Bearing Friction

In this work, a novel bearing test rig was used to evaluate the impact of oil viscoelasticity on friction torque and oil film thickness in a hydrodynamic journal bearing. The test rig used an electric motor to rotate a test journal, while a hydraulic actuator applied radial load to the connecting rod bearing. Lubrication of the journal bearing was accomplished via a series of axial and radial drillings in the test shaft and journal, replicating oil delivery in a conventional engine crankshaft. Journal bearing inserts from a commercial, medium duty diesel engine (Cummins ISB) were used. Oil film thickness was measured using high precision eddy current sensors. Oil film thickness measurements were taken at two locations, allowing for calculation of minimum oil film thickness. A high-precision, in-line torque meter was used to measure friction torque. Four test oils were prepared and evaluated. The first was a monograde, Newtonian oil, while the remaining three oils were multigrade oils having varying levels of viscoelasticity. Importantly, each test oil was carefully blended to ensure similar kinematic and high temperature high shear viscosities, isolating viscoelasticity as the only variable. Viscoelasticity was quantified as Trouton ratio (ratio of extensional to shear viscosity), and ranged from approximately 64 to 162, for the viscoelastic oils. Results for bearing friction and oil film thickness are presented at various operating speeds and loads. All multigrade oils were observed to produce lower friction torque compared to the monograde baseline. Among the multigrade oils, minimum oil film thickness was observed to increase with increasing viscoelasticity. However, only a single multigrade oil (highest viscoelasticity) resulted in a larger minimum oil film thickness compared to the monograde baseline.