Implications of Simultaneous Mechanical and Thermal Loads on the Rheological Properties of the Grease in Ah-64 Helicopter Gearboxes

This paper deals with changes in the rheological properties of the grease used in the tail rotor and intermediate gearboxes of the Apache AH-64 helicopter when subjected to simultaneous mechanical and thermal loads within the operating temperature limits. From the steady shear rate sweep experiments on grease samples extracted from gearboxes, it was observed that there is a significant reduction in apparent viscosity over the shear rate range tested. This reduction in apparent viscosity might lead to inadequate lubrication in contact regions between teeth thus accelerating tooth wear. To verify if the change in properties was due to temperature alone, experiments were conducted on grease samples heated in convection oven at 195°F±3°F. From the results it was evident that the effect is not solely due to temperature. However when simultaneous mechanical and thermal loads are applied, there is a dramatic change in the rheological properties of the grease. From the experiments it was also observed that the grease reaches a stable or a metastable state and thereafter it does not seem to have much change in apparent viscosity either with operating conditions or time. This is evident from the similar flow curves for the TGB and the IGB grease samples even though they had different time histories. The grease in this stable or metastable state has significantly different rheological properties than the virgin grease. However, it still exhibits non-Newtonian behavior but power-law is not a good model to describe the flow curve for the TGB and IGB grease samples.