Experiments on Grease Performance in Aircraft Landing Gear Pin Joints

A pin joint simulation machine has been built to test a real landing gear pin joint under realistic loading and reciprocation conditions. The pin is loaded hydraulically using a hydraulic actuator to apply a fixed displacement cycle whilst measuring the reactive torque. The machine was used to measure the torque cycle (and hence friction coefficient) required to operate the joint. In this work a method of evaluating different formulation greases has been proven. This involved measuring their frictional torque and also evaluating performance using a Sommerfeld type approach that displays the different lubrication regimes in the joint for different conditions. Measured friction coefficients were in the region of 0.02 to 0.12 depending on the joint load and articulating speed. In actual gear the surface sliding speed is low and so the joint operates in the boundary regime. The required torque and coefficient of friction have been related to the lubrication mechanisms occurring as a function of articulation angle, reciprocal frequency and applied axial load for lubrication starvation in a reciprocating journal bearing.