Contact Stress, Frictional Torque and Lubrication in Articulating Pin Joints

Pin joints are used in many items of industrial machinery where two structural members are required to articulate. A pin is assembled between typically four bushes. As the members articulate the pin rotates relatively slowly inside the bushes. A hydraulic test machine has been developed to load and articulate a 56 mm diameter pin held in four bushes, typical of that found in earth moving machinery and aircraft landing gear. The rig is capable of applying realistic loads and speeds of articulation, whilst measuring torque and angular displacement. The test machine design and operation is described. The pin joint is lubricated with mineral oil based grease. Joint rotation is relatively slow and hydrodynamic calculations predict that the joint operates in the boundary regime. During a single cycle the torque remains largely constant, with a static friction peak at the beginning and end of the cycle. Measurements of the torque during articulation demonstrate friction coefficients varying from 0.02 to 0.12 depending on the rotational speed. The contact stress distribution between the pin and bushes was estimated by an ultrasonic method. An ultrasonic pulse was sent through the hollow pin and reflected back from the pin-bush interface. High contact pressures causes conformity between the pin and bush and so the wave passes through. The proportion of the sound wave reflected was used to estimate the contact stress distribution. The results showed how the stress is maximum in the outer bushes and over a 60° arc of the pin. The results of the study can be used to understand how pin joints function and how they may be optimised.