Experiments on the pressure distribution and frictional torque in articulating pin joints

Abstract

In its simplest form, aircraft landing gear consists of large structural members connected by pin joints that allow articulation, and hydraulic actuators for deployment. The design of these pin joints is critical to successful operation. In this article, the authors explore two related aspects of pin joint design: first the contact pressure distribution within the joint and second the frictional torque required to rotate the joint. The former is important in stress analysis and the latter in defining the required actuation force. The joint consists of a pin located within four bushes that are fixed into the structural member. Grease is fed into the bearing cavity. A purpose-built test rig was designed and built to hydraulically load and articulate a specimen pin within its bushes. A novel ultrasonic method was used to measure the contact pressure between the pin and bushes when the joint is subjected to a constant radial load. The method is based on recording the proportion of an ultrasonic pulse that reflects from the pin—bush contact. When there is no contact the pulse is fully reflected and when contact takes place the pulse is partially reflected. The proportion of the pulse reflected depends on the conformity of the surfaces and hence the contact pressure. The pressure profiles measured in this way were found to be approximately cosinusoidal, extending over an arc of±60° regardless of the radial load. The rotational torque and angular displacement were also measured during joint articulation cycles for a range of applied lateral pin loads. The results showed that articulation torques ranging from 20 to 150 kN m were required to rotate the pin as the lateral load was increased from 5 to 40 kN. An estimate of the friction coefficient between pin and bush can be obtained directly from this lateral load and torque data. However, an improved measurement, which includes the effect of the radial component of the lateral loading force, was obtained by combining the pressure distribution data with the torque data. Friction coefficients in the range 0.08–0.11 were deduced in this way and were found to increase slightly with load. This indicates that the joint operates in boundary-lubricated regime and that grease entrainment is an important factor.