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 paper we explore two related aspects of pin joint design; firstly the contact pressure distribution within the joint, and secondly 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 that is 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 are and hence the contact pressure. The pressure profiles measured in this way were found to be approximately cosinusoidal, extending over an arc of ±60º 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 articulation torques ranging from 20 to 150 kNm 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, that 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 to 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.

Determination of coefficient of friction for a novel bed mechanism

“…The authors have provided two methods for calculation, but none is preferred. Some groups of researchers [14,15] have also discussed the experiment set up to find frictional torque. They developed a hydraulic test machine to load and articulate a large diameter pin held in four bushes, typically found in earthmoving machinery and aircraft landing gear.…”

“…The mechanisms in the above works [13][14][15] were either for high power, had well lubricated single pin joints, or the machines and techniques used were sophisticated [16][17][18]. However, the mechanism discussed in this paper is complex in nature and runs at a low speed.…”

Model and experiments to determine lubricant film formation and frictional torque in aircraft landing gear pin joints

“…Note that equation (11) in fact represents an approximation for the torque from the pressure distribution. Ref Zhu et al 9 and Colbert et al 10 describe in detail how the tangential pressure components reduce this torque. However, the effect is relatively small (leading to a reduction of less than 10%) and so for simplicity is neglected here.…”

“…Strictly, this kind of contact violates the Hertz principle because the contact area is not small compared with the radius of the contacting bodies. However, an experimental analysis 9 shows that the approximation is not too severe. The mean and peak contact pressures and half-contact width are then given by where R ′ is the reduced radius where R and R b are the outer radius of pin and inner radius of bush, respectively, and E ′ is the effective modulus …”

An ultrasonic method for measurement of oil films in reciprocating rubber O-ring seals

“…Recently developed ultrasonic methods have been used for monitoring tribological contacts, including lubricant film thickness [28][29][30], contact pressure and size [31] and friction [32]. These provide a promising approach for inspecting the hydraulic reciprocating seal without modification to the sealing structures.…”