Paper: Analytical study of the extrusion of rectangular elastomeric seals for linear hydraulic actuators.
Author: George K. Nikas
Where: Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2003, 217(5), 365-373. Available from the publisher.
Rectangular elastomeric seals used on reciprocating piston rods in high-pressure hydraulic actuators often suffer from extrusion damage at the low-pressure side of the actuators. The extrusion takes place at the narrow clearance between the rod and the actuator where the seal develops a “nip” under conditions of high sealed pressure and/or high friction with the rod, which is amplified in the absence of a back-up ring. This form of strain can lead to permanent damage of the seal and impair the sealing performance of the system. This paper deals with the modelling of this kind of seal extrusion. Algebraic equations were developed to describe the shape and contact pressure of the extruded part of the seal with the rod. A study is presented about the effects of various operating parameters on the extent of seal extrusion in order to minimize the risks of damage. It was found that only the use of a back-up ring can adequately cancel the seal extrusion mechanism.
Some figures from this work
The set-up for the model is shown in Fig. 1. The hydraulic actuator contains fluid under pressure. The piston rod reciprocates and fluid leaks under the seal and escapes to the air side of the system. Owing to the sealed pressure, the lower right seal corner is distorted as shown in the figure and a small nip is created, especially during the outstroke.
Fig. 1. Configuration for the seal model.
Examples of the shape of the extruded part of the seal during an outstroke for various sealed pressures pcyl and corner radii r of the seal are shown in Fig. 2 below, referring to rod-actuator clearance of 0.2 mm. The horizontal axis represents the length of the extruded part inside the narrow gap between the piston rod and the seal housing. These have been computed analytically.
Fig. 2. Examples of the shape of the extruded part of the seal during an outstroke.
Fig. 3 shows examples of the contact pressure and shape of the extruded part. These have been computed analytically from the equations developed in the paper. Notice that the extruded part is only partially in contact with the piston rod and the contact pressure rises sharply over a very small distance, until it reaches the seal contact pressure, which is approximately equal to the sealed pressure of the actuator. Such an abrupt pressure rise and corresponding stress concentration could cause the removal of the extruded part from the seal, which will then create a local defect, affecting the sealing performance of the system.
Fig. 3. Examples of the contact pressure on the extruded part.
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