Contact Hydraulics in The Sealing Footprint — Effects on Deformation, Leakage and Friction of Soft Seals

Abstract

An interactive treatment of conditions within the sealing footprint of a soft seal pressed against a relatively rigid wall is presented, based on the assumption of some seepage, however small. This allows the effects of pressure and friction on sealing of O-rings, lip seals, or other forms of soft seals to be analyzed. It has importance for design improvements to sealing effectiveness, extrusion resistance, friction reduction in sliding seals and assessment of the influences of transients or assembly procedures. An example of its application to elastomeric O-rings is given. Other materials, composites and shapes for which an adequate deformation model exists can be treated similarly.For O-rings, quasi-static finite element analysis has been used previously to calculate deformed shape and total normal reaction load across each sealing footprint. A portion of this normal load is carried by hydraulic pressure of the seepage; the remainder is contact. The hydraulic pressure gradient across a sealing footprint is therefore interactively dependent on the contact pressure distribution according to a relationship that can be found empirically (independently of seal geometry) for particular materials and conditions. This relationship must then be included in the analytical model if hydraulic and contact pressures across the footprint are to be separated, seepage calculated, and friction applied only to the contact component. Otherwise, the pressure-assist and friction behaviour will not be well predicted.In previous analyses of O-rings it was assumed either that there was no hydraulic pressure in the footprint, or that its gradient was constant. In fact, its gradient will be greatest where the contact pressure is greatest, and least at zero contact (i.e., close to either end of the footprint). An iterative methodology to analyze any relationship between hydraulic pressure gradient and contact pressure is described, and is used in concert with a general purpose finite element code. Linear, power law and exponential cases are compared. Constants or exponents for particular conditions are derived from results for nitrogen leakage between a flat, rubber washer and a rough, metal surface.Results show the pressure-assist sealing behaviour of a typical O-ring to be less than was previously calculated. Wall friction, however, is also less. For designers, it is shown how such modelling can be used to explore means to improve the behaviour of O-rings and provide a powerful capability for soft seal optimization.KeywordsContact PressureLeak RateContact Pressure DistributionLeakage CoefficientRectangular GrooveThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.