Characterising the friction coefficient between rubber O-rings and a rigid surface under extreme pressures

James Jc Busfield,Eduardo Yanes,Benjamin Berryhill,Nicola M Pugno,Julien Ramier

doi:10.1016/j.polymertesting.2021.107378

Abstract

Previous research into the friction behaviour of elastomers has typically focused on the effects of velocity, contact pressure, counter surface and lubrication on the coefficient of friction. O-ring type elastomer seals are common in many different industries. Friction plays a critical role during the setting and in service of these components. An experimental O-ring friction testing rig has been developed that can measure the effects of sliding speed and hydrostatic pressure on elastomer friction. Finite element analysis (FEA) packages can adopt fixed friction coefficients or ones that are pressure dependent. For the latter case, the dependence of the frictional behaviour is typically obtained from the instantaneous stress response at any given pressure and then related to the normal force response. The friction rig described in this paper uses industry standard dimensions for the O-ring gland, the pre-compression levels, extrusion gap size and pressure rating. The coefficient of friction is derived by dividing the measured friction force by the normal force, which was determined using an FEA modelling approach, as it could not be measured directly. Finally, a relationship between the frictional velocity and surface roughness is obtained in order to provide a frequency dependent Coefficient of Friction (CoF) that is easily translatable between surfaces.

Highlights

The mechanical properties of elastomers and how they change under a hydrostatic pressure has been studied previously (Pai, [1])
The surface profile effects on rubber friction are well known (Gabriel, [4]) so the piston is characterised with profilometry and analysed with a height difference correlation function (HDC)
Characterisation of the surface roughness was done in the vertical direction to reflect the piston motion

Summary

Introduction

The mechanical properties of elastomers and how they change under a hydrostatic pressure has been studied previously (Pai, [1]). It is understood that high pressure reduces the polymer chain mobility which increases the effective elastic modulus of the material. This modulus increase can significantly change the sealing effectiveness. Pressure increase for frictional studies has always taken place with a direct load on the elastomer; this works well when the pressure range does not cause changes in the glass transition temperature (Tg) or the elastic modulus of the material. Oilfield conditions require seals to have service pressure ratings upwards of 70 MPa. Use of regular indenters or normal forces at these pressures will cause severe problems such as the puncturing or fracture of the sample resulting from very large local deformations. In practice Oring seals are used in these high fluid pressure conditions, which makes them ideal for this investigation

Methods

Results

Conclusion