An Empirical Study on the Friction of Reciprocating Rod Seals at Predefined Lubrication Conditions and Shear Rates

Abstract

A key factor influencing the friction of rod seals is a thin oil film, which is dragged into the sealing gap at outstroke and instroke. Accurate determination of oil film thickness in the sealing gap of rod seals is a challenging task since it is only in the range of a few nanometers. A novel measurement procedure to analyze the friction of common reciprocating sealing systems in direct relation to the shear rate and film thickness is introduced in this paper. Results from a first empirical study with film thicknesses in the range of a few nanometers and shear rates up to γ˙=107 s−1 were used to compare the friction of practically relevant polyurethane U-cups. The U-cups differ in their geometry and surface roughness. It is seen that even at such thin films, the measured friction of those seals can be approximated by Newtonian fluid friction (speed, film thickness, viscosity, contact area). In general, the novel measurement procedure is useful in a scientific and technical context, since it offers a new perspective on tribological mechanisms at thin film lubrication conditions.