Frictional characteristics of elastomeric radial lip seals at extremely low temperatures

A novel low-temperature radial lip seal test rig was set up, allowing for simultaneous non-contact, telemetric measurements of both radial lip seal contact temperature and seal friction torque. Extreme cooling of the sealing contact zone down to below -50 °C was achieved by continuously feeding carbon dioxide snow pellets into the bore of a hollow seal counterface adaptor. Though this setup enables a simultaneous evaluation of two identical test seals, where the inner space is filled with lubricant, in this work, the frictional characteristics of wetted, single test seals were investigated. This eliminates the churning losses of the oil-filled ring gap that, due to the drastic increase of the lubricant viscosity at low temperatures, would severely hinder the correct determination of the seal friction torque. Experiments were conducted with plain radial lip seals made of NBR and FKM, using two polyglycol oils with different viscosity grades VG 220 and VG 46, respectively. Starting from a stationary state at 100 rpm (0.419 m/s) with contact temperatures in the range of 45 °C to 60 °C, the sealing contacts were subsequently cooled down, finally reaching steady state seal contact temperatures as low as approx. -50 °C. Thus, during cool-down of the sealing systems, both seal elastomers pass through the glass transition, and both lubricants pass through their pour points. In contrast to warm operating conditions, where speed-step dependent seal friction changes could be accurately predicted based on soft micro-elastohydrodynamic asperity lubrication theory, speed step experiments at such extremely low temperatures revealed that there was no viscous friction response, i. e. the seal friction was due to Coulomb-type friction. While showing larger fluctuations, the overall level of this Coulomb-type friction was comparable to the steady-state seal friction measured under warm operating conditions. When warming the systems up, the original lubrication mode was reestablished.