Behaviour of several lubricants for space applications under transient speed conditions

Theoretical work has underlined the importance of squeeze effects in non-steady-state elastohydrodynamic (EHD) lubrication when parameters such as load, speed, or geometry vary cyclically. The present paper reports on the results of an experimental investigation into EHD film thickness under start-stop, ball-on-flat contact conditions for four liquid lubricants used in space applications. The conditions studied are representative of those occurring in stepper motor bearing contacts. The lubricants investigated included a multiply alkylated cyclopentane (Pennzane™ SHF 2000), a branched perfluoropolyether (Krytox™ 143 AC), a polyalphaolefin (Nye™ 182), and a linear perfluoropolyether (Braycote™ BXL 413). Measurements were made at two frequencies (2 and 50 Hz) and two temperatures (25 and 80°C). At 50 Hz, corresponding to stop-start time intervals of 10 ms, there was insufficient time during the stationary periods between motion for any significant collapse of the central film thickness to occur. However, partial collapse did take place at the contact peripheries. Since this collapse occurred in the inlet, it often produced a pronounced reduction in central film thickness during start-up. It was also found that, during start-up, a series of film thickness oscillations occurred, resulting in the film thickness becoming successively greater and less than the steady-state value. This may have a significant effect on the pressure experienced by the contact. Although the four lubricants exhibited similar overall patterns of behaviour, there were considerable differences in the rate of film collapse and in the details of the film thickness reduction and oscillation.