Liquid cryogenic lubricant
For some time there has been a need for reliable bearings for short term use, in cryogenic systems, particularly high speed rocket turbo-pumps. Now fluorinated polyethers have been shown to be suitable and effective lubricants for rolling element bearings in cryogenic systems. Their lubrication effectiveness is comparable to that of super-refined mineral oil lubricants operating at room temperature.
A study and test programme were conducted using a different approach to the problem of lubrication in cryogenic systems. The new concept involved the use of liquid lubricants. To be an effective lubricant in a cryogenic system, a fluid must be a liquid in the cryogenic temperature range, capable of operating at the maximum system temperature without evaporating; it must be chemically insert, not susceptible to water absorption, and must have good heat transfer properties. Additionally such a fluid must be able to form a fluid film in the high pressure region of a rolling element contact.
Four fluids, each having a different viscosity, were evaluated in two phases: first to compare the lubricating characteristics of fluorinated polyether fluids at outer race temperatures of 89 to 227 K with those of a mineral oil at room temperature; and second, to determine the effect of fluid viscosity, maximum Hertz stress, and contact angle on the system temperature. Lubrication effectiveness was analysed with respect to wear and the deformation of the ball running track and compared with the effectiveness of a super-refined mineral oil at room temperature.
Rolling contact under these conditions results in an alteration of the rolling-element surfaces. This effect is usually indicated in three basic forms: (1) elastic deformation, (2) plastic deformation, and (3) wear. Deformation and wear were essentially the same for the specimens run in fluorinated polyethers at cryogenic temperatures as for those specimens run (under the same working viscosity) in super-refined mineral oil at room temperature or slightly higher. These results indicate that the fluorinated polyethers as lubricants in the range of 89 to 227 K are comparable to mineral oil lubricants at moderate temperature, that is, 311 to 478 K and also verify the fact that the fluorinated polyether lubricants can form elastohydrodynamic films at the cryogenic temperatures tested. They show that the fluorinated polyetbers could be used in many cryogenic applications, providing longer life and high reliability.