The most basic characteristic of a mechanism is the relative motion of faying surfaces under load. Points, lines, and surfaces of contact are assessed by calculating deformations and the resulting contact stresses1. The objectives of the analysis are to 1) estimate the forces due to friction and 2) predict whether damage occurs2.
Types of damage due to excessive contact stress include galling, friction welding, vacuum welding, fretting and ploughing. All of these modes can generate particulates, which is of special concern in close proximity to high-vacuum seals.
The design process can affect outcomes with respect to the two objectives through the selection of materials, surface roughness and surface treatment. Lubrication3 of the faying surfaces can also be employed to reduce the coefficient of friction and to reduce wear. See Conley, op. cit., Ch. 5 for additional information on this general subject.
Many surface combinations behave differently under vacuum than in standard atmosphere. This is particularly true when they’re lubricated. The differences are not limited to coefficient of friction. Many liquid lubricants outgas under vacuum, all of them will migrate away from the faying surfaces to some extent, and particulates like to stick to them even more than under atmosphere. Dry lubricants are susceptible to moisture absorption at their surfaces during fabrication, which can lead to deterioration of the material once under vacuum and (therefore) generation of particulates. They also cannot be replenished as they wear out, without replacement of one or more parts. In addition to preceding references to Conley, see NASA TM-86556, “Lubrication Handbook for the Space Industry”4.
Footnotes