5.1 Relevance of the Spiral Orbit Tribometer (SOT)-The SOT was designed to evaluate the relative degradation rates of liquid lubricants in a contact environment similar to that in an angular contact bearing operating in the boundary lubrication regime. It functions as a screening device to quickly select the lubricants, evaluate the ability of various components of a lubricant (base oil, thickener, or additive) to lubricate a contact in rolling, pivoting, and sliding conditions simultaneously, and study their chemical decomposition if necessary. The SOT provides a means to study the tribological behavior of oils and greases during operation, while they undergo changes as a function of typical parameters encountered in the lubrication field (temperature, environment, materials used, load applied, and speed). Test conclusion is defined to be when a friction coefficient limit (typically an increase of 0.1 above the steady state value) is surpassed. Normalized lubricant lifetime is then defined as the number of orbits completed divided by the initial amount of lubricant used (in μg). The SOT was initially developed to evaluate lubricants for space applications, but is also relevant for conventional environments. Some results in vacuum are presented (Fig. 1). At this time, no data for tests in ambient conditions have been published (see Fig. 2). The user of this test method should determine to their own satisfaction whether results of this test procedure correlate with field performance or other bench test procedures.

FIG. 1 Relative lifetimes of three typical space lubricants at 23°C in vacuum on 52100 steel

Pepper, S.V., Kingsbury, E.P., "Spiral Orbit Tribometry - Part II: Evaluation of Three Liquid Lubricants in Vacuum", Tribo. Trans., V 46, 1, pp 65-69, 2003

FIG. 2 Comparison between full scale bearing tests** and SOT data at 23°C on 440C steel.

Bazinet, D.G., Espinosa, M.A., Loewenthal, S.H., Gschwender, L., Jones, W.R., Jr., Predmore, R.E., "Life of Scanner Bearings with Four Space Liquid Lubricants", Proc. 37^th Aerospace Mech. Symp., Johnson Space Center, May 19-21, 2004