Tech Briefs

Friction and lubricant degradation rate can be quantified rapidly.

The spiral orbit tribometer (SOT) bridges the gap between full-scale life testing and typically unrealistic accelerated life testing of ball-bearing lubricants in conjunction with bearing ball and race materials. The SOT operates under realistic conditions and quickly produces results, thereby providing information that can guide the selection of lubricant, ball, and race materials early in a design process.

The SOT is based upon a simplified, retainerless thrust bearing comprising one ball between flat races (see figure). The SOT measures lubricant consumption and degradation rates and friction coefficients in boundary lubricated rolling and pivoting contacts.

The Spiral-Orbit Tribometer is based on a retainerless flat thrust bearing. The ball is squeezed between the upper and lower flat races while the lower flat race is rotated, causing the ball to orbit in an opening spiral.

The ball is pressed between the lower and upper races with a controlled force and the lower plate is rotated. The combination of load and rotation causes the ball to move in a nearly circular orbit that is, more precisely, an opening spiral. The spiral’s pitch is directly related to the friction coefficient. At the end of the orbit, the ball contacts the guide plate, restoring the orbit to its original radius. The orbit is repeatable throughout the entire test. A force transducer, mounted in-line with the guide plate, measures the force between the ball and the guide plate, which directly relates to the friction coefficient. The SOT, shown in the figure, can operate in under ultra-high vacuum (10–9 Torr) or in a variety of gases at atmospheric pressure. The load force can be adjusted between 45 and 450 N. By varying the load force and ball diameter, mean Hertzian stresses between 0.5 and 5.0 GPa can be obtained. The ball’s orbital speed range is between 1 and 100 rpm.

For most of the orbit, the ball undergoes pure rolling with pivot; however, when the ball contacts the guide plate, sliding also occurs. The period of contact with the guide plate, termed the “scrub,” is the most tribologically severe part of the orbit and is when the majority of the lubricant’s tribo-degradation occurs.

Typically, a small amount of lubricant (≈50 μg) is applied to the ball at the beginning of a test. Such a minute lubricant amount usually degrades within one or two days. The test duration can be varied by adjusting the initial amount of lubricant and/or the load force. A test is terminated when the lubricant fails. Failure is defined when the friction coefficient rises beyond a predefined limit, which is usually two to three times the starting friction coefficient. The lifetime of the lubricant is then quantified as the total number of ball orbits completed divided by initial amount of lubricant on the ball. Typically, four tests are done for each condition and their results averaged for statistical significance in the test data. Relative lifetimes for various materials can be compared and can be directly correlated with relative lifetimes in real applications.

This work was done by Stephen V. Pepper and William R. Jones, Jr. of Glenn Research Center, Edward Kingsbury of Interesting Rolling Contact, and Mark J. Jansen of the University of Toledo. For further information, access the Technical Support Package (TSP) free on-line at under the Machinery/Automation category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17912-1.

This Brief includes a Technical Support Package (TSP).

Spiral Orbit Tribometer (reference LEW-17912-1) is currently available for download from the TSP library.

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