A three-ball tribometer has been developed for use in evaluating the performances of oils and greases as lubricants in a vacuum at room temperature. This apparatus differs from the one described in "Vacuum Four-Ball Tribometer for Testing Liquid Lubricants" (LEW-16194), NASA Tech Briefs, Vol. 21, No. 4 (April 1997), page 64. The present apparatus is designed especially for experiments on the tribodegradation of liquid lubricants under conditions similar to those observed in preloaded angular-contact ball bearings operating in the boundary-lubrication regime in a vacuum. Tribodegradation can include both physical and chemical changes caused by a combination of contact stresses and aggressive ambient conditions. The chemical changes typically involve chemical reactions between lubricants and bearing surfaces, decomposition of lubricants, and darkening of lubricants associated with the formation of products denoted generally as "friction polymers."
The tribometer is housed in a stainless steel vacuum chamber. The main tribological assembly is a retainerless steel thrust bearing with three balls of 0.5-in. (12.7-mm) diameter placed symmetrically between flat races. The balls and plates are made of 440C stainless steel, which is commonly used in instrument ball bearings. The bottom plate is mounted on a shaft that passes out of the vacuum chamber through a steel bellows and is connected via a load cell to a deadweight cantilever device that pushes the bottom plate upward to apply the preload. A preload of 100 lbf (445 N) provides a mean Hertz (contact) stress of 1.39 GPa, which is typical for a preloaded instrument bearing. The bottom plate does not rotate. Driven by a synchronous gearmotor via a ferrofluidic rotary feedthrough, the top plate rotates at 4 rpm. Such a low speed helps to ensure the desired room-temperature test condition.
The rotation of the top plate causes the balls to roll on the bottom plate in an almost circular orbit of about 21-mm radius. More precisely, the balls gradually spiral outward from their initial radii with a pitch of about 0.5 mm per revolution and would eventually fall out if allowed to continue rolling without the restraint provided by a guide plate. The balls eventually make contact with the guide plate for a distance of about 5 mm, where they move along a straight line called the "scrub" and are forced back to the initial, slightly smaller orbit radius. Together, the spiral and scrub portions of the orbit constitute a track (see figure) that is stable and repeatable and is traversed thousands of times by the balls during an experiment. The scrub is also utilized to measure the coefficient of friction; this measurement is accomplished by use of a load cell that supports the guide plate and measures the force exerted on the guide plate by each passing ball. To ensure that bulk temperatures remain near room temperature and to prevent metallic wear, an experiment is stopped at the first sign of increased friction.
Once the vacuum chamber has been evacuated and an experiment is in progress, a residual-gas analyzer with a line of sight to the balls and plates is used to determine the composition of residual gas from the chamber and of any molecular species that evolve through tribodegradation of the lubricant. After an experiment, the degraded lubricant can easily be examined by use of such surface- and thin-film-analysis techniques as optical and electron microscopy, photoelectron spectroscopy, and infrared and Raman microspectroscopy.
The design of this tribometer offers several advantages. It provides a more credible simulation of preloaded angular-contact ball bearings operating in the boundary-lubrication regime than does a tribometer containing a ball sliding on a plate. Retainerless operation eliminates forces associated with balls sliding in retainer pockets and thereby enables a simple yet rigorous analysis of stresses to which a lubricant is subjected. The absence of a porous retainer eliminates the uncertainties associated with a supply of lubricant; the bearing is forced to operate with an extremely small lubricant charge, so that there is maximum opportunity for the lubricant to be tribologically exercised to make it undergo tribodegradation. The balls and plates are inexpensive. Surface analyses of tracks can be performed more easily on the flat plates of this tribometer than on the curved surfaces of ordinary bearing races.
This work was done by Stephen V. Pepper and Ben T. Ebihara of Lewis Research Center and Edward Kingsbury of Interesting Rolling Contact.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7-3
21000 Brookpark Road
Refer to LEW-16550.