Ball Bearing Performance in UHV – Test Summary
DRAFT
E. Doyle 5/1/00
Background
The NLC Beam Delivery collimators, as presently conceived, will require precision mechanisms
for operation in UHV (10-7 Torr or better) and high radiation (>105 r/hr). The applications are all
low speed (0 to 2 rpm) and low cycle (104 to 106).
The standard failure scenario in UHV begins with wear of the oxide film which prevents metal
contact at the atomic level. In air, the oxide can re-form very quickly if worn away but not in UHV.
Once atomic metal-metal contact is possible, localized cold welding between balls, races and
retainers begins to occur. The mechanism experiences sudden torque spikes as microscopic
weld zones between contacting components are torn apart, leaving damaged rolling surfaces.
Eventually the elements weld so strongly that the torque required to break free exceeds the
mechanism’s capacity. Or, particles of debris jam the balls, preventing motion.
A literature search revealed several possible solutions to delay the onset of atomic metal contact:
inorganic dry film lubricants, soft metallic film lubricants and ceramic balls and/or races. They act
in various ways by preventing atomic metal contact between rolling elements.
The most promising solution for the collimator application appeared to be ion sputtered film of
MoS2 on the races of 440C stainless steel bearings. Unfortunately no load/life data could be
found for any of the options. For this reason, we decided to test some bearings in UHV to gain
experience with the actual problems involved. (During the testing, a paper was found containing
test data for bearings, slightly larger than ours, with dry film lubricants and running at 9000 rpm.
The results were encouraging in regard to our requirements. Discussion of these results may be
found at the end of this article.)
Test Apparatus and Procedure
The bearing chosen for testing is one intended for positioning the collimation rotors in the rotary
consumable collimator. The anticipated service is basically static, but the positioning rollers will
rotate incrementally as the rotor is indexed. Also, each time the collimation aperture is adjusted,
the rollers will rotate slightly. This could result in several thousand cycles of oscillating motion in
the lifetime of the bearing. The proposed bearing (Berg p/n B1-26-Q3) is .188” I.D. and .500”
O.D. with a static radial load capacity of 67 lb. Balls, races and cage are 440C stainless.
To establish a baseline condition, we planned to test some bearings with all lubrication removed
and cleaned for UHV. Then we would test bearings prepared with the dry lubricant.
The test chamber is shown in Figure 1. Radial load is applied to the bearing by means of dead
weight acting through a bellows feed through. Rotation is through a bellows-type rotary feed
through. Figure 2 shows the interior. Two identical bearings are mounted such that a radial load
on one creates an equal and opposite load on the other.
The test regime consisted of a fairly crude measurement of the torque required to turn the
bearings. This was done by wrapping a thin teflon tape around the manual turning knob on the
rotary feed-through. The tape was allowed to hang down and a hook was attached to the end.
Weights were hung sequentially on the hook until there was enough torque to start the knob
turning. Usually, it would not continue to move but would stop as if a bearing contained an
internal obstacle. At that point, more weight was added until the knob moved continuously. This
weight was recorded. Then weight was added to the loading platform to increase the radial load
on the bearings and the process was repeated. After reaching a predetermined maximum radial
load, weights were removed incrementally and the torque readings were repeated as the radial
load was reduced.
This test was done at atmospheric pressure and repeated over a period of about three weeks
after the system had been pumped down below 10-7 Torr. The system ultimately pumped below
10-8 Torr.
Results
Figure 3 shows the torque vs radial load in air and under vacuum.
Even with no radial load, there is a measurable torque to turn the shaft, due to the bearings in the
rotary feed-through and the energy needed to flex its bellows. In the first week under vacuum,
there seemed to be an increase in average torque. Then after three and four weeks, it was lower.
When the crudeness of the experiment is taken into account, it’s hard to assign any significance
to these trends.
The other test consisted of cycling the bearings under load. This was at first done manually,
oscillating the knob through an arc of about 150o. This was done in two sessions, applying about
6000 fatigue cycles, total. The first session (3000 cycles) was at a radial load of 32.2 lb, about
half of the max static load. The second session (3000 cycles) was at 45.2 lb, or 2/3 of the max
static load. Then the chamber was baked at 125oC for nine days and about 9000 complete
revolutions applied with an electric drill motor. The revolution count was approximate. The
bearings were held at full radial load during the final two series of tests and during the baking. At
intervals of 1500 rev, the friction torque was measured. The results are plotted in Figure 4.
After the continuous rotation tests, the bearings were removed and inspected. They were both
noted to feel very rough when turned by hand. The outer races were cut in half with a small
abrasive cutting disc. One bearing was found to have a large amount of dark colored solid
contamination in the races. The contaminant was stuck to the surface but could be scraped off
with a hard wooden stick. It’s likely that this material was some kind of residue from the original
grease, incompletely cleaned before testing. Possibly the continuous rotation of the bearing
continuously dislodged this material. As it accumulated in the races, the friction torque increased.
The bearing races were inspected under a microscope. The races of both bearings showed a
somewhat “sand blasted” appearing band around the bearing in the bottoms of the grooves,
distinct from the mostly polished surface of the race. In addition, there are isolated stripes cutting
across the first band with a similar surface.
Figure 5 shows these wear bands. The transverse bands may have been created during long
periods under steady load. Without axial preload, ball bearings are very “soft” axially. Vibration
from the vacuum pump, which was sitting on the table with the test chamber, could have caused
microscopic motion resulting in “fretting” type wear.
No pits or major surface damage were observed.
TRANSVERSE
WEAR BANDS
CIRCUMFERENTIAL
WEAR BAND
FIGURE 5. Bearing outer race after testing.
Inspection revealed nothing but the lubricant residue to account for the increasing torque.
Nothing was observed which warned of impending failure. The magnitude of the applied radial
load was sufficient to create stresses near yield in the races. It can be reasonably concluded
from our test that 440C stainless is not very susceptible to cold welding.
Discussion
The paper referred to above 10 contained results very encouraging for our application. The
researchers tested ball bearings (full compliment, no retainer) of a slightly larger size (8mm shaft,
22mm o.d.) at high speeds (9000rpm), high vacuum (8e-7 Torr) and high temperature (300oC)
with a variety of lubricants (MoS2 , Ag and Pb). Balls and races were high speed tool steel (AISI
T5). The bearings were lightly loaded axially (20N or 4.5 lb, which is ~2% of the max static
loading for a bearing of this size, using Timken S38R for comparison).
MoS2 exhibited lower running torque but shorter life than the metallic lubricants. The shortest life
reported was ~10e6 revs, two orders of magnitude greater than our requirements. The metal
lubricants had lives from ~75e6 to ~150e6 revs. A bearing was apparently tested with no film, but
the actual life is not given. Scaling from the graph is risky due to the small value, but the life thus
obtained is about 1e6 revs.
The conclusion is: if we load the bearing to a few percent of its max static load and lubricate with
MoS2 or soft metals we should expect bearing life in millions of revolutions. This should suffice
even for the continuously rotating renewable collimator (@ 2rpm it would turn 1e6 revs per year).
References
1. E.E. Bisson & W. J. Anderson, Advanced Bearing Technology, NASA SP-38, TJ1061 N3
[SLAC]
2. P.D. Fleischauer, Tribology in the Space Environment, NASA SMC-TR-98-35
3. Metals Handbook Vol 18, “Lubricants for High Vacuum Applications
4. T. Spalvins & J. Przybyszewski, Deposition of Sputtered Molybdenum Disulfide Films and
Friction Characteristics of Such Films in Vacuum, NASA TN D-4269
5. S. H. Loewenthal & R.G. Chou, Evaluation of Ion Sputtered Moly Disulfide Bearings for
Spacecraft Gimbals, Tribology Transactions, v 37 (1994) , 3, 505-515
6. J.R. Lince & P.D. Fleischauer, “Solid Lubricants”, Ch 7 of Space Vehicle Mechanisms, P.L.
Conley, ed, TL875 C663 1998 [SU]
7. P. J. Lawlor, The Actuated Latch Pin and its Development, (NASA citation misplaced)
8. D. H. Buckley et al, Influence of Crystal Structure on the Friction and Wear of Titanium and
Ti Alloys in Vacuum, NASA TN D-2671
9. D. W. Wisander, Friction and Wear of Titanium Alloys sliding Against Ti – 6% Al-4%-Vd Alloy
in Air at 430C, NASA TN D-8207
10. Takashi Ogawa et al, Development of Ball Bearings with Solid Film for High-Vacuum, High
Temperature, High-Speed Application, Lubrication Engineering, Vol 49, 4, 291-299, April
1993 (Journal of the Soc of Tribologists and Lub Engineers)