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National Advisory Committee for Aeronautics, Technical Notes - Effect of Lubricant Base Stock on Rolling - Contact Fatigue Life

Five lubricants of different base stock were tested using groups of
l/Z-inch air-melt AISI M—l tool—steel balls under rolling-contact fatigue
conditions in the fatigue spin rig. A methyl silicone, a mineral oil, a
glycol, a sebacate, and an adipate were used. The particular fluids
chosen all had about the same atmospheric pressure viscosity (lO centi—
stokes) at the test temperature of 100° F. Other test conditions such
as ball loading were held constant. Ball loading was held at that level
necessary to produce a maximum.theoretical Hertz stress of 725,000 pounds
per square inch in compression at the contacting surfaces and 225,000
pounds per square inch in shear at a depth of 0.009 inch below the surfaces.
This investigation studies the effect of lubricant base stock upon rolling-
contact fatigue life and correlates any observed differences in life re—
sults with unique properties of the different base-stock fluids.

The tests showed differences in rolling-contact fatigue life for the
five different base-stock fluids tested. The observed lives appeared to
correlate with the pressure-viscosity coefficients of lubricants of the
same base stock. Lubricants whose viscosities were increased the greatest
by pressure produced the longest fatigue lives. However, other lubricant
properties, such as bulk modulus and chemical activity, may‘well influence
fatigue life, and more data are required to determine their relative
importance. Chemical activity did not appear to be significant in these
tests, and the spells obtained in all tests compared closely'with those
obtained in full-scale bearings. Metallurgical transformation in the
material was consistent for all test runs.

The ever increasing operating temperatures of aircraft gas-turbine
engines have created a demand for rolling-contact bearings capable of
sustaining more severe operating conditions than possible with long
established materials and lubricants. Development of new lubricant-
material combinations must be carried out in order to meet successfully
present and anticipated demands of engine designers for satisfactory
bearing performance under more severe operating conditions.