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

A series of rolling-contact fatigue tests was conducted in a bench
rig developed at the NACA Lewis laboratory. Four paraffinrbase mineral
oils of varying viscosity (at atmospheric pressure) were used as lubri-
cants. Ball specimens were AISI Mel tool steel (air melt). Test temr
perature was 100° F, and a calculated Hertz compressive stress level of
725,000 psi was maintained.

A continuous trend toward longer life was Observed with increasing
lubricant viscosity over the range studied (5 to 120 centistokes at
1000 F). This trend holds at any percentage of specimens failed. The
life scatter remained about constant at each viscosity level studied.

A plot of log of life at any survival level against log of lubri-
cant viscosity produces a reasonably straight line. This line indicates
that rolling-contact fatigue life is a function of approximately the 0.2
power of lubricant viscosity.

One of the primary considerations in developing a bearing capable
of sustaining the high temperatures encountered in present and antici-
pated aircraft gas-turbine engines is the rolling-contact fatigue life
of the bearing elements. Aside from bearing design and loading, the
fatigue life is affected by the materials used in the bearing elements
and the substance used to provide lubrication.

During highpspeed rolling contact, the lubricant, in addition to
reducing sliding friction and cooling the bearing, affects the pressure
distribution in the contact zone through hydrodynamic action. The
theoretical calculations of stress described in appendix.A are for static
loading only. At high rolling speeds these may not be entirely correct.
A precise three-dimensional analysis of this phenomenon would'be very
complicated, but a general discussion will illustrate the point.

The force necessary to maintain this rate of shear depends upon the vis—
cosity of the fluid. Thus a more viscous fluid would require a greater
shear force. Since this shear force is resolved from the pressure be—
tween the adJacent rolling—element surfaces, a portion of the ball load
is borne by that portion of the fluid which is outside of the contact
area that would eXist if no lubricant were present. Thus the effective
contact area is increased and the maximum contact pressure is reduced.
For a given rolling speed the maximum pressure would decrease with in-
creasing lubricant viscosity.