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National Advisory Committee for Aeronautics, Technical Notes - Influence of Lubricant Viscosity on Operating Temperatures of 75 Millimeter Bore Cylindrical Roller Bearing at High Speeds

The influence of oil viscosity on the effectiveness of cooling and
lubricating high-speed rolling-contact bearings is of particular signifi-
cance in turbojet and turboprop engine design. Not only must the high-
speed engine bearings be lubricated, but a large portion of the bearing
heat must be removed by the lubricant; the heat absorbed by the lubricant
must, in turn, be removed to maintain safe operating temperatures. In
order that the heat to be removed from the lubricant will be a minimum,
it is desirable that as little heat as possible be generated by shearing
the lubricant in the process of cooling and lubricating the bearings.

The information available in the literature on the bearing cooling
effectiveness and the heat absorption characteristics of lubricants as
functions of lubricant viscosity-deals with bearing performance at
relatively low speeds. The results therefore cannot be applied to the
solution of bearing problems in aircraft gas-turbines because of the high
operating speeds. The effect of very low oil flows on friction torque is
reported in reference 1. The effects of higher oil flows on friction
torque are considered in references 2 and 5. Information on the effect
of oil viscosity and oil flow on operating temperatures, friction torque,
and.power dissipated at low speeds is contained in references 4 to 6.

High-viscosity oils are preferred for lubricating gears because of
the greater load—carrying capacity. In many instances, however, the use
of highdviscosity oils as lubricants for aircraft gas-turbine engines is
prohibited by the wide temperature ranges over which these oils must
function as lubricants and coolants. Whether or not a specific oil may
be used depends on whether it is pumpable at the lovitemperature limit of
operation and on whether its high—temperature stability is satisfactory:

If oil is supplied to a bearing through a single small—diameter Jet
normal to the bearing face and directed at the cage-locating surface, a
portion of the oil will be deflected and a portion of the oil will be
transmitted through the bearing (reference 7). The deflected oil serves
more specifically as a coolant than as a lubricant. (A small amount of
oil may enter the bearing and then exit on the deflected-oil side.) The
oil transmitted through the bearing serves both to lubricate and to cool
the bearing; in addition, it may be a source of heat due to churning.
The total heat rejected to the oil (for a specific bearing, DH, and load)
as affected by oil inlet viscosity, oil inlet temperature, and oil flow
is important to the engine designer.