2. Cooling. Lubricating oil must cool moving
grade) were used in early jet engines. This oil was
parts by carrying heat away from gears and
distributed in two grades--1010 for normal
bearings. This is an important function consider-
use and 1005 for extremely low temperatures.
ing the many parts located next to burners or
MIL-L-6081 grade 1010 is still used as a pre-
turbine wheels, where temperatures are over
servation oil in fuel systems.
1700F.
The types of lubricants used in the engines of
Liquid lubricants cool by pumping or spraying
today are different from the lubricants used years
ago. As the power output of jet engines increased,
absorbs the heat and later dissipates it through
aircraft were able to fly higher. The result of jet
oil coolers.
engines operating at these higher, colder altitudes
3. Cleaning. Another major function of a
and higher engine temperatures created greater
lubricating oil is cleaning. Oil carries dirt, small
demands on lubricating oils. This, in turn,
carbon and metal particles, and gum and varnish
required the development of synthetic lubricants
to filters. This has become increasingly important
that could withstand these higher bearing
with the higher compression ratios, engine speeds,
temperatures.
operating temperatures, and closer tolerances
MIL-L-7808 was the first synthetic oil
between parts in newer engines.
developed to meet these demands. Today, most
jet engines use another synthetic-based oil,
DESIGNATIONS OF
MIL-L-23699. These two oils are completely
LUBRICATING OILS
compatible and may be mixed when necessary.
However, certain 23699 characteristics are
All lubricating oils used by the Navy have a
downgraded in proportion to the quantity of 7808
classification number, which shows the grade and
oil, if mixed. Synthetic oils are based on acids
intended use of the oil. Aircraft engine lubricating
and other chemicals; therefore, they are not
oils are given a four-digit grade number, such as
compatible with the mineral- or petroleum-based
1065. The Navy and the Air Force use the Saybolt
oils.
scale for designating the viscosity of oil. The
NOTE: You should consult the applicable
designation consists of four digits. The first
technical instructions for the grade number
digit designates the use of the oil; the 1 indicates
or MILSPEC of oil recommended for use
aviation engine lubrication. The last three digits
in an engine. Reciprocating engines use
give the viscosity using the Saybolt scale.
MIL-L-228S1, W-120, or E-120 oil, which
is not compatible with the turbojet engine.
NOTE: You are probably more familiar
with the Society of Automotive Engineers
FUNCTIONS OF JET ENGINE OILS
(SAE) numbers for grading viscosity. If
you want a comparison between the two
Lubricating oils must perform three basic
systems, take the 3 numbers for the Saybolt
functions in a jet engine: (1) lubrication, (2) cool-
system, divide by 2, and round to the
ing, and (3) cleaning.
nearest multiple of 10. For example, 1065
has an SAE rating of 30.
1. Lubrication. Oils should have the following
characteristics to lubricate properly:
Synthetic oils use military specification
a. It must be of low enough viscosity to
numbers for references. See table 5-1.
flow readily between closely fitted, rapidly moving
parts. It must also have a high enough viscosity
to prevent metal-to-metal wear.
LUBRICATING OILS
b. It must not break down under high
temperatures and pressures.
Contaminated fuel in the lubricating system
c. It must have a low enough pour point
of an engine can be disastrous to engine opera-
to flow readily when starting under extremely low
tion. Lubricating oils can be contaminated
temperatures.
through operational conditions (dusty or sandy
d. It must have a high enough flash and
places, high operating temperatures), faulty
fire point so it doesn't burn or vaporize under high
maintenance practices, and part failures.
heat.
An example of operational contamination is
e. It should not form and deposit excessive
carbon. Carbon forms when oil evaporates,
amounts of carbon, varnish, or gum deposits.
5-2
