TM 1-410. Technical Manual, Airplane Structures 1941: Section 18 - Lubricants

SECTION XVIII: LUBRICANTS

68. General.--a. These instructions generally cover classes and uses of aircraft lubricants (other than for engines), and are furnished for use as reference and information only in cases that are not covered in the handbook of instructions pertaining to particular items of  equipment.

b. Grease and oil containers when left open collect grit, lint, etc., which renders the lubricant unsatisfactory for use. In certain cases the rapid wearing of parts has been traced to dirty grease. To avoid this contamination all grease and oil containers are to be kept tightly closed except when actually being used, and they should never be opened for use when sand or grit are likely to be blown into them.

69. Classification.-Lubricants stocked by the Air Corps are classified according to their specified use as

a. Greases.-(1) Low temperature aircraft.-For plain or antifriction (ball or roller) bearings operating at moderate temperatures and moderate loads.

(2) Graphite cup.-For plain bearings operating under heavy loads and for screws and gears when a graphited grease is specified by the manufacturer.

(3) High, melting point.-For plain and antifriction bearings operating at high temperatures.

(4) Aluminum soap.-For antifriction bearings operating under moderately heavy loads.

(5) Lead soap base.-For plain and antifriction bearings operating under very heavy loads.

(6) Compound, antiseize, white lead base.-Used as a general thread lubricant.

b. Oils.-(l) Nopcompounded.-For all antifriction bearings designed to retain a fluid lubricant, lightly loaded gear and screw mechanism of aircraft control systems, and lubrication of exposed parts which must operate at low temperatures. Those used for aircraft lubrication are as follows:

(a) Aircraft engine oil, grade 98 (SAE 50).

(b) Lubricating oil, class D, grade 20 (SAE 20).

(c) Aircraft instrument oil.

(2) Compounded.-For heavily loaded gears such as landing gear, retracting mechanism, propeller drives, and control column gears.

(a) Retracting mechanism gear lubricant (heavy SAE 110), equal in engine oil viscosity to SAE 70.

(b) Oil lubricating E. P. gear, low temperature (light SAE 80) equal in engine oil viscosity to SAE 20.

(c) Petrolatum.

70. Selection of grades.-In consideration of grades of lubricants summer refers to periods in which normal minimum daily temperature is above -7° C. (20° F.) ; winter refers to periods in which normal minimum daily temperatures range from -1° C. (30° F.) to -29° C. (-20° F.) ; extreme cold refers to periods during which temperatures below -29° C. (-20° F.) are likely to be encountered. Too stiff or too heavy lubricants should be avoided more than too thin or too light lubricants. Change of grade of lubricants should be made at a regular 40-hour inspection period whenever normal seasonal changes of temperature occur.

a. Summer.-(l) Medium grade of the various greases should be used except for small, well-housed antifriction bearings, for which the soft grades are satisfactory for both summer and winter.

(2) For heavily loaded gears use gear lubricant or thread lubricant, the latter only if a fluid lubricant is not retained and frequent lubrication is not convenient.

(3) Nonstructural parts such as steps, seats, and loosely fitted hinged joints will be lubricated with aluminum soap grease or graphite grease, Medium.

(4) Engine oil or petrolatum will be used for oil-lubricated parts, the petrolatum when rust prevention is also desired.

 (5) Hard grades of the various greases may be used for parts having limited movement or where the grease is not retained well

b. Winter.-(1) Soft grades of the various greases are used for all structural bearings. Low temperature grease is used when available, except when high melting point grease is indicated in specific instructions. Equal parts of a medium grade grease and aircraft instrument oil thoroughly mixed at room temperature will provide a useful substitute for the soft grades, having less stiffness at low temperatures but greater leakage if the temperature rises.

(2) Gear lubricant is satisfactory for all gear lubrication at low temperatures.

(3) For oil lubrication, lubricating oil, class D, is used.

c. Extreme cold.-(1) No lubricant resistant to flow at ordinary summer temperatures will permit easy operation below - 29° C. ( - 20° F.). For this reason all hand and electrically operated mechanisms having lightly loaded bearings, gears, and screws are lubricated with aircraft instrument oil. Those heavily loaded are lubricated with oil, lubricating, E. P. low temperature.

(2) Most ball bearings on aircraft control surfaces and pulleys on the control system are tightly inclosed or sealed except for a narrow clearance around the inner race which is filled with a soft grease. This is not changed except for extreme cold operation when the grease should be washed out with -aircraft instrument oil. The aircraft instrument oil remaining in the bearing will be sufficient and satisfactory lubricant for extreme cold condition.

(3) In all cases where thinned greases or oil have been substituted for standard greases or oil for low temperature operations, the substituted lubricant is replaced with standard lubricants as soon as the temperature above those classified as extreme cold prevail.

71. Thread lubrication.-a. The material specified in paragraph 69a, (6) for use on threaded parts is a lubricant which has the necessary antiseize properties to permit assembling parts to a tight fit with, a minimum of torque and with a maximum freedom from seizure. Seizures in general are due to frictional flowing or yielding of the metal on contacting threaded surfaces and are particularly likely to occur whenever unlubricated threaded parts are screwed together without proper thread clearance, or when tightened or adjusted while stressed as in the case of tie rods under tension.

b. The lubricating compound should be stirred well before being applied. Care should be exercised not to apply it in excessive quan tities and application should be made on the male fitting only. It is recommended for use on the following assemblies:

(1) Where aluminum or aluminum alloy pipe threads are used. In this case the male thread should be of dissimilar material as no satisfactory thread lubricant is known which will prevent thread seizure in pipe threads where both the male and female parts are aluminum or aluminum alloy. In special cases where both male and female parts are of these materials, it is not intended that they be disassembled during life of the unit.

(2) On steel threaded parts (plated or unplated) having a considerable stress imposed upon the threads, or which are subject to frequent adjustment.

(3) For assembling threaded fittings of the various piping systems except those used on vacuum pump lines.

(4) In assembling aircraft instruments and as a general purpose thread lubricant, except where a nut embodying a self -locking feature is installed.

c. Proper selection of mating threaded connections, use of the proper lubricant as specified, and proper assembly of parts will result in satisfactory service operation. No sealing compounds (resinous materials to prevent leaks in improperly fabricated or assembled joints) have been specified. Such materials are considered unnecessary and undesirable due to the possibility of plugging the lines.