Downloaded from https://www.everyspec.com FM 1-506 frictional resistance of the moving parts and high antiwear properties to resist the wearing action that occurs during engine operation. Moisture. Corrosion of bearing metals is almost always due to moisture in the oil. Although it is possible for corrosion to occur from acidity, it is unlikely unless moisture is also present. Therefore, to prevent corrosion, it is important to eliminate moisture from the oil as much as possible. This is especially true at high temperatures because they increase the rate of corrosion.
Cooling Ability Lubricating oil should have maximum cooling ability in order to absorb as much heat as possible from all lubricated surfaces. Because of the accumulation of these harmful sub- stances common practice is to drain the entire lubrica- tion system at regular intervals and refill it with new oil. The time between oil changes varies with each make and model aircraft and engine combination. Chemical Stability The extreme operating conditions and high dollar value of aircraft engines make it necessary to use lubricat- ing oil of the very best quality. The following chemical transformations can occur in a lubricating oil and make it unfit for service.
WARNING When handling oil used in gas turbine engines, do not allow oil to remain on skin any longer than necessary. It contains a toxic additive that is readily absorbed through skin. Acidity. Acidity in oils is dangerous chiefly when high temperatures and moisture are present as is the case in aircraft engines. The results of a high acid content are corrosion of metal and the formation of sludges, emul- sions, and deposits in the oil system. Oxidation. All lubricating oils tend to oxidize when in contact with air. The compounds formed by oxidation are undesirable and harmful since they are generally of a gummy or acid character.
GAS TURBINE ENGINE LUBRICANTS There are many requirements for turbine engine lubricating oils; but because of the small number of moving parts and the complete absence of reciprocating motion, lubrication problems are less complex in the turbine engine. This, together with the use of ball and roller bearings, requires a less viscous lubricant. The turboprop engine, while using essentially the same type of oil as the turbojet, must use a higher-viscosity oil because of the higher bearing pressures introduced by the highly loaded propeller reduction gearing. Sulfur. Sulfur may occur in lubricating oil as free sulfur or as sulfonates.
Free sulfur may be present through careless or improper refining or by actual addition. Free sulfur is corrosive in nature and impairs the stability of the oil. The presence of sulfonates indicates overtreat- ment with sulfuric acid or inadequate washing of the oil to free it of chemicals during the refining process. Sul- fonates are strong emulsifying agents that tend to promote sludge formation in the oil system. Lubricating oils containing high percentages of sulfur oxidize more easily than those with low sulfur content. Gas turbine engine oil must have high viscosity for good load-carrying ability but must also have viscosity low enough to provide good flow ability.
It must also be of low volatility to prevent loss by evaporation at the high al- titudes at which the engine operates. In addition, the oil should not foam and should be essentially nondestructive to natural or synthetic rubber seals in the lubricating system. Also, with high-speed antifriction bearings, the formation of carbons or varnishes must be held to a minimum. Carbon Residue. Petroleum lubricating oils are complex mixtures of hydrocarbons that vary widely in their physical and chemical properties. Owing to these difference some oils may vaporize under atmospheric conditions without leaving any appreciable residues. Other oils leave a nonvolatile carbon residue upon vaporization.
This carbon residue is the result of a partial breakdown of the oil by heat, which is caused by destruc- tive distillation of the oil without air entering into the reaction. Many parts in the engine operate at a tempera- ture high enough to cause this reaction and to form carbon deposits. These deposits are undesirable as they may restrict passages. In reciprocating engines they may cause sticking piston rings and sticking valves. The many requirements for lubricating oils are met in the synthetic oils developed specifically for turbine engines. Synthetic oil has two principal advantages over petroleum oil: it tends to deposit less lacquer and coke and evaporates less at high temperatures.
Its principal disadvantage is that it tends to blister or remove paint wherever it is spilled Painted surfaces should be wiped clean with petroleum solvent after spills. 1-6