Aircraft technical Basics: TM 1-405, Aircraft Aircraft Engines, 1941: V - Engine Lubrication

SECTION V - ENGINE LUBRICA'T'ION

45. Lubrication requirements. - a. When one dry metallic surface is moved over another, a high resistance or friction is encountered which results in the generation of heat and excessive wear. If a layer of greasy substance or lubricant. is placed between the two metallic surfaces, the wear on the metal is practically eliminated, and heat is reduced to a minimum. Friction between metallic surfaces can be classified as sliding, rolling, or ball-bearing friction. Sliding friction such as encountered in the majority of engines in main and connecting-rod bearings, requires more complete lubrication than rolling friction encountered in roller bearings. Of the three, ball-bearing friction requires the least amount of lubricant for proper lubrication.

b. In aircraft engines, the lubrication system is designed to meet the problems of high temperatures, high bearing stresses, and proper functioning in all flight attitudes of the aircraft, except the inverted.

c. The high temperature of the various engine parts tends to thin out the lubricant (lower its viscosity) which decreases its effectiveness in overcoming metallic friction; therefore, provisions must be made to cool the oil externally. The cooled oil, on reentering the lubrication system of the engine, materially assists in reducing the high temperatures of the various parts, particularly the bearings.

d. The high bearing loads, especially in connecting rods, result in metallic friction which is reduced to a minimum by allowing sufficient clearance between the bearing surfaces to accommodate a heavy film of oil. This film of oil introduces a cushioning effect which considerably reduces the strains of shock- loading.

e. The oil pressure pump and its distributing lines and passages circulate the oil under pressure to the various working parts of the engine as long as the oil is supplied to the inlet side of the oil pump. The external part of the oiling system furnishes this supply under all conditions of engine operation except inverted flight. The scavenging system must thoroughly drain the crankcase of surplus oil. This is accomplished by the use of various drains and sumps incorporated in the lowest section of the engine. In prolonged steep dives, steep climbs, vertical banks, and inverted flight, there is a probability that the scavenging system will cease to function, resulting in surplus oil accumulating in the crankcase. Under such conditions, provisions must be made in the design and location of the crankcase oil breathers to prevent this surplus oil from flowing out of the engine.

f. The system that supplies oil under pressure to the operating parts and completely scavenges the crankcase of surplus oil is known as the force or pressure dry sump type of lubricating system. Although the pressure part of the system is more or less independent of the scavenging part, both parts must function properly to insure proper lubrication.

g. The pressure part of the system consists of the pressure pump, pressure relief valve, and distributing pipes or passages leading to the various parts to be lubricated. In addition to those parts to be lubricated under pressure, there are numerous other parts, such as pistons, cylinder walls, valve mechanism, accessory drive gears, etc., which are lubricated by the splash or spray of oil agitated by the revolving crankshaft and connecting rods. An oil pressure relief valve is installed in some part of the pressure system and is adjusted to maintain a specific oil pressure, shunting the surplus oil back into the scavenging system, or to the inlet side of the pressure pump. In instances where hydraulically controlled propellers are used, two oil pressure relief valves, or a compensating relief valve, may be employed to insure positive pressure to the propeller shaft. An oil-pressure gage connection is also installed in some part of the pressure system for connecting a line leading to the oil pressure indicator. A suitable oil screen or strainer is incorporated in the system to remove foreign particles, sediment. etc., from the oil before, it reaches the bearing surfaces.

h. The scavenging part of the system consists of a scavenging pump, drain sumps, screens, and the necessary oil return lines. One or more drain sumps are required to collect the surplus oil from the engine crankcase and are located in the lowest section of the engine. Screens are incorporated in the sumps to prevent foreign particles, sediment, etc., from circulating through the system. The oil return lines or passages connect the sump chambers to the intake side of the scavenging pump which returns the oil to the supply tank for recirculation to the pressure part of the system.

i. For complete details of the lubrication system employed in a specific engine, reference must be made to the lubrication chart in the technical publication for the engine involved.

46. Radial engine lubrication.  The circulation of oil through a typical radial-type engine is shown in figure 41.

a. In the pressure part of the system, the oil enters the pump (A), which forces it through the screen or strainer (B), to the rear and front crankshaft journals. The oil is forced into the drilled passages in the rear crankshaft journal which lead to the crankpin, master connecting-rod bearing, and short-rod knuckle pins. One or more lines or passages branch from the rear crankshaft journal and lead the oil to the various rear section drive shaft bearings (C). The line branching off the front crankshaft journal leads the oil under pressure to a control valve (D), by which a hydraulic, controllable pitch propeller may be operated by the engine oil pressure directed to the connection E). The oil pressure relief valve (F) is adjusted to obtain the desired pressure in the system, shunting surplus oil into the scavenging system, or to the inlet side of the pressure pump. The dotted arrows designate oil forced out at various points in the pressure system which lubricate, by splash or spray, such parts as pistons, rings, cylinder walls, accessory drive gears, etc., before draining into the scavenging system sumps.

b. In the scavenging system, an oil sump (G), located in the lowest sect ion of the crankcase, collects all the surplus oil drained from the pressure system. The screen (H), located in the sump, prevents foreign particles from restricting the flow of oil returning to the scavenging pump and is accessible for periodical removal and cleaning. The scavenging pump (I) removes the oil from the sump and forces it back into the external oiling system for recirculation.

FIGURE 41.-Typical radial engine lubricating system.

c. In connection with the lubrication system, a crankcase breather (J) is installed at some point on the upper part of the crankcase to relieve internal pressure resulting from high temperatures and high-speed piston operation. Provisions are made in the construction of the breather to permit escape of the pressure without loss of oil, usually by the use of internal baffles. In some breathers a gravity-operated valve remains open in normal attitudes of the engine and closes when the engine is inverted. This is done to prevent oil from flowing out of the breather in inverted flight.

47. In-line and V-type engine lubrication. - The circulation of oil through a typical in-lane or V-type engine is illustrated in figure 42.

a. In the pressure part of the system the oil enters the pump (A). which forces it through the screen or strainer (B) and distributing pipe to each of the crankshaft journals. From the rear crankshaft journal the oil is forced to the hollow camshaft (C), furnishing lubrication to the overhead valve mechanism. From the front crankshaft journal a pressure line leads through an oil-control valve (I) to a connection (E) for operating a hydraulic, controllable pitch propeller. In geared engines, a pressure line leads to the propeller-shaft reduction gear. The oil-pressure relief valve (F) is adjusted to obtain the desired pressure in the system, shunting surplus oil to the. intake side of the pressure pump. The oil which is forced into the crankshaft journals is thrown by centrifugal force to the crankpins and connecting-rod bearings. The dotted arrows designate oil thrown out at various points in the pressure system, which lubricates by splash or spray such parts as pistons, rings, cylinder walls, valves, tappets, accessory drive gears, and bearings before draining into the scavenging sumps.

b. In the scavenging system two scavenging pumps (I) are shown, one of which drains the front engine sump (G), through the screen (H), and the other drains the oil from the rear sump (G) and forces it back into the external supply for recirculation.

c. The oil breather (J) is usually located at the front end of the engine and extends a certain distance into the crankcase to prevent loss of oil through the breather at certain attitudes of engine operation. Several baffles may be incorporated in the breather which aid in preventing escape of oil mist under all operating conditions.

FIGURE 42. - Typical in-line or V-type engine lubricating system.