Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 8. Oil Pressure Gage

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION VIII. - OIL PRESSURE GAGE

31. Purpose and use.-a. Oil pressure gages are required on all types of aircraft engines to show the pressure at which the lubricant is being forced to the bearings and the various other points of the lubricating system.

b. Some of the specific uses of the oil pressure gage are to

(1) Warn the pilot of impending engine failure due to exhausted oil supply, failure of the oil pump, burned-out bearings, broken oil leads, and other causes indicated by loss of pressure.

(2) Indicate to the pilot or mechanic that the oil is circulating under proper pressure before the take-off.

(3) Provide a means of measuring oil pressure during adjustment of the oil relief valve.

32. Description.-The standard type oil pressure gage (fig. 12) is a differential pressure measuring instrument having a Bourdon tube mechanism enclosed in a 1 7/8-inch bakelite case. The range of this gage is from 0 to 200 pounds per square inch with the scale marked in 10-pound per square inch graduations. The gage is provided with a single connection on the back of the case which leads directly into the Bourdon tube and has a 3-volt light, the light receptacle being molded integral with the instrument case.

33. Operation.-a. All aircraft engines are provided with engine-driven oil pumps so that whenever the engine is running, oil is forced through the engine under pressure. This pressure is controlled by a pressure relief valve which is adjustable and is set for the pressure recommended for the particular type of engine involved. The gage is connected into the system at a point between the relief valve and the engine.

b. Operation of the gage is the same as for any Bourdon tube instrument with one exception; the opening X into the Bourdon tube (fig. 6) is very small compared with other pressure gages of this type. This restriction prevents the surging action of the pump from damaging the gage. Unless this provision is made, the pointer on the gage will oscillate violently through wide ranges so that it cannot be read accurately. Due to this restricted opening, under extremely cold-weather conditions and with the heavy Oils that are used, the gage fails to indicate properly any pressure when the engine is first started. Under these conditions, the accuracy of the gage should not be doubted nor its operation questioned until the temperature of the oil reaches the lowest point on the minimum marking of the oil thermometer which will occur after a few minutes running.

FIGURE 12.-Oil pre ssure gage and surge chamber.

c. Engines which operate at pressures above 100 pounds per square inch must also have a surge chamber (fig. 12) connected into the line. The air that is trapped in the surge chamber when the line is connected to the instrument serves as a cushion for the pulsations in the oil pressure caused by the pump. This eliminates the hammer effect on the mechanism of the gage and prevents oscillation of the pointer.

34. Installation.-a. Surge chambers should be installed in the oil pressure gage line on all installations where specified.

b. The general instructions for installation of aircraft instruments given in section III apply to this gage.

35. Maintenance.-a. When sluggish operation of the oil pressure gage as evidenced by lag in indication is noticed during engine operation in cold weather, the line leading to the gage is disconnected, thoroughly drained, and refilled with aircraft instrument oil. This procedure is performed as often as necessary.

b. The general instructions on maintenance of airplane instruments given in section II apply to this gage.