Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 11. Chronometric Tachometers

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 XI - CHRONOMETRIC TACHOMETERS

46. Purpose and use.-a. Chronometric tachometers are used on airplanes to measure the speed of the engine crankshaft. All standard types of this instrument are calibrated in revolutions per minute. The chronometric tachometer is very rugged, extremely accurate, and gives long periods of service with a minimum of maintenance and repair. However, its use is limited to single-engine airplanes because it is shaft-driven. Unsatisfactory operation results when the shaft length exceeds 20 feet or when the shaft must have an excessive amount and number of bends.

b. With the advent and use of adjustable pitch and constant speed propellers, actual uses of the tachometer are limited to the determination of the propeller speed. When using constant speed types of propellers, the instrument is used to check operation of propeller governors and controls during take-off and during variable altitude and density conditions. When using adjustable types of propellers, the tachometer is used as a check when the propeller is changed from low to high pitch, or vice versa.

47. Description.-a. Standard types of chronometric tachometers (fig. 15) have a range of 0 to 3,500 rpm. All types are designed to be driven at one-half crankshaft speed, and are provided with a reversing mechanism to prevent damage due to engine kick-back. This mechanism also makes it possible for the drive unit to be driven either clockwise or counterclockwise. Since direction of rotation is optional, this feature need not be considered when connecting the instrument with the engine. The later types of chronometric tachometers are provided with two-piece cases, a 3-volt individual light, and a drive shaft "take-off" nipple which is located in the back of the case. The latter feature eliminates the necessity of having a 90° bend in the shaft.

(1) Front view.  (2) Side view.
FIGURE 15.-Chronometric tachometer.

b. The mechanism of the chronometric tachometer (fig. 16) consists of the following four principal units:

(1) The driving mechanism A which takes the power from the flexible shaft and transmits it to the power plant.

(2) The power plant B which furnishes the power for driving the watch mechanism, counting mechanism, and synchronizing cams which time the action of the counting mechanism.

(3) The watch mechanism C which times the action of the synchronizing cams.

(4) The counting mechanism D which counts the revolutions transmitted to it during periods of 1 second each and indicates the results on the instrument dial.

FIGURE 16.- Chronometric tachometer mechanism.

c. The drive shaft assembly (fig. 17) consists of the shaft proper which is made of hard drawn bronze wire, shaped in a coil form with suitable tangs soldered to each end for attachment with the engine and the instrument. The shaft in turn is placed in a braided copper-covered metal casing which serves as a bearing in which the shaft can rotate. All parts are nonmagnetic to eliminate interference with compasses and the covering on the casing serves as a shield to prevent interference with radio.

48. Operation. This tachometer actually totals the revolutions (and fractions thereof) that occur during each alternate second. The seconds are automatically measured by a special watch escapement. The drive system, which consists of a chain of gears driven by the flexible drive shaft from the engine, operates the escapement cam and the counting system. The escapement cam being actuated by a friction drive causes a gear driven by the drive system to become meshed with the counting gear intermittently for 1-second intervals. During the 1-second intervals that these gears are in mesh, the large gear rotates a distance, proportional to the speed of the drive system. A stud on the lower side of the counting gear pushing against a stud on a wheel directly below the pointer gear rotates the indicator hand through the same distance as the counting gear. At the end of the second the counting gear is disengaged and re-turned quickly to its starting position by a spring, but the indicator hand is held stationary during this time. During the next second, if the engine has run faster than during the previous second, the counter gear pushes the indicator farther around the dial. If the engine speed has been less, the indicator is released and a spring causes it to drop back to the position of the counter gear at the end of the l-second counting period. This arrangement causes the indicator hand to move by jerks and the indicated reading at any instant is the rate of speed of the engine during the previous second. As long as the drive shaft continues to rotate this cycle of operation continues automatically. After rotation of the drive shaft ceases, there will usually be heard a rhythmic ticking which is normal and is caused by the escapement continuing to beat under the tension of the mainspring until the energy stored up in the latter is expended. Under normal conditions this may be as long as 30 seconds.

FIGURE 17.-Tachometer drive shaft and accessories.

49. Installation.   a. The general points on installation of instruments as given in section III are applicable to this instrument.

b. The following specific points will be observed when installing chronometric tachometers:

(1) That the correct length drive shaft is used.

(2) The drive shaft has as few bends as possible. In no case is the radius of any bend to be less than 6 inches. When sharp bends at the engine or instrument end of the shaft cannot be avoided in any other way, the 90° adapters as shown in figure 17 will be used.

(3) The drive shaft retaining washer (fig. 17) is always placed on the engine end of the shaft before the coupling nut is attached to the drive on the engine.

(4) The tachometer is always placed on the instrument panel so that the drive shaft "take-off" is at the bottom.

50. Maintenance.-a. All general points on maintenance of instruments as given in section II are applicable to tachometers.

b. The following specific points of maintenance procedure are applicable to chronometric tachometers:

(1) If the pointer jumps or acts erratically, the fault will often be found in the flexible shaft connection or whipping of the cable in the casing due to a "tight" radius, a dry flexible shaft, or misfit or worn-out shaft connections.

(2) If the instrument fails to indicate. it may be due to a broken cable, faulty connections, or drive gear at the engine connection. Care must be exercised in connecting the flexible shaft, as any force used will result in "cocked" connections and the breaking of cables.

(3) In replacing broken cover glasses, care must be exercised to avoid particles of dust or glass entering the mechanism. If such exposure has been unavoidable, the instrument should be carefully examined and tested before sealing with a new glass.

(4) When the tachometer is at zero or normal rest, the hand should point vertically and centrally through the zero graduation mark.

(5) Under no circumstances are these instruments to be lubricated by service activities.

(6) It is possible to reset the pointer on zero by removing the pointer screw and pointer and then resetting it at the proper position. Care should be taken when doing this not to injure the surface of the dial.

(7) If the flexible drive shaft becomes defective, it is replaced with a new one rather than repaired or lengthened.