2-8
The output frequency of electric power generators is checked when the engine is started and
during preventive maintenance routines.
Equipment that operates in the af range must be adjusted to operate at the correct frequencies.
Radio transmitters must be accurately tuned to the assigned frequencies to provide reliable
communications and to avoid interference with radio circuits operating on other frequencies.
Radar sets must be properly tuned to obtain satisfactory performance.
As you can see from the above examples, frequency measurement does indeed play a valuable role in
maintenance. These measurements can be divided into two broad categories: MECHANICAL-
ROTATION FREQUENCY measurement and ELECTRICAL-OUTPUT FREQUENCY measurement.
Depending upon your job and/or the type of command to which you are assigned, you may be tasked with
performing one or both of these types of measurements.
MECHANICAL-ROTATION FREQUENCY MEASUREMENT
The rotating frequency (speed in revolutions per minute) of armatures in electric motors and engine-
driven generators, as well as the blade speed in turbines, is measured with devices called
TACHOMETERS, STROBOSCOPES, and STROBOTACS.
Tachometer
A tachometer is an instrument that measures the rate at which a shaft is turning. Although
tachometers are installed on machinery, such as generators and engines, you may need to determine the
speed of a rotating machine that is not equipped with a tachometer. In these instances, you will be
required to use a PORTABLE TACHOMETER. Portable hand-held tachometers measure speed by direct
contact with the shaft of the measured unit. Portable tachometers are for use only during testing and
should not be used continuously. The common types of portable tachometers are the CENTRIFUGAL
and the CHRONOMETRIC.
CENTRIFUGAL TACHOMETER.—A centrifugal-type tachometer is illustrated in figure 2-5,
view A. View B shows the internal arrangement of the centrifugal tachometer; refer to view B in this
discussion. In the centrifugal tachometer, centrifugal force acts upon fly weights that are connected by
links to upper and lower collars. The upper collar is affixed to a drive shaft; the lower collar is free to
move up and down the shaft. A spring, which fits over the shaft, connects the upper and lower collars.
