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tachometer(tăkŏm`ətər), instrument that indicates the speed, usually in revolutions per minute, at which an engine shaft is rotating. Some tachometers, especially those used in automobiles, are similar in construction and operation to automotive speedometersspeedometer,
instrument that indicates speed. A cable from an automotive speedometer is attached to the rear of the transmission of an automobile; the cable turns at a rate proportional to the speed of the car.
..... Click the link for more information. . Other types, often connected directly to the shaft whose speed they indicate, are small electric generators whose output voltage is proportional to speed. This voltage is applied to a voltmeter whose dial is calibrated in speed units. Another type, used only with engines having an ignition system, operates by counting the pulsations of current or voltage in the ignition system, the number of these being proportional to the speed of the shaft.
an instrument for measuring the speed of rotation of the shafts of machines and mechanisms. Centrifugal, mechanical, eddy-current, and electric tachometers are most common; pneumatic and velocity-head, or hydraulic, tachometers are used less frequently.
In a mechanical centrifugal tachometer (Figure 1), a sliding
coupling is mounted on a shaft. The coupling has hinged arms carrying weights that spread apart when the shaft rotates, moving the sliding coupling along the shaft against a counterbalancing spring. The position of the coupling on the shaft depends on the speed of rotation and is transmitted by an arm mechanism to an indicator pointer; the indicator dial is calibrated in revolutions per minute. The tachometer shaft may be driven directly, by the controlled mechanism, or indirectly, by a flexible shaft.
An eddy-current tachometer (Figure 2) uses the interaction of the magnetic fields generated by a permanent magnet and a rotor, whose speed of rotation is proportional to the eddy currents generated. The currents tend to deflect a disk, which is mounted on the shaft and restrained by a spring, through a certain angle. The deflection of the disk, which is rigidly connected to a pointer, is indicated on a dial.
Electric tachometers may be of the generator or impulse type. In tachometer generators the electromotive force of a DC or AC generator is proportional to the angular velocity, from which the shaft speed can be determined; the readings are transmitted to a remote measuring instrument. The operation of impulse tachometers is based on conversion of pulses generated in the primary circuit of an ignition system by the opening of interrupter contacts into a current that is fed to a permanent-magnet indicator. The frequency of pulses in the primary circuit is proportional to the speed of rotation of the engine shaft.
A. A. SABININ