An instrument for measuring the angular
velocity of a rotating shaft. The simplest is a timed revolution
counter. Other mechanical tachometers included the centrifugal
tachometer, similar to the flyball governor;
the vibrating-reed tachometer, a group of reeds of different
lengths which is held against the shaft housing so that the reed whose natural
vibration frequency equals the rotation frequency of the shaft vibrates
by resonance; and the velocity-head
tachometer, in which a pump or fan on the shaft produces a measured
air pressure. Electrical tachometers are usually electric generators or electric impulse counters. The eddy-current tachometer is used as a speedometer.
The centrifugal tachometer (see illustration) measures the instantaneous rate of rotation of haft. It is capable of accuracy to ±%1 of full scale value.
- Fixed bush (cylindrical sleeve).
- Moveable bush.
- Rotating shaft.
- Weights held in central position by bushes. As the shaft rotates, the weights move outward due to centrifugal force. The pointer then moves, indicating speed.
- Rack and pinion. When the moveable bush slides up the shaft, as the weights move outward, the rack and pinion mechanism operates and the pointer moves.
- Pointer and graduated scale.