Tachometers, or speed indicators, indicate the speed directly and thus include the time element. The principal types are centrifugal, liquid, reed, and electrical. In the centrifugal type, a revolving weight on the end of a lever moves under the action of centrifugal force in proportion to the speed, as in a flyball governor.

This movement is indicated by a pointer which moves over a graduated scale. In the portable or hand type, the tachometer shaft is held in contact with the end of the shaft being measured, and in the stationary type, the instrument is either geared or belted.

In the liquid tachometer of the Veeder type, a small centrifugal pump is driven by a belt consisting of a light cord or string. This pump discharges a colored liquid into a vertical tube, the height of the column being a measure of the speed.

Reed tachometers are similar to reed-type frequency indicators, the reeds being set in resonant vibration corresponding to the speed of the machine. The instrument may be set on the bed frame of the machine, where any slight vibration due to the unbalancing of the reciprocating or evolving member will set the corresponding reed in vibration.

Some forms are belted to the revolving shaft and the vibrations imparted by a mechanical device. Electrical tachometers may be either reed instruments operated electrically from small alternators geared or belted to the machine being measured or ordinary voltmeters connected to small permanent magnet dc generators driven by the machine being tested.

Chronographs are speed-recording instruments in which a graphic record of speed is made. In the usual forms, the record paper is placed on the surface of a drum which is driven at a certain definite and exact speed by clockwork or weights, combined with a speed-control device so that 1 in on the paper represents a definite time.

The pens which make the record are attached to the armature of electromagnets. With the pens in contact with the paper and making a straight line, an impulse of current causes the pen to make a slight lateral motion and, therefore, a sharp indication in the record.

This impulse can be sent automatically by a suitable contact mechanism on the shaft of the machine or by a key operated by hand. The time per revolution is then determined directly from the distance between marks.

Stroboscopic methods are especially suitable for measuring the speed of small-power rotating machines where even the small power required to drive an ordinary speed counter or tachometer would change the speed, also for determining the speed of machine parts which are not readily accessible or where it is not practicable to use mechanical methods or where the speed is variable.

One convenient form of stroboscopic tachometer employs a neon lamp connected to an oscillating circuit supplied from a 60-Hz circuit, which is adjusted to “flash” the neon lamp at the frequency necessary to make the moving part that the lamp illuminates appear to stand still. Speeds from a few hundred to many thousands of revolutions per minute can be very conveniently measured.

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