TACHOMETER, an instrument for measuring the speed of rotation of shafts and machine elements. Most tachometers have a rotating part which must be connected with the element whose speed is to be measured. With the hand-type this driving relation is maintained only long enough to take a reading, while with the types that are mounted on a machine the instrument is perma nently connected to the rotating part and indicates the speed as long as that part is in motion. The hand-operated type usually has a stem provided with a vulcanized rubber tip of conical form which can be pressed into the centre-hole of the rotating shaft. Permanently mounted tachometers are driven by belt or flexible shaft, or through a mechanical coupling.
There are quite a number of different principles upon which the design of a tachometer may be based. All of the principles used in speedometers (q.v.) lend themselves equally well to the design of tachometers. In fact, a speedometer is a tachometer placed in driving connection with a road wheel and graduated in miles per hour instead of in revolutions per minute. Thus, there are centrifugal, magnetic drag, clockwork and magneto type tachometers. In addition there is the principle of vibrat ing reeds which seems to have been used for tachometers only. It is based on the fact that in most machines having large re volving parts, such as steam turbines and electric generators, vibration of the frame is synchronous with rotation of the re volving part.
The Frahm vibrating reed tachometer consists of a series of calibrated steel reeds of different lengths, having one of their ends mounted in a brass block and a short length at their other end turned at right angles and enamelled white. These enamelled ends are located side by side in a rectangular opening in the case. With the instrument mounted on the machine (or merely held against it by hand), and the machine in operation, one or more of the reeds will respond visibly to the vibration of the machine. The speed of rotation of the shaft can be gauged to a much closer limit than the difference between the speeds indicated by adjacent vibrating reeds, usually 5o revolutions per minute. Reed tachometers are made for speeds of from Soo to 12,000 revolu tions per minute. Preferably the range of any particular instru ment should not exceed an octave, as if it does, and the actual speed corresponds to a reed near the lower end of the scale, both the reed located there and a reed corresponding to twice this speed will vibrate.