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Tachometers are instruments used to measure rotational speed, typically gauged in rotations per minute, or angular speed of a rotating shaft. They are employed in a wide number of applications, from measuring conveyor drive speeds to gauging the speed of rotating parts on manufacturing machines, such as rolling mills or injection molders. Other uses include the measurement of automobile engine crankshaft rotation, rail vehicle speed, and tape speed for playback audio functions.

Displays for automobile tachometers are regularly found with other instrumentation on the vehicle’s dashboard, generally in an analog format, although digital displays are also available. Two types of tachometers are produced by manufacturers: contact and noncontact tachometers. Contact tachometers use a part in direct contact with the rotating component to measure its speed; these types of devices are well suited to applications encountering large amounts of invasive particles, as they are less subject to such contamination.

A variety of sensor devices are used for noncontact tachometers, including infrared, optical and proximity sensors. These sensors use different light beams and measurement devices to calculate rotations per minute. An infrared sensor, for example, uses infrared light to tally the number of gear teeth passing through a selected area, thereby measuring the speed of the gear. Optical sensors use a rotating reflector or similar device to determine rotations. Audio tachometers are useful in order to regulate the speed of tapes during playback, which is necessary due to the changing size of the reel.

Tachometers are typically synced with various control units to regulate machine speed. This is particularly useful in machining operations where either continuous or alternating shaft speed is important. In plastic injection molding, for example, the screw will encounter varying resistances, but must be kept at a constant speed; the tachometer relays whether more or less force is need from the engine to achieve this speed.