A tachometer is an instrument designed to measure the speed of rotation of an axis or disk. Tachometers generally measure rotations per minute (RPM), although some models also serve as speed and / or totalizers. Measuring the rotational speed of a rotating object is important for several reasons. The parts are often classified at a maximum rotational speed beyond which they may be subject to failure or breakdown of the system. Other parts can operate more efficiently at a particular speed.
Tachometers are available in a number of styles. While many are handheld models intended for spot checking conditions, others are intended to be permanently mounted, constantly monitoring the speed of rotation and capable of triggering an alarm if speeds are too high or too low.
Tachometers can essentially be divided into two groups:
1. Contact tachometers: Contact tachometers operate by placing a wheel freely rotating in direct contact with a rotating shaft or disc. The shaft or disk drives the wheel creating pulses that are read by the tachometer and converted to RPM. Linear velocity and distance can also be calculated.
2. Non-contact tachometers: Non-contact tachometers use a light source, usually a laser or infrared light, to measure the speed of rotation. This does not require any physical contact between the tachometer and the target. The tachometer is directed to the spinning target to which a piece reflective tape has been attached. When the light source reaches the target, it is reflected from the tape and back to the tachometer. The tachometer measures the speed at which the light pulses are reflected back in the tachometer, which is proportional to the speed of rotation of the shaft or disk.
Like many instruments, tachometers are becoming more and more digital. Although it is still possible to find analog tachometers, especially in cars, digital models are much more frequent. Digital tachometers offer a number of advantages over their analog cousins, mainly in the advanced functions available with each type. Analog tachometers are not available for functions such as memory, or statistical functions such as average or change of units of measure. These functions are common in most digital instruments.
Stroboscopes
Stroboscopes, commonly called stroboscopes, use a flash lamp - usually xenon or LED - driven by an oscillator to inspect or measure the rotational speed of rotating objects. The oscillator activates the lamp in a stable state that can be set at a flash rate every few seconds to hundreds of flashes per second. The flashing light creates the perception of stop motion, which is ideal for diagnostic inspections of moving machinery and can also be used as a measuring instrument to determine the cyclic speed measured in rotations per minute (RPM).
The strobing strobe light creates an optical illusion known as aliasing. As the strobe flashes it provides intermittent glimpses of the moving target. When the stroboscope flash rate matches the target rotation speed, each flash illuminates the target at the same position in its rotation cycle creating the perception that the object is stationary. Once this point is reached, the flash rate can be increased or decreased to make it look like the object is rotating forward or backward, whose speed depends on how much the flash rate of the stroboscope varies from the target rotation speed.
The ability to "visually stop and start" a rotating piece of machinery or adjust its turning speed and direction is an important tool for inspection and troubleshooting. Technicians can identify damage to gears, shafts, pulleys and belts without having to stop machinery. In addition, any oscillation or alignment problems can be noticed quickly before the problem becomes severe.
Although the ability of a stroboscope to visually stop the movement of a piece of machinery for inspection is the main use of the instrument, it is also very useful as a tool capable of measuring the speed of rotation. Measuring the speed of rotation involves matching the strobe flash rate with the speed of rotation of the equipment being tested. Some kind of reference mark, such as reflective tape, makes this easier. Once the rates match, the stroboscope can convert the number of flashes per second into RPM.