Information About Tachometer

What Is a Tachometer?

A tachometer (revolution-counter, Tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolution per minate (RPM) on a calibrated analogue dial, but digital displays are increasingly common. The word comes from Greek Ταχος, tachos, "speed", and metron, "to measure".

How Does a Tachometer Work

  A magnet that magnetizes "teeth" of an iron gear located on the motor. In general... The magnet magnitizes the "teeth" as the approach then de-magnetizes as they move away.. which creates an etectric field.. The wire attached to the magnet connects to the tachometer which displays the rpm, hope this helps someone.. -- Another User: There are multiple types out there. The modern, most exact, is to have a laser or infra red light with a receiver on the opposite side. When the wheel ( or shaft rotates) it has a mirror or a tab that obstructs the path of the light every time it revovles ... then there is simply a chip to count the number of obstructions per minute. This one is extremely accurate and can handle some of the highest speeds. But there are a host of older versions ... many still in use. One is quite the same as above except there is a magnet on the shaft and a sensor ( Google 'reed switch' for example ) nearby that detects the magnet passing. Another way is a chip attached to a switch that is physically pushed or contacted by a cog on the shaft. Still another is to have a generator attached to the shaft and the Tach is actually a voltage meter telling the reading off that generator. Finalley ... saved this one for last ... chances are you looked up this question because you wanted to know how the oldest Tachs worked back when electrical gear wasn't commonly used in cars and such. How did they pull it off back then? Well here goes: The end of the shaft had 2+ arms attached to the side that are weighted and would swing outward further and further, the faster the shaft rotated. The weights had a collar attached below them that would rise upward, the faster the shaft ran. Still with me? Check out this graphic if you need http:/www.railway-technical.comgovernor.gif the wieghts are the round black objects and the rising collar is green. So now all that is left is to have a hook ( with wheel to reduce friction ) that is hooked in the path of the collar so that it will get pulled up with the collar. The hook is attached to a cable ... on the other end of that cable is ( drum roll ) the sneedle of the Tach ... tugging it against a spring, over more and more as the shaft spins faster and faster. No electricity and no chips involved! Obviously this is somthing of a percision device and only as accurate as its most recent callibration ... but it worked enough. There were even hand held versions, very precise instruments that machinist would press against the axels of gears and shafts to get RPM readings. Now they have little hand held laser devices that are pocket size and much more accurate.

Learn more about Tachometer

    • Tachometers, in their most basic forms, are devices that measure the speed of an object. Most commonly, they measure the rotation of a mechanism, like the engine shaft in a car. Traditionally, tachometers are dials with a needle pointing to the current speed in RPMs (revolutions per minute). However, with the onset of new reading systems, the use of digital tachometers has risen sharply.

    Dial

    • The dial tells the driver the tachometer's reading. In a car, it is located on the dashboard. The instrument itself measures the RPMs of the engine drive shaft. The device is necessary in order to regulate how hard the engine is being worked. The way in which the measurements themselves are taken, though, can vary.

    Generator

    • Engines with ignition systems usually utilize a small generator attached to the engine drive shaft. In this case, the tachometer is actually a voltage meter, meaning that it counts the pulsations of voltage in the ignition system. The output voltage is proportional to the shaft's speed so measuring voltage is converted into an accurate measurement in RPMs.

    • The voltage is generated via a permanent magnet on the shaft. There is a toothed wheel made of iron, which becomes magnetized as the magnet passes the teeth. Then, as the magnet rotates away from the teeth, the wheel becomes de-magnetized. As these changes occur, an electric field forms around the permanent magnet. This field affects the electric charges in a wire coil that surrounds the magnet, generating electricity. As the tooth approaches the magnet, the current flows one way in the coil. As the tooth moves away from the magnet, the coil's current switches direction. The tachometer reads the frequency with which the coil's current changes direction. Additionally, if the engine turns more quickly, the change in the magnetic field becomes more radical, generating higher voltage. The tachometer also uses this information to inform its reading.

    Sparks

    • A simpler--yet less common--method is to measure the rate at which sparks are released into the engine's cylinders. This is obviously only useful in a gasoline engine, which uses spark plugs to provide the explosive heat energy that moves the vehicle.

    Laser

    • A newer version that is quickly gaining popularity due to convenience and accuracy is the laser tachometer. This type requires no physical contact between the tachometer and the engine shaft. Basically, it beams infrared light at the shaft. One place on the rotating shaft is reflective. The tachometer measures the rate at which the light is reflected back onto the tachometer.

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20th Jan 2015

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