Engineering in Kenya

Mechanical Tachometers

Posted by on Nov 11, 2011 in Mechanical Engineering | 0 comments

Mechanical Tachometers are the devices used to count revolutions. They have been used for different functions on various machines including water wheels, steam engines and various rotating machinery to maintain safe speeds and for checking repair cycles. Engineering in Kenya has more articles.

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Mechanical Tachometers Mechanical Tachometers

Mechanical Tachometers

Difference Between Speedometers and Mechanical Tachometers

Mechanical Tachometers are usually cable driven and use magnets to assemble and attach the indicator. Their design resembles speedometers but differ in that mechanical tachometers are calibrated to indicate the engine revolutions X 1000.

The tachometer cable is driven from the distributor shaft making the ends to turn the central assemble of the gauge.  The magnet in the centre assembly exerts pressure on the indicator metal causing movement, the faster the cable turns, and the greater the magnetic pull making the reading from the Mechanical Tachometers dial.

Disadvantage of Mechanical Tachometers

The disadvantages with the Mechanical Tachometers include twisting of the cable or breaking due to lack of proper maintenance practice such as poor lubrication. Poor lubrication could cause the internal assembly to bind resulting to irritating noise. The non linear mechanical aspect for mechanical tachometers has reduced their accuracy and may require extra time assembling the parts that will compromise the drive mechanism.

Speed in industry is the term used to define the revolutions per minute of all rotating equipment. It is the scalar unit that is equal to the magnitude of the velocity.  Mechanical Tachometers happens to be one of the various methods to measure the angular speed. It is usually done in revolutions per minute (rpm). Calibrations do differ depending on the application such as feet per minute, miles per hour, and yards per minute or production units per time.

Types of Mechanical Tachometers

There are four main types of Mechanical Tachometers namely, revolution counters, centrifugal force tachometers and resonance tachometers.

Revolution counter is used to measure an average of rational speed instead of instantaneous rotational speed. It consists of a worm gear that is usually attached to a spindle. It has two dials, an inner one and an outer one. The inner dials represent one revolution of the outer dials and the outer dials represent on revolution of the spindle. The tachometer has a stopwatch attached to the revolution counter and is used to indicated time.

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Resonance tachometer operates by a vibrating reed type speed. It has an indicator that detects the repeated machine vibrations frequency. It consists of tuned steel reeds of varying lengths. One side of the reed is attached to moving parts of the machine and the other side to the reeds gets attached to the bottom of the calibrated scale giving reading directly per rpm.

As the speed of the machine increases, vibrations at the base machine assembly also increase and the base of the tachometer starts vibrating. The change in the vibrations corresponds to that of the speed of the engine or a machine. The range of this type of tachometer ranges from 600 to 100,000rpm. Its accuracy is said to be 0.5%.

This type of tachometers is notorious in reading accuracy. The older they get, the lower the accuracy they get. To calibrate them, one should set the timer to tachometer and set the rpm to its configuration. Then the time taken is divided by the actual elapsed speed. Then the result is multiplied by 2310, to give the actual rpm of the Mechanical Tachometers.

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