The term “tachometer” came from two Greek words, namely: tachos which means speed; and metron which means “measure.” Hence, etymologically, the word “tachometer” means “measure of speed” or “speed measurement.” Well, the etymological meaning of tachometer tells something about what it does, for a tachometer basically measures the rotational speed of a disk or shaft. It shows this measurement in revolutions per minute (RPM) on an analogue dial, although digital tachometers are increasingly becoming popular and common. The tachometer is also referred to as revolution-counter, rev-counter, RPM gauge, and tach.
The tachometer is specifically a device that indicates the angular speed or rotary speed of any rotating shaft. However, the term sometimes only refers to the electrical and mechanical instruments that show the instantaneous values of speed in RPM but do not indicate the average values of speed for a given interval.
Since the tachometers used on aircraft, cars, and other vehicles only indicate the rate of the engine’s crankshaft’s rotation, but they can surely show the safe range of rotational speeds. With the data a tachometer provides, a driver can readily select the specific gear and throttle settings for any given driving experience and condition. Thus, the tachometer can prompt a driver to slow down when he/she is continuously moving at high speed, for prolonged rate use of crankshaft at high speed can surely cause problems like overheating, inadequate lubrication, damage to sub-parts of the engine. The onset of these damages can eventually lead to the partial or total breakdown of the engine.
Drivers whose cars are with manual transmissions, for example, would surely benefit from the data provided by the tachometer. The analogue tachometer, for example, would prompt a red signal on the gauge area indicating that the driver is operating at unsafe speed. On the other hand, in cars with automatic transmissions, the presence of the red zone is already superfluous because automatic vehicles are already equipped with a revolution limiter that electronically prompts or limits the engine speed when the car is speeding beyond its engine limitation. Thus, you would not find this red line in cars that are equipped with this revolution limiter feature. Moreover, you would also not find this redline in machines with diesel engines that are equipped with mechanical injector systems that have an integral governor, for this integral governor is designed to prevent the engine from over-speeding.
Purposes of Using Tachometers on Small Engines, Tractors, Trucks & Other Vehicles
Trucks and tractors are usually equipped with tachometers that have other markings such as that of a green arc that indicates the range of speed wherein the engine can produce maximum torque. Operators of the abovementioned vehicles would surely benefit from this green arc feature of the tachometer. Some tractors that are equipped with a power take-off system are also fitted with tachometers that indicate the necessary engine speed for rotating the PTO. It basically indicates the standardized speed required for most PTO-driven equipment.
Regulations vary from country to country with regards to the requirements for a tachometer. Many countries, for example, require tractors to be fitted with a speedometer. Since it would be superfluous to have another dial that indicates speed, the tachometers are usually fitted with a second scale that indicates the unit of speed, and this second scale is fairly sufficient for practical use on the road. On the other hand, tractors that have multiple gears are usually equipped with multiple speed scales. For aircraft tachometers, however, a green arc is generally fitted into the tachometers that indicates the standard cruising speed range of the aircraft’s engine.
Different Types of Tachometers That You Can Use On Small Engine
It has been mentioned above that there are analog tachometers and digital tachometers. However, there are also contact and non-contact tachometers and time and frequency-measuring tachometers. Here is an in-depth description of these different types of tachometers:
1) Analog Tachometers
These tachometers feature a dial-type interface and a needle. They, however, does not come with storage for readings and are incapable of computing the average and deviation in RPM. The analog tachometers make use of a mechanism that converts speed into voltage. The voltage is eventually displayed on the tachometer.
2) Digital Tachometers
The digital tachometers feature either a LED readout or an LCD readout with concomitant memory for storage. These tachometers can do statistical operations and can provide precise monitoring and measurement of whatever type of time-based quantities. There is increasing use of digital tachometers nowadays, and they are becoming fairly common for they are easy to read with their numerical readings.
3) Contact and Non-contact Tachometers
The contact type of tachometers features a direct contact with the rotating shaft. They are equipped with a magnetic sensor or an optical encoder. On the other hand, the non-contact type is suitable for mobile applications. It also makes use of an optical or laser disk. These two types of tachometers are basically ideal for use if you want to have precise measurements and they are useful for data acquisition purposes.
4) Time and Frequency Measuring Tachometers
These tachometers relied heavily on measurement methods. The time-measuring tachometers, for example, readily calculate the speed by merely measuring the temporal interval or lapse between two incoming impulses. These tachometers are ideal for use in measuring low speed. On the other hand, the frequency-measuring tachometers calculate the speed by merely measuring the incoming pulses’ frequency. Lastly, the frequency-measuring device is suitable for measuring high-speed rate.