Tachometers and stroboscopes belong to a class of instruments that measure the rotational speed of a shaft or disc such as those found in motors, conveyors, and other machinery. Stroboscopes have the added ability to serve as an inspection device for rotational machinery while in motion. Knowing the rotational speed of a device, or being able to inspect the device while in motion, is important to ensure that machinery is running within established parameters.
Tachometers
A tachometer is an instrument designed to measure the rotation speed of a shaft or disk. Tachometers generally measure rotations per minute (RPM) though some models also serve as rate meters and/or totalizers. Measuring the rotational speed of a spinning object is important for a number of reasons. Parts are often rated to a maximum rotational speed beyond which they may be subject to failure or system breakdown. Other parts may operate more efficiently at a particular speed.
Tachometers are available in a number of styles. While many are handheld models meant for spot-checking conditions, others are meant to be permanently mounted, constantly monitoring rotational speed and able to trigger an alarm if speeds are too high or too low.
Tachometers can, essentially, be divided into two groups:
- Contact tachometers: Contact tachometers work by placing a freely spinning wheel in direct contact with a rotating shaft or disc. The shaft or disc propels the wheel creating pulses that are read by the tachometer and converted into RPM. Linear speed and distance can also be calculated.
- Non-Contact tachometers: Non-contact tachometers use a light source, generally a laser or infrared light, to measure rotational speed. This requires no physical contact between the tachometer and the target. The tachometer is aimed at the spinning target to which a piece reflective tape has been affixed. As the light source hits the target it is reflected off the tape and back to the tachometer. The tachometer measures the rate at which light pulses are reflected back to the tachometer which is proportional to the rotating speed of the shaft or disc.
Like many instruments, tachometers are becoming increasingly digital. Though it’s still possible to find analog tachometers, especially in cars, digital models are much more prevalent. Digital tachometers offer a number of advantages over their analog cousins, primarily in the advanced functions available with each type. Analog tachometers have no provision for features such as memory, or statistical functions such as averaging or changing measurement units. These functions are common on most digital instruments.
Stroboscopes
Stroboscopes, commonly referred to as strobes, use a flash lamp—generally xenon or LED— driven by an oscillator to inspect or measure the rotational speed of spinning objects. The oscillator activates the lamp at a steady state which can be set at rates from a flash 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 for determining cyclic speed as measured in rotations per minute (RPM).
The flashing light of the strobe creates an optical illusion known as aliasing. As the strobe flashes it provides intermittent glimpses of the moving target. When the flash rate of the stroboscope matches the rotational rate of the target, each flash illuminates the target at the same position in its rotational cycle creating the perception that the object is stationary. Once this point is reached the flash rate can increased or decreased to make it appear as if the object is rotating forward or backwards—the speed of which is dependent upon how much the flash rate of the stroboscope varies from the rotational rate of the target.
The ability to visually “stop and start” a spinning piece of machinery or adjust its rotational speed and direction is an important inspection and troubleshooting tool. Technicians can identify damage to gears, shafts, pulleys, and belts without having to stop the machinery. Additionally, any wobbles or alignment problems can be quickly noticed before the problem becomes severe.
Though the ability of a stroboscope to visually stop the motion of a piece of machinery for inspection is the primary use of the instrument, it is also quite useful as a tool capable of measuring rotational speed. Measuring rotational speed involves matching the flash rate of the strobe to the rotational rate of the equipment being tested. Some sort of reference mark, such as reflective tape, makes this easier. Once the rates match, the stroboscope can convert the number of flashes per second to RPM.
Applications for Tachometers and Stroboscopes:
Tachometers and stroboscopes are valuable tools for measuring the spinning speed (and inspecting in the case of stroboscopes) of rotating equipment. This is important for a wide range of equipment including:
- Centrifuges
- Saw blades
- Grinders
- Elevators/escalators
- Engines
- Motors
- Conveyor belts
- Fans
- Propellers
Things to Consider When Selecting a Stroboscope or Tachometer:
- Does the application involve rotational analysis or only rotational speed measurement?
- Is spot testing or permanent placement needed?
- What is the maximum RPM the instrument will measure?
- What level of accuracy is required?
- Is contact or non-contact measurement preferred?
- What price range is most attractive?
If you have any questions regarding tachometers or stroboscopes please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
