Question

1. What is a phototachometer and why would you use it to measure the rotational speed of a given shaft? (3 Marks)
2. What is a stroboscope and what are the benefits of using a stroboscope to validate the findings? (3 Marks)

Why would you record the measurements at different shaft speeds? (2 Marks)

Answer 1

(1)A PHOTOTACHOMETER is used to measure rotational speed.The device contains a light source – typically a laser, which can create a focused, coherent beam of light. This beam of light is capable of bouncing off of reflective objects.

To use the photo tachometer, you start by creating a reflective mark on the rotating shaft you wish to measure. You must also ensure the rest of the rotating surface is comparatively non-reflective. An easy way to do this would be to create a white spot with a paint marker on an otherwise black or dull-metal shaft. Then, aim the beam of light from the phototachometer at the mark on the spinning shaft. Each time the reflective spot passes the beam of the photo-tachometer, some light is reflected back towards the device, where it is picked up by a light sensor. By counting the number of times the light sensor is triggered in a given time, it’s possible to determine the rotational speed of the shaft under test.

Advatages of phototachometer

*it is a non-contact type tachometer.

*easy and accurate

*no need of much modifications on the shaft.

(2)STROBOSCOPE, also known as strobe, is an instrument used to make moving object appear to be stationary or slow moving.It is used for the study of rotating,reciprocating,oscillating or vibrating objects.Stroboscopic instrument is employed in the observation of periodicmotions. Such instruments are used forexample, to measure oscillation frequencies of mechanical and electronic systems, to measure resonance frequencies, to study the vibrations of various bodies, and to visually monitor rapidly moving parts of machineries.Principle of operation of stroboscopic instruments is as follows: the object performingperiodic motion is

illuminatedand made visible in separate time intervals that are very small bycomparison with the period of the object’s motion. If the

frequency of the light pulses is the same as the frequency of period of the object’s motion, then the object appears stationary.

When these two frequencies are somewhat different, the object appears to be executing a motionthat is slower than the actual motion.

Thefrequency F of the slowed motion is the differencebetween the

two frequencies

that is, F = f

Advantages of stroboscope:

*low wear LED lightning trigger
*illumination level is 300 x 500 mm
*high intensity of light
*adjustable light frequency
*rotary device parallel operation

Measurements are made at different shaft speeds to:

*increase accuracy of values

*better performance of instruments

*lower occurance of errors

Answer 2

i. Phototachometer

• Definition: A phototachometer is a non-contact device that measures rotational speed (RPM) using light (laser or LED) reflected off a rotating shaft.

• Why Use It?

• Non-invasive: No physical contact avoids load interference.

• High Accuracy: Suitable for precision applications (e.g., motors, turbines).

• Safety: Ideal for hazardous or high-speed environments.

Key Formula:

$$\text{RPM}=\frac{\text{Number of reflections per minute}}{\text{Number of markings on shaft}}$$

(No calculation needed unless specific reflection data is provided.)

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ii. Stroboscope

• Definition: A device emitting flashing light at adjustable frequencies to "freeze" motion visually.

• Benefits for Validation:

• Cross-Verification: Confirms phototachometer readings by synchronizing flashes with shaft rotation.

• Visual Inspection: Detects irregularities (e.g., wobbling, misalignment).

Key Formula:

$$\text{Shaft RPM}=\text{Stroboscope flash frequency (Hz)}\times 60$$

(Example: 50 Hz flashing = 3,000 RPM.)

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Final Answers:

1. Phototachometer: $\boxed{{\displaystyle \text{Non-contact RPM measurement using light reflections}}}$.

2. Stroboscope: $\boxed{{\displaystyle \text{Visual validation via synchronized flashing}}}$.

3. Variable speeds: $\boxed{{\displaystyle \text{To assess performance and safety across operational ranges}}}$