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5.0 USING THE STROBOSCOPE TO MEASURE RPM
The primary use for a stroboscope is to stop motion for diagnostic inspection
purposes. However the stroboscope can be used to measure speed (in
RPM/RPS). In order to do this several factors need to be considered. First,
the object being measured should be visible for all 360° of rotation (e.g.
The end of a shaft). Second, the object should have some unique part on
it, like a bolt, keyway or imperfection to use as a reference point. If the
object being viewed is perfectly symmetrical, then the user needs to mark
the object with a piece of tape or paint in a single location to be used as a
reference point.
Look only at the reference point.
If the speed of rotation is within the range of the stroboscope, start at the
highest flash rate and adjust the flash rate down. At some point you will
stop the motion with only a single reference point of the object in view.
Note that at a flash rate twice the actual speed of the image, you will see
two images (reference points). As you approach the correct speed, you
may see three, four or more images at harmonics of the actual speed. The
first SINGLE image you see is the true speed. To confirm the true speed,
note the reading and adjust the stroboscope to exactly half this reading, or
just press the ÷2 button. You should again see a single image (which may
be phase shifted with respect to the first image seen).
For example, when viewing a shaft with a single keyway you will see one
stationary image of the keyway at the actual speed and at 1/2, 1/3, 1/4,
etc, of the actual speed. You will see 2 images of the keyway at 2 times
the actual speed, 3 keyway at 3 times, etc.
The Flashes Per Minute (FPM)
equals the shaft’s Revolutions Per Minute (RPM) at the highest flash rate
that gives only one stationary image of the keyway.
