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For example, when viewing a shaft with a single key way 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.
Example: Object Rotating at 5000 RPM
If the speed is outside the full scale range of the stroboscope (20,000
FPM), it can be measured using the method of harmonics and multipoint
calculation. Start at the highest flash rate and adjust the flash rate down.
You will encounter multiple images so be aware of these. Note the flash
rate of the first SINGLE image you encounter, call this speed “A”. Continue
decreasing the flash rate until you encounter a second SINGLE image. Note
this speed as “B”. Continue decreasing the speed until you reach a third
SINGLE image at speed “C”.
For a two-point calculation the actual speed is given by:
RPM = AB/(A-B)
For a three-point calculation:
RPM = 2XY(X+Y)/(X-Y)
2
where
X = (A-B) and
Y = (B-C)
Stopped Image
1/4 times
1/2 times
1 time
2 times
3 times
4 times
Flash Rate (FPM)
1250
2500
5000
10000
15000
20000
