7
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 images at 3 times, etc. The FPM equals the shafts
Revolutions Per Minute (RPM) at the highest flash rate that gives only one
stationary image of the keyway.
Stopped Ima
ge
1/4 times
1/2 times
1 time
2 times
3 times
4 times
Flash Rate (FPM)
750
1500
3000
6000
9000
12000
Actual RPM
Example: Object rating @ 3000 RPM
If the speed is outside the full scale range of the stroboscope (500,000
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.
Be aware that you will encounter multiple images. Note the flash rate of
the first SINGLE image you encounter and call this speed “A”. Continue
decreasing the flash rate until you encounter a second SINGLE image and
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)
In instances when you can shut down the device and install a piece of
reflective tape, then an optical tachometer is easier to use for RPM
measurement.
Stroboscopes must be used when you can’t shut down
the device.
The human eye is not easily tricked into seeing a stopped
image by a stroboscope when the flash rate is slower than 300 FPM.
Therefore, stroboscopes are just about impossible to use below 300 FPM
for inspection or to measure RPM.
