Determining The Actual Rotational Speed Of An Object; Useful Information - SKF TKRS 41 Mode D'emploi

7. Determining the actual rotational
speed of an object
The stroboscope can be used as a digital revolution
indicator to determine an object's actual rotational
speed and/or the frequency of cyclical movements.
The stroboscope does this by visually "freezing" the
object's movement and then taking a reading of the
rotational speed or frequency from the LCD display.
As is the case with all stroboscopes, it is vital to
ensure that this "frozen" image is not a harmonic of
the object's actual rotational speed.
Example 1 (marking required):
This example shows the importance of using identiication markings. Say you want to determine the actual
rotational speed of this ventilator.
The only thing you know is that its rotational speed is less than 3,500 rpm. The following "frozen" images will appear if
you reduce the lashing rate based on 3,500 FPM (lashes per minute):
Image No.
Flashing rate
Image No.
Flashing rate
What is the actual rotational speed of the fan?
Images 1, 3, 5, 6 and 8 correspond to the original one, which means the rotational speed could be 3 300, 1 650,
1 100, 825 or 550 rpm.
1 2
3 300 2 200
5 6
1 100 825

Useful information:

• It's helpful to have a rough idea of the object's
rotational speed beforehand.
• Regular shaped objects, e.g. a fan with several
vanes or a motor shaft, must be afixed with
an identiication marking (using colour or
a relective strip etc.) in order to be able to
differentiate its orientation of movement.
• A still image always appears exactly at integer
division of the speed of the object's actual
rotational speed!
3
1 650
7
733,3
4
1 320
8
550
EN 19