SKF TKRS 41, Instructions For Use Manual

Page: 22 / 164

20 EN
Which is correct?
To determine the fan’s actual rotational speed, one of the ventilator vanes is attached with a marking and
the test is repeated.
Image No. 1 2 3 4
Flashing rate 3 300 2 200 1 650 1 320
Image No. 5 6 7 8
Flashing rate 1 100 825 733,3 550
The orientation marking c
onirms that the images at 3 300, 1 650 and 825 rpm are harmonic multiple images.
Three identiication marks appear in each of these images.
Still images appear at 1 100 rpm and again 550 rpm, each displaying just one mark. Remember that a still image
always appears exactly at integer division of the speed of the actual rotational speed of an object. 550 is half of 1 100.
This means that the rotational speed of the ventilator must be 1 100 rpm.
Example 2 (no marking required):
This example shows how the actual rotational speed of an object can be determined without using an orientation
marking. This is only possible for suitably shaped objects.
Let’s assume the only thing we know about this cam is that it rotates at less than 7,000 rpm.
Its clear shape eliminates the need for an orientation marking.
The following “frozen” images will appear if the lashing rate of 7,000 is reduced:
Image No. 1 2 3 4
Flashing rate 6 000 4 000 3 000 1 500
The images showing 6 000 and 4 000 rpm are double or multiple images rather than single images.
Still images appear at 3 000 and again at 1 500 rpm. 1 500 is half of 3 000.
This means that the actual rotational speed is 3 000 rpm.