User Manual

10. Application information
17
Using the orientation mark, it is now clear that the images
appearing at 3,300, 1,650 and 825 RPM are multiple-
image harmonics. In each of these cases, three identifica-
tion marks appear. On the other hand, a singular image
appears at 1,100 and again at 550.
Here, only one mark appears. Recall that “a singular image
always appears at exactly one half of the object’s true
RPM”. 550 is one half of 1,100. Therefore, the rate of the
fan must be 1,100 RPM.
Example 2 (no mark needed):
This example illustrates how the actual speed of an object
can be determined without the use of an orientation mark -
provided that the object has a suitable shape.
Assume that the speed of this cam is known only to be
less than 7,000 RPM. Because it has a unique shape, it
does not need an identifying mark. As the flash rate is
lowered from 7,000, the following harmonic images appear:
Image No.: 1 2 3 4
Flash Rate: 6,000 4,000 3,000 1,500
The harmonic images at 6,000 and 4,000 RPM are not sin-
gular, but double and quadruple. A singular image does
appear at 3,000 and again at 1,500 RPM.
1,500 is one half of 3,000. Therefore, the rate is
3,000 RPM.