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2.8.2 SUBMULTIPLE SPEED MEASUREMENTS.
If the Type 1531 Strobotac is set to flash at an integral submultiple
of the fundamental speed of a rotating object, a single image will be ob -
served, just as it will at the fundamental speed. At flashing rates between
these submultiple, multiple images will be observed. Table 2-2, for
example, shows the number of images that will be obtained at various
stroboscope flashing rates (below the fundamental speed) of a device
rotating at exactly 1800 rpm.
Note the numerical relationship between the numerator of the sub-
multiple fraction and the corresponding number of images seen. This
relationship will always hold true, regardless of the speeds involved.
Table 2-2 lists only a few of the more useful submultiple speeds and
corresponding images; many other multiple images are possible (for
example, five images will be seen at 5/7, 5/8, etc.).
Submultiple flashing is necessaryo to observe or measure the speed
of objects moving at rates above 25,000 rpm. Refer to paragraph 2.8.3
for the method of determining the fundamental speed when submultiple
operation is necessary.
SUBMULTIPLE SPEED/lMAGE RELATIONSHIP
Sumbmultiples of
Fundamental
Number 0f
RPM Dial
Speed (1800 rpm)
images Seen*
Setting
1
1
1800
5
5/6
1500
4
4/5
1440
3
3/4
1350
2
2/3
1200
3
1080
3/5
1
900
1/2
2
720
2/5
1
600
1/3
1
l/4
450
1
360
1/5
1
300
l/6
*At dial settings above fundamental speed, multiple images
always occur. Refer to Table 4-1.
2.8.3 MEASUREMENT OF SPEEDS ABOVE 25.000 RPM.
Speeds up to about 250,000 rpm can be accurately determined by
calculations based on submultiple measurements. The procedure is as
follows :
9

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