SPEED MEASUREMENT.

where T = time (in seconds) for one cycle of CAL indicator lamp

Example - if the CAL lamp period equals 2 seconds, the line frequency

is 60 c/s, and the RPM dial is set at 900; the SELF TEST error is:

900

-- . 7.5 rpm

60X2

2-8 SPEED MEASUREMENT.

2.8.1 FUNDAMENTAL SPEED.

When measuring the rotational speed of an object, it is recommended

that the RPM dial be set initially at a higher flashing rate than the speed

of the object. Then, slowly reduce the flashing rate until the first single

image is observed. At this point, the flashing rate of the stroboscope

will be equal to the rotational speed of the object, and the speed can be

read directly from the RPM dial.

when using the middle - or low-speed ranges, simply switch to the

next higher range without moving the RPM dial setting to check whether

the stroboscope is flashing at the fundamental speed of the object. Since

the ratio between ranges is exactly 6:1, six images will appear at the

next higher range, if the stroboscope has been set to the fundamental

speed. If only three images appear, for example, then the stroboscope

has been set to one-half the correct frequency. On the high-speed range,

it is necessary to double the speed setting to check for fundamental-speed

operation. A double image should occur when the frequency setting is

doubled. If the fundamental speed of the device being measured is above

12,500 rpm, it is not possible to check for correct speed setting by the

method outlined above. In this case, refer to paragraph 2.8.3.

(3) With practice, and especially when the approximate speed of the

object can be estimated, an operator can accurately measure the speed of

rotating objects quickly, and with confidence. It is necessary, however,

to thoroughly understand the following basic principles involved in making

speed measurements with a stroboscope:

The operator must distinguish between single and multiple images.

Odd shaped objects usually cause little difficult y, but objects which are

Symmetrical in shape (gear, disc, fan, etc) must be marked to provide

a visiblee reference (refer to PRINCIPLES OF OPERATION, paragraph

Multiple images will always be observed when the stroboscope

flashing rate is set to a multiple of the fundamental speed of the object.

When reducing the flashing rate from a rate higher than the funda -

mental speed of the object, the first single image will be seen when the

flashing rate is equal to the fundamental speed.

When the flashing rate is below the fundamental speed of the object,

single and multiple images will be observed. The single images will

always occur at integral submultipIes of the fundamental speed of the