3-29
Stroboscope Methods
When using a stroboscope to measure the speed of rotating or reciprocating mechanisms, hold the
instrument so that the light from the stroboscope lamp falls directly on the part to be observed. If the part
is uniform, or symmetrical, place an identification mark with chalk or a grease pencil on the portion to be
observed. This method provides a positive means of identification, because if only one reference mark is
observed during measurement, you can be sure that either the fundamental synchronization or a
submultiple thereof has been obtained. If the approximate speed of rotation is known, the stroboscope
controls may be set to the appropriate positions prior to actual measurement. The main frequency control
that determines the rate of the flashing light is then varied until the reference mark on the moving part
appears to be standing still. The calibrated scale of the stroboscope will then show the speed directly in
revolutions per minute (rpm).
If you have no idea of the speed of the moving part, it is best to start the measurement procedure at
the highest frequency that the stroboscope can deliver. The flashing rate of the stroboscope can then be
gradually reduced until a single stationary image of the reference mark is obtained. This is the point of
fundamental synchronism that corresponds to the speed of the moving part. Do not continue to reduce the
flashing rate of the instrument beyond this point without a valid reason for doing so. If you do continue
the reduction, a stationary image will still be observed, but the stroboscope will indicate a submultiple of
the true rotational speed; thus, a measurement error will be introduced.
Stroboscopes generally have a high- and low-range switch. The typical low range is from 600 to
3,600 rpm, and the upper range is from 3,600 to 15,000 rpm; there is a slight overlap in ranges to ensure
reliable frequency coverage. In view of the limitation imposed by flasher tube life, the stroboscope should
always be operated at a flashing rate that is as low as possible, consistent with the rotational speed of the
observed part. If you should be required to operate this instrument over a long period of time, use a
submultiple of the fundamental synchronous speed. The pattern will remain just as stationary, and the
tube life will be greatly extended. In addition, the quality of the light is better at the lower ranges than at
the upper end of the scale. Sometimes you will encounter a rotating or vibrating device that is moving
faster (or slower) than the measuring range of the stroboscope will accommodate. Although such speeds
can still be measured, you must use the multiple or submultiple synchronism points.
There are two methods of measuring high speeds. The first method is to obtain a single stationary
image of the rotating object at a subharmonic speed relationship and to record that value as A. Then
obtain a second single stationary image at the next lower subharmonic speed relationship, and record this
value as B. The unknown speed may then be computed from the following formula:
For example, assume reading A was 4,000 rpm and reading B was 3,500 rpm. The computation
would be as follows:
