Monarch Instrument Vibration Strobe - Manual (Page 7)

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The Narrow and Wide Bandwidth selector on the strobe’s front panel can be used to optimize its’
ability to lock onto the vibration signal at any speed. The Wide bandwidth will allow the strobe to
track the signal over a fairly wide range of speed change and should be used when tracking the
fundamental frequency (in simple systems).
On machines such as a gear drives, reciprocating engines, or any drive with multiple fundamental
frequency excitation, the strobe could encounter difficulty tracking the designated signal, due to the
tracking filter’s normal bandwidth. If substantial levels of multiple fundamental frequencies, or
fundamentals with harmonics occur within a selected bandwidth, the shaft reference may appear to
oscillate, or drift substantially, when viewed with the Vibration Strobe. Also, if the accelerometer
cannot be positioned on the primary point of interest, a fundamental frequency of something other
than that of the primary point of interest may be predominant. In these cases, use the Internal (INT)
mode and flash rate adjust to stop the motion of the primary point of interest, switch the Bandwidth
switch to Narrow, and then switch to the Locking (LOCK/TRACK) mode. The filter is far more
sensitive and will better be able to discriminate the required signal. However, it will not be able to
track over widely varying speed changes, which will require retuning in the Internal (INT) mode.
After the image has been locked and the Narrow or Wide Bandwidth has been optimized for the
speed and/or background noise on the sensor signal, the Phase knob can be used to adjust the phase
of the output pulse (and image) - refer to the Phase Analysis section later in this manual for adjusting
the Phase Angle control.
MEASURING RPM WITH THE Vibration Strobe
To read and measure RPM (speed) with the Vibration Strobe, select a permanent or semi-permanent
object or mark on the shaft to use as a visual reference. Most technicians choose to select an
unopposed shaft keyway, or other marking on the shaft, that is not duplicated on the opposite side
(180 degrees) of the shaft. If it is possible to stop the machine, mark the shaft with a center punch,
paint or permanent marker, liquid correction fluid, etc. When marking, it is a good idea to use
marks on the shaft 180 degrees apart, such as a horizontal line (-) at one point, and a vertical line (I)
180 degrees away. This will give an indication of a “+” when the flash rate (and LCD display) is at
two times actual running speed. This serves as a useful indicator.
It is suggested that the shaft mark/object should be observed at as much of a 360 degree rotation as
possible, however, normally a range of 180-200 degrees is sufficient. After placing the Mode
Selector Switch to INT, direct the flash toward the mark/object on the shaft. If the speed of rotation
is within the range of the strobe, start at the highest flash rate and adjust the flash rate slower until a
single image of the dedicated reference is observed. Note that at a flash rate twice the speed of the
image, that two images may be observed 180 degrees apart from each other. As you near the
correct speed you will see 3, 4 or more images at harmonics of the running speed. The first single
image you observe should be the true running speed. To verify this, simply slow the flash rate to half
of what was noted at one times speed. A single image should again appear. Readjust the flash rate
again, as closely as possible back to running speed, the next higher single image. As the mark is
locked-in, the LCD display will read out the true RPM (speed) of the machine part being observed.
BALANCING AND PHASE MEASUREMENT
The Vibration Strobe contains an internally tuned filter that is incorporated in a phase shifting
network, which allows the shaft reference mark to be directed at any convenient location on the
machine while balancing or performing motion studies (phase measurement) along a machine’s
casing or along a machine train. Examples of a convenient location are the machine’s horizontal
split line, top or bottom dead center, or the plane of the reference transducer.
NOTE: It is advisable to “LOG” this reference location, so that it may be utilized in future studies
or balancing procedures.
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Checking for unbalance:
1. If the vibration transducer is moved from the vertical position on the bearing to another
radial position, the reference mark will move to the new position if there is a significant
amount of unbalance. The reference mark will follow the vibration transducer around the
clock only if the once per turn vibration is caused by unbalance.
Checking for alignment:
1. Alignment checks can be made by moving the vibration transducer to each end of the
machines in a train. The vibration transducer is positioned axially at these locations for
these measurements. Keep track of the phase for each measurement location.
2. As the vibration transducer is moved to each location, it’s direction keeps changing from
North to South, for example. When it changes direction, phase will shift from 12:00
o’clock to 6:00 o’clock.
3. If you move across a coupling and phase does not change as expected, a misalignment
condition can be the cause.
MOTION STUDIES
The Vibration Strobe can be a useful tool to determine how a mechanical support or a piping system
is moving. It allows the user to find points of maximum motion as well as minimum motion. This is
important if a new pipe hanger is to be installed or if a brace is going to be added to dampen a
vibration condition.
For these tests the strobe light should be set on a tripod. A long cable will be needed for the sensor.
The sensor should be mounted on a magnetic base.
Follow the same 5 steps outlined in the previous section for General Preparation for phase analysis,
and then follow these steps:
1. Mark the piping system off in given intervals and note the location of supports or hangers.
For example, use 6-foot intervals. Draw a simple diagram on paper.
2. Start at the machine end, adjust the strobe light to running speed, and note the phase
reading using the clock face method. (For reference, adjust the phase marker to the 12:00
o’clock). Move the vibration transducer to each location, noting the phase reading
at each.
NOTE: Do not move the strobe light, just the vibration sensor. Do not change the Phase Angle
control knob after the initial reference has been set.
3. The phase markings should “walk” around the clock face as the sensor is moved to each
location. Each time the marking is at or near 12:00 o’clock, the motion is at or near
maximum and is in phase with the reference point. Each time the marking is at or near
6:00 o’clock, the motion is at or near maximum in the opposite direction and is out of
phase with the reference point.