| Trouble-shooting Belt Driven Systems |
| Our machine is a fan, belt-driven by an electric
motor. As the unit is running, tune the Stroboscope to die operating frequency
(running speed) of the drive pulley/sheave. Visually stop the motion. Inspect
the belts and sheaves for signs of wear, cracks and frays. Read the exact
part numbers if replacement is required. |
| If you are able to shut die fan off, use chalk
or a paint stick to draw a line across all the belts (figure la). Draw
another line along the entire edge of the outboard belt (figure 2a). Safely
start the machine. Again tune the stroboscope to running speed, now observe
which belts are actually pulling the load (figure lb). You should be able
to spot the loose and stretched belts immediately. |
| Next slightly de-tune the stroboscope, say
50-100 cpm (rpm), and observe the outboard belt. If it draws a shape similar
to a peanut (figure 2b), check your sheave alignment of the unit. |
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Figure 1a
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Figure 1b
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With the machine off, draw a reference line across all the belts. Re-start
the machine and observe the reference lines. If the alignment is good the
lines should remain as pictured. |
Shoot the belts as they run. Outboard belt is pulling the load due to misalignment.
The mark can be seen at running speed. Each progressive inboard belt is
slipping and the marks lag behind and move slower than running speed. |
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Figure 2a
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Figure 2b
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Stop
machine. Mace chalk on outer edge and slowly hand turn the sheaves. This
will leave a mark the entire length of the belt. |
As the machine runs, tune to running speed. Slightly de-tune stroboscope
and view belt profile. If the peanut shape is observed, check alignment. |
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| Can't shut it down? |
| To check for belt slippage without shutting
off the unit: Tune the stroboscope the exact running speed of the sheave
you want to inspect carefully view the belts at the sheave. The belts and
the sheave should all appear stopped. If there is any slippage you will
see a belt slowly creeping while the sheave is "frozen." |
| Taperlock Sheaves |
| Many belt systems make use of the taperlock
bushing on the sheave. Because unequal pull on the installing bolts can
cause these sheaves to "tip", use the following technique to check for
axial concentric. Tune the stroboscope to operating speed and observe the
sheave. Next, double the speed an observe the maximum displacement of the
sheave. If the sheave appears to move a great deal, check the run-out with
a dial indicator and re-align the sheave. This method will work for any
rotating component when checking for maximum displacement. Belts, pulleys,
blades and shafts may be observed with this method. |
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Actual Running Speed
1500 rpm Side view
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2x Actual Running
Speed 3000 rpm Observe maximum displacement
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| Proper Tuning |
| Stroboscopes should be used with caution.
For safety, inform those around you that the shaft or rotating part is
still moving but the strobing light makes it "appear" to be stopped. When
tuning the stroboscope there are several times when the object will appear
to be stationary. The highest CPM/RPM giving a single image is the exact
running speed. Always check the drive unit name plate to confirm the actual
running speed range. Other single images will occur at sub-harmonic rates
(i.e. 1/2 x, 1/4x, 1/8x running speed). Similar images can be seen by tuning
to harmonics of operating speed (i.e. 2x, 4x, 8x, running speed). If looking
at a shaft with one key way, tuned to 3x running speed or the 3rd harmonic,
three keys will be visible. It is important that you find the fundamental
operating frequency for most inspection techniques. |
| Triggering a Stroboscope |
| A stroboscope can be triggered typically from
a TTL pulse signal. When the pulse hits the input the stroboscope will
flash- Various sensors and vibration analyzers can provide this type of
pulse. When the stroboscope is triggered externally, the flash rate and
phase will automatically be synchronized with the trigger signal. |
| Phase Shifting |
| Internal phase shifting is a feature used
on advanced Stroboscopes which allows the user to visually shift a reference
mark to a different viewing position. This technique is useful when the
reference mark is hidden or obstructed. Moving the reference mark is usually
accomplished via a jog button which relocates die image. |
| External phase shifting allows the tracking
of variable speed machinery. External phase shift also allows a fixed delay
in rotational degrees or time (usec). The stroboscope flash is being delayed
from an external trigger signal. This feature is used to look at an instant
of a cyclical operation that differs from the timing of the trigger signal.
For instance you may be triggering off an engine at Top Dead Center (TDC)
and the valves are open. If you wish to see the valves closed, you can
shift the phase 180 degrees with the electronics of the Stroboscope. |
| Selecting a Stroboscope |
| Chose the type of power: Mains or re-chargeable
batteries. When selecting a Stroboscope consider the speed of the equipment
you wish to observe, the type of environment how bright the light should
be and the triggering options. As just discussed, consider the need for
phase shifting. Do you need the stroboscope to function as a tachometer
with an external sensor? There are a wide variety of instrument available. |
| Summary |
| This article does not even begin to outline
all the uses of the Stroboscope in both maintenance and production. Stroboscopes
can be used with video inspection systems for high speed packaging, with
vibration data collectors to provide phase studies and operational deflection
shape (ODS) analysis, to aid in dynamic balancing and the list goes on.
The Stroboscope is a very cost effective maintenance and quality assurance
tool. It can save you time and money while providing real-time information
in a safe and repeatable method. |
