Excessive Roll
Deflection
The force of the web causes deflection in a roll as it changes
direction. When the web starts to apply force, or load, to the roll
surface, the roll bows, or deflects. The greater the load, the greater
the force that can result in roll deflection. If the deflection
is excessive the result can be wrinkling or bagging.
Roll deflection is often identified when the web bunches near the
midsection of the web directly around the roller. However, the presence
of deflection may be much more subtle. Consider a situation in which
you have twenty thousandths of an inch deflection on an idler, and
50 idlers in the system. By the time the web gets from one end of
the machine to the other, the web on the outside has to travel one
inch more than the web on the inside. This is an example of accumulated
effect due to deflection. It's not the effect of single roll that
causes the problem - it's the effect of the total system.
Like so many other situations, the extent of the problem will depend
greatly upon the web characteristics. Some webs recover quickly
edge to edge when stretched slightly, others do not.
Small-diameter idlers with long face lengths should raise a red
flag for any qualified roll supplier. Deflection is, perhaps, the
single most important factor that determines roll construction.
Roll material (steel, aluminum, or carbon fiber), roll diameter,
wall thickness, face length, web width, shaft sizing, and bearing
location all have to take load, or deflection, into consideration.
Excessive deflection can also lower critical speed-the point at
which harmonic vibrations occur in the roll. A lower critical speed
will force you to run the line slower. Be aware that two identical
rollers could have different critical speeds based on different
degrees of web wrap load, deflection, and/or mounting.
To avoid problems of this kind, be sure your rolls are properly
specified. When ordering, you should know the amount of web wrap
(usually stated in terms such as 10 o'clock to 3 o'clock), web tension
(PLI), any nip load (PLI), web width, and maximum expected line
speed. If you haven't thought of them in advance, a Webex applications
engineer will walk you through these questions and can even help
you troubleshoot much of this over the phone.
Shaft deflection is just as critical as roll deflection. The problems
surface in several forms. The shaft may start vibrating inside the
roll or the angular misalignment of the shaft through the bearing
can also cause premature bearing failure.
Sometimes changing the mounting to a more rigid set-up can help.
If you have a short-faced roll with an extra-long shaft, like a
60-inch roll with a 120-inch shaft, you may be better off using
an 80-inch or 90-inch roll. The resulting angle is going to be less,
and you'll have a stiffer shaft/roll configuration. The bearings
will also last longer because they experience less angular misalignment.
In other cases, using a larger tubular shaft within the roller may
reduce shaft deflection significantly.
Getting the right shaft is as important as the right roll. Count
on Webex to help you determine the specifications based on the same
criteria: wrap, load, speed, and deflection.
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