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 Technical discussion on slack loops from Coe Press Equipment
It
is generally safe to assume that a feeder can respond much more quickly
than most unwinders can. That's because the unwinder is starting and stopping
the coil which represents a much larger inertial load. In some cases the
unwinder is also required to do the work of straightening the material
as well. For it to have enough power to respond as quickly as the feeder
does, it would require a much larger drive and the ability to position,
as is the case with a combination unwinder/feeder/straightener. This explains
why most systems have a slack loop.
The purpose of the loop is to provide a reservoir of slack material that
the much quicker feed can draw from upon start up while the less responsive
unwinder accelerates to line speed and in turn to absorb the material
that is unwound while the unwinder decelerates at its slower rate when
the line stops. It also allows the unwinder to run at a fairly constant
speed during continuous operation even though the rate of consumption
is constantly changing due to the intermittent feed motion.
Slack loops
can take a variety of forms, the most common of which are shown in Figure
4. The overhead loop configuration is used to conserve floor space by
storing material vertically and above the unwinder. This method is generally
used with either inline cradles, or with centering reels in cases where
the material is of heavier gauge and would require considerable floor
space if it weren't stored overhead. The material must also be stiff enough
to resist buckling under its own weight as it's pushed up and overhead,
which can be a limiting factor in the amount of storage.
 The
paddle loop configuration stores material in the form of loose wraps around
the coil and may be used in conjunction with a feeder/straightener combination,
or with a pull through straightener at the feeder or when straightening
isn't required. This style requires the unwinder to be located as close
as possible to the feed and that the material be stiff enough to resist
buckling or looping between them. Since storage is somewhat limited with
a paddle loop it is generally used only when shorter feed lengths are
the norm.
Although it consumes the greatest amount floor space, the standard horizontal
loop configuration is by far the most common and versatile variety. To
accommodate long feed lengths or aid in achieving higher line speeds,
the addition of a pit can greatly increase the amount of material storage
in a horizontal loop. To avoid inducing set in the material it is extremely
important that the strip be supported in the correct radius entering and
exiting the loop if straightening is done prior to the loop. This is accomplished
by using a series of catenary support rollers, or by using a chute arrangement
with the correct minimum radius for the thickest gauge material.
The loop storage requirement for any application is equal to the amount
of material that is consumed by the feeder while the unwinder accelerates
to line speed, minus the amount of material that is unwound during that
acceleration period. For example, an operation requiring a 12 inch feed
progression running at 120 strokes per minute results in a net line speed
of 120 feet per minute or 24 inches per second. If the unwinder is capable
of one foot per second squared acceleration then it will take 2 seconds
to accelerate to the 120 FPM line speed. During that acceleration period
only 12 inches of material will be unwound while the feeder will use 48
Inches. The result is that 48 inches of material will have been consumed
but only 12 inches will have been replenished leaving a deficit of 36
inches. If there isn't already more than 36 inches of material stored
in the loop before starting, then the system can never reach line speed
without first depleting all of the material in the loop. Of course there
must be some additional buffer beyond the 36 inches required to overcome
the deficit of material in order to avoid having the loop become completely
tight before reaching line speed. This means that the total storage requirement
should probably be at least 48 inches for smooth operation. If the loop
contains less material storage than that, it will necessitate that the
press speed be reduced to allow the unwinder to keep up.
The amount of material in the full loop, minus the straight line or tight
length of the loop. The actual storage is determined by the depth of a
loop, not by its length. In fact, the longer the length of a loop, the
less material that is actually stored in it for a given depth. The shortest
loop length for a given depth yields the greatest amount of material storage.
The minimum permissible length of a horizontal loop is determined in part
by whether straightening is done prior to the loop or if it is done after
the loop. If straightening is performed after the loop, as in the case
of a feeder/straightener combination, or a pull through straightener at
the feeder, then the minimum loop length is determined by the minimum
distance that the heaviest gauge of material will loop into without being
forced. This will probably result in the material taking some additional
set but it really doesn't matter because it will be straightened at the
feeder anyway.
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On the other
hand, if straightening is done before the loop, as is the case with power
straighteners, the minimum permissible length of the loop is determined
by the maximum material thickness that will be run on the line factored
with the depth of the loop. This is due to a factor called the "minimum
bend radius" or MBR for that material. MBR is the minimum radius
that pre-straightened material can be bent into, not exceeding its yield
point, so that it will return to a flat condition, after exiting the loop.
If material is bent beyond its yield point into a radius smaller than
its MBR then it will take set and won't return to a flat state when it
exits the loop. This is true regardless of the loop configuration. With
a horizontal loop the entry and exit cascade sections must be configured
for the MBR of the thickest material to prevent the weight of the loop
from bending the material around the exit straightening roll as it enters
the loop or over the lower feed roller as it exits.
See
Chart Below
The MBR
for a loop is determined by the composition and maximum thickness of material
that will be run in the loop. The thicker or softer the material, the
larger its MBR will be. It will vary with the material composition, but
a good rule of thumb is the MBR for mild steel will be approximately 360
times its thickness. For example, with 1/4 inch thick mild steel the MBR
is .25 x 360, or 90 inches. If it is bent into a radius smaller than 90
inches it will retain some set after it exits the loop which means that
it may not easily slide through the die, or produce a good part. Given
that the MBR for a material cannot be exceeded then the length of the
loop will be determined by it. Therefore, for a given loop depth, the
greater the maximum material thickness, the longer the loop length must
be. And in turn the greater the depth, the longer the length requirement.
This is true until a depth of two times the MBR is reached. At this point
the length of the loop will be equal to four times the MBR. Once this
length and depth are reached any added depth results in optimum vertical
storage with no further horizontal length being required. At this point
every added inch of loop depth yields two additional inches of actual
stored material.
Once the storage requirement has been calculated based on the maximum
progression and strokes per minute factored with the response time of
the unwinder, use the loop storage chart in Figure 5 to determine the
space requirement for a standard horizontal loop. The "Length"
figure is the loop length requirement for that material thickness and
loop depth. The "Stored" figure indicates the amount of material
that will be stored in a full loop. With overhead or paddle loop configurations
it is somewhat more difficult to determine the actual storage in the loop.
Consult the equipment manufacturer to make certain that the storage available
will be adequate for the system performance.
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