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OUR
CUSTOMERS "When I presented my challenges
to Accra-Wire Controls, it was apparent that the technology
used in tension control and coil handling could really help
us. We are now able to load a shift's production at one time,
eliminating up to nine coil changes." - Monty
BrennerPlant Manager
Pullman Industries
Michigan, USA
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"Sizing
up pallet decoilers" by Michael
Grabow
Magazine
The primary reason for purchasing a decoiler is to increase production.
By correctly sizing the machine for your needs, you can increase production
and the bottom line. |
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Buying a decoiler that is too small for your jobs
won't increase production and may actually cost more in downtime
and maintenance. Some of these maintenance costs are replacement
of the motor or controller because of frequent power overload, bent
guide arms, and increased bearing wear. However, acquiring a decoiler
larger than necessary drains money that could be better spent in
other areas and uses valuable floor space.
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Size
Does Matter
It would be nice to be able to purchase just any
decoiler and put on it any size coil or weight. However, the real
world does not work this way, and neither would the decoiler.
Coil size and weight determine the required decoiler
motor horsepower. If the horsepower is too little, the machine won't
be able to accelerate to running speed in the required amount of
time. To start your research, consider the following coil size and
weight formula:
Inertia = ((ri^2 + ro^2) * Weight)/2
where: Inertia = ft.-lb.2
ri = inside radius in ft.
ro = outside radius in ft.
Weight = weight of coil in pounds.
For example, given a spool size of 36 in. OD and
16 in. ID that weighs 1,000 lbs., the inertia can be found as follows:
First, convert the spool diameters into radii
and inches into feet:
36 in. OD = (36/2)/12 = 1.5-ft. radius
16 in. ID = (16/2)/12 = 0.667-ft. radius
Then, plug in the numbers:
Inertia = ((0.667^2 + 1.5^2) * 1,000 / 2 = 1,347.44
ft.-lbs.2
This value gives an indication of the momentum that the motor torque
must overcome to move the load. Remember, the greater the inertia,
the more torque is needed to move the load.
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Weighty
Issues
A larger coil diameter or heavier coil weight increases
package inertia, which requires a higher-horsepower motor. Decreasing
a coil's ID reduces the inertia. It's easy to see how coil weight
can speed up or slow down a machine, but it may be less obvious
what effect the OD has on inertia. Even less apparent is the role
ID plays.
As coil ID increases, more weight is distributed
to the outside of the coil. For example, consider holding a rope
tied to a brick. If the rope is short, it's easy to swing the brick
around in a circle. However, if the brick is on a long rope, it
will be much harder to start or stop swinging. The longer the rope,
the more difficult the task of slowing down or speeding up the moving
brick.
In most stamping applications it is undesirable
for the table speed to be the same as the motor speed because this
causes the table to rotate too fast. To make sure the speed is reduced,
a gearbox is placed after the motor to reduce table speed.
Gearbox output can provide more torque than is
output by the motor alone, much like a step-down transformer: The
speed is reduced, but the torque is increased. The gearbox also
must be properly sized to handle the inertia of the spinning coil.
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Gauging Proper Motor Size
If a motor is undersized, it can't control the spin of the table
properly. In effect, the coil takes control of the job rather than
the motor, which can cause overtravel of the gearbox on deceleration
that can cause damage.
Ramping-up and Stopping Time. The running speed of a 1-HP
motor can be as fast as a 5-HP motor. The difference comes in during
load acceleration and deceleration. Therefore, faster acceleration
time requires more horsepower.
The time required to stop a coil is as important as getting it
up to speed. This is especially true in the event of an emergency
stop. A too sudden stop can damage a decoiler's gearbox. Likewise,
not stopping quickly enough causes strip material to uncoil uncontrollably,
possibly even causing injuries to workers nearby.
A controlled stop is more desirable than letting the load coast
to a stop. Ideally, the decoiler should stop within one turn of
the table. This keeps the material on the coil and prevents machine
damage.
For applications in which rapid acceleration to a high speed is
required, an accumulation system is needed. An accumulation system
is a series of pulleys that keep a certain amount of strip material
in reserve. This reserve strip can be pulled and used without turning
the decoiler. This gives the decoiler the necessary time to ramp
up to speed. A speed sensor prompts the accumulator to supply material
while the decoiler is given time to accelerate to line speed
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Control Arm Types
Most pallet decoilers have a speed control mechanism called a
dancer arm. One type of control arm can't handle all material types.
Material type and strip width dictate the type of arm needed to
control the feed speed. Three common styles of dancer arms are:
- Mandrel (loop system)
This arm type can be used for long feed lengths (more than 24
in.) and is good for wide (8 to 32 in.) or soft material such
as aluminum (see Figure 1).
- Vertical Dancer Arm
Consider this type if you use a heavy-gauge or wide material (8
to 24 in.).
- Side Swing
This control mechanism is suitable for thin material (0.002 to
0.187 in.) and can be used with many types of metal, such as steel
and aluminum (see Figure 2).
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Figure 1
A mandrel (loop system) can be used for long feed
lengths (more than 24 in.) and is good for wide (8 to 32 in.) or
soft material such as aluminum.
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Figure 2
A side swing mechanism is suitable for thin material
(0.002 to 0.187 in.) and can be used with many types of metal, such
as steel and aluminum.
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Table
Size
For table size, the rule again is not too big or too small. Look
for a machine that can hold your material, but isn't so large as
to put a strain on the motor. The platform should hold an entire
skid with no part of the coil package or the pallet protruding beyond
its edge. This can help prevent injuries to workers.
Make sure the table is thick enough to hold the weight of the
load. The table is subject to much abuse by the loading and unloading
of pallets.
The law of inertia mentioned previously applies to tables that
are too large. The larger the table, the heavier and farther out
the load becomes. This also increases required horsepower to accelerate
and decelerate the entire load.
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Constant
Tension
Most processes do not just pull on the material, and some may
even reverse the feed.
A decoiler ensures not only that the material comes off the coil,
but also that it stays on the coil until needed.
Keeping some tension on the material is vital because the strip
can fall when it becomes loose. If this loose strip catches on something,
the machine will jam up. Coils also can clockspring, which is the
tendency to unwind when left unbound. Keeping a constant tension
on the strip keeps the coil from coming loose.
Knowing the exact specifications of your job, such as coil ID,
OD, and weight, as well as acceleration and run speed, will make
it possible for you to find a machine that suits your needs and
will provide your company with many years of increased productivity
and profits.
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REPRINTED FROM THE MAY 2004 ISSUE OF
Stamping Journal MAGAZINE
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Consult
AWC for the right choice for YOUR application. |
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