Choosing the proper motor table sizes saves time, money
May 4, 2004
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.
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.
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 = ((ri2 + ro2) 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 = ((0.6672 + 1.52) 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.
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.
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 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.
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.
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:
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.
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.
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.