How one stamping plant automated scrap handling.
Stamping scrap, almost by definition, is one of the most difficult things in the world to handle automatically. Sharp, tangled, low in value, and high in volume, it seems to make a mockery of today's high-speed presses and sophisticated coil-feeding equipment. Some of the most expensive and efficient stamping equipment available is periodically stopped while a forklift driver plays "checkers' with tote bins.
Moving into a completely new plant in 1981, we decided to do something consttructive about the downstream end of our high-volume stamping operation. It was a learning experience involving three conveyor systems, a lot of grief and a considerable amount of money before we got the smooth, trouble-free system that's now in our plant.
Producing an extensive line of filter units for automatic transmissions, we work primarily with draw-quality steel in the range of 1/16 to 3/16 and wire cloth. The steel stampings are produced in our main press line (Figure 1) consisting of three 200-ton and one 100-ton straight-sided presses. Wire cloth and some parts are formed in a line of six smaller gap presses.
As business grew in our plant, presses were installed wherever they would fit. That developed into a helter-skelter arrangement that defeated anything beyond the most rudimentary attempts toward scrap-handling automation.
Each press was equipped with an elevating conveyor that loaded scrap into a tote bin. These were emptied as the need dicated, but the inevitable spillage involved a lot of housekeeping.
Designed entire shop . . .
The opportunities offered by a completely new building were too ideal to be ignored. We set out to engineer the whole shop, from raw material to packing and shipping, as a state-of-the-art facility. Naturally it included a centralized scrap collection and disposal system. Our goal was to avoid the manual handling of scrap. We wanted to leave that to our scrap dealer.
Designing the system and choosing the right equipment developed into a three-year-long education.
We did some checking around, and then sat down with a company that had supplied some individual press conveyors in our old plant. They recommend a hinged steel belt running in a pit through the length of the pressroom.
Small secondary hinged-steel-belt conveyors, under each press, fed the central conveyor. And, at the delivery end, an S-design hinged steel belt elevated the scrap through a wall-opening into the scrap dealer's gondolas.
This system was grief from the beginning, a result of basic design faults.
The main conveyor was 200-ft long and 30 wide, involving about 400 ft of 4-pitch belting that weighed 16,000 lb. With one central power package to move the belt at over 20 fpm, it presented drive, clutch, and tracking problems from the time it started to run.
Stretching the belt under these loads was the first headache. We found ourselves taking up the stretch, taking out links, and taking up more stretch constantly.
As a result of this, the scrap-removal system wasn't running half the time, and that presented real problems. We no longer had elevating conveyors removing scrap from individual presses into tote bins.
When the system was down, we were reduced to manually digging scrap from under the presses. This was a dirty, time-consuming, and expensive job.
Then we had a catastrophe. The system simply stopped. We opened it up to identify the problem, and found the main conveyor had practically destroyed itself.
Unbeknownst to us, the carry-over of scrap on the return run of the hinged steel belt had collected under the conveyor, walked the belting off the drive sprockets, and bent the conveyor frame. Axles had worn through the side frame in spots and, in other places, axles had run along the frame to cut themselves off.
We dug about 80,000 lb of scrap, mostly 3/4-dia slugs, from the floor of the conveyor pit. They had gradually built up and compacted during one year of intermittent operation.
This problem could probably have been alleviated by cleaning the pit periodically, had we known about it. But the condition of the equipment was now so bad that a complete rebuilding project had to be launched.
. . . and redesigned . . .
Drawing on hard-won experience, we re-engineered the complete system. Intermediate tracking sprockets were added to both top and bottom flights of the belting. A motion-detection device was added so that we knew when the conveyor was jammed. Side frames were rebuilt and strengthened.
We did all of the belt-and-suspender things. When finished, we had a much better-built, better-engineered hingedsteel-belt conveyor system. But we hadn't changed the concept. After four or five months, the flaws in the concept began to make themselves known again.
The conveyor belt started to stretch and catch every type of scrap. It got between the flights of belting, it got under the belting, it caught in the hinges and began to eat the tracking sprockets. The conveyor jammed everyday.
Again, we were catching the scrap in pans placed under the presses, loading it into tote bins and hauling it out with forklifts. We were worse off than we were in the old plant.
At this point, we contacted several conveyor companies to find something that would really work in a modern stamping plant. Three types of conveyors were suggested: magnetic, rubber belt, and oscillators.
Magnetic conveyors had a lot to recommend them. Given material of a uniform size, they do a beautiful job. And, if one judges quality on the basis of price, they must be awfully good since they are certainly expensive.
But the material in our plant presented some problems. While the scrap that drops down from our presses is mostly small slugs, we also chop the coil scrap into pieces, ranging up to 16 long, which are fed into the central conveyor. When these longer pieces happen to bridge the spacing between magnets, the scrap stands still, creating a dam.
We also collect scrap from six smaller presses into drums or tote bins. Light in weight, and often wire cloth, this scrap was to be manually dumped into the collection system from time to time.
When the people selling magnetic conveyors saw this, they turned green. Sharp and tangled, this miscellaneous fine scrap became a lump, tying into itself because of the magnetic field.
One place where magnetic conveyors proved to be superior is under the presses where small slugs drop through. Falling about 3 ft, these slugs tend to bounce off anything other than a magnetic conveyor.
Because of their uniform size, the slugs present no bridging problem. A magnetic conveyor captures them and moves them beautifully.
The troughing belt simply couldn't stand up in our tests. While there are some very durable belt materials available, none of them could stand a constant stream of razor blades in a three-shift operation. We happen to use a great number of belt conveyors in our assembly and packaging operations, and were very confortable with the concept, but it didn't suit pressroom demands.
. . . and found a solution
It was at this research stage that we visited with a representative of Prab Conveyors Inc, Kalamazoo, MI, who suggested a combination of conveyors and took us to examine an oscillating conveyor being used in a difficult stamping operation.
This particular oscillating conveyor was moving a considerable volume of fine motor lamination scrap and doing a beautiful job. From a distance, the tiny pieces of scrap flowed so smoothly that they seemed to be liquid.
There were no joints or hinges in the conveyor pan to catch scrap. The motion of the conveyor was almost undetectable. And the unit was quiet.
Prab's application engineers designed a scrap-collection system for our plant and submitted a quotation. In this design (Figure 2), the 200-ft run of the central collection conveyor is made of three flights of oscillating conveyors, each feeding into a slightly wider section.
A sealed-bed magnetic conveyor is located under each of the large presses, catching slugs as they drop and discharging into the central oscillator. These units were proposed to replace the existing messy and troublesome steel-belt conveyors.
A Prab pivot-belt (Figure 3) conveyor elevates the scrap at the end of the run, raising it into the scrap dealer's gondolas. This unit is really a modification of the familiar hinged-steel-belt conveyor, but only the leading edge of each flight is hinged on an axle. The trailing edge of each flight lies over the following hinge.
This design provides two advantages. First, because the hinges are protected, the stamping scrap can't get into them. There's no jamming, no build up of material.
Secondly, as the belt rotates over the delivery end of the conveyor, the individual flights swing free with a flipping motion that discards any material that might be adhering to the surface of the belting.
This self-protecting, self-cleaning belting eliminates the problem of scrap carry-over and all of the problems that develop from a build up of scrap on the return side of the conveyor run.
Had the original installation been designed with Prab's pivot belt, most of the problems we suffered for two years would never had occurred. It is not a gimmick. It works!
The gains continue
The advantages we have gained from this combination of conveyors began with the installation and continues today.
The complete system was designed to fit into existing pits and under presses where they stood. The only construction involved the enlargement of an opening in the concrete block wall where the pivot belt conveyor goes out of the plant to load the scrap dealer's gondolas.
This modification was necessary because Prab engineers recommended a slightly lower incline for better conveying. That was accomplished by removing a few blocks and bricks.
All installation was accomplished on weekends. We never shut the pressroom down. At planned times, we would take an individual press out of service to install and wire its magnetic conveyor. Metal chutes were also constructed under each press to direct the small slugs toward the conveyor.
We also got one bonus that hadn't been part of the previous system design: scrap removal for our six small presses.
This scrap, collected in drums or small tote bins, had been moved out of the plant by forklift trucks. Prab engineers designed and built a dumping station at the lead end of the oscillating conveyor where we now empty these drums as the need arises. Small, lightweight and tangled, the lumps of scrap are gradually spread into the main flow of material as it is carried along.
Early on, we had a problem with scrap being produced by our 100-ton press, but that involved a communication problem, not a design fault.
Flat scrap stuck
Forming some metal gaskets of double-weight tin-coat material, we produce very flat scrap about the weight of a TV dinner tray. This slightly oily scrap would hit the underpress magnetic conveyor and stick like a suction cup. Unfortunately, we hadn't told the Prab engineers about this material, so they hadn't made allowances for it.
The solution was simple. We replaced the surface of that conveyor with rigidized stainless, and the problem disappeared. The slight texture broke the adhesion.
One other problem, which we are in the process of resolving, is a direct result of the system's smooth operation.
The gondolas that receive the scrap are housed in a separate room adjoining the main plant. If someone doesn't pay attention, the gondolas will overflow, filling the scrap room to the roof.
That has happened, particularly on the night shift when production is running smoothly. It didn't happen with the previous system because it gave us so much trouble we were always watching it.
We are installing a bin-level indicator that will flash a light or sound a horn, letting the foreman know it's time to call the scrap dealer for a change of gondolas.
Having worked for other manufacturers and in the contract stamping industry, I have never approached this kind of scrap-handling efficiency. The system is operatorless. The combination of conveyors, the design, and the control system simply work the way that are supposed to--without problems or supervision.
We are stamping about 9-million lb of steel annually, and we never touch the scrap. The scrap dealer has a key to the storage building; our sole involvement is to call him when a load is ready.
Aside from that, the system works just like the lights. We turn it on and off.
Photo: 1. The main press line. The scrap-handling conveyor system--consisting of three flights of oscillating conveyor totaling 200 ft--is under the metal plates at lower left in the photo.
Photo: 2. Plan-view drawing of the present scrap-handling system.
Photo: 4. This pneumatically operated diverter shunts the scrap into one of two gondolas in the scrap room, which is walled off from the main plant.
Photo: 3. Scrap being transferred from the oscillating conveyor, lower left, onto the inclined pivot-belt conveyor.
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|Author:||Tamburinno, Robert M.|
|Publication:||Tooling & Production|
|Date:||Sep 1, 1984|
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