I. THE PIRANHA DOWN!
Big Mike, owner of the largest southern Wisconsin area scrap metal facility, called his mammoth shredder, "The Piranha." It was a monster machine with a voracious appetite that swallowed large chunks of scrap metal and spit them out in the form of fist-sized steaming nuggets of steel, the kind of dense, clean, usable product steel mills always welcomed.
It was in December of 1989 that the Piranha suddenly had taken ill, no longer able to accept and digest steel scrap. Big Mike was looking 'for engineering medical help and had asked their electrical P.E. for advice. He recommended that Big Mike call on my services to get the Piranha running again. He did.
When I first arrived at the Big Mike's 50th street yard, snow was trying to hide the disabled shredder, scattering its roof in white quilted patches, cloaking the variegated bleakness of a yard in winter, lending a surreal cast of intermittent paleness to the moribund machine and its surrounding equipment.
On that gray and dreary morning, the shredder roof cover, its hardened steel sections corroded redly from inactivity, was being removed to allow observation of the initial impact chamber, the area where most of the damage had taken place.
From the control tower platform I shot some video of the roof removal, watching as the crane men cut away the angle braces on the superstructure, attaching the crane's lifting chains to the roof's lifting lugs. The crane made a trial lift. Still there were hidden angle braces tenaciously clutching the roof to the shredder. Again the connected angles were cut away by Dan, the yard manager, and again the riggers attached the lifting chains, eye-balling the four lengths, trying to make certain the assembly would be lifted evenly.
The crane operator revved the engine, feeding power to the crane system hydraulics, slowly winding back the cable drums, drawing in the cables to raise the roof section. The cables went taut again; the roof wobbled but would not yet clear, still being hooked on somewhere.
The riggers checked again to see if all fasteners had been removed. One hidden brace to a guard platform was found welded to the main feeder roll housing, so Dan torched off the angle between the two. Again the crane tugged tentatively at the chained lifting lugs; however, one last welded spar at the opposite side clung to the roof section. In a few moments it was severed from the housing.
When all welded brace and platform elements had finally been cleared the crane began hoisting the heavy roof section. Reluctantly the roof began to move up from the bearing housing, catching momentarily as the roof shifted, then breaking away as the crane men rotated the roof slightly.
A glimmer of light began to show between the top of the shredder body and the roof section, the opening coming larger' and larger as the roof was moved slowly upwards. When the roof had cleared the shredder walls the crane operator leisurely moved the section horizontally until it cleared, gently lowering it to ground level at the east side of the shredder complex, just off the roadway.
The inside was a discouraging mess. The two breaker arms that had been mounted opposed, high and low, had been torn off in two ugly gouges from the 48" diameter shaft assembly. The breaker arms, with their hammers and cutter bars, were jammed at the clearance areas of the shredder wall where they had fallen, bringing the shredder to an abrupt, tortured halt.
Rays of winter sunlight penetrated the wall cracks, illuminating the area where the cutter plates had been knocked off and the outside supporting walls pierced. Over the usually work-polished internals lay a coating of fresh rust. On the massive shaft in the initial impact chamber could be seen the remains of the breaker arm welds, fracture points where dynamic forces in the chamber had ripped and gouged steel away from steel.
The cheeriness of the sunlight did not compensate for the dismalness of the sight....
II. WORLD CLASS SHREDDER.
1. The Monster.
Yes, it was indeed a monster, a King Piranha if you will, reputed to be the largest vertical automobile shredder in the world. It was driven by four 500 HP General Electric motors, each' running at a no-load draw of 200 amps, each motor capable of pulling nearly 400 amps before automatic cut-off, all four motors interconnected through a gearing system. The drive setup was designed to allow continuous synchronous production operation on only three motors.
The huge 60 ton rotor assembly rested on heavy-duty Torrington bearings, the lower one capable of 3.45 million pounds static load, and 2.5 million pounds dynamic load; the upper bearing could handle 700,000 pounds static load and 1.26 million pounds dynamic load. The breaker arm's design center speed of rotation was 180 rpm.
The initial shredding of the material fed into the machine was effected by the two steel breaker arms on the 48" diameter upper shaft. For the operation at Big Mike's yard, an arm had been re-welded to each side of the shaft, one breaker arm high, the other low.
On the ends of the breaker arms were welded large hardened 'hammers, each hammer the base for hardened steel cutter bars which were bolted on the hammers, one bar vertical, the other bar horizontal. Part of the hammer shape itself was shaped to be a cutting surface.
At the throat of the upper chamber where the breaker arms rotated was a powerful hydraulic feed roller, rotatable in either direction, able to move up and down as the operator controlled it. A gravity slide chute let autos and other scrap gravity feed into the maw of the feed roller as the material dropped from the end of the feed conveyor into the chute.
The feed conveyor ran from 10 feet above ground level to a height over 60 feet. Autos, barrels, white goods, and miscellaneous scrap were set on the conveyor using an American crane with a clam bucket. Lugs on the steel feed conveyor belt caused the scrap to travel up to the end of the conveyor where it dropped off into the slide chute.
The operator was able to control the rate of feed from the control tower next to the shredder by regulating the speed and action of the hydraulic roller as the scrap came down the slide chute into the roller throat.
The material entered the initial impact chamber and was first smashed by the breaker arms rotating cutter bars. The bars tore the incoming material into pieces small enough to allow their falling into the space between the subsequent horizontal cutting bar layers and the hardened steel cutter edged walls. As the material passed downward, each of the five levels of bars had a part in converting the shredded steel pieces into smaller and smaller nodules.
While the shredder process was in operation an exhaust blower drew an air stream through the mill section, collecting most of the particulates from developed by the shredding process, sending them to a cyclone separator.
Completely processed nodules of 3" to 4" size eventually collected at the bottom of the shredder where sweeper plates flung them onto a vibrating shaker trough. At the end of the shaker trough was a rotating magnetic separator which attracted the steel nodules but allowed the non-ferrous metals and debris to fall onto a smaller scrap conveyor.
The steel nodules were carried over by the magnetic separator and dropped onto the loadout conveyor that conveyed them to a holding area, ready for magnetic crane loading onto a carrier.
2. The Breakdown.
Big Mike had bought the shredder from a firm in the Twin Cities area. At the time of the sale the two large breaker arms normally mounted on the shaft had previously been snapped off. The seller stated the breaker arms had failed because a van had been sent through the machine, a van with a large section of thick steel stabilizing plate in it which caused the breaker arms to fracture. The company told Big Mike that the shredder could be made operable merely by rewelding the breaker arms back on. Mike had this welding work done by a local boiler firm.
While in Mike's scrap yard, the shredder had operated successfully through months of processing steel drums, white goods, and a few small autos. However, on one tragic morning, the shredder had been started and was being brought to running speed; no material was in the shredder.
Abruptly there was a horrendous crashing noise; something, perhaps a weld failure, had caused a breaker arm, likely the upper one, to come off, ripping out the lower breaker arm in the process. Powerful centrifugal forces hurled the breaker arm assemblies against the sides of the shredder, cracking the walls and dislodging cutter plates. When the breaker arms and plates jammed, the shaft rotation was brought to a halt. The shredder was rendered inoperable; a new breaker arm design was needed.
3. Analyzing the original breaker arm design.
Big Mike furnished the shredder drawings. Careful study of the original design seemed to indicate that the obvious suspect points of the breaker arm/shaft assembly were at the area of the weld. The thickness of the old breaker arm was 5 inches, the total arc length of the weld, according to the drawings, was about 33 1/2 inches on top and bottom. Considering a weld all around a breaker arm, this resulted in 77 inches of total weld to fasten each breaker arm to the shaft.
This led to the inevitable conclusion that the breaker arms as originally designed and mounted to the shaft would be unable to meet continuous auto shredding production demands, prone to failure after a finite period of operation. Further, if the breaker arms were re-welded after each failure, the welds areas probably would continue to fail.
This analysis was borne out in conversation with one of the original design team, who also had told Big Mike that they could almost predict the time when the breaker arms would fail during normal shredder production. He also stated that if large metal masses were accidentally fed into the shredder, the breaker arms as designed might break off immediately.
4. Parameters for redesign.
In the preliminary stages of the new breaker arm design it was decided that no attempt would be made to design and attach a set of similar breaker arms, mounted high. and low as before, extending out from, and welded onto, the central shaft. Even if the original geometry were retained, and the breaker arm sections and welds further strengthened, the possibility of stress fracture at the joints would always exist.
The development of my new design culminated in a decision to fabricate a single breaker arm, for dual impact operation, From a single 6" thick piece of steel. Instead of attaching the breaker arms high and low, the plan was to have each arm end's hammer and cutter bars swing at the same level, with the entire breaker arm rotating precisely in the center line of the feed-in throat.
The breaker arm would completely encircle the shaft, with twelve inches of six inch thick metal between the shaft and the outside of the breaker arm's narrowest part. The theory was that when the cutter bars and hammer would strike the incoming scrap, one radial 72 square inch section of the breaker arm would be in tension, with the opposite radial 72 square inch section in compression.
Instead of simply stressing a 77 inch weld between the shaft and a single breaker arm, most of the stress would be accommodated by the mass and velocity of the breaker arm itself, with the all-around weld length of the new breaker offering much greater strength.
5. Initial strike-point.
At the side of the feed throat is a hardened steel anvil plate. In the original design, it seemed as though the cutter bars were to first strike the scrap against this anvil, providing the initial shearing impact.
However, as the breaker arms kept failing and being rewelded, the process kept successively shortening the breaker arms so that the swing diameter was less than when originally fabricated; and, as the swing diameter grew shorter, the clearance space grew wider. The cutter bars began to strike both the anvil area and the wall just past the anvil area simultaneously, instead of just the anvil. This caused undue wear and cracking in the wall, and premature loosening of the wall bolts.
In the new arm design, the swing diameter of the cutter bars was at the 10' original swing dimension. This made the clearance between cutter bar and anvil average 5 inches where the initial strike contact takes place.
6. Material and Specifications.
I made a design decision to fabricate the breaker arm from a single piece of A36 steel, 6" thick, a structural type carbon steel with good qualities of strength and weldability for use in welded constructions, when added strength is required per unit weight. A36 has a tensile strength which ranges from 58,000 to 80,000 psi, and a yield strength of 36,000 psi.
For a 72 square inch cross section at the narrowest point, using a conservative tensile value of 70,000 psi, total tensile would be about 5 million psi; yield would be 2.6 million psi. The estimated weight of the breaker arm, with the hammers and braces welded on, and the cutter bar bars bolted on, approximates 8,000 pounds.
7. Steel vendor.
Because experience and accuracy in flame-cutting was critical in this application, I decided to order the A35 steel blank from Ryerson Steel in Milwaukee, and have them flame-cut the breaker arm to size in their Chicago plant.
The arm would be delivered to Big Mike with the 48" diameter shaft mounting hole already in it, ready for the hammers and the gusset plates to be welded on. To enhance weld strength, a 1" bevel would be flame-cut into the upper and lower diameter of the shaft mounting hole. The material was ordered out, shaped in Ryerson' 5 Chicago plant, and shipped to Big Mike in three weeks.
8. Welding the hammers and gusset plates.
The Grunau Company, a Milwaukee corporation known for its fine work in process piping for breweries and industry, had been awarded the welding contract. When the Grunau crew arrived on the job site, the new breaker arm had already been set up on a flat bed trailer in the yard, adjacent to the shredder, ready for the critical welding work to began.
The welders started by supporting the heavy hammers in proper position. Before the hammer could be fitted onto the face of the breaker arm, a rib section on the hammer had to be slotted out to fit. This operation gave additional support to the hammer in the slot area.
After the hammer was set, the welders began to heat the work, after which they tacked the hammer to the arm. Next the six rectangular gusset plate supports also were tacked in place, three above, and three below each arm. All tacking and final welding on the arms was specified to be done with 7018 rod.
9. Stress relief
As is usual with such built-up assemblies, the multi-pass welding could be expected to build up complex stresses in the structure. To ensure that the critical hammer weld assemblies would maintain their integrity during shock loading, it was decided that the breaker arm would undergo stress relieving.
One of the few companies in the area with large enough furnaces to treat entire breaker arm at once was the Milwaukee Steel Treating Company. The arm was placed in their furnace and brought to a temperature of about 1400 degrees F., held for 12 hours, and allowed to slowly cool.
10. Safety keeper bolts.
In the design of the new breaker arms, the existing L-shaped hardened steel hammers were to be mounted at the ends of the breaker arms, and the cutter bars bolted onto the hammers. The center hole of the vertical cutter bar fell on the centerline of the six inch thick arm. It was decided to drill and tap the arm behind the hole so that a 1 1/2-12 bolt, could be installed which not only would held hold the cutter bar secure to the hammer, but also the hammer to the shaft.
The hole was drilled and tapped by Neilson Machine of Racine, who did the work while the breaker arm was on the trailer. They used a drill anchored magnetically to the arm for boring the hole, and painstakingly cut the threads manually. It was intricate work.
11. Cleaning up the existing shaft.
When the original breaker arms failed and fell off, large steel gouges and projections were left in the fracture areas of the upper and lower sections of the shaft. The gouged areas had to be adequately filled, and any projections ground off flush, so the shaft would have an acceptable rotational balance.
12. Plywood pattern.
The new breaker arm was heavy and clumsy to work with; it had to fit in place on the first try. Because a heavy duty crane would be required to lift it and set it into place on the central shaft, everything had to be sized correctly. An error in the arm/shaft diameters would mean that the grinding work would have to be repeated until a fit was achieved, with the crane on expensive standby during the rework activity.
A 6 foot by 6 foot by 1" thick marine plywood pattern was fabricated for use as a gauge, with the center hole cut into it as close as possible to a 48" maximum diameter, the same as the nominal ID of the breaker arm. After each grinding and filling operation during the shaft refinishing, the pattern was placed over the shaft to check fit.
The pattern indicated any high points on the outer surface of the shaft that would need more grinding and polishing, or any low points in the gouged areas that required more fill-in. Eventually the shaft neatly accommodated the 'gauge,' thus indicating a good fit with the new arm.
13. Spacers.
The breaker arm was to be placed strategically in the 23 inch free area between the top of the shaft and the first shelf of the shaft assembly, an area which constituted the initial impact chamber. The centerline of the breaker arm was about 12" above the shelf surface, and to maintain this height, eight spacers, each 3" x 3~ x 9" were set at equal points and welded into place. The breaker arm would be set on these spacers.
14. Setting the new breaker arm.
The heat treated breaker arm was delivered to Big Mike. The yard crew fastened the cutter bars into place with the 1 1/2-12 grade 8 bolts and nuts, torquing them to 3200 foot-pounds.
The shaft was covered with lubricant to ease installation. The Grunau riggers then attached the chains, keeping them as evenly in length as possible. Slowly and deliberately the crane operator raised the breaker arm, clearing the top of the shredder, then swung the breaker arm across to a point directly above the waiting shaft. Then, even more slowly and gently, operator began lowering the arm to the point where the riggers could guide it into position.
As the breaker arm inched down there was a brief hangup. The breaker arm was not precisely square to the shaft, having a slight degree of cant because of variables in the length of the rigging chains. The riggers adjusted the chain's turnbuckles, lengthening one, shortening another, squaring up the relationship between the breaker arm and the central shaft. The arm moved still lower, finally settling over the end of the shaft, moving down it bit by bit.
Another hangup occurred; the riggers stood atop the breaker arm, jiggling it into place, until it aligned itself squarely on the shaft, descending into place atop the spacers with a satisfyingly solid thunk.
15. Welding the breaker arm to the shaft.
The Grunau welders manually rotated the breaker arm about the shaft, on the bearing surface provided by the spacers, to make sure the ends of the breaker arm cleared the walls on all sides. On the breaker arm/shaft contact area itself, as a result of the flame-cutting operation there was a 1/16 to 1/64 inch space between the breaker arm and the shaft, so the welders shimmed the breaker arm and shaft into concentricity.
Next the breaker arm was securely tacked to the shaft on both top and bottom to prevent either of units from moving when the root weld was made. After the root weld was completed, the finishing passes began, each separate pass welded shaft opposite so as to maintain evenness. The weld length was 151 inches at each bevel, a total of 352 inches overall.
At this point the breaker arm rested on spacers, but was not welded to the spacer tops. In the remote possibility that the bevel welds would fail, the breaker arm would simply rotate on top the spacers. Even if both the breaker arm/shaft welds and the spacers failed, the breaker arm still would merely rotate loosely, free of the shaft. It would be almost impossible for the new breaker arm to fly off the shaft even if the breaker arm cracked at one narrow section of the breaker arm.
16. Repairing and repositioning the walls.
With the failure of 'the old breaker arms, and the subsequent damage to the integrity and concentricity of the walls, Big Mike asked Grunau to bring the shredder walls back into operating condition. To accomplish this, a crew of Grunau iron workers had been busy with wall repair. They used heavy duty come-alongs, pulling the walls back in place, heating and welding the area to keep them there. Some of the shims had been broken out of the joints where the three 120 degree wall sections met; these also were replaced.
As mentioned before, when the breaker arms had battered the walls, large cracks were made in the one inch steel wall backing plate. All of those cracks had to be heated, straightened, filled with weld, then ground. Over the wall sections were installed new hardened steel cutter plates, fastened in place at a bolt/nut torque of 3200 pounds.
17. Reinstalling the roof.
When the walls were finished, and the breaker arm was welded on, the roof with its hardened steel under-surfaces was craned into place. Because of the difference in crane technique between roof removal and replacement, other parts of the structure interfered with setting the roof straight down. It had to be swung in on an angle, which didn't offer a smooth transition onto the upper portion of the shaft bearing housing. Only by liberal lubrication of the bearing housing did the roof go back on properly so it could be securely bolted into place.
18. Bringing the lube system on line.
Integral and critical to the operation of the shredder was the forced lubrication system which pumped oil to the upper and lower bearings. It usually took about 20 minutes to get the lube system into full operation. It took much longer to achieve any flow as the hoses, pipes, and filters in the dual system were filled with congealed oil, because of non-use, and had to be flushed out and cleaned before full flow was maintained.
There were some minor problems with the lube system. When the system got to full flow it was noted that there was leakage around the lower seal of the upper bearing. The company installing the pressure seal had put it in upside down, and instead of the lube oil's pressure holding the seal to the shaft, the oil bypassed the seal. Reversing the seal put a stop to the leaking.
The shredder control system was instrumented with safety features built in, including one for the lube system. For the shredder motors to run, the lube systems had to be on and at full flow.
19. Bolt check and initial start-up.
Prior to initial start-up, the breaker arm welds and the bolts on the cutter bars were checked, as was the drive assembly power train. The four 500 hp motors were lubricated per GE specifications. Next, the main lubrication system was actuated, and in a few minutes the green lights that indicated proper pressure and flow all lit.
Number one motor was started. It groaned as it began to turn the shaft and breaker arm slowly, the current rising from 0 to over the 200 amp no-load operating level. As the speed rose, the current would peak to about 350 amps, then drop td 200 as the motor reset, ever increasing the rotation. At last the shaft was turning at design speed on #1 motor, about 180 rpm. At this point the three other motor-ready lights came on and the other motors were powered up.
All four motors came in at full rpm; the shredder was operating smoothly, and the usual small sub-sonic vibrational component could be sensed, indicating that the balance of the breaker arm was satisfactory.
This initial run test continued for about 15 minutes; the power was then turned off, the machine coasting to a stop, which took about 30 minutes from cut-off to final halt.
20. More run-in testing.
After the shredder had come to a halt, the cutter bar bolts were again checked and found to be under the 3200 foot-pound torque requirement. Again the shredder was put through the startup procedure, this time running at normal operating speed for about 30 minutes. The motors were cut off again and the shredder breaker arm coasted to a stop. Cutter bar bolts were checked and found to have lost about 300 foot pounds.
Another light test run was made; the shredder running for about 45 minutes, and this time the cutter bar bolts had lost about 200 foot pounds, still relatively tight. The shredder was ready for testing with scrap steel material.
21. Testing on empty drums.
Big Mike's company processes 55 gallon steel drums discarded by a Cranberry company in Kenosha. The scrap food product drums were of light steel, an ideal material on which to begin the final testing series.
Before the test run, the cutter bar bolts on the breaker arms were again brought to a tightness of 3200 foot pounds, after which the shredder start-up sequence was begun. The American crane used its clamshell bucket to load the empty drums on the feed conveyor which brought them up to the load-off end of the conveyor and dropped onto the gravity slide chute. The feed roller was turned on, ready to crunch the drums and roll them into the shredder for the first test of the breaker arms.
The shredder operator caught the drums one by one with the lugs of the hydraulic feed roller, flattened them slightly, and fed them steadily into the initial impact chamber. The racket could be heard over the whirring of the shredder as the rotating breaker arms hit the steel drums. The drums were being smashed apart by the impact of cutter bars traveling at 94 feet per second, and at a developed force of over 420,000 foot pounds. As the torn shreds of steel passed down through the other cutter bar levels of the mill the finished shredded product began to emerge onto the shaker section.
The drums had been converted into small, almost spherical, nodules of hot metal, which were fed to the shaker section by the rotating floor plates. From the shaker the nodules then went into the area of attraction of the magnetic drum, and were drawn onto the drum by the magnetic field. The nodules fell onto the loadout conveyor as the section of the drum over the conveyor was demagnetized. The nodules were then carried on the belt of the loadout conveyor to a storage pile at the drop-off end.
The shredder was run for about 45 minutes in this first test with actual material. When checked, the cutter bar bolts had loosened slightly, so were tightened again for the next run. More of the empty drums were run through the machine until all on hand had been processed. The first test of the shredder had been successfully completed.
22. Testing on white goods.
The next test consisted of running the so-called white goods through the shredder. White goods consist of washers, dryers, refrigerators, water heaters, etc., so named because of the white porcelain exteriors.
The run was on three washers and three dryers. The feed conveyor slowly dragged them up to the end where they fell into the slide chute. When the operator started the hydraulic feed roller; the first washer was drawn into the feed roll throat, and then into the initial impact chamber where the breaker arm began chopping apart the washer as though it were tinfoil.
Next came the dryer. It too was crushed down and fed into the initial impact chamber, passing through the hammer levels, with the now familiar steel nodules emerging from the shaker section.
Then the rest of the white goods were methodically fed into the shredder, making their way through without incident. Some time later, a few water heaters were put into the shredder, one after the other. In process, the toughly made heaters caused the motors to draw more current, a phenomenon found to be typical for water heaters of all makes.
The steel from the appliances were caught by the magnetic drum; however, the foam insulation and the aluminum from the units were not attracted by the magnet, and were dropped off onto the dirt conveyor which ran at right angles to the loadout conveyor. More white goods were fed into the shredder, among them refrigerators and stoves. All went through easily in a run that had lasted about an hour.
The cutter bar bolts were again checked and tightened where necessary. A second run was made which exhausted the supply of available scrap; about 20 tons of scrap had been processed into small nodules very desirable in steel mill remelt application. The next day saw a 90 minute run, the limit of availability for the white goods on hand. All operations were completed without incident.
23. Dust control.
It became apparent from the amount of dust from the shredder, that a means to remove particulate matter from the exit of the machine had to be installed, so a large cyclone separator was set in place adjacent to the shredder. The high capacity blower that had come with the shredder was put in running condition, an 18" exhaust line was run from the blower to the side of the separator, and a dust collection line was connected to the separator dome.
At the cone bottom of the separator, a cut-off was connected which could be opened at the end of a run, allowing captured dust to be dropped into containers. The surplus air and dust exiting from the dome top of the separator was captured by a long bag filter.
24. Testing on autos.
The next phase, the critical phase, of the shredder testing process was to begin sending autos through the machine. We started small, namely, a chassis from a compact auto which was mostly skeletonized. The motor, transmission, and gas tank had been removed, as is usual for any auto to run through, but much of the body metal also was missing.
The body was craned onto the feed conveyor, making its way up to the end where it dropped into the slide chute. The hydraulic roll grabbed the auto body and began to feed it into the initial impact chamber. As the auto was fed into the impact chamber, more noise was produced than that from the steel drums and white goods, and power draw rose to about 300 amps. In seconds the entire auto had passed through the chamber.
Then a larger auto was sent through less the engine, transmission, and gas tank. It too entered the impact chamber and was shredded without any apparent strain. Another large auto, an Oldsmobile sedan, was maneuvered into the impact chamber, passing through the roll in about 30 seconds.
At this point the operation was halted and the shredder allowed to empty itself preparatory to shutting down and checking the cutter bar bolts. This time most of the ten cutter bar bolts had loosened considerably and had to be retightened.
25. Exhaust fan over loadout conveyor.
One of the negative effects experienced during the time auto bodies were being run through the shredder was the significant amount of non-metallic material generated. There was foam rubber, plastics, small pieces of tires, and other materials, such as paper, that were too light to fall onto the dirt conveyor. A small portion of the debris associated with these items 9not mixed in with the product, went over the magnetic drum, and got carried along with the steel nodule loadout flow of the conveyor, ultimately falling into the storage pile.
In order to remove such light debris from the final product, an exhaust fan was mounted over a point on the first third of the loadout conveyor. As the product passed under the mouth of the fan the vacuum removed most of the light stuff and allowed the cleaned product to continue on.
26. Testing white goods and autos mixed.
Big Mike brought in another batch of auto bodies, and a test run was made with auto bodies alternating in sequence with white goods and empty drums. For the 90 minute run, an auto would be put through, then some white goods, even some household scrap from the municipal drop-off station was mixed in with the rest.
After this last test run it was felt that the design and performance of the new breaker arm was sound, and that Big Mike could start normal scrap production. However, I thought further testing was needed to determine the length of time the machine could be run before the cutter bar bolts became dangerously loose, which could cause problems. Because the bolt sets loosened as they did, I wanted to develop an effective bolt checking schedule as part of regular maintenance.
27. Resolving the bolt loosening problem.
During the 90 minute test period the bolts again had loosened. We were paying close attention to the loosening, and for good reason. Tight bolts on the hammers and cutter bars were critical to a safe process sequence.
Back when Big Mike had purchased the shredder, it was supplied with a number of 1 l/2"-12 grade 8 bolts for the cutter bars. The theory of tightening the bolts to a given torque had been explained by the manufacturer.
Mike was told that when new bolts and nuts were installed and torqued to 3200 psi, the bolts would stretch during operations and would have to be re-tightened on each use for a short period. However, when the bolts had achieved a certain "stretch limit," they would stretch no further, and the bolts would remain tight after this limit had been reached. I didn't believe it.
But Mike found it was common for the bolts supplied with the machine to loosen from a torque level of 3200 foot pounds down to 1800 foot pounds and even lower, and if left untightened would gradually loosen completely and possibly vibrate off. The yard was were under the impression that this was the way the bolt and nut combination was to work. In fact, some bolts and nuts had already been loosened and lost through the machine.
It became necessary to order replacement bolts for safety's sake, and I discussed the requirements and specifications with a vendor closer to the Big Mike's plant, in this case, the Big Bolt Company of Itasca, Illinois.
During the meetings with Big Bolt it was learned that many of the wall bolts and cutter bar bolts that had been provided with the shredder were of too loose a fit to meet bolt standards. Using a regular go/no-go ring gauge for the 1 1/2-12 bolt, it was found that the bolts that had come with the machine not only would be screwable onto the 'go' gauge, but could also be screwed onto the no-go gauge, with room to spare.
When the bolt/nut combinations furnished with the machine were screwed together the fit was very loose, and there was significant wobble, suggesting that the set did not have enough effective thread-to-thread interface for strength in thread-area contact. This lack of thread-area contact must have allowed the bolts to loosen so quickly.
After the Big Bolt corporation's nuts and bolts were used, loosening of the sets during shredder use was brought to an absolute minimum. Once the bolts had been stretched and retightened, they stayed tight.
28. Overloads to the shredder.
The overload tests of the shredder system were inadvertent. The yard operating procedure was that all of the scrap to be run through the shredder was to be screened as it was repiled next to the feed conveyor.
But, unseen in the yard, some just underground, some mixed in with the scrap, were heavy steel objects, such as die blanks from a local brass casting company. Although careful screening was done as a matter of course in the test runs, pieces such as die blanks could be missed.
Of course one was indeed missed, a billet piece of hardened steel about 5x3x16' that got fed into the shredder. However, it passed without incident through the cutters, making a good deal of noise on its way.
Another such piece went through the system, and even small sections of I-beam made it through without damage to the breaker arms. It was Dan's opinion that had such pieces of steel entered the system with the old arm design, the arms would have failed.
29. Final test; production operations begin.
The final test of the shredder involved sending through a string of scrap auto bodies in sequence, one right after another. The motors, gas tanks, and transmissions had been removed from the large autos; the 4-cylinder smaller motors and transmissions in the compact cars were drained of oils and fluids and left in the bodies.
The bodies were conveyed up the feed conveyor, passing down into the slide chute, and then fed into the shredder one by one. All of the bodies went through without a hitch, taking an average of 20 seconds to pass through the initial impact chamber.
The shredder operator in the control tower kept the feed even. As the auto bodies went through he watched the motor current drain so it would not become excessive so as to blow a breaker. Shredding the auto bodies generated a substantial amount of heat in the machine internals, so the operator shredder's water spray system to cool inside. The spray also reduced the unrecovered dust which sometimes blew out a small amount through the shaker section.
After the autos had passed through the shredder, white goods and drums began to move through. The storage pile beneath the end of the loadout conveyor became ever higher, piled with tons of hot clean product, ready to be loaded on a truck for hauling to the mill. The steel mills had been impressed by the size, density, and cleanliness of the product made by the shredder.
When the final test run was over, two hours later, the cutter bar bolts were checked again, and it was found that the torque settings on the new fasteners obtained from Big Bolt company had remained secure.
30. Back in normal production operation.
Since the shredder was successfully tested, it has been operating six days a week, running problem free at full capacity, the continuity of its operation subject only to availability of scrap. Because of the reliability of the new bolt sets, it is required only to check cutter bar bolts daily, just before each run. The wall bolts require checking even less often.
III. THE RETURN OF THE PIRANHA!
It was approaching November and the Piranha was again viciously chewing up scrap and spitting out hot grist for the steel mills. The operator worked the hydraulic levers, controlling the feed of autos into the maw of the shredder, occasionally glancing out towards the eastern horizon where Kenosha's Lake Michigan harbor reached out to the autumn blues and greens of water and sky meeting in the hazy distance.
The shredder pounded along, shaking dried brown leaves from the yard's cottonwood trees into drifting down, some slowly settling onto the feed conveyor where a steady flow of scrap was making its way up the conveyor.
It was another, better morning, a working morning, ten long and hard months later, the beginning of a clear October day, with the giant breaker arm spinning in clattering song, the entire shredder complex rumbling in familiar and comforting sub-sonics, the dust removal system in easy respiration, the odor of hot metal and cranberries, the white water vapor rising in the sunshine, the magnetic drum turning slowly and effectively, and the loadout conveyor bringing the shiny steel nodules out to the ever-growing storage pile.
Big Mike noted the scene with much satisfaction. He was oblivious to the noise and jumble of the scrap yard, sensing only the inherent beauty of his massive machine.
The Piranha was back in business, hungry as ever!
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Email Address: drer@execpc.com
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