Current State of the Art in Shredding Plants (and Beyond)

People quite often ask me what is new or exciting in shredding procedures or equipment, and that is a favorite topic of discussion with me. I am constantly surprised to find that some people are far behind the current knowledge and that they are trying to “reinvent the wheel” in some areas while at the same time pushing the development envelope in other areas.

The best thing to do, of course, is to understand what has already been invented, retain the useful qualities, and continue to make improvements in other areas.

Not everything that will be discussed will be new and exciting, but you may be sure that most of us can learn a great deal from trying to understand what others are doing and which ideas they are trying.

In order to conduct an orderly and complete discussion regarding a shredding plant, I believe that it will be useful to start at the feeding side of the equipment and continue through the process.

Infeed Conveyor

The most popular type of infeed conveyor is made using caterpillar-type rollers and a chain, which carry a wide track pad to make a steel pan conveyor. This conveyor should be manufactured from heavy steel components, and the sideboards should be especially strong at the feeding area.

The track pads are generally supported with rail throughout the entire length of the conveyor, so that if a heavy piece of scrap is dropped onto the conveyor, the pad will only deflect a short distance and will not be damaged.

This type of infeed conveyor is normally driven in one of two ways. It is mechanically driven with a fixed speed or it is hydraulically driven with a variable speed.

The variable speed design is believed to be the most productive, particularly when the power system will allow the conveyor to operate at much higher speeds than those that are normally required. This lets the shredder process scrap at a high tonnage rate, even when very light or bulky material is being shredded.

Very high production shredders need the infeed conveyor to operate at speeds of up to 100 feet per minute.

Feeding Device

One preferred design of feeding devices is constructed of very heavy steel components and has replaceable liners on the sides, at least up to the feeding rollers system.

Almost all high production shredders use some variation of the double feed roller system, whether mechanically or hydraulically driven. At Newell Industries Inc. (NII) we provide either system depending upon the wishes of the operator. For high production shredding plants, a Super Double Feed Roller (SDFR) is highly recommended.

An SDFR features a hydraulically driven unit that has the ability to over-feed the shredder. This requires roller speeds of ten RPMs or higher, which requires a 100 to 150 horsepower hydraulic power unit. The speed of the feed roller is tied to the amperage requirements of the main electric motor, so that when the motor is under-utilized, the feed roller speeds up. When the motor is fully utilized, the feed roller either slows down or stops. The electrical connection is able to do this better through a PLC much faster than a human and it is much more consistent. The PLC never has a bad day.

SDFR units also feature larger shafts and bearings than the previous designs of feed rollers. This is seen as important to reduce down time and to provide for longer life.

In the category “and beyond,” we have installed a shredder with a feed roller that has two different size rollers. The upper roller is 60 inches in diameter, and the lower roller is the standard 36 inches in diameter. This was done to try to improve the ability of the feed system to work without very much human intervention. The larger diameter roller is able to “walk” up on incoming scrap and crush it without the operator needing to use the hydraulic cylinders to raise the feed roller. Transportation vehicles have increased in size over the past few years and a very high percentage of automobiles and sport utility vehicles are in use. In order to get these larger vehicles into a modern shredding plant, it is necessary to deal with the greater height of the incoming material. The larger top feed roller addresses this issue.

Shredder

It is not often disputed that Super Heavy-Duty shredders are the current state of the art in shredding equipment. The SHD shredder is designed to accept unshreddables with a minimum chance of damage to the equipment. This is accomplished through the use of a very heavy housing constructed with four-inch steel plate, strong reinforcements, special liner plate designs, an effective reject door system, large 14-inch to 18-inch main bearings for the rotor, special designs of rotors, and with hammers designed to take the heavy hits that occur when a massive unshreddable is hammered.

SHD shredders allow the operator to process a much wider range of material than traditional shredders. This can allow the operator to enjoy higher tonnages of material that is available at lower after-shrinkage costs, to produce a product that is cleaner and denser at a lower cost per ton.

Current state of the art shredders feature oil circulation systems for the main rotor bearings with monitors for oil temperature and flow. This keeps the bearing cool and clean and gives early warnings when problems occur. In the “and beyond” category, some sophisticated operators are now having oil in this system tested and analyzed on a regular basis. This is used to predict bearing performance and to allow the operator to change the bearing before a failure occurs that could lead to consequential damage.

NII continues to believe that the Battle Rotor provides the best service when shredding with an SHD shredder, but there are some knowledgeable operators who prefer a spider rotor, even for this application. There are some heavy-duty spider rotors being built by NII and by others which feature heavier spiders and heavier castings that are used to cap the spiders.

Although the reject door has been a part of shredders for many years, there have been improvements made to reject doors in SHD shredders. The reject door has been lengthened and the effective opening area has been increased. This has been done to increase the chances of earlier rejection of unshreddables, which helps to minimize the chance for serious damage to the shredder.

During the past year or so, significant improvements have been made to the design of the cast grates that are used in these shredder systems. These designs are available for all manufacturers’ designs of shredders.

Observation of the wear pattern on grates often indicates that the first few bars wear out before the remaining grate bars. The entire set is normally changed at the same time. Thus there is a significant amount of usable grate material that is often thrown away.

We have designed sets of grates so that the first few grate bars of the set are cast with thicker webs than the remainder of the set. This significantly raises the tonnage that a set of grates can process before it needs to be changed. We have also designed the grates so that the holes on the upper part of the grate have parallel sides for the usable life of the grate. This solves the problem of the shredded product losing density as the grate set wears.

Electric Motor Systems

There are a number of direct current electric motor installations being made to power shredding plants. The initial results seem to be encouraging, but it is too early to categorically claim that the direct current motor system is clearly better than the alternating current electric motor.

The direct current (DC) motor allows the operator to vary the speed of the rotor, but our observation is that once the operator has decided what speed is desirable, the RPMs usually stay at that point.

The DC motor also allows the operator to precisely set the maximum demand that will be allowed. This can result in lower power bills. The offset to that is that the lower demand also means that there is less power available to rapidly accelerate the rotor back to the desired RPM range.

Direct current motors seem to spend a considerable amount of time running at less that the desired RPM range. This reduces the striking force available from the hammers. While the complete effect of this is not yet clear in terms of tons per hour that are possible, it is believed that operating the rotor at less than the optimum speed will result in lower tons per hour production.

The direct current motor costs less than a new AC motor, but most of the direct current motors that are being used are rebuilt motors. If a new direct current motor were to be purchased, the cost would be higher than for an AC motor.

There is some belief that the direct current motor may require more maintenance than an AC motor.

The above observations do not lead to a final conclusion but rather indicate that this is an issue that should continue to be studied.

Conveyor System

All conveyor applications must be designed to eliminate spillage, to carry the proper amount of material, to keep the material wide and thin and to be properly safeguarded. Conveyors must have all pinch points protected, lockouts must be available and pull chords should be accessible in all places where a person could be working.

In conveyor applications where the material being transported needs to be wide and thin, for example, when the material is about to be separated in some way, we prefer to manufacture a flat conveyor with steel pan sideboards.

When material is being transported to a stockpile, it is probably best to use a deep trough conveyor, which can be less expensive to buy and to operate.

World’s Best Practice Management

Perhaps nothing will improve the operation of a shredding plant more quickly and at less cost that the implementation of a World’s Best Practice Management system. This is a very simple concept. It means elimination of waste and adopting a system of making continuous improvements.

The author is former chairman and CEO of Newell Industries Inc., San Antonio, Texas.