Low Speed Shredders: An Option for Some Recyclers

Low-speed shredders can accomplish a number of recycling tasks, sometimes while using less energy and making less noise.

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July 26, 2001

The increasing awareness that the Earth’s material and energy resources are finite is beginning to make some industries rethink the economics of conservation.

In some industry segments the raw goods and energy required to make products is still considered cheap and plentiful, and land is still considered an inexpensive resource that can be used for the disposal of wastes that companies produce. But in other segments (and parts of the world), spiraling raw material costs and a drastic reduction in landfill space has sparked a movement to conserve through recycling and resource recovery.

Industries that do have an economic incentive to conserve are now looking for ways to improve their profitability and remain competitive by reducing both raw material and waste disposal costs. With ever-increasing success, entities ranging from municipalities to manufacturers are investing in aggressive recycling and resource recovery programs that lower disposal costs while providing an alternative (and less expensive) source of raw materials.

MAKING PROCESSING COST-EFFECTIVE

To date, one of the greatest impediments to recycling certain waste streams has been the difficulty of separating waste from reusable materials. Historically, this process has been labor intensive and dangerous. In some instances, destruction of waste is performed manually using band saws, chain saws and hammers. High-speed shredders can improve speed, but in some cases can produce unacceptably high noise levels.

A method of product separation and volume reduction that has proved suitable for some corporations, municipalities, and recyclers is low-speed, high-torque shredding. Makers of these types of shredders tout lower vibration and noise levels, lower power consumption, and reduced installation costs as the advantages of the machines.

One manufacturer of low-speed shredders is SSI Shredding Systems, Wilsonville, Ore. The company, which has been making low-speed, high-torque shredders since the early 1980s, offers a line of hydraulic and electric rotary shears ranging in size from five horsepower to 800 horsepower units.

“We are frequently compared to granulators or hammer mills that run from 250 rpm to 1,000 rpm,” says SSI senior engineer Chris Nichols. “Typically, a hammer mill has to be in the 1,000 hp to 1,500 hp range to deal with heavy material that our shredders process using only 150 hp to 200 hp,” he notes.

Pointing to one of the advantages of low-speed shredders, Nichols adds that high-speed machines “are large pieces of equipment with high dynamic loading [that] require substantial foundations and vibration isolation. We typically run at low speeds and can put our equipment on normal floors. This can save a great deal in the installation cost of our shredders.”

HOW THEY WORK

The low-speed shredders made by SSI process materials by continuously drawing feedstock past two counter-rotating cutter shafts. The shafts, outfitted with cutting discs separated by spacers, rotate at five to 30 rpm. The cutters have one or more beak-like hooks, or knives, that intermesh with cutters on the adjacent shaft. The hooks grab, shear and tear the material in a continuous scissoring motion.

When overfeeding occurs, or when non-shreddable matter is introduced into the feed hopper, the machine automatically shifts into a reversing mode, and clears the overload condition. This feature minimizes machine damage and wear and reduces downtime and repair costs.

WHY THEY’RE USED

Industrial shredders are often used to destroy defective, confidential, or date-expired materials and products. Destroying these products eliminates potential security risks and product liability problems that could result from theft during product storage, transportation or disposal.

Art Van, a furniture sales company based in Detroit, employs low-speed shredders to economically dispose of old box springs, mattresses, couches, and debris from its 26 stores. As a customer service, Art Van picks up and disposes of old mattresses for customers who have made new purchases. This assures that the old mattresses are not collected, re-covered and sold on the “gray” market.

“SSI solved a container problem for us,” says Art Van maintenance supervisor Ed Michael. “We were using open-top containers from disposal companies that cost about $400 each, and we were using about 15 of them a day. By shredding the mattresses, we can get away with three to four per day. The shredder helps us provide a service to our customer without overloading landfills.”

While this application entails reducing the cubic volume of waste, other applications can aid in recycling efforts. For example, shoe manufacturers can shred and granulate defective and worn tennis shoes that can then be reprocessed into running track surfaces. Industrial shredders are also be used in the tire recycling arena, turning scrap tires into two-inch square chips for burning, or further reduced into crumb rubber for recycling into road mix or playground cover.

Shredding is increasingly used as a first processing step. The efficiency of granulating, pulping, or blending of baled, bulky or otherwise difficult to process material can be greatly improved by shredding it first, shredding advocates say.

“Our shredders are ideal for the pre-processing of materials requiring size reduction, volume reduction, or liberation of contaminants,” says Nichols. “And they improve the ability to fully utilize downstream equipment.”

Shredders are also used as part of a multi-step recycling process to recapture the precious metals commonly embedded in electronic scrap from computers, circuit boards, motors and switching devices.

Low-speed, high-torque shredders are critical to the operation of Butler-MacDonald, Indianapolis, a company involved in the recycling of scrap electronic and telecommunications equipment. Initially, they shred or pulverize the material to segregate steel, aluminum, plastics and nonferrous copper-bearing materials. Later, they use pulverizers and granulators for further size reduction, prior to final material separation.

“Taking a large product and trying to reduce it to half-inch or smaller pieces can’t be done in one bite. SSI shredders are very effective at taking that first bite and giving us a sized product that the downstream machines can digest,” says Mike Thierault, production manager at Butler-MacDonald.

A PLASTIC EXAMPLE

Sundance Products, Gainesville, Ga., a custom compounder of recycled polypropylene resin, has made low-speed, high-torque shredders a key part of its operations. The company purchases scrap polypropylene in all shapes and sizes, processes the material and then extrudes the plastic into pellets for resale to injection molding busineses. Approximately 80% of the scrap material they purchase must undergo a series of size reduction steps. The initial step in the process is shredding.

“We drop about 160,000 to 200,000 pounds a day through our shredders,” says Steve Hungerford, vice president of raw materials procurement and sales for Sundance Products. “The shredder allows us to take large plastic sheets in different forms and bring them down to a size that can be handled through the separation processes that follow. Throughout the years, we have made adaptations to the teeth that shred and tear the material. SSI works closely with us to determine efficient tooth configurations. The teeth can also be changed if the product stream changes,” he adds.

“Some companies focus only on power and the size of the cutter,” Hungerford continues. “With us the cutter configuration is very important. We look at how aggressive the hook is, how much material it can grab, the hook height and width, cutter material, and the number of hooks per wheel. We place a lot of importance in the cutter configuration so the shredder doesn’t take more than it can handle.”

In addition to the configuration of the cutter, the shredder’s power supply can also be tailored to the application. Hydraulic shredders with up to 800 hp are available for applications where overly large or tough material is being fed, or for conditions that require frequent reversal of the shredder blades.

Low-speed shredders have found homes at hundreds of industrial, waste handling and recycling work sites worldwide, performing key material processing tasks.

This article was supplied by Jeff Elliott of Power PR on behalf of SSI Shredding Systems Inc.

 

Locked in Place

Two features recycling production manager Mike Thierault of Butler-MacDonald, Indianapolis, likes about his shredding system are the shaft design and the way the cutters are locked down. The cutter stacks for shredders may be mounted on hexagonal shafts or round-keyed shafts. The Advanced Cutter Locking System (ACLS) from SSI Shredding Systems, Wilsonville, Ore., secures the cutters onto the shaft with a force of approximately 100,000 pounds. This keeps the cutters stacked more tightly on the shaft and eliminates labor time to retighten a threaded nut, and minimizes the possibility of shaft damage or failure due to movement of the cutter wheels.

“We offer both round and hexagonal shafts because with the ACLS we can keep the cutter stack tight,” says Chris Nichols. “In the past, a loose stack on a hexagonal shaft would cause it to look like a corn cob. If a stack loosens on a keyed shaft, it might wear a little bit and indent the key, but the keys were easily replaced. Now, our ACLS assures that the cutter stack will remain tight and allows our customers to select hexagonal shafts for extremely high impact applications.”

The labor savings involved with the ACLS set-up can be important to some companies. “For example, one of our companies had a severe application where they were running their machine 24 hours a day. The old bearing nuts were loosening every seven to eight hours and had to be retightened. Now, with the ACLS, they tighten the stack only when new cutters are installed.”