Low-Speed Shredders
Jun 9, 2014, 09:16 AM
March/April 2003
For size-reduction tasks, there’s nothing like a rotary shear shredder, which uses low speed and high torque to process.
By Kent Kiser
There’s more to shredding than just high-speed, high-impact, hammermill-type automobile shredders. There are also low-speed, low-impact, high-torque alternatives in the form of rotary shear shredders. While these machines can’t pound a car hulk to bits in seconds, they can handle an impressive array of other shredding challenges.
As with most shredders, the main purpose of rotary shear shredders is to reduce material to manageable size. Such size reduction can be beneficial or necessary for several reasons, including (1) to process material into an acceptable—and more valuable—recyclable commodity, (2) to prepare material for further processing like granulation, (3) to make material easier and more cost-effective to handle, store, transport, and/or discard, and (4) to destroy defective, confidential, or date-expired materials and products.
Many rotary shear shredders are “generalists”—that is, they’re designed to process a range of industrial, municipal, and hazardous scrap and waste, including tires, pallets, C&D debris, metal scrap, white goods, paper scrap, 55-gallon drums, asphalt, concrete slabs (with rebar), glass, plastics, stumps, railroad ties, furniture, mattresses, and more. Other rotary shear shredders are “specialists” in that they’re designed to process certain materials—such as scrap tires—particularly well.
While rotary shear shredders—like their hammermill-type counterparts—exist to size-reduce material, the difference lies in how they do it. Hammermill shredders feature a single large rotor equipped with hammers that swing out as the rotor turns at high rpms and high horsepower, processing material against a fixed anvil.
In contrast, rotary shear shredders can feature one to four shafts equipped with cutting blades that rotate at low speed and high torque, processing material against a stationary plate or other rotating blades. This low-speed, high-torque approach makes rotary shear shredders a better choice than hammermill shredders for certain materials and processing jobs.
Covering the Basics
To further grasp how rotary shear shredders work, it can be helpful to start at the beginning of the process, when material is fed into the system:
Feeding Factors. There are two ways to feed rotary shear shredders—batch and meter. In the first case, batches of material are fed directly into the shredder’s hopper by a skid-steer loader, forklift, front-end loader, or material handler via a grapple. With meter feeding, material is automatically fed into the machine via a conveyor whose rate is controlled by the shredder’s programmable logic control (PLC) system. While meter feeding is more efficient than batch feeding and while it can minimize overloading of the shredder, it isn’t practical for all operations and materials.
After entering the hopper, material is drawn into the cutting chamber past one, two, three, or four shafts. The shafts, equipped with knives and spacers, rotate at low speed to achieve maximum torque and cutting ability. While the definition of “low speed” is open to debate, many manufacturers maintain that true rotary shear shredders operate at 6 to 50 rpms. In general, the slower the shaft’s rotation, the greater its torque and the less “impact load” it generates as the knives encounter the material. The knives typically have one or more beak-like hooks that grab material and pull it through the rotating blades, thus processing the material.
The Shaft Situation. Regarding the number of shafts, a general rule is that the more shafts in a rotary shear shredder, the smaller the processed pieces it can produce. By this rule, therefore, single-shaft machines, whose rotating knives reduce material against a fixed knife or plate, produce the roughest cut and largest processed pieces. (The horizontal-feed single-shaft shredder with integrated screen is an exception to this rule, however. This design has gained broad acceptance among plastic recyclers and has virtually displaced dual-shaft shredders in some processing niches, says one manufacturer.)
Two-shaft shredders are the next step up. Their dual counter-rotating shafts have cutting discs that intermesh like fingers. As the hooks on the discs pull material into the chamber, the discs cut the material along their circumference edges. Dual-shaft shredders can process material to smaller pieces than single-shaft machines, especially when equipped with a sizing screen under the cutting chamber. They should be used in applications where difficult or unshreddable materials may be encountered.
Three- and four-shaft shredders are the most effective at processing material to a small, uniform size. They do this by using a sizing screen under the shafts that enables the bottom cutters to recirculate material into the cutting chamber until it is small enough to pass through the holes in the screen. These multi-shaft shredders are also effective at liberating materials that are entrapped in other materials, such as metal pieces in plastic components.
In multi-shaft shredders, the shafts can turn at the same speed or at different speeds with differentials from 10 to 50 percent to enhance the unit’s power distribution as well as its grabbing and cutting ability.
Another shaft consideration is its diameter in inches. The general size range goes from 5 to 111/2 inches, with smaller shafts being more flexible and thus more appropriate for less-demanding jobs and thicker shafts capable of processing tougher materials and keeping the cutting knives in closer tolerance to each other.
Knife Knowledge. Among rotary shear shredders, there are two major knife designs—hook knives and Holman replaceable knife inserts. Hook knives are solid one-piece blades with beak-like hooks that can be mounted on a hexagonal or round shaft. Holman systems, in contrast, have a solid rotor with raised, serrated knife seats that are covered with bolt-on replaceable knife inserts. Hook knives are reportedly better for processing a variety of scrap materials, while Holman systems are more specialized for processing materials such as scrap tires because they offer a closer knife-to-knife tolerance and, thus, cleaner cuts.
Hook knives are available in various thicknesses, diameters, hook profiles, and number of hooks—all of which affect the grabbing and processing abilities of the shredder. In these regards, the following considerations and rules apply:
• Knife Thickness—This refers to the thickness, or width, of the cutting blade itself. The width of hook-knife blades can vary from 1/2 inch on smaller shredders to 4 inches on bigger units. Notably, there’s a limit to how thin a blade can be and still withstand the shredder’s torque load. The appropriate thickness depends on the material being shredded and the desired thickness of the processed material. If you want a 2-inch particle size, for instance, one manufacturer recommends using knives with a 11/2-to-13/4-inch width since the processed particle size will increase as the knives lose their edge.
One manufacturer also notes that a greater knife thickness can increase a shredder’s production rate due to the knife’s increased grabbing ability and other factors.
With Holman knives, the knife inserts can range in width from 1.7 to 2.2 inches in single-rotor shredders and 3.2 to 4.4 inches in two-rotor—or “double-stack”—shredders;
• Hook Profile—This refers to the shape of the hook on the knife, particularly its overall length and curve. As you’d expect, the smaller the hook, the less material it grabs on each rotation and the less material the shredder will process on each pass. When shredding aluminum scrap and plastic film, for instance, a smaller hook can be prudent so the hook nibbles at the material rather than taking big bites of it. For softer scrap, a larger hook would be more appropriate to feed the shredder more aggressively; and
• Number of Hooks—Hook knives can have many hooks, though most feature anywhere from one to five hooks. Generally, the greater the number of hooks, the smaller the final particle size. If the blade has too many hooks, however, it can attempt to do too much grabbing and overtorque the machine, says one manufacturer. Too many hooks can also lower the knives’ grabbing ability due to the decrease in the peaks and valleys across the entire stack of cutters.
Holman-type knives also vary in the number of serrated knife seats they offer, with 18 being common for primary shredders and 14 for standard and secondary shredders.
Another important knife consideration is how they’re arranged on the shaft—the so-called stack configuration. Though there are numerous configuration options, two of the most common are open and spiral. In the open approach, all knife hooks are aligned down the length of the shaft. In the spiral configuration, the hooks are arranged in a staggered, spiral pattern around the shaft. There are pros and cons to each configuration. For instance, more material can be grabbed with the open configuration, but that reduces the available cutting force per hook, explains a manufacturer. (Arranging the hooks in a chevron or V arrangement, however, is an effective design that allows open cutting without aligning all the hooks, notes another manufacturer.) In the spiral configuration, meanwhile, less material can be grabbed, but the cutting force per hook increases.
Feeling the Power
Rotary shear shredders can draw their processing power from hydraulic or electric drives that, in turn, can be powered by electric or diesel motors or generators. While some manufacturers power their multi-shaft shredders with one motor—let’s say a 100-hp model—others opt to achieve the same power by using two smaller motors such as two 50-hp units. With single-motor shredders, the drive motor must split the available torque between the cutting shafts, while two-motor shredders provide separate power to each shaft. The two-motor rationale is that two motors divide the power load and can therefore share the stress load when the shredder is overloaded or encounters an unshreddable.
Hydraulic vs. Electric. In general, hydraulic drive systems are preferred in applications that involve dense, heavy materials, a wide variety of materials, or a high frequency of unshreddables, as well as in large batch-feeding situations in which materials are so large that the shredder could instantly overload. Hydraulic drives are preferred in these circumstances, in large part, because of their ability to handle a higher frequency of reversals than electric drives. Also, hydraulic drives can cycle from forward to reverse to forward in less time—a reported 3 seconds vs. 20 seconds for electric drives.
One manufacturer takes advantage of this reversing ability—it offers a feature that allows the user to program the hydraulic drive to follow a specific forward-and-reverse cycle, enabling the machine to process material to a smaller size than could be achieved through straightforward shredding. Among their other advantages, hydraulic drives are suitable for reduced voltage starting and don’t require starting and stopping of an electric motor for reversals. In addition, a hydraulic power unit can be installed a substantial distance from the shredder and enclosed for noise isolation and ease of servicing. That said, the overall preventive and ongoing maintenance costs of a hydraulic system can be greater than those for an electric system.
Electric drives, in contrast, are generally smaller and more energy-efficient than hydraulic drives. That’s because hydraulic systems have an up-front calculated loss of 5 to 15 percent efficiency (some say even more) when electric motor energy is converted to hydraulic energy. Among the benefits of electric drives, they are quieter, typically less expensive to install, and require less floor space than hydraulic units. Electric drives also have fewer operating components and therefore require less maintenance. They are a good choice for applications in which material is meter-fed and the absence of unshreddables can be assured.
Direct vs. Indirect. Rotary shear shredders can receive the power from the drive motor either directly or indirectly. In direct drive, the hydraulic motor and/or reducer from an electric motor is mounted directly to one of the cutter shafts. There are many advantages to direct drive, one manufacturer notes, including transferring the motor’s torque directly to the cutter shaft for 100-percent efficiency. Direct drive reportedly offers greater shredder performance and may increase shredder reliability. It does this by eliminating a high-stress input shaft/gear assembly and replacing it with a planetary gearbox.
The indirect-drive arrangement—also called a three-shaft design—connects the hydraulic motor and/or electric-motor reducer to an input shaft via a mechanical coupling. The input shaft turns a gear that transfers power to a gear on one of the cutter shafts. Two additional gears between the cutter shafts provide counter-rotation and load sharing on single-motor shredders. These counter-rotation shared gears are common on all single-motor shredder designs. While indirect-drive systems are less expensive than direct drives, their multi-step transfer of energy from gear to gear makes them less efficient—though they can help develop more cutting force than direct drives, asserts one manufacturer.
Whichever type of drive is used, the shredder should be designed to grab only as much material as it has the power to shred, while providing protection against unprocessable items. In other words, once the shredder has grabbed the material, it must have enough torque or power to complete the shearing process without overloading the drive system.
Preventing Problems. In practice, of course, it’s common for rotary shear shredders to encounter processing problems due to overfeeding or the introduction of an unshreddable object into the hopper.
Thankfully, one of the signature traits of many rotary shear shredders is their ability to automatically shift into a reversing or unjamming mode when they run into trouble. (It should be noted that many shredders with Holman-type knives don’t have an auto-reverse feature, though they can feature a shear-pin coupling to disengage the drive or an ampere load sensor to shut down the machine if a problem develops.
If a processing problem does develop in electric-drive shredders, the unit’s PLC system detects the problem through an increase in amperage draw. In hydraulic-drive shredders, the machine’s hydraulic system detects the problem through high oil pressure.
To prevent overloads and instantaneous stops from damaging the shredder’s drive components, the shredder’s drive system seeks to release the so-called shock load energy. In electric-drive systems, this is accomplished when the torque coupling slips momentarily—like a clutch in a car. With hydraulic drives, the hydraulic oil reaches a predetermined peak pressure point, which triggers the opening of a relief valve that dissipates the pressure. The shredder is then signaled to shift into reverse to clear the cutting chamber. Some units even have an automatic shutdown feature that stops the shredder if multiple reversals occur within a predetermined time period.
The abilities to detect problems and reverse the shaft serve to minimize machine damage and wear while also reducing potential downtime and its related costs. Manufacturers stress, however, that these abilities are protective features and are not intended to be a mode of normal operating conditions.
Collecting the Goods. The final step in the rotary shear shredding process is deciding how the processed material will be collected, contained, removed, and/or stored. With many shredders, material simply passes through the shredding knives and falls out the open bottom of the machine onto the ground, into a collection container, or onto a conveyor. Some shredders (as noted previously) have sizing screens under the cutting chamber that force material to be reduced to a certain size before it can exit the chamber.
Whichever type of shredder you use, it’s important to have a plan for handling the processed material to prevent it from accumulating and interfering with the machine.
Shredder Shopping
If you’re in the market for a rotary shear shredder, answering the following questions can help you find the best machine for your needs, manufacturers say. (For a partial list of firms that make rotary shear shredders, see “The Low-Speed Lot” on page 120.)
• Do you want a stationary or mobile system?
• What type(s) of material will you process?
• What are the maximum physical dimensions—length, width, and height—of the material?
• Will the material have any notable traits in terms of density, moisture, hazardous constituents, and so on?
• How will the material be fed into the shredder—batch or meter?
• What throughput, or processing volume, do you need from the shredder?
• What finished particle size do you want?
• What will happen downstream—will the material be processed further or collected for storage, shipment, or disposal?
• Do you need any special features? (These could include remote-control operation, a ram for the infeed hopper to positively feed material into the shredder, or an enclosed hopper to contain materials that are prone to shatter when shredded.)
When comparing shredders from different manufacturers, consider each machine’s ease of operation as well as its ease of maintenance.
While cost is certainly a consideration, it shouldn’t be the most important factor. Operational costs of a misapplied shredder can quickly offset any savings in machine price. After all, what’s the sense of spending less on a system that lacks the necessary power and features to accomplish your processing task? As one manufacturer asserts, “We’ve never heard a customer complain that the shredder they purchased was too large; however, many wished that they had a larger unit.” In sizing a shredder, the manufacturer says, the goal is for the shredder to be capable of achieving the desired result and throughput rate using 65 percent of its available energy.
Since there’s no magic formula for selecting the right shredder, many manufacturers offer to process a sample of a customer’s material and begin the selection process from there.
The Low-Speed Lot
American Pulverizer Co., 314/781-6100, www.ampulverizer.com
Barclay Roto-Shred Inc., 209/466-1209, www.tireshredders.com
Columbus McKinnon Corp., 800/848-1071 or 941/755-2621, www.cmshredders.com
Franklin Miller Inc., 800/932-0599 or 973/535-9200, www.franklinmiller.com
Garb-Oil & Power Corp., 801/832-9871, www.garb-oil.com
Granutech Saturn Systems, 877/582-7800 or 972/790-7800, www.granutech.com, www.saturn-shredders.com
Jacobson L.L.C., 763/571-1000, www.jacobsonmn.com
Komar Industries Inc., 614/836-2366, www.komarindustries.com
MTB Recycling, 33/474-929-936 or 303/670-4141, www.mtb-recycling.fr
Magnatech Engineering Inc., 913/845-3553 of 636/949-0096, www.magnatech.org
Marathon Equipment Co., 888/733-8248 or 205/699-6022, www.marathonequipment.com
Pallmann Pulverizers Co. Inc., 973/471-1450, www.pallmannpulverizers.com
Processing Systems Technologies L.L.C., 800/413-1161 or 360/435-8955, www.shredandgrind.com
Protoworks Inc., 519/882-3700, www.schredmax.com
SSI Shredding Systems Inc., 800/537-4733 or 503/682-3633, www.ssiworld.com
Shred Pax Inc., 800/962-7888 or 630/694-1100, www.shredpax.com
Shred-Tech Ltd., 800/465-3214 or 519/621-3560, www.shred-tech.com
Tryco/UNTHA International, 217/864-4541, www.tryco.com
Weima America Inc., 888/440-7170 or 803/802-7170, www.weimaamerica.com
Wendt Corp., 888/936-3826 or 716/873-2211, www.wendtcorp.com
Williams Patent Crusher Inc., 314/621-3348, www.williamscrusher.com •
Kent Kiser is editor and associate publisher of Scrap.
For size-reduction tasks, there’s nothing like a rotary shear shredder, which uses low speed and high torque to process.