SSI News: How to select a Shredder

How to select a Shredder

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When it comes to selecting volume reduction equipment for material recycling, most material processors would agree that the low-speed, high torque rotary shear shredder is a good choice. Low energy usage, reduced noise, and low dust generation are some of its advantages.

In addition, the low capital and operating costs of this equipment allow even the small operator to enjoy the benefits of shredding.

All low-speed rotary shear shredders operate on the same basic principle: Materials are drawn past two or more counter rotating cutter shafts. The shafts, outfitted with cutting blades and spacers, rotate from approximately 10 to 50 rotations per minute. The cutting blades, available in various thicknesses and diameters, typically have one or more hooks for size control. These hooks intermesh with the cutter blades on the adjacent or opposing shafts to grab, then shear, fracture or tear the in-feed materials.

When properly designed, operated, and maintained, low-speed shredders help to automate the volume reduction process, increase product value, and maximize transportation capacity.

The relative newness of the low-speed rotary shear shredder in the commercial processing market is evident in the limited database of available purchasing information. Unfortunately, when processors improperly select shredding equipment, they can limit the effectiveness of the processing system. They can also expose downstream equipment to potential damage, resulting in lost revenues.

Know what you need

How can processors avoid these problems? They must take the time to completely understand and review the entire processing system, including auxiliary equipment. Processors who understand their needs will be better prepared to evaluate the numerous types of low-speed shredders available.

Before selecting a shredder, one must analyze the size, density, and volume of the materials to be processed.

Of course, other considerations include the different types of materials and combinations to be processed and the rate at which they are to be shredded. Are ferrous, non-ferrous, wood, plastic, or paper materials being shredded? What are the dimensions (length, width, height) of the materials? How many tons per hour should be reduced?

Ultimately, the shredding system selected must provide a useful service, produce material at the correct production rate and particle size, and maintain maximum system efficiency and on-line reliability.

The best way to start this process is to consider the desired end result: the successful shredding of the intended materials. To accomplish this, first properly size the shredder to the job.

What to look for

The basic rotary shear shredding principle is: The shredder should be designed to grab only as much of the intended material as it has the power to shred, while providing protection against non-processable items.

This translates to a shredder that has:

- an adequate in-feed opening to accept materials;

- an adequate cutter diameter and correct cutter configuration to grab materials;

- adequate power to shred the materials–with a self-metering design;

- a cutter configured to produce the desired particle size; and

- built-in protection against overload and non-shreddable materials.

Processors also need to ensure that the cutter hook, configuration—this is, cutter thickness, hook profile, and number of hooks—and its corresponding power suit their needs. A common rule of thumb is that the smaller the cut and the more hooks per cutter, the smaller the end particle size.

Enough power?

Another important element in shredder selection is power. Once the shredder has grabbed the intended materials, it must have enough torque or power to complete the shearing process without overloading the drive system. The power requirement must be matched to the desired feed rate without compromising the service factor of the equipment.

All equipment has design limitations.

The same equipment must also be able to detect when something is non-processable or when overloading has occurred. A shredder should be designed to automatically shift into a reversing or un-jamming mode to clear the overload condition. This will minimize machine damage and war and reduce potential downtown and cost. This is characteristic of the low-speed rotary shredder.

Shock protection is necessary to cushion against excessive shock loads. Otherwise, instantaneous stops damage the shredder’s drive components such as shafts, gears, and couplings. Between the two types of shredder drive arrangements—hydraulic and electric—the hydraulic drive generally offers better shockload protection. The electric drive is more energy efficient and mechanically less complicated, but is limited in the amount of uncontrolled feed stock that it can process.

Single-pass reduction may not be appropriate for high-capacity processing when a small shredded end product is required. Multi-stage shredders provide processing of materials prior to subsequent reduction. They also reduce potential overload conditions, lessen power requirements for secondary equipment, and achieve smaller particle size.

Feeding the shredder

There are two methods to consider when feeding a shredder: meter and batch feeding. Meter feeding is achieved with a conveyor belt. Batch feeding uses a forklift or grapple loader.

Meter feeding is the most common choice, as the shredder can control the rate at which it receives materials. This helps minimize overload conditions. However, in some cases, meter feeding is inappropriate.

Discharge of materials is also a factor to consider in the system design. Materials need to be removed at an appropriate rate in order to avoid interference or accumulation.

Accessories

While sizing, power requirements, production rate, and particle size are the primary criteria in shredder selection, processors should also look at other accessory features, such as the following:

Explosion-proof components: Hazardous material processing may require these of explosion detection and suppression systems, explosion- proof panels and controls, and shredding chamber ventilation systems.

Ram feed: In-feed hopper rams provide positive positioning of bulky in-feed materials and use the shredder and the system’s horsepower better.

Enclosed hopper: Enclosable hoppers contain materials within the shredding chamber that are prone to shatter when shredded.

The success of any processing system depends on how well matched the components are to the materials to be processed. The planning and selection criteria for the system are extensive: material analysis; system sizing, power, and production rate; particle size; and in-feed and discharge methods. With the proper planning and correct system selection, processing systems can be greatly improved.