The shear shredder – properly described as a high torque, low speed industrial shear shredder – is the new kid on the block when it comes to reduction technology. With greater versatility, lower energy consumption, and lower levels of noise and dust, the industrial shear shredder poses some advantages over its older relatives, the hammermill, wood hog and granulator.
The industrial shear shredder has two or more counter-rotating shafts equipped with hooked knives capable of reducing a wide variety of materials, including tires, municipal solid waste, bulky wastes such as furniture, stoves, refrigerators, 55-gallon drums, outdated computers, telecommunications equipment, carpet, baled metals such as aluminum and magnesium – the list is virtually endless. Destruction of off-specification and/or outdated consumer products is also possible.
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When purchasing a shear shredder, there are a number of factors to consider. While the drive type is the single most important issue (see discussion on page 10), it is by no means the only question determining shear shredder selection. Questions such as whether a keyed shaft or a hex shaft is superior have no simple answer, as there are benefits and drawbacks to both designs. Either shaft, if properly designed and applied, will perform adequately. Debating a single drive versus a dual drive is a more complex issue. For the most part, shredders under 200 horsepower have a single drive and shredders over 200 horsepower have a dual drive. Naturally, there are exceptions to all rules, but economical drive components are readily available for either electric or hydraulic units up to 200 horsepower, so single drive can be used. For machines over 200 horsepower, the drive system options are fewer and much costlier, so manufacturers often opt to install two 200-horsepower drives rather than one 400-horsepower drive. The choice of open loop or closed loop hydraulic drive systems is a debate with no correct answer. An open loop hydraulic system pumps oil from a reservoir through a valve package, out to the hydraulic motor and back to the reservoir. A directional valve controls flow direction. A closed loop hydraulic system maintains a continuous flow of oil between the hydraulic pump and the hydraulic motor. A small amount of oil is continuously drained from the loop and cooled, and is replenished by a charge pump adding oil into the loop. Flow direction is controlled by a swash plate in the hydraulic pump. The primary features of a closed loop system are the ability to pressurize the oil to more than 4,000 pounds per square inch which permits the use of a smaller pump and motor to perform equivalent work, and the ability to vary the flow of oil which varies the cutter shaft speeds. A major drawback to the closed loop system is the substantially higher component costs, and the need for a higher level of oil maintenance and service. The primary advantages to an open loop system are the lower cost, readily available components (pump, motor, filter), and the lesser degree of oil maintenance. In the past, the inability to vary the flow was a drawback, but there are now variable speed open loop hydraulic drive packages for shredders. By Mike Hinsey, director of sales for MAC/Saturn, Grand Prairie, Texas. |
FACTORS TO CONSIDER
When selecting a shredder, there are many factors to consider. A common error is equating horsepower and shredder performance. Although horsepower is a rough guide to the size of the shredder, factors such as maximum knife tip force and shredder shaft speed are much more reliable indicators of a shredder’s capability and capacity.
For example, to shred baled aluminum extrusions, 240,000 pounds of knife tip force is recommended. A shredder with less cutting force available will probably not have sufficient power to get the job done.
A variety of factors and design features in a shredder can combine to determine available knife tip force, and horsepower is only one single contributing variable. In fact, a 200-horsepower shredder with low speed and high torque could generate more knife tip force than a 300-horsepower shredder and therefore out-perform it.
In addition, a 300-horsepower shredder would cost more to operate and maintain than a shredder powered by a 200-horsepower motor.
Shredder shaft speed can also indicate the capacity of a shredder on a particular material. For example, Shredder A with a shaft speed of 28 rotations per minute (rpm) may process 20 tons per hour of municipal solid waste, whereas Shredder B, with a shaft speed of 20 rpm, might only process 15 tons per hour of municipal solid waste. This is assuming the knife tip force of the two units are similar.
Horsepower is one variable which contributes to determine recommended shaft speed; but horsepower is not a direct indicator of how much material a shredder may or may not be able to process.
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The single primary difference in shredders is the technology used for the drive system, which could be either direct electric or hydraulic. The application for which the shredder will be used should be the only criteria used in determining the drive technology. DIRECT ELECTRIC DRIVE Direct electric shredders dominate the market for applications that require 100 horsepower or less. For the most part, these are relatively light duty, low volume applications where there is little to no chance of the shredder reversing. The reason for this is the inability of the direct electric shredder to stall in an overload condition and reverse rapidly, then resume forward rotation. The time needed to stop the electric motor, reverse the contactor, stop the motor again, then resume forward motion could exceed 10 to 15 seconds and result in a significant temperature rise in the electric motor. If this happens frequently, you risk overheating the electric motor and damaging the shredder. Many applications are ideal for direct electric shredders. Some examples include: 2 The destruction of consumer goods like clothing, footwear and food items 2 Shredding of waste paper 2 Document destruction 2 Secondary shredding of tires. HYDRAULIC DRIVE While electric drive shredders dominate in the lower horsepower market segment, hydraulic drive shredders are generally the preferred choice in the higher horsepower market segment. Using a hydraulic drive motor powered by a pump, connected to an electric motor of diesel engine, the shredder can sense an overload condition, reverse away from the load and resume forward rotation in less than three seconds. This has obvious productivity advantages. Due to the higher volume requirements and the rapid reversal feature, the hydraulic drive shredder dominates the market in applications requiring more than 100 horsepower in power. The shredder’s ability to reverse rapidly reduces prolonged stress on the machine and increases capacity due to the increased forward rotation time. An application with known frequent reversing, or a waste stream with a high likelihood of unshreddable materials, requires a hydraulic drive shredder. Common applications for hydraulic drive shredders include: 2 Tires, especially truck tires 2 Nonferrous metal 2 Plastics, both rigid and film 2 Oversized, bulky waste 2 Hazardous waste materials 2 Polyurethane foam. By Mike Hinsey, director of sales for MAC/Saturn, Grand Prairie, Texas. |
KNIFE DESIGN
Knife tip force and shaft speed are not the only factors to consider when investigating industrial shear shredders. Other design features to consider are knife design, configuration of knives on the shredder shafts, method of feed (i.e., front end loader or conveyor, design of feed hopper; whether or not a ram assist is needed to feed cylindrical shapes, etc.).
In addition, knife design can play a critical role in how a shredder will perform with a given type of material. The width of the knife, the number of hooks per knife, and the length and angle of the hooks are a few of the variables engineers consider when designing a knife for a specific application. An aggressive knife – one with multiple, long, forward angled hooks – would be used to shred light duty material that is difficult for the shredder to grab.
The same knife may be inappropriate for densely packed, difficult to shred materials such as plastic film. Aggressive knives will tend to pull too much material at one time into the bite of the shredder, causing the unit to overload. A knife with single, shortened, laidback hooks would be much more effective for reducing plastic film.
Although these are particularly extreme examples, choosing the correct shredder blade for a particular application requires considerable experience and expertise.
SHREDDER FEED OPENING
The size of the shredder feed opening also plays an important role in a successful installation. A particular product must easily fit within the shredder feed opening to allow the shredder knives to grab the item or items to be shredded.
The manner in which materials are fed into the shredder also affects shredder performance. Conveyor feeding and bulk loading are two of the most common forms of feeding industrial shear shredders. The ability to bulk feed shear shredders using overhead cranes or front-end loaders is an advantage not offered by hammermills and granulators. This advantage allows for greater flexibility when incorporating a shear shredder into a turnkey system installation. Bulk feeding can also result in significant labor savings and decrease the risk of injury caused by heavy lifting and twisting.
Although high torque shear shredders are generally easier and less costly to maintain than hammermills, granulators and replaceable tip shredders, it is still important to look at the specific design features offered by shredder manufacturers. Features such as hexagonal shredder shafts, adequate bearing and seal protection, stack tightening systems and shredder drive configurations will help to keep maintenance and operating costs to a minimum.
Other intangible factors must be considered when selecting an industrial shear shredder. A manufacturer’s reputation for parts availability, service and general product support is important. Product range is also key, as is having the choice of hydraulic or electric drives.
By Rob Glass, sales and marketing manager for Shred-Tech, Cambridge, Ontario.
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