The Basics: Focus on Shredding
By Rafael Reveles
For most operations, the shredder is the heart of the operation. By choosing the right type of machine and knowing how to customize it, shredding performance can be maximized further. An industry shredding expert describes some of the key aspects to realizing the maximum performance from a shredding asset.
The choices available in the security shredding equipment market have never been more diverse and numerous. Most manufacturers have seen just about every application and have a solution for it. Understanding the types of shredders available and criteria for evaluating the best choice is a good practice before making such a large purchase. Many manufacturers are willing to facilitate a test as buyers perform their due diligence to ensure they make the best selection for their exact material. The shredders highlighted here are not an exhaustive list, but the most common ones used for paper and confidential hardware destruction. Likewise, there is not one “correct” way to shred. Selecting a shredder is a very business-specific choice depending on many factors and not one choice will fit every business.
Types of common shredders
There are a multitude of shredders available but they generally fall into two categories of either slow or high-speed machines. High-speed machines cut or fractionate faster due to the higher RPM of their rotors or shafts, which also leads to higher heat generated by more friction in some cases as well as higher dust generation. They are often times the right machines when heavy items need to be broken apart because extreme liberation is needed, or particle sizes need to be small. Slow-speed machines rely on high torque shear cutting and generally run cooler, produce less fines and are quieter. Whether slow or fast, the shredder chosen will need to have the right umbilical systems including provisions for noise, dust, fire and maintenance depending on what is being shredded.
For the high-speed machines, mills, whether hammer or ring type, are a longtime staple of size reduction and best apply to hard drives or heavy confidential metallic or bulky materials. These mills use high speeds to spin rings or hammers that fractionate the product on impact until small enough to fall through a sizing grate. Rings are a suitable choice when better liberation and less fines are desired. The rings internal diameter is about 2.5 times larger than the diameter of the rings retaining shaft which allows for deflection and rotation of the ring. In the case of hammers, multiple types are available including chisel types, notched, and plain bars. The hammers are especially useful in reducing heavy or hard-to-shred items. The geometry of these mills adds to the shredding mechanism by means of the sweep of the hammers/rings. As the hammer passes the screen or grate, the distance between the two progressively gets closer to encourage more grinding action and further reduction. There are also some variations on mills where the rotors and shafts are vertical. The material is ground against bars and textured wear plates as it falls towards the bottom of the mill. Some vertical machines have several stages of size reduction implements such as pre-breaker bars followed by rings below. Rather than a sizing screen, an adjustable orifice at the bottom of the mill’s tub controls the volume of the exit and how long the material recirculates inside the vessel. Other types of vertical mills use large chains to fractionate materials where a coarse shred is needed or where large tolerance is required for a very large piece to be introduced. Maintenance and wear can be higher for mills in certain cases requiring owners to keep removable wear plates and surfaces in stock. These mills are not good candidates for applications involving paper or easily flammable materials due to the heat and friction developed.
Single Rotor Shredders
There are two main types of single rotor on the market that either accept whole product or are designed to be secondary and intake pre-shredded materials. These machines are high speed and rotate from 125-250 rpm depending on design. To be efficient they rely on momentum, the flywheel effect and use solid, heavy rotors to enable more efficient shredding. Multiple blades affixed to the rotor shaft shear against stationary blades or anvils attached to the granulator frame. A sizing screen allows the material to recirculate until small enough to pass and meet security requirements. Maintaining the correct manufacturer’s specification on shear gap (distance between rotor blade and stationary blade interface) is a key aspect to enabling proper and efficient cutting. Most machines allow for a blade flip where the cutter can be unbolted and turned over to use an unworn face. Many anvils can be sharpened as they wear making these machines very serviceable over time. Some machines have a hydraulic ram and a large hopper that takes on whole product and pushes it into the rotor cutters horizontally. A PLC (programmable logic controller) monitors the demand on the drive motor and can modulate how much the ram feeds material into the cutting area which is especially helpful for paper applications. Other machines are designed for smaller pieces including pre-shredded materials and rely on gravity for feeding into the rotor chamber and seen in applications requiring high volume (two stage shredding). Similar to mills, these machines offer high quality liberation and throughput. They are challenged when dealing with steel and non-ferrous metals and shine when processing batches of plastics, circuit boards and paper.
Shear-type, slow-speed, high-torque shredders are the most common shredders in the secure shredding industry and fairly economical to operate. The interlocking cutters are disk-like and have hooks integrated into their circumference. They size reduce on the principal of shear cutting against cutters on the opposing rotating shaft that have an interference gap of 0.010”-0.030”. They are offered in two, three or four shaft models with either electric or hydraulic driven options. For units with more than two shafts, sizing screens are placed under the shredder cutters to produce a defined particle size. For two shaft models, cleaning fingers are used for stripping off the shredded materials from the cutter stack and prevent shaft deflection or damage. The two shaft shredder models are affordable, but are limited to produce mostly a long strip cut based on the width of the cutters. For those wishing to produce a tighter particle size, multi-shaft shredders deliver a defined piece and reasonable throughput based on the screen opening size. Care must be used when choosing the cutter thickness and screen size since throughput can be reduced by excessive re-circulation. Having about a 10 percent larger hole versus cutter width can be a winning approach for using a four shaft shredder.
Shear Shredder Customizing
All the shredders mentioned above have their own ways of customizing based on application, but we will focus on specific ways to optimize shear shredders specifically. The cutter is one of the most influential ways to define shredder performance. The overall radius or diameter of the cutter directly affects the available cutting force since force = torque / distance. Based on this simple physics equation, one can see that reducing distance, in this case the radius of the cutter, improves the amount of force developed. There is of course a limit to how much this trick can work since the cutter overall size has to be large enough to geometrically grab whatever is being shredded. Now consider how the height of the hook plays a role in cutting ability since force is also proportional to surface area and pressure. A smaller surface area as seen in a cutter with a shorter hook will exhibit more pressure that enables better shearing. Whatever cutter configuration is chosen, the quantities and patterns of cutters placed in the shredder chamber is also significant. The total number of hooks engaging together at the same time will all equally draw on the available cutting force, so only having one hook engaging at a time is always going to be the strongest option. For higher throughput, more hooks need to engage at once and spiral or chevron patterns of the cutter hooks are often seen on most manufacturer’s offerings. The cutter alloy is also a key variable and can influence operational costs due to cutter wear rates. Common cutter materials include D2 tool steel, 4140 heat treated and hardox variants such as 600. The brittleness of the cutter must be balanced with the difficulty of the materials to shred to ensure cutter cracking and failure is not likely. Lastly, if it is not possible to make a cutter change and the shredder does have a planetary or similar gearbox, the gear ratio can be changed to adjust the shredder shaft speed with slower speeds yielding more torque. On occasion, just an internal section of the planetary can be switched out at small expense to make the ratio change.
Shredder Drive Systems
There are either hydraulic or electric drives usually offered with most types of shredders and both have a role to play with their applications. Hydraulic systems are mainly used for heavy duty stationary plant processing or situations requiring frequent reversal. Hydraulic systems are also found on shredder trucks often and use a PTO driven pump to reduce weight. Hydraulic-mechanical systems have a compound drive where a hydraulic motor turns a planetary gearbox to achieve a desired torque with a lower-cost motor in some cases. Many hydraulic pump systems allow the volume the pump displaces to be adjusted to control the speed of the shredder shafts, while others only allow for one fixed speed and are less expensive. Hydraulics are a proven technique, but do require by-the-book maintenance to keep them running well to avoid replacement of expensive pumps prematurely. Electric motor directly driven shredders are popular due to their efficiency and simpler to maintain nature. A dry clutch or belt system is common to reduce gearbox damage caused by hard jams and reversals. The planetary gearbox however will require frequent oil changes and monitoring since that is the most likely failure point in these types of drives. When using an electric drive, external options can allow for more shredder customization such as a variable frequency drive (VFD). VFD’s allow the motor and shredder speed to be decreased and adjusted on the fly by reducing the frequency of the power. They can also be used to speed up shafts as well, but do lose motor torque in those scenarios once the motor frequency rises above the standard 60 hertz. Using the shredder’s drive motor amperage, a special controls routine can be developed to run a shredder at speeds above normal motor speed and then slow down once more shredding amperage load is detected to ensure peak torque is realized when truly needed. Such a setup can drastically increase throughput for various types of materials.
There has never been so many great choices in shredding equipment and customizing your shredder to meet the exact needs of your job pays dividends. Selecting the right cutter or hammer materials and configuration, design, screen size and drive system play key roles in performance. Selecting a shredder style and platform that can be easily re-configured and adjusted for changing business needs is a good investment. Step one in the search for a shredder should always be to arrange a test with the manufacturer with your materials and speak to several suppliers.
Rafael Reveles is an engineer and president of Converge Engineering and has 18 years of experience designing and building shredding systems. He can be contacted at email@example.com www.convergeengineering.com