Shear-type, slow-speed, high-torque shredders are some of the most common shredders in the recycling 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.

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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, material can be conveyed to an external screener with oversized pieces then being re-fed into the same shredder in circumstances where throughput does not need to be high.

Multi-shaft shredders deliver a defined particle size 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 shredding e-scrap for example. Overall, the most adaptable shredder for recycling is the 4-shaft shredder which is a top performing and flexible platform.

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, 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, the gear ratio may 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. Check with your OEM for feasibility.

In some circumstances, splitting the shredder chamber into two can be a good fit when volumes do not justify two shredders.

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 processing or situations requiring frequent reversal. 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. Some 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.

Conclusions

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 materials and configuration, design, screen size and drive system play key roles in performance. Using advances in drive systems taking in shredder load feedback to dynamically adjust the speed can be a game changer. 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. We can help.  

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Rafael Reveles

Metal Shredders: Processes and Advantages

Chapter 1: What are Metal Shredders?

To recycle scrap metal efficiently, it must be processed and prepared for reuse. Metal shredders are used to handle various types of metal scraps, breaking them down into standardized shapes and sizes for sorting and further processing in recycling operations. Their robust design ensures long-lasting performance, allowing both ferrous and non-ferrous metals to be crushed before being sent to steel mills for recycling. Additionally, shredders enable the destruction of metallic materials when necessary.

Scrap metals are often bulky and difficult to handle in their original form. For example, an old car is cumbersome but becomes easier to move and process once shredded. Shredding reduces the volume of scrap metal into smaller, more manageable pieces, making it easier to separate ferrous and non-ferrous metals, as well as other materials like rubber, plastics, and fabrics. Some shredders can even distinguish between magnetic and non-magnetic materials to maximize the value of recycling.

Here are some components of metal shredders and their functions:

• Rotor: A shredder's rotor has saw teeth cutters that mesh with knives to shred scrap and waste.
  
• Blades: Blades are an essential component of a shredder, but they must be carefully chosen to match the material to be shredded because not all blades work with all materials. Different blades are utilized for different types of metals to be shredded. However, they can be coated to extend their usable life and are composed of hardened alloyed steel.
  
• Housing: The housing houses all of the shredder's parts and is constructed of sturdy metal.
• Motor: A shredder's motor runs at a low speed to reduce strain. The shredder can shred various materials thanks to its modest spinning speed and strong torque.

Chapter 2: What are the different processes involved in metal shredding?

• The two different kinds of shredders are hydraulic and electric power systems. Electric systems are favored in applications that require less room while still having the ability to handle the majority of materials. However, hydraulic power is preferable for heavy-duty processing because it can manage batch-feeding overloads.
• The most popular feeding techniques for metal shredders are batch and meter feeding. Batch feeding is intended to shred huge amounts of material at once. Metered feeding involves putting materials into the shredder at a consistent flow and typically uses a conveyor.
• "Grabbing." is the process where material is pulled into the cutting blades. The texture and weight of the material, and the shape of the cutter’s hook, all influence how the grabbing process works.
• Due to technological advancements, modern industrial shredders have touch-panel control systems. These systems enable in-process adjustments to shred materials, which prevents the need to directly access the mechanism to make changes and adjustments.

Step-by-Step Process of Metal Shredding

• A shredder is fed with the material that needs to be processed through its hopper (where the scrap metal is dropped in from the top); some systems feature a conveyor belt.
• In a typical paper shredder, the paper is seized by the blades and forced through the knives or cutters. Industrial shredders can forcefully shred a car, making the process a little more difficult. In either situation, the cutting mechanism grabs the material, dragging it through the rotating blades to chop it into smaller pieces. Depending on the metal being shredded and its design, the material is cut into various sizes as it moves through the shredding machine. A secondary shredder or granulator machine may be used with some materials to reduce a particle’s size further.
• Sizing or discharge screens are used to sort materials and send oversize bits back through the cutting blades in order to reduce material to a predetermined size with bulk shredders. Screens come in a wide range of sizes and are made to accommodate the type of metal. The cutter is designed only to work when the screens are properly aligned. Air, ballistic devices, over-belt magnetics, and eddy current separators trap pollutants, including sand, stones, glass, ceramics, and metals, that are trapped and removed during the screening process.
• Each type of shredding mechanism, shredded material, and intended use of the particles collected requires a particular method for collecting the shredded materials. A conveyor system is sometimes employed to gather the materials used as raw materials for recycling. The shredding procedure aims to avoid the disposal of recyclable materials in landfills and prepare them for remanufacturing.

Post-Processing of Shredded Metal

Once shredded, metal becomes much easier to transport to a facility for further processing. The shredding process also separates non-ferrous materials such as plastics, foam, glass, aggregates, wood, rubber, textiles, and electrical components. These materials are then directed to recycling streams to prevent them from ending up in landfills.

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Shredded ferrous metals are sent to a furnace where they are melted down. After melting, the metal undergoes refining to ensure its quality and remove impurities. Electrolysis is a common refining technique, using a direct electric current to induce a chemical reaction. Post-electrolysis, the metal is cooled and solidified, ready for use in producing new products.

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Chapter 3: What are metal crushers and how do they work?

In the crushing process, four main external forces are applied: rolling, shearing, impact, and grinding. Press rolling is effective for coarse and medium crushing, especially for hard and large materials, as it involves reducing the thickness of metal stock by passing it under one or more rollers.

Shearing is employed for fine crushing and is particularly useful for breaking down tough materials. Impact forces are used to crush brittle materials and are commonly applied in medium crushing, fine grinding, and ultra-fine grinding.

Grinding is used for breaking down small pieces and fine particles, specifically in fine and ultra-fine grinding stages. Often, the crushing process incorporates multiple external forces working together simultaneously.

Working Principle of Metal Crushers

The core principle of a metal crusher involves hammering. The main machine's rotor, driven by a high-speed, high-torque motor, spins as hammers strike the material, breaking it into smaller pieces within the crushing chamber. Crushers can be categorized into dry, wet, or semi-wet systems based on their operation. A dry system includes a dust removal feature and uses an air cyclone sorting system to separate metal from non-metal materials. In contrast, a wet system applies water during the crushing process to simultaneously wash and sort the materials. The semi-wet system applies a minimal amount of water only as needed to reduce dust while crushing.

Chapter 4: What are the different types of metal shredders?

Single-Shaft Metal Shredders

Single-shaft metal shredders have a rotor and hydraulic ram used to press garbage and scrap against the rotor, which starts the cutting process. Pressure sensors allow the ram to travel backward and forwards, preventing jamming and overheating of the material. Large bulk materials can be broken down into particles as small as 10 mm using these shredders, which can be equipped with various screen sizes. All single-shaft metal shredders have a classifier screen that provide a user with variable measurement settings that output units can be sized at.

Double-Shaft Metal Shredders

The double-shaft metal shredder is a favored choice in industrial applications due to its low-speed operation, high torque, and substantial throughput capabilities, making it effective for processing a wide range of materials. These shredders, which can be driven by either electric motors or hydraulic systems, are designed for volumetric reduction, helping to minimize the space occupied by waste materials. Despite often lacking a classifying grid, the distinctive blade design and controlled production parameters of double-shaft shredders allow for effective size control of the output material. These advanced primary shredders offer versatility and durability for heavy-duty use. Depending on the material type and desired output size, double-shaft metal shredders can be equipped with various components or used in conjunction with other shredding stages to achieve the required material size.

Three-Shaft Metal Shredders

Three-shaft metal shredders can be used either independently or as part of specialized shredding systems. They come in three models (800--mm) and offer capacities ranging from 1.5 to 3.5 tons per hour, equipped with a screen for separating the output material. The processed material can be reduced to sizes between 30x30mm and 50x50mm. These shredders feature a robust cutting system with durable rotor blades and a sweeper shaft. The cutter discs are individually replaceable, and the shredder includes a pivotable screen basket and protective bulkhead seals for the shaft bearing. They are available with either mechanical or hydraulic drive systems and offer automatic power-down and reverse functions.

Four-Shaft Metal Shredders

Four-shaft metal shredders feature two drive rotors that push scrap material onto additional cutting rotors located below. These cutting rotors, powered by separate gear motors, are designed to pre-shred materials before further size reduction or processing with a granulator. Comparable in size to twin-shaft shredders, four-shaft models are often used for tougher waste, such as cars, that cannot be processed by twin-shaft shredders. They include replaceable classifying grids and are available in both electric and hydraulic versions, allowing for versatile material processing and precise output size control. Four-shaft shredders represent advanced technology aimed at creating highly efficient and adaptable equipment. With anti-wear treatments on grids and interchangeable shaft technology, these shredders significantly lower management costs and reduce maintenance requirements.

Horizontal and Vertical Hammer Mill Metal Shredders

Horizontal and vertical hammer mill metal shredders operate by repeatedly striking feed material with heavy hammers, which can weigh between 250 and 1,000 pounds and rotate at speeds of 500 to 700 revolutions per minute (rpm). These hammers are driven by motors ranging from 2,000 to 10,000 horsepower. In a hammer mill shredder, material is introduced into a rapidly rotating steel drum, where it is struck by hammers on a shaft, breaking it into smaller pieces. Horizontal models feature hammers that strike horizontally, whereas vertical models have hammers striking vertically.

Despite their design differences, all hammer mill metal shredders operate on the same principle: large hammers attached to a rotor shred vehicles and other heavy metal items. These shredders are equipped with multiple outboard flywheels and either electric or diesel motors that power the spinning hammers. Large hammer mill shredders also include features like rejecting doors and manganese liners for expelling uncrushable materials. At peak performance, they can process up to 350 tons of material, or 450 automobiles, per hour, and annually produce 2.5 million tons of high-density, uniformly fragmented metal scrap.

Chapter 5: How Metal Shredders Contribute to the Economy and Considerations When Choosing a Metal Shredder?

How Metal Shredders Contribute to the Economy

While it might not be immediately obvious, industrial metal shredders, especially those used in scrap metal recycling, play a crucial role in environmental protection. Without metal shredders, processing large volumes of scrap metal efficiently would be challenging. Their absence would mean longer processing times to meet the rising global demand for scrap metal and a slower reduction of scrap metal accumulation worldwide.

To safeguard the Earth's natural resources, it is essential to minimize the use of new metal as raw material and instead increase the recycling of metal. The shredding process also allows for the effective separation of scrap metal from non-metal and non-ferrous materials, helping to prevent these materials from ending up in landfills.

Considerations When Choosing a Metal Shredder

Capacity: The effectiveness of a metal-shredding facility hinges on choosing an industrial shredder with adequate capacity. Factors such as the volume of material, its physical dimensions, weight, and bulk density all impact a shredder's capacity. The size of the cutting chamber may limit a facility's overall capacity. It is crucial to review a shredder’s capacity rating carefully and consider having some additional capacity beyond estimates. Operating a shredder too close to its maximum capacity can shorten its lifespan, while an oversized shredder might consume excessive power and occupy more floor space than necessary.

Output Specification: Before purchasing a shredder, operators need to clearly understand its intended use and capabilities. For some, the primary goal is not to process metal but to reduce the size of bulky items that are difficult to store. In these cases, the shredder’s output might be less critical. However, for others, particularly those facing strict compliance regulations, output performance is crucial. Additionally, some organizations have specific requirements for their shredders. Therefore, it is essential to choose a shredder that meets the desired output specifications. Ideally, a metal shredder should be versatile enough to adapt to changing needs.

Environmental Consciousness: There are considerable differences in attitudes toward recycling and trash management in the US. Operators must therefore prioritize their performance standards when purchasing shredders that are appropriate for the job required. Due to their reduced carbon footprint, electric motors, rather than diesel hydraulic drives, are used to power several contemporary types of metal shredders because this technology uses less energy. If the expense of the manufacturing process is detrimental to the environment, there is little value in turning rubbish into raw materials for new products. Investing in more "green" machinery also makes financial sense because fuel-guzzling shredders can be expensive due to their high energy requirements. These energy requirements may reduce the potential ROI (return on investment) of a metal shredder and metal-shredding operation.

Types of Material to be Processed: Different shredder designs are optimized for various materials. For instance, high-speed chippers are ideal for shredding tree branches, while plastics are typically processed in granulators (rotary knife cutters), though bulky plastic items may first go through a shredder. When shredding vehicles, a hammer mill metal shredder is often used after initial shredding. While four-shaft shredders can handle specific tasks more quickly, twin-shaft shredders might be preferred for their adaptability and quieter, less dusty operation across diverse waste materials.

Maintenance: Metal shredders endure significant stress and require regular maintenance to ensure proper operation. Components such as cutters, blades, or hammers will become dull and need sharpening, adjustment, or replacement. When choosing a shredder, consider factors like ease of upkeep, repair requirements, and the availability of replacement parts. Maintenance accessibility should be a key consideration in equipment selection.

Safety: Industrial shredding can be hazardous, prompting manufacturers to design safer equipment. Modern shredders incorporate safety features such as protective mechanisms to prevent exposure to moving parts and proactive diagnostic controls that reduce the need for manual intervention. Enhanced safety includes auto-stop functions for foreign objects, low-noise operation to minimize noise exposure, and advanced detection systems (UV, infrared, heat, and spark) to prevent fires. Ergonomic designs also ensure that maintenance can be performed safely and comfortably, reducing the risk of injury.

Chapter 6: What are the advantages and disadvantages of metal shredders, and what maintenance practices are required for them?

Advantages of Using Metal Shredders

• Environmental Conservation: Utilizing metal shredders lessens the impact of scrap metal on the environment. As was already indicated, the metal shredded in a metal shredder can be recycled or utilized again. This recycled material guarantees unused metal won't land near water supplies or dampening sites. Using scrap metal shredders eliminates metals’ negative effects on the soil, groundwater, and landscape. Additionally, recycled metal lessens environmental risks like air pollution.
• It is Cost-Effective: Utilizing metal shredders is quite cost-effective. These devices provide a less-expensive option for handling scrap garbage. In addition, a metal shredder doesn't produce any chemicals.
• A metal shredder facilitates the separation of damaged scrap metal. Any metal shredder typically separates any metal into ferrous and non-ferrous elements. Reusing metal is made simpler by using a metal shredder. Additionally, a metal shredder ensures a metal is safer to use after being processed.
• Recycling: Using metal shredders to facilitate metal recycling is one of its main purposes. Those who work in the recycling sector invest in metal shredding equipment as an essential component of their operations. When scrap metals are crushed inside metal shredders, new, pure metal is created that can be used in various ways. The resulting tiny metal fragments can be melted to become molten metal. This molten material can be used to create new, usable pieces. As a result, users won't need to bother with buying new metal to make additional products.
• A metal shredder is simple to operate as metal is shredded and the volume of material is lowered. Additionally, a metal shredder typically takes up little room for a facility and rarely requires additional personnel to manage. Due to their modest size, transportation costs are minimal.
• One of the main advantages of a metal shredder is its elimination of contaminants from metal. Thus, this treatment raises the metal's purity and value.
• Most electronic devices, and the parts that make them up, are made of precious metals like silver, gold, and platinum. Low-speed metal shredders can separate and recover these metals during the pre-processing phase of shredding.
• Due to the pressure on their financial resources, project developers are looking for ways to save costs while still producing livable and high-quality structures. Metal shredders produce shredded which can be turned into products, materials for landscaping, and cheaper insulation for buildings. In addition, when waste is converted into scrap, a recycling company's fees are significantly lowered if they are also hired to collect waste from a construction project. Therefore, project developers include a permanently installed, on-site shredder in their proposals for most projects.

Disadvantages of Metal Shredders

• Metal Jams: Metal shredders have a bad reputation for jamming, but more expensive models have additional measures to stop this from occurring. One should never insert more feed at any given time than a metal shredder manufacturer recommends in order to avoid a metal jam. Most metal shredders have a reverse button to clear a metal jam if a jam happens. Other, more expensive shredders have technology that prevents jams. These shredders have powerful motors that provide extra power when a shredder is jammed.

Maintenance Procedures and Safety Measures of Metal Shredders

• Metal shredders typically employ grease or oil lubrication techniques. The area around metal shredders can be reasonably simplified using grease lubrication. Grease lubrication makes the motion of all moving components smooth on the metal shredder. Oil provides the best lubricant for metal shredders. Still, regardless of the type of lubrication used, it is important to pay attention to the quantity and consistency of the lubricant to maximize a shredder’s performance.

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• Routine maintenance of metal shredders is essential, particularly in the waste metal business. The best thing one can do for their business is to clean and maintain the shredding chamber of their shredding equipment routinely to avoid lengthy downtime. For metal shredders to remain sharp and operate at their best, the regular inspection and replacement of blades are crucial. Blades for metal shredders can be resharpened periodically to ensure efficient metal shredding. If the blades are worn out and can no longer be sharpened further, they can be replaced. If not handled promptly, one seriously damaged knife can shut down an entire system. Regular belt inspection and replacement are also recommended to help avoid unpleasant surprises while a shredder is in use.
• It has been stated that hindsight is 20/20, and there is no time more evident of the truthfulness of this adage than amid an unanticipated maintenance issue. Regular preventive maintenance of a metal shredder will reduce the likelihood of an unforeseen problem. Additionally, being able to foresee issues before they arise enables one to add precautions to the waste-to-energy system that will keep a business operating efficiently and the shredder operating.
• Lock-out/tag-out procedures should always be followed before attempting to access the shredding machine's cutting chamber. Access doors to the cutting chamber are included with every shredder, making it simpler to clean the rotor, rotate or change the knives, and change screens. A safety switch stops the machine from being turned on while the door is open out of concern for user safety. This switch is intended to safeguard staff as they carry out important preventative maintenance procedures like knife rotation and replacement and fundamental debris removal.