BioCycle January 2006, Vol. 47, No. 1, p. 26
Interviews with manufacturers of grinders, shredders and chippers provide insights into the latest designs and innovations.
Nora Goldstein

MANY factors are put into the equation when deciding on what type of size reduction equipment to purchase or lease. But perhaps the single most important is productivity – how to process the most material for the targeted end market(s) with the least amount of downtime for maintenance. Matching the equipment to the type of material being processed is a key factor in maximizing productivity. For example, a site processing feedstocks that are likely to contain significant amounts of metal, such as mixed construction and demolition debris, may opt for a machine with slower revolutions per minute (RPM). Conversely, a site handling primarily clean wood fractions might opt for a machine with higher productivity.
Along with the increased diversion and recycling of the woody fraction of the waste stream has come increasing sophistication in the size reduction equipment to process it. BioCycle interviewed manufacturers of grinders, chippers and shredders to learn more about the latest innovations and trends in equipment design and markets. This article highlights such developments as computer controls to detect contaminants and monitor feed rates, the addition of track-mounted models to optimize materials handling and facilitate wood processing in forestry applications, and the introduction of heavier-duty grinders to service construction and demolition debris processing as well as storm-generated debris.
A FEW FUNDAMENTALS
A quick review of some size reduction terminology and fundamentals will assist in understanding the various features and innovations in the marketplace. First up is an explanation of the basic equipment categories: grinders, shredders and chippers. At the risk of oversimplifying, the following definitions are used in this article:
Grinders: Size reduction takes place in these high speed machines by repetitive pounding of particles into smaller and smaller pieces through a combination of tensile, shear and compressive forces. Hammers, attached to a rotor, hit the material, smashing it into smaller pieces. Screens assist in compressing the material being ground.
Shredders: The slow speed shredding action essentially tears materials apart, versus smashing them. Many shredders use a pair of counter rotating shafts with cutting devices. Material is drawn into the rotating shafts, then pulled apart.
Chippers: Chippers cut the feedstock being processed, versus smashing or tearing it apart. Chippers have a set of knives mounted on a rotating drum or disk that slice the material.
Very generally speaking, shredders often are used as a first step in a size reduction processing train, especially when the feedstock is known to have metal contamination. “We have a customer in California, for example, who is processing railroad ties for boiler fuel,” says Dan Brandon at Morbark. “Because the railroad ties contain a lot of steel, they are first run through a slow speed, high torque shredder, breaking them up into 8- to 10-inch chunks and exposing the steel. A cross belt magnet removes the steel and from there, the material is fed into a horizontal grinder to get it down to the size needed for the boiler fuel market.”
With most grinders, mounted screens work in tandem with the rotor and hammers in the smashing action – essentially trapping material so that it continues to be struck by the hammers. The last screen in the configuration ultimately determines the size of the particles produced. Companies offer a range of screen plates and hole sizes (shape of the holes vary as well) and methods to change out the screens vary by manufacturer. For example, Rotochopper screens are changed at ground level using a hoist and rail system installed in the machine.
To guide customers through the wide range of equipment designs and features, Morbark asks a minimum of five questions to help focus their selection. The questions are asked in the context of the customer’s current and future business operations, notes John Foote, Vice President of Sales and Marketing. “The first question is what are they grinding? Is it C&D, green waste, palms, logs, stumps? Second is what kind of volume do they need to grind? Third is what end product are they looking for? Is it compost, biomass fuel, mulch? Fourth, what is their budget range? And last, are they talking stationary, portable, track-mounted, diesel-powered, electric-powered? All five questions need to be answered before talking about which machine is right.”
ROTORS, CUTTERS, SCREENS – AND PARTICLE SIZING
During the interviews, it became apparent that rotors and screens are at the heart of the grinding technology. Ancillary to that are the attached hammers, and, with certain designs, the anvil. What seems to distinguish one brand of equipment from another are the weight of the rotors, the direction in which they turn, the configurations of the screens and tub versus horizontal designs. For example, one vendor noted that the weight of the rotor has to be a heavier steel than what might be encountered in the feedstock, otherwise the “contaminant will win.” Heavier rotors are more suited to applications with a high likelihood of metal contamination in the feedstocks, thus they turn at a slower speed. Other vendors rely more on shear pins and torque limiters to protect equipment against contaminants. Clearly, when researching size reduction equipment, there are tradeoffs between weight/durability of parts, horsepower and productivity. But knowing the primary feedstocks to be processed, and the end markets being targeted, will help guide that decision.
One clear product distinction, at least with horizontal grinders, is the availability of upturn and downturn rotors. An upturn rotor turns clockwise, and the hammers strike the material several times in the rotation before it actually meets the anvil, where the major compressing action takes place. Conversely, a downturn rotor turns counterclockwise, and the material hits the anvil almost immediately. “The difference in rotors and speed are critical to the job at hand,” explains Tim Griffing, an engineer with Continental Biomass Industries (CBI). “If there is heavy metal in the incoming stream, a downturn rotor at 900 or 700 RPMs is a much more efficient way to grind. If a site is producing a mulch product from a fairly clean wood waste stream, an upturn rotor at 1400 RPM is better.”
Peterson Pacific Corp. notes that its upstroke models also can result in higher productivity with less horsepower. “We can produce as much with a 400 hp upstroke than with a 600 hp downstroke,” notes Cody Peterson, adding that an upstroke grinding system typically produces a lower density mulch than down-cutting rotors. “The upstroke system provides an initial fragmentation of material, which then moves back to the anvil area, where it is struck again and then screened,” he adds. “The upstroke actually draws material into the machine, making it an integrated part of the feeding system. We have found this design results in fewer fines in the mulch. For mulch coloring operations, fewer fines require less colorant to achieve a consistent, uniformly colored product. And many fuel specifications also require fines to be minimized, depending on the boiler type.”
Bandit Industries markets a line of horizontal grinders that utilize a cutting technology instead of hammer action. For example, its 5680 model has a cuttermill that is 83-feet long by 67-inches in diameter. The machine uses Bandit’s breakaway sawtooth holder design that controls the length of cut while providing protection from damaging contaminants. “The sawtooth design creates versatility in the machine because we can add or remove cutting tools depending on the feedstock being processed and the desired particle size,” notes Jerry Morey of Bandit. “In addition, our machines are designed so that product sizing is done in the initial cutting of the material, versus on the screens.”
He adds that its Beast line of equipment has been redesigned so that the speed of the infeed conveyor also factors into particle sizing. “The slower the machine is fed, the finer the particle size that results. In addition, the design of our conveyor anvil chain eliminates the hang-up of material between the infeed conveyor and the anvil. We use the downturn rotors, which aids in the feed of the material.”
In grinders, the screens – or grates – in a machine serve the dual function of assisting in actual size reduction, as well as final particle sizing. “Every manufacturer has their own recipe when it comes to screens,” says Griffing. “With high speed units, all offer many different screens sizes and hole shapes, and customers can mix and match – even to the extent of what species of wood is being ground, e.g., hemlock or spruce. Typically these machines have three screen sections. Most grinding is done using the first section, so that screen needs to be the smallest. The back screen is sized to get the product out.” Generally speaking, he adds, the smaller the desired particle size, the smaller the screen hole required (and the higher the wear) and thus production can be lower. “Horsepower is the key to get around that situation.”
Mike Byram with Vermeer Manufacturing Co. notes a host of factors come into play when it comes to achieving the desired particle size during grinding: “Grinder operators achieve the desired material size by trial and error using the variables including the cutters, screens and controls such as infeed speed. Different material types, the condition of these materials – moisture, age, shape – and loading and general operation of the grinder affect the end product particle size.”
When it comes to tub grinders, Keith Hermanson of DuraTech Industries offers this rule of thumb: “Seventy-five percent of material in the ground pile will be half of the screen size used. For example, two 4-inch round screens will give you 75 percent 2-inch material with various sizes in the other 25 percent. The only way to get a good constant sized product is to do a regrind. In this process, DuraTech takes one 4-inch by 7-inch, and one 6-inch by 9-inch, on the initial grind for a production rate of up to 400 yards per hour. After the initial grind is done and the regrind begins, we generally put one 2-inch and one 3-inch round to finish the product at a rate of up to 450 yards per hour. Throughput can always be reduced or increased by tub rotation. The faster the tub turns, the more product the customer puts on the ground. The trick is to match the speed of the tub with the desired end product and not compromise for more production.”
On Peterson models, half of the rotor is surrounded by the screen system. “A large screen area is a big advantage in production – the more area there is to discharge, the more efficient the machine is,” says Peterson. “We utilize four individual grates with different hole configurations, e.g., square, rectangle and round, and we can stagger the hole size. The further the material travels around the rotor, the less likely it is to shoot out as a stick. The first screen gets the brunt of the wear, as that is where the real hard sizing takes place.”
Rotochopper grinders employ a baffled screen, which helps to reduce spears in the ground product. “The baffle is essentially a louver on the back of screen so that wood cannot pass directly through the hole,” explains Vince Hundt, Sales Manager at Rotochopper. “A piece of wood that is the right dimension but is still 6-inches long may try to pass through the screen with 1.5 inch square holes. The baffle prevents the wood from getting through, and then the next tooth comes around and splinters it, achieving the desired particle size.”
A fairly new line of equipment available in North America is the Seko Bio-Chopper series of low-speed, high torque shredders. The Constellation Series range in capacity from 300 to 600 cy/hour of production capacity. Known as “choppers,” these units do not employ screens. Instead, there are rotating shafts with a series of teeth that fit through a cutting bar, which results in a coarser product. “We sell a secondary attachment that is similar to a high-speed hammermill that is run off of the same piece of equipment,” says Lyndell Pate with North 40 Compost Technologies, which markets the SEKO equipment. “This is used to reduce the material to the size needed.”
To facilitate particle size reduction and to remove contaminants, some equipment vendors suggest screening feedstocks ahead of the grinder. For example, CBI markets a Prescreener to remove dirt and other abrasive materials from landclearing debris and yard trimmings prior to grinding. “Dirt is the highest wear factor in grinders, as are glass and plaster,” says Griffing.
Byram of Vermeer adds that screening wood waste ahead of grinding also improves the quality of the end product. “For example, biomass fuel materials must be clean to achieve the best end value,” he says. “The ideal time to screen unwanted materials from the wood waste is prior to grinding. It keeps the end product cleaner and reduces machine wear.”
In terms of protecting machinery, Dura-Tech pointed out several features on its grinders that help minimize damage from dust and dirt. “To address overheating issues and contamination of the radiator, we developed a self-cleaning design that both sweeps material off of the screening area and creates a vacuum that clears the material from the screen,” explains Hermanson. “To minimize contamination build-up on the engine, its housing is completely enclosed, which also creates a blowing effect that cools the engine. The other effect, which our engineers didn’t count on, was the impact on the engine decibel rating. It turns out the blowing effect around the engine throws the sound down to the ground, damping the noise level during operation.”
SMART MACHINES
Among the newer features on size reduction equipment are computerized sensors and diagnostic systems. The sensors can be used to detect contaminants in the feedstock that could damage the equipment. Others are designed to adjust the speed of the infeed system, depending on what is being ground.
Rotochopper introduced the “StopWatch” metal alert and diagnostic system that is designed to stop the feed belt when a metal object is detected. “Essentially, the system monitors the ‘harmonics’ of the machine,” explains Hundt. “The second a piece of metal touches the tooth, the monitoring system hears it and reverses the infeed mechanism so there is no damage to the teeth or rotor. That system also is capable of giving the machine owner a wealth of information, e.g., how many hours it has been operating and the condition of the teeth, so the owner knows when the teeth should be replaced.”
Tim Griffing notes that CBI’s computer-controlled feed system allows the feed to slow down when the RPMs of the engine drop, then accelerates once the engine has come back up to speed, which improves productivity over the course of a day of processing. “The system allows the feed to continue instead of stopping altogether, even if the RPM of the engine starts to die down.”
In 1995, Peterson Pacific Corp. introduced the patented Impact Release System that automatically opens the anvil and first grate if a severe impact occurs. This allows the contaminant to be ejected. The anvil housing then pivots back into an operating position. The most recent design of this system enables it to operate in three different modes, depending on how much uncrushable material is in the feed, and what the desired end product is, e.g., mulch, boiler fuel. Peterson also introduced the third generation of its Adaptive Control System, which monitors the grinding load and varies the speed of the infeed system to keep the engine working at the top of its power curve. “It automatically accelerates the feed system when the engine load is light, and slows when the load is high,” explains Cody Peterson. “The system also monitors the engine temperature and varies the cooling fan speed to minimize power usage and reduce fuel consumption.”
Morbark has designed a smart feed system, known as IQAN, that optimizes the operation of the hammermill and horsepower of the engine based on the type of material being processed using preset parameters. “If logs and stumps are being ground, one operating mode is selected; if it’s a regrind operation, a different mode is selected,” explains Lucky Robison, Morbark’s Vice President of Engineering. “In the ‘old days,’ material was fed in until the engine bogged down to a certain RPM level. Then the feed system would back out, allowing the engine to recover power. With this new system, you never hear anything stop – it is a continuous feed of material, using the horsepower to its maximum efficiency all the time.”
In 2004, Morbark introduced a satellite linked diagnostic feature, where a customer who is experiencing a problem with a horizontal or tub grinder can dial in via modem to the company’s headquarters in Winn, Michigan. “We can tell them where the problem is, as well as make adjustments,” adds Robison.
TRACK MOUNTS
Increasingly, size reduction equipment manufacturers are offering track-mounted systems, catering to jobs where it is more efficient to move the machine to the material versus the material to the machine. “We are seeing a much greater interest in self-propelled machines,” notes Jerry Morey of Bandit Industries, adding that his company was one of the first to introduce this option. “The advantage, especially in a land clearing operation, is you eliminate the need to forward the material. Instead, the chipper can move to it, process it and leave. Another application is at composting or mulch sites where windrows or piles of chips are being built. Essentially, self-propelled machines eliminate multiple handling of piles.”
Peterson Pacific introduced a track-mounted model in 1999. “We recognized the efficiency of track-mounted units,” explains Peterson. “If we have a machine with an infeed of 1,000 cy/hour, the operator can run out of material to feed it within 20 minutes. With a trailer-mounted unit, you must clean out the area around the grinder, then hook it up and move it to where the next pile is. With a track machine, the operator can keep advancing the unit right into the pile of material to be ground. The unit also works well in wet weather and winter applications.”
One caveat with track-mounted units is the cost, says Griffing. “They are becoming more popular, but they are more expensive than wheeled units. But if it’s going to stay at a site, and the operator wants to move the unit around, especially if it is remote-control-operated, there are advantages. We are seeing their use primary on the wood side, for going after slash – the tops of trees and the limbs that lumber or paper companies don’t use in their mills. That wood is a viable boiler fuel.”
BIOMASS MARKET
Supplying processed wood as boiler fuel is definitely generating new business for size reduction equipment manufacturers. “The biomass market is one thing that has all the grinder companies excited,” says Hundt of Rotochopper. “We came out with a machine about a year-and-a-half ago which we market as the Forestry Package. It is a higher clearance machine with a very aggressive infeed mechanism for the purpose of attacking brushy material. There are parts of the country, for example, that have a problem with red cedar taking over thousands of acres of forest. The red cedar has stiff lateral branches that spread over 10-feet from the tree base. To enable whole tree field grinding, more horsepower was installed on the power feed drum, providing more down pressure – hence more pulling power or traction – to process the cedar. This enabled market-direct grinding in one pass.”
Peterson says that it isn’t just wood processors who are trying to tap the biomass fuel market, but logging contractors as well. “We see situations where logging companies are trying to recover their own waste to tap that market,” he explains. “Anyone trying to tap the cogeneration market needs to clearly understand the fuel specifications, however. There are length restrictions, and certain species of wood that aren’t allowed. Some plants won’t accept product less than 4-inches in size, as the finer material can cause flashes when it enters the boiler.”
Bandit Industries is introducing a new unit next month that is modeled after its Beast Recycler but has a 40-inch diameter infeed capacity. “This machine is targeted for the biofuel market, although we expect it will find its way into markets where grinders and waste reduction machines are used” says Morey. “This chipper has a drum head and knives and counter knives that will last longer and make a more dimensional chip. It can be used for processing orchard waste, land clearing operations and tree removal – the above-ground portion of the tree. The machine is designed to process at 100-feet/minute, whereas waste reduction machines process at amaximum of 40 to 50-feet/minute. The downside is that the equipment is not tolerant of most contaminants, although it will tolerate a certain amount of dirt.”
He adds that in terms of particle size specifications for biomass fuel, there are plants in Europe that require a very fine product under an inch. “This fine material is mixed with coal dust and fed into coal-fired power plants,” Morey explains. “We’ve been able to meet this specification in a single pass with the Beast Recyclers.”
HURRICANE FORCE EQUIPMENT
While most of the manufacturers interviewed have seen their equipment used for hurricane and ice storm debris clean-up, the destruction from Hurricane Katrina has put many of these machines to the test, especially because of the mix of materials in the debris. “We designed our high torque, slow speed shredders for processing materials like those generated by Katrina and other storms,” says Dan Brandon of Morbark. “They work better for highly contaminated materials with a lot of metal. Along the Gulf Coast and in Florida, contractors also have been dealing with the more traditional hurricane-related materials.” He adds that Morbark just introduced the Model 1600, a “beefed up” tub grinder with a 1,200 horsepower engine. “It’s designed to handle large volumes presented by storm debris and other really big jobs.”
In their experience, high volume machines with 860 to 1000 hp engines tend to be tolerant against contamination, and meet the process requirements of the Army Corps of Engineers and the Federal Emergency Management Agency (FEMA). “With disaster clean-ups, you have to be running 24/7,” notes Brandon. “These machines must perform continuously, with limited time allowed each day for maintenance and lubing. Our new satellite assistance program has been a help for our customers on the Gulf Coast. We can diagnose any problems and we sent spare parts to the region early on so contractors can access replacement parts quickly.”
CBI’s Model 4800 shredder (now renumbered as the 8400), has passed OSHA and FEMA requirements for use in Katrina clean-up. “The machine has a heavy-duty demolition grate design for metal contamination,” says Griffing. “It is capable of doing very high tonnages.” The machine has a belt feed system with upper and lower feed rollers. “In this type of application, belt feeds are better than drag chains, especially when the feedstock includes wire, cables, and metal strapping that can get caught in the chain,” he adds. “The up-time is better and there is no spillage under the machine.”
Several people interviewed noted that horizontal grinders and shredders with covered feed belts and tarped discharge chutes are preferred when debris that may contain asbestos or lead paint are being processed. Tub grinders are not as common, they note, because of the confined spaces where processing takes place and the release of airborne particulates.
It also was noted during the interviews that machines designed to process mixed construction and demolition debris are well-suited to waste streams generated by Hurricane Katrina. These machines tend to operate at slower speeds and are designed to tolerate materials such as metal chunks and concrete.
The ability to easily transport grinders and shredders from site to site is also a consideration, notes Byram of Vermeer. “Mobility of grinders for ease of transportation from location to location is important. Training efforts are also critical as some new customers may not be familiar with machine operations and maintenance.”
Morey of Bandit Industries says their customers operating on the Gulf Coast have found that the most effective strategy in handling Katrina debris is presorting materials to be processed. “Our first operator on the ground in New Orleans didn’t run his grinders for two weeks,” he notes. “Instead, his company was gathering, hauling and sorting the materials. This strategy reduced equipment downtime for maintenance as a result of not processing significant volumes of contaminants.” He adds that it also is critical to have plenty of spare parts immediately available in the early phase of responding as transportation systems are disrupted and it is difficult to get items to the processing sites. “FEMA lifted all the road regulations for oversized and overweight machines,” says Morey, “which quickened the process of getting machines delivered.” Bandit grinders are FEMA-approved.
NICHE FOR SMALLER UNITS
Sundance Grinders, with only one model available, targets its marketing to smaller-scale jobs, including curbside grinding services. “We have the smallest horizontal grinder on the market, so we sell portability,” says Doug Lloyd, who acquired Sundance Grinders in 2004 and brought the Sundance Kid III back into production. The horizontal grinder is compact enough to be towed behind a three-quarter ton pick-up truck. “Not long after we reintroduced the grinder, there was a tremendous ice storm that did a lot of damage to trees in Kansas, where we are located,” recalls Lloyd. “We had all sorts of opportunities to test and demonstrate the machine for curbside grinding of tree limbs and other organic material!”
The unit employs four rows of 11 swinging hammers, and does not use screens or grates to facilitate grinding. “In between the hammers are two rows of fixed stationary fingers, so the material hits the hammers initially, then over two more stages, goes through the fingers, which provides fairly good control of particle size,” he adds. “This design allows operators to handle wet material, whereas with screens, the holes can get plugged up. In our design, there is nothing to plug up – feedstocks either go through or they don’t.” Throughput of the machine is about 20 tons/hour. Because the feed roll will only allow a maximum of 8-inch material to enter the grinding chamber, the ideal size material for continuous grinding is 3- to 6-inches in diameter.
In addition to its shredder line, SEKO manufactures a Biochopper-Mixer (the SAM-5), which grinds and mixes feedstocks at the same time. The machine has a side conveyor, so composters can discharge the ground and mixed feedstocks directly into windrows, explains Lyndell Pate. “The unit also has weigh-scale sensors, so the unit can be installed on a roll-off truck and used for collection of food residuals from grocery stores or a food processing facility. Weight tickets can be printed out for customers.” Pate adds that the shredder-mixer also is used to grind more nontraditional feedstocks such as mortalities and wallboard. “The idea is to add as much flexibility as possible for smaller processors, allowing them to handle a broader range of materials in their recycling or composting facility. We’re also working on some batch mixing programs so operators can optimize the compost mix.”