Recycled tires are the source for a growing number of exciting new products today. In fact, turning post-consumer tires into a valuable new resource is big business, and it all begins with a tire shredder.

For most tire recycling operations, choosing the right shredder can be a "make or break" decision. The market has plenty to choose from with a variety of options to match buyers' needs with equipment capabilities as closely as possible.

Operators considering a purchase should first look at the end product they're after. "It's a competitive market, so buyers need equipment that positions them favorably as a low cost provider," said Michael Weible, marketing specialist for Diamond Z Manufacturing of Caldwell, Idaho. "Having the right equipment is the key," he said.

To recycle tires, there are two distinct types of shredder designs - high-speed, tub-style grinders, and low-speed, high-torque rotary shear shredders. Either type works, but there are some important differences.

Tub-Style Grinders
Though not in widespread use for tires, tub-style grinders feature a rotating "hopper" where scrap tires are placed. As the tub rotates, guides direct the tires toward the center where a cutting bar holds them in contact with the hammermill. Through an arrangement of rotating hammers - each one of which can weigh in excess of 100 lbs. - the hammermill begins the process of reduction into a various mix of shred, or "grind," depending upon the end product in question.

"The tips of each hammer are equipped with replaceable carbide cutting teeth, said Mike Weible. "When the hammers are through, the primary grind is a mix of shredded rubber ranging from six-inch pieces to less. Further reduction takes place by grinding a second time, or better yet, by directing the primary output through a secondary rasper, granulator, or cracker mill," he added.

High horsepower, low-torque motors, typically drive tub-style grinders. The horsepower is necessary because the hammermill needs velocity to effectively break down the rubber of the tires. Water lubricates the tires as they're consumed by the hammermill.

The impact of the hammers also begins to separate the rubber from the steel wire of the beads, belts and sidewalls, depending on the tires being shredded. In-line magnets remove most of the steel wire as the shred is further processed.

One advantage of the tub-style grinder is throughput. According to Diamond Z, "Larger tub grinders can handle up to 4,000 tires per hour, producing shred of 6" or less. Advanced controls allow a single operator to run the machine over an eight hour shift."

Rotary Shear Shredders
The most popular type of shredder used for tires is known as the rotary shear. Within the rotary shear group though, there are the versatile, "hook-shear" type shredders, and those dedicated to tire recycling, known as the Holman-type shredder.

With a hook-shear shredder, conveyors typically deliver scrap tires to a chute-like hopper. At the bottom of the hopper are parallel shafts with hook shaped blades, or knives. Spacers on the shafts, which are "nested" with blades on adjoining shafts, separate the blades. As the shafts rotate toward one another, the hook-shaped blades grab the tire and draw it down through the blades, shredding the carcass of the tire as it passes through. Hook-shear shredders are driven by low-speed, high-torque drive systems.

According to Glenn Newton, president of Granutech-Saturn, makers of hook-shear type shredders, "Most hook-shear shredders feature blades made from heat-treated 4140 grade steel alloy. The blades, and spacers slide on to the shaft, which is either hex-shaped or keyed, depending on customer preference. The assembly is held in place on the shaft by a locking nut or collar. The primary advantage to this design is flexibility," he said. "Worn blades can be re-welded, sharpened, and they can be configured on the shafts in different ways to shred a variety of materials - from tires, to wood to nearly all types of solid waste," he said.

Some rotary-shear shredders are purpose-built for tire recycling, however. Equipment dedicated to tire recycling is known as the Holman-type shredder. Charles Astafan, general manager of Columbus-McKinnon's Sarasota, Florida operation said, "The Holman shredder was specifically designed for processing steel belted scrap tires. It is similar in design to the hook-shear shredder, but it uses knife rotors and rotor spacers that are permanently fixed to the shafts by means of heat shrinking. The actual knife is then attached to the rotor by bolting it in place. Wear plates or liners are also secured to each side of the rotor to protect them from wear," he said.

Holman-type shredders use blades made from hardened tool steel. When they are press-fit onto the shafts, they are aligned at extremely close tolerances to the blades on the opposite shafts. According to Charlie Astafan, "Since there's no deflection at the blade tips, it enables the shredder to cut tires more like a pair of scissors. The payoff to the operator is in reduced maintenance costs on blades, per ton of tires processed. It also produces a cleaner cut, with no wire exposed on the sides," he said.

It is also important to note that the Holman design uses feeder rollers to align and feed tires to the knives, unlike the hook and shear shredder that uses the hooks on the knives to capture and pull the tire between the edges of the knives. Notes Astafan, "The feeder rollers are very simple, reliable and inexpensive. They promote tire feed and reduce the chance of any jams when feeding whole tires into the shredder, and they eliminate tire bounce," he added.

Downstream from the primary shredder, operators can choose from a wide variety of additional equipment to further process the recycled rubber. Tom Wendt, Jr., Eldan product manager for Wendt Corporation, of Tonawanda, New York said, "From the primary shred, material is then further processed in a heavy rasper that produces a much smaller chip, roughly 5/8", which is nearly wire-free. Magnetic separation completes the steel removal, and then a classifier further reduces chip size and begins the fiber removal process. Size reduction continues with granulators which can produce a wide range of crumb rubber sizes specific to each end-user's specification."

Mark Diemunsch, president of Barclay Roto-Shred in Stockton, California, makers of Holman-type tire shredders observes, "There's lots of different ways recycled tires can be used today, but three applications outnumber the rest. There's shred for landfill acceptance, or use as a daily alternative cover. There's the 2-inch chip for TDF (tire derived fuel), drain fill or civil engineering applications. And there's crumb rubber," he said. "Tire recycling isn't for small operators anymore. You have to be big to be in this business today. It takes trucking and transportation, and an established market for your output. Then, of course, it takes the right equipment."