Shredder Guide -- Medium Speed Grinders

There is a growing market for a new medium-speed series of shredders. These machines, a single-rotor design, operate between 100 rpm and 400 rpm.

Unlike slow speed, shear type shredders, which have two shafts operated at slow speed with high torque, the medium speed grinder consists of a round drum with multiple replaceable cutter inserts which cut against a fixed bed knife. The machine generates a consistently sized product because material is fed through a screen on the discharge side.

Within the medium-speed machinery, there are two main types of machines. The first type is a long-established machine of European design that is only beginning to be accepted in the United States market. Widely used throughout Europe, this single-rotor, electric-drive grinder is increasingly used in place of small electric drive shredders.

The machine consists of a solid machined-steel rotor with relatively small cutter tips (1½ to 2 inches), a charge hopper, a hydraulic feed ram and an electric motor with reducer.

The more advanced designs incorporate a load-sensing hydraulic control on the ram to maximize production and minimize rotor stall. While these machines are typically powered by less than 100 horsepower, models designed in the U.S. are entering the market with capacities up to 200 horsepower.

Operated with and without screens, medium-speed grinders are used to shred plastics, electronic scrap, wood waste and nonferrous metals. They are also used for product destruction.

A common application for this machine is the plastic molder. Rejected bulky molded plastic parts can be batch-fed into the machine, processed by the rotor through the screen and fed back into an extruder for repelletizing.

Likewise, small producers of scrap wood (such as furniture manufacturers or pallet remanufacturers) use these medium speed grinders to produce a fuel-quality product.

The larger series of medium-speed grinders appeals to an entirely different market than the smaller European-based machines. Using up to 400 horsepower drives and rotor weights of up to 20,000 pounds, these machines are primarily intended to be secondary processors. They rely on vertical feed, and, much like a hammermill, the rotor inertia is used to produce the work. But unlike a hammermill, a true cutting process occurs within the machine.

A rapidly growing application for this series of machines is secondary processing of shredded tires. Medium-speed grinders will reduce rough-shredded tires to nearly steel-free chips less than ¾ inch in size. An added feature of the process is the machine’s ability to separate bead and tread wire from the rubber in a clean enough form to permit the sale of the steel as a scrap product.

By reducing the tires to small, nearly steel-free chips, final process granulators may see maintenance wear reduced by 200 percent to 400 percent while increasing production significantly.

Another application for the large medium-speed grinders is the reduction of thin-film plastics to extruder-quality feedstock without the need for pre-shredding or further granulation.

Emerging markets for the grinder are the reclamation of shingles and as a secondary processor for wire and cable. It has also been used to shred carpet.