All torn up: "full-box shredding" and good record keeping can help operators ease wear on their shredding plants

Recycling Today, April, 2005 by Jackie Gubeno

The automobile shredder has one of the toughest jobs in recycling--and the hammermill's wear parts reflect just how much abuse the machine takes in doing its job.

No matter how hard the alloy, shredder wear parts are quickly worn down, especially the hammers. Additionally, the metal panel wall gets beat up fast, as do the rotor and the grates at the bottom of the shredding chamber.

With hammers supplying the greatest burden in an auto shredder, wear parts represent a large chunk of a shredding plant's operating costs. Estimating strictly on parts purchased and delivered and not accounting for variables like electricity and waste disposal charges, wear parts can account for 60 percent to 70 percent of a shredding plant's costs, says Kevin Toil, general manager at Riverside Engineering Inc., San Antonio. And while auto shredder operators can't get around wear part replacement, they can take steps to decrease their costs and to prolong the lives of their parts.

SLOWING WEAR. While there haven't been many recent technological or operating breakthroughs when it comes to reducing wear on hammermills, some old tricks of the trade are becoming more widely accepted and practiced by operators trying to squeeze a little extra life out of their shredder wear parts, says Jim Schwartz of Texas Shredder Inc., San Antonio.

"The ideas, in principle, are very old," Schwartz says. "Something that's been known for a long time by the sharpest shredders is happening more often--if you keep the shredder box full of scrap, wear parts last longer."

Keeping the box full takes some of the pressure off the hammers as scrap shreds in part against itself, saving some wear and tear on the cast parts, he says.

"Full-box shredding results in higher wear part life and denser scrap," Schwartz says.

Toft agrees. "Keeping the shredder full at all times is the best way to get the full performance from the shredder and the wear parts in the shredder," he says. "When the shredder is kept full of material, the motor is constantly loaded, the risk of unshreddables is reduced and the throughput of the shredder is increased," Toft says.

As automated feed systems have become more common, keeping the chamber full has become easier to do, says John Martinek of Levand Steel & Supply, Irondale, Ala. "It's hard to keep it full using a manual system," he says. "Computer programs help maintain a full mill the whole time--and production goes up."

Increasing "scrap-on-scrap" shredding is a time-tested way to prolong shredder wear part life, says Randy Brace, also of Riverside Engineering.

In addition to keeping the shredder full, Brace also suggests mixing heavier scrap with lighter scrap to generate better production rates and the lowest wear rates.

Brace also says reducing the amount of dirt that is processed with the scrap can help reduce wear on parts. In addition, Brace recommends not shredding scrap to a higher density than needed. "Wear part consumption in the shredder goes up exponentially with an increase in density," he says.

Some say bigger parts are better, Schwartz says. Many argue if a hammer has more material, it will wear away more slowly and need to be replaced less frequently, he says.

"There's a school of thought that says narrow hammers cut better," Swartz says. "But wider [hammers] tend to last longer--many hammers come up to 6 inches wide."

Schwartz also says many manufacturers choose to use bridge grates as opposed to multi-support grates on the bottom of the shredder, saying that they're built to last longer.

While on first consideration, it may sound like multi-support grates would be better on wear, they are generally thinner than bridge grates and actually tend to wear away more quickly, Schwartz says.

"Just about all manufacturers are retrofitting old machines with bridge or trust grates," he says. "They tend to last longer and not break as often."

Toft says there have been a number of revisions to casting chemistry made in the last decade designed to prolong wear part life for shredders. He says the ease of maintenance is now playing a larger part in wear part design than in the past.

"This is easily seen in front wall castings and bottom grate castings," he says. "These pieces are now originally designed to be easier to replace in the mill for less downtime due to maintenance. There have also been changes to spider cap design for increased life and better fit."

KEEPING TRACK. While some principles of wear parts maintenance--like full-box shredding--apply pretty evenly to any shredding operations, others are a little more case-specific, Brace says.

"The recipe for maximizing the money spent on wear parts is a little different for every yard," he says.

For instance, Brace says that a high-production yard will typically choose to replace hammers with fewer production tons on them because a hammer's productivity drops as it wears out. In tailoring a replacement program, shredding plant operators should be aware of their yards' wear parts consumption rates and the effect of waiting too long to replace a casting, Brace says.