July/August 2003
Which style of rotor—spider or disc—works best in a shredder? It’s a controversial question, with no single or simple answer.
By Robert L. Reid
Want to start a heated debate among shredder operators? Forget religion or politics—let’s talk rotors.
“As soon as someone mentions rotors, it will be the most contentious discussion you’ll ever see,” says international shredder expert Trevor Masters. “Everybody has such vast differences of opinion on everything.”
Jay Robinovitz, vice president of operations for Schnitzer Steel Industries Inc. (Tacoma, Wash.), agrees, noting that “the rotor issue generates a lot of passion—it’s the core of your shredding.”
Indeed, the rotor is what spins inside a shredder’s chamber, slamming its attached hammers into flattened car bodies, old appliances, bales/logs, or other scrap and reducing it to small pieces. Hence, the rotor you choose affects “the whole dynamics of your shredding”—from feeding the machine to production rates to both scheduled and unscheduled maintenance, Robinovitz states.
Most of the rotor issue centers on which type works best. In North America, two types are widely used—spider rotors and disc rotors (especially the somewhat newer “no-weld” or low-maintenance models).
A third type—a barrel or capped-disc rotor—is popular in Europe. Though no North American processor is using a barrel rotor today, at least one manufacturer expects to sell one here in the next year.
Spider rotors are so-named because they have a series of “arms” around the diameter of the rotor, with rows of hammers in the spaces between the arms. Disc rotors, in contrast, have a series of solid round plates with hammers in between them. The barrel rotor is essentially a fully enclosed disc rotor, with open spaces only for the hammers.
Accentuating the Positive
Disc and spider rotors definitely have their strong supporters among processors and equipment manufacturers, though most of this passion gets expressed praising the benefits of one style rather than badmouthing the other type. When processors and manufacturers do directly compare rotor styles, they tend to focus on the different conditions—from the infeed material to the shredder’s design and horsepower—that best suit the different rotors.
One reason, of course, is that shredder and rotor manufacturers often make and/or sell both spider and disc rotors. Thus, they’d be shooting themselves in the financial foot if they criticized either style too heavily. For instance, Javier Herrera, vice president of business development and technology for Harris Shredders (Peachtree City, Ga.), which sells both styles, says that spider rotors can offer better production rates on car bodies and lighter scrap while disc rotors are best for heavier scrap such as plate-and-structural steel and bales.
Likewise, Mike Woodward, technical manager for Wendt Corp. (Tonawanda, N.Y.)—which sells roughly equal numbers of spider and disc rotors—is “partial to the spider rotor” because it works best in Wendt’s own shredders, which rely heavily on the cutter bar to do the actual shredding. Wendt also sells rotors for other shredders that do most of their processing against the bottom grates—a condition that favors disc rotors, Woodward says.
Jim Schwartz, vice president of Texas Shredder Inc. (San Antonio, Texas), offers another opinion, asserting that “so long as it’s well-designed and well-made, there’s no such thing as a bad rotor. A disc rotor will shred light stuff just as well as a spider, and a spider rotor will shred heavy stuff just as well as a disc.”
At the same time, Schwartz notes, “some rotors seem to offer higher production, lower maintenance, and fewer problems.” So while Texas Shredder has worked with different rotor suppliers and will sell a customer either style, the firm recommends the no-weld disc or, if the shredder is designed for it, the barrel rotor.
Limited Experience
One key point in the rotor debate is the fact that most shredder operators have only used one type of rotor. If they use a spider today, it’s often because they’ve always used spiders, and the same goes for processors using discs.
“They simply don’t have experience with another type of rotor to know which they’d like,” says Wendt’s Woodward.
Even Trevor Masters, who operated shredders for nearly 30 years and who now has his own U.K.-based consulting firm, notes that he only has firsthand knowledge of disc rotors.
“There are no schools of shredding,” he says, “so all the experience you get is from the school of hard knocks—things like when and how to change out the parts, whose parts to buy, how and how not to fit them.” After going through such hard knocks, he explains, “I think people are reluctant to go through it again, so they tend to stick with the same rotor if they can.”
Moreover, your current operator might not be the person who bought the rotor, which can make him hesitant to change. Switching rotors is like owning a Chevy and having someone suggest you put a Ford engine under the hood, says Wendt’s Woodward. “You might be thinking: ‘I’m not sure I want to change the manufacturer’s parts and pieces.’ So when they’ve got to replace that part, they simply go with whatever was originally purchased.”
Of course, some processors do take the risk to change rotor styles. Their experiences—together with the views of several manufacturers—offer a variety of opinions (albeit sometimes contradictory) on the perceived strengths and weaknesses of each rotor.
Switching to Spider
At Schnitzer Steel in Tacoma, the installation of a new megashredder in early 2000 also meant trying out a new type of rotor. Until then, all Schnitzer facilities had used spider rotors, notes Jay Robinovitz, but the Tacoma site’s Texas Shredder 122/108 machine came with a weldable disc rotor.
“This was our first go-around with a disc, and we were really looking for tons per hour and throughput,” he says. The results were disappointing, however, with lower-than-expected production and failures such as broken end discs, excessive wear in the hammer pin areas, and a “significant” number of problems with bearings that forced Schnitzer to reevaluate the rotor.
“We run very aggressive,” Robinovitz says. “We run some very large products, and the disc rotor actually limited the rate of feed that we could put through the machine.” Specifically, Schnitzer had trouble processing bales with the disc rotor—exactly the opposite of what some other processors have found. When it comes to rotors, though, individual experiences can be slightly different.
For instance, some processors like the fact that unshreddables have a place to “hide” between the arms of a spider rotor, while others note that a disc rotor tends to keep unshreddables out of the chamber entirely. Those same qualities, however, can make the spider jam, some users report, while an unshreddable that does get inside a disc unit might force the rotor up, damaging the bearing housings—as Schnitzer discovered in Tacoma, Robinovitz says.
After trying various changes to make the disc work and even visiting the sites of other processors with megashredders to get their advice, the Tacoma facility switched last year to a four-arm spider rotor. “We spent a lot of hours and sleepless nights” before making the change, Robinovitz recalls. Throughout the process, some people within Schnitzer objected because installing a spider rotor went against what the shredder manufacturer recommended. This shift also involved changing the parts supply for the shredding operation because the spider rotor required caps to protect its arms as well as a different hammer configuration, different pins, and a relocated pin-puller.
What’s more, the switch didn’t even solve the Tacoma site’s shredding problems—at least not right away. Instead, Schnitzer had to work with the rotor manufacturer to redesign its spider rotor for the Tacoma plant’s heavy-duty processing approach.
Today, Schnitzer’s Tacoma facility is processing 10 to 14 percent more material than it did with the disc rotor thanks to “the more aggressive feed pattern the spider gave us,” Robinovitz says. Switching rotors, he concludes, was “the right decision for our operation.”
OmniSource Corp.’s Global Shredding Technologies (Baldwin, Fla.) also switched to a spider rotor after years of working with discs, notes Steve Garber, general manager. He prefers a four-arm spider because it requires much less maintenance—just changing parts such as the protective caps.
In contrast, keeping a disc rotor running meant keeping a highly trained welding crew on hand and contracting for a welding service on weekends. Even the supposedly no-weld disc rotor that Garber tried required a considerable amount of maintenance, especially when hard pieces of scrap knocked a chunk out of the unit. “We were welding on it continually just to keep it going,” he says.
The time and expense of welding on disc rotors is often cited by processors who prefer spiders, though in fairness some processors also complain about how much they had to weld to repair or maintain spider arms.
Switching to the spider rotor did involve a learning curve for the operators of Global Shredding’s 90/104 Newell machine because the spider is more susceptible to jamming, Garber says. Still, the result after some two years has been a roughly 5-percent increase in production, plus the fact that “the spider has lasted twice as long as any of the disc rotors we’ve had, and it’s not yet at the end of its life,” he notes. Ultimately, Garber hopes to get between 1 million and 2 million tons of production from his current spider.
Another OmniSource facility also switched from a disc rotor to a four-arm spider. The firm’s Fort Wayne, Ind., shredding plant had performed well with disc rotors on various machines, explains Michael Richmond, plant manager. But when the most recent disc on the plant’s 98/104 Newell shredder needed to be replaced at the end of 2002, the site’s managers decided to try a spider rotor “just to see how it worked for us,” he says. “We were hearing a lot of things about spider rotors and people using them—that they ran longer and got more tons out of them than the discs—so we chose to try one to see how it would do.”
Installed in January 2003, the four-arm spider hasn’t been in use long enough for Richmond and his team to fully evaluate the results. They figure it will take six months to a year of operation to determine which rotor style best meets their operation’s needs. They do feel, though, that they’re gathering useful information on the pros and cons of spiders versus discs.
In the spider’s favor, the Fort Wayne site is seeing slightly higher throughput. “It seems that the loose items—your miscellaneous items such as washers, dryers, and other white goods—seem to get into the mill a lot easier,” says shredding manager Dale Baker. By contrast, the site had to “force feed” such material into its disc rotors.
The spider rotor is also credited with increasing hammer life on the Fort Wayne shredder—up to 7,500 to 8,000 tons per hammer now compared with about 5,500 tons per hammer in the disc rotor, Baker says. That’s because the spider arms themselves help the hammers grind up material, whereas on disc rotors the hammers “are the only edge inside the mill—they do 100 percent of the grinding.”
On the plus side for disc rotors, Baker says, the spider seems to stall the shredder far more easily. When the unit stalls, the mill must be locked out, opened up, and cleared manually, which generally requires about three hours of unscheduled downtime. As a result, certain materials that had been shredded with the disc rotor—such as wires and cables—had to be processed by other equipment after the spider rotor was installed, Richmond says.
In addition, it takes roughly two hours to change the spider’s protective caps, which can need replacing anywhere from every other day to weekly “depending on what you’re processing and whether there are any heavies in the mill—it can vary quite a bit,” Baker explains.
Given these pros and cons, the Fort Wayne facility is “still up in the air” regarding which rotor it likes better, Richmond says. “We need to run this [spider] rotor a little bit longer to get a little better feel before we make that decision.”
Deciding on Discs
At Sioux City Compressed Steel Co. (Sioux City, Iowa), the decision in 2000 to switch from a spider rotor to a disc was somewhat unplanned. The old four-arm spider rotor broke down soon after the firm’s managers had seen a no-weld disc rotor at the ReMA convention in San Antonio—a disc that was available immediately.
“We’d been curious about a disc rotor for a long time—curious enough that this was a chance to buy it and try it,” says Norman Bernstein, owner of Sioux City Compressed Steel.
Though Sioux City didn’t have any major complaints about spider rotors, it has been pleased with the switch. For one reason, bales were more likely to jam with the spider rotor, notes engineer Mike Potash. That’s because the spider would take in whole bales at a time, which could plug up the machine, while the disc rotor gradually breaks up bales as it draws them into the chamber, Potash says.
While the two styles cost roughly the same, the disc rotor’s production “is at least as good if not a little bit better” than the spider rotor’s throughput, Potash says. While he concedes that spiders tend to last longer—perhaps twice as long, though Sioux City hasn’t run its disc long enough to tell—Potash considers the disc to be just as economical. In fact, he estimates that the switch is saving Sioux City 75 cents to $1 on each ton of scrap shredded thanks to reduced maintenance costs, especially because the facility no longer has to worry about the parts and labor for replacing caps on spider arms.
Sioux City’s disc rotor does have replaceable caps on its end discs, but then so did the spider rotor. There are no-weld end discs available, Bernstein notes, which sounds appealing for the future—a rotor you can run until it’s worn out, then throw it away. He also believes that the disc rotor weighs more than the spider, thus providing greater inertia—a key factor in shredding.
This higher inertia gives disc rotors an advantage over spiders because disc machines lose fewer rpm when you feed in material, especially difficult-to-shred scrap such as rebar and bales, explains Randy Brace, vice president of engineering for Riverside Engineering Inc. (San Antonio), which mostly sells spider rotors. Inertia might not be a problem for large shredders with considerable horsepower, he adds, but processors with smaller machines must operate spider rotors “much more carefully” to avoid pulling rpm down too low. In some cases, smaller machines will perform better with discs than spiders, Brace concludes.
At Silver Dollar Recycling L.L.C. (Las Vegas), a change in feedstock dictated a change to a disc rotor. Though Silver Dollar had previously used six-arm spiders to process car bodies, the installation of a Harris 98/115 heavy-duty shredder with a no-weld disc last year coincided with the firm’s decision to reduce its reliance on highly competitive auto hulks and instead focus on cheaper but heavier scrap such as No. 2 iron, says Jim Brewer, operations manager.
“I have no complaints about either type—it’s just the application that you use the rotor in,” he explains. “If you’re going to run car bodies and a light tin mix, then I’d absolutely stay with a spider. But seeing that I wanted to run so much more No. 2 iron with light tin and less car bodies, I went with the disc rotor.”
Ironically, Brewer had tried some disc rotors in the 1980s that required considerable welding—and he hated them. Other processors offered tales of similar bad experiences, warning him away from disc rotors. But Brewer had also broken off spider arms, which likewise required a great deal of maintenance and welding to repair. So when it was time to buy a new shredder, he “decided to take some chances with this machine” because the promise of a truly low-maintenance rotor had great appeal. So far, the new no-weld disc is living up to his expectations: “It just takes a beating and keeps on going,” says Brewer.
Maintenance on spider arms was also a headache for Bill Bukevicz, executive vice president of Davis Industries Inc. (Lorton, Va.). Specifically, the spider arms cracked, needed a lot of buildup under the caps to sit solidly, and were difficult to keep balanced, he recounts.
Since the company switched two years ago to a no-weld disc rotor in its 80/104 Texas Shredder system, however, those problems have mostly disappeared.
The no-weld rotor does need some welding—such as when heavy scrap knocks out a chunk—but the company’s overall rotor-maintenance costs are down, especially because it no longer has to buy spider caps.
“On the disc rotor, the only thing you need to do is put in pin protectors, which you need on spiders too,” Bukevicz notes. Fortunately for Davis Industries, it found that many of its spider rotor replacement parts such as pin protectors were interchangeable with the disc unit.
No Easy Answers
In the end, the choice of which rotor works best seems more anecdotal than scientific—and certainly not universal. After all, users cite some of the same advantages and disadvantages for each style.
For Trevor Masters, the biggest problem in comparing rotors is a lack of hard data and the fact that most shredders are operating well below their capacity. For example, Masters recently worked with a shredder operator who planned to switch from a six-arm spider to a disc rotor in hopes of boosting production. But he helped the firm squeeze out an extra 25 tons an hour even before changing rotors.
The potential for that extra production “was probably there all the time,” Masters says, “so we would never have known if it was the rotor change or something else.”
That’s why he believes “it will be very difficult to ever assess which is the best. You can’t have somebody operating their shredder at, say, 50 to 60 percent efficiency and then swapping out one style of rotor for another and commenting on whether it may be better or worse.” •
Robert L. Reid is managing editor of Scrap.
Which style of rotor—spider or disc—works best in a shredder? It’s a controversial question, with no single or simple answer.