July/August 1992
The Fastener Factor
Here's how one scrap recycling company has reduced its maintenance worries and increased its productivity by using more reliable conveyor belt fasteners.
In the high-production, high-maintenance business of shredding, a certain amount of downtime is inevitable—even necessary. At North Star Steel Co.'s shredding plant in St. Paul , Minn. , for example, says Jerry Herzog, raw materials maintenance supervisor, "We count on running the system two-thirds of the day and maintaining it during the other third. When we hit that goal we're still way above the industry average."
Most of the operation's downtime is related to the shredder itself, a 6,000-horsepower machine that churns out a little more than a ton of fragmented steel—or frag—a minute. Operating 16 hours per day, five days a week, that's a production rate of about 5,000 tons per month.
But problems with other parts of the shredding system, including the conveyor system, also contribute to equipment downtime. The fact is that shredded scrap—a hodgepodge of crumpled steel sheets, gnarled wire, and twisted pipe with sharp, jagged edges—is murder on conveyor belts and belt-splice fasteners, frequently damaging conveyor components badly enough to bring a shredding operation to a halt. "Recently, a piece of rebar got caught on a wing pulley and tore through the belt like a knife," Herzog says. "Although the motor tripped out before the tear got too far, that section of belt had to be replaced."
While North Star officials concede that some conveyor belt damage—and resulting downtime—is unavoidable in a shredding system that takes as much abuse as it dishes out, they consider one problem area—belt-splice failures—unacceptable. As a result, the company recently embarked on a search for a solution, which they found in a switch to heavy-duty, rivet-attached mechanical fasteners.
Conveyor Belt Blues
North Star's conveyor system—in effect, the shredder's circulatory system—features nine belts, a mix of three- and four-ply weave with top covers ranging from 3/16- to 3/8-inch thick. After being shredded, scrap is conveyed across a shaker table and rides a 54-inch-wide conveyor belt to a large rotating tumbler-drum. Lightweight materials such as foam rubber, dirt, and upholstery fabric are air-separated by 600-horsepower fans and are carried off by a 48-inch-wide conveyor belt.
The remaining scrap travels via two 48-inch-wide belts—one inclined and one flat—to the system's second separation stage. Crossing another shaker table, the scrap is fed onto a 60-inch-wide "under mag" belt, so called because the scrap passes under a magnetic drum, which separates ferrous frag from other shredded materials.
A 36-inch-wide belt discharges the nonferrous-bearing residue, with residual ferrous pieces culled from the nonferrous stream by one final magnet at the end of the nonferrous belt. These pieces are conveyed by a 36-inch-wide reclaim belt back to the under mag belt. Finally, clean ferrous frag drops onto a radial stacker belt, which dumps the scrap into gondola cars for a short swing around the plant to the minimill.
In addition to scrap abuse, these conveyor belts must withstand other operating conditions that have contributed to North Star's belt damage and belt-splice failures. For instance, the 3/16-inch top-cover of the 36-inch-wide radial stacker belt recently began to separate from its plies after becoming impregnated with residual oil from shredded auto parts. About the same time, North Star experienced a rash of splice failures due to a combination of oil impregnation, high belt tension, and small pulley diameters.
In addition, all of these factors have contributed to weakening the belt carcass around the North Star system's bolt-attached mechanical fasteners. Because bolt-splice installation requires a uniform row of bolt holes across the belt, the company found that so many of the belt's load-bearing fibers were severed that oil-weakened belts with high load weights and tensions were unable to hold onto the splice. Either the bolts would come out through the belt ends, or the belt would split along the row of bolt holes as it flexed around the pulleys. During one five-month period, North Star experienced 20 splice failures.
"When a splice fails or the belt tears, we can't simply cut out that section and adjust the take-up," says Doug Duren, North Star's raw materials superintendent. Conveyor take-ups are used strictly for regulating slack and tension, and they can accommodate a maximum of only 24 inches of belt. Thus, a new section of belt must be spliced in to replace each damaged section taken out—a time-consuming process with bolt-attached fasteners.
Finding a Better Splice
Looking for a solution, North Star's shredder crew consulted local system-equipment distributor Baldwin Supply Co. (Minneapolis) and considered a number of alternatives, with vulcanized splicing and oil-resistant belting ranking high on the list. Both were tested and ultimately rejected as cost-prohibitive. "Because of our short belt life," Durensays, "specialized belting simply costs too much to be repeatedly cutting out damaged sections. The same holds true for vulcanization—it's not economically feasible, and replacement and repair times would be too long for our needs."
With these concerns in mind, Baldwin Supply recommended a switch to the new fasteners—without a change in belting—for all system conveyors. The hinged-plate SR size R6 fasteners, made by Flexible Steel Lacing Co. (Flexco) (Downers Grove, Ill.), attach to the belt with hammer-driven, self-setting rivets that are preassembled with patented, breakaway pilot nails. The pilot nails guide the rivets between rather than through the belt-carcass fibers and, therefore, prevent the rivets from damaging or weakening the belt. In addition, the fasteners don't require any hole punching or drilling, so no belt material is removed.
The SR design combines 1 1/2-inch-wide top and bottom plates joined at one end by two hinge loops, through which a nylon-covered armored cable hinge pin is threaded. Each pair of plates, secured to the belt with five rivets arranged in a staggered pattern, sandwiches the belt ends. Fortunately, the R6—the largest of Flexco's rivet-hinged fasteners—was able to operate smoothly around North Star's 16-inch-diameter pulleys. (Normal minimum pulley diameter for the R6 is 18 inches, but due to North Star's lower operating tensions, the splices could work with the smaller pulley diameter.)
As an added benefit to using the R6, North Star found that the same hammer blows that set the rivets also compress the fastener plates into the belt cover. This simultaneous compressing and peening action holds each increase in compression, giving the plates a tight, permanent grip that prevents the belt from pulling solely against the rivets. Equally important, the leading and trailing edges of each plate extend beyond the rivets to prevent the belt from flexing around them during operation. These factors have virtually eliminated North Star's splice failures during normal operating conditions, the company reports.
Enjoying Easier Repairs
In addition to a stronger, more durable splice, the new fasteners also have enabled North Star to cut repair times in half during operating shifts. To speed installation of fresh belt sections, the maintenance crew uses Flexco's MSRT multi-rivet driving system, in which a preloaded guide block holds 20 rivets in alignment over the rivet holes of four SR fastener plates. A special five-pronged drive rod, inserted into the top of the loaded guide block, enables five rivets to be hammer-driven and set simultaneously.
Because the rivets are self-setting, they allow North Star to quickly perform the total splicing job from the top side of the belt, without removing the belt from the conveyor. An anvil plate, positioned beneath the splice while rivets are hammered in, captures the pilot nails as they emerge below the lower fastener plate. The head of each pilot nail then works in conjunction with the anvil plate to peen the rivets automatically, with the pilot nails subsequently released.
The ease with which the rivet-hinged fasteners can be installed has enabled North Star to replace damaged belts on a moment's notice. "Sometimes, when the shredder goes down for one reason or another, I'll have somebody make up a 20- or 30-foot section of belt and we'll splice it right in there," says Herzog. Quickly handling such preventive maintenance "kills two birds with one stone," he explains, and eliminates subsequent system downtime.
To speed replacement of whole conveyor belts, North Star's shredder employees keep full spares on hand, cut to length. Anticipating the relatively short life of the belts, "each conveyor has a replacement belt with pre-installed splices," Herzog says. "During the midnight maintenance shift, we can simply string a belt around the conveyor, slip in a hinge pin, and it's ready to go."
Plates for the SR fastener are supplied in continuous strips in lengths that fit standard belt widths, including the 36-, 42-, 48-, 54-, and 60-inch widths in North Star's shredder operation. Fastener strips assure proper plate spacing and alignment, and make it easy to install a straight, uniform splice. "We can put an SR splice into a 60-inch belt in about an hour," Herzog notes.
Considering the number of benefits it's gained, North Star can attest that looking for a solution to something as seemingly small as belt-splice failure can have a big effect on the productivity of a shredding operation. By resolving this problem through more dependable fasteners, North Star's shredder has been able to more reliably meet its minimill's needs while also reducing its own maintenance headaches and costs. •
Here's how one scrap recycling company has reduced its maintenance worries and increased its productivity by using more reliable conveyor belt fasteners.