January/February 2005
By maintaining the magnetic poles of your lifting magnets, you can extend their useful life, increase their productivity, reduce repair costs, and—ultimately—enhance your bottom line.
By Jim Fowler
Several years ago, a scrap processor ordered a new lifting magnet. When the magnet arrived, he took one look at it and called the manufacturer.
“What’s wrong?” the vendor asked.
“My old magnet was flat on the bottom,” the upset processor replied. “On this new one, the center core and outer ring are sticking out from the bottom. What kind of inferior magnet is this?’”
The manufacturer immediately understood the problem. There was nothing wrong with the magnet, of course. A new lifting magnet is supposed to have a raised center core and outer ring on the bottom. The problem was that the processor had worn down his old magnet to the point where the bottom surface was flat. In short, he had worn off the magnet’s poles, which explains why he had to buy a new magnet in the first place.
This cautionary tale points out the importance of magnet pole maintenance and the need for processors to understand the concept of “wear depth”—the depth that the inner and outer poles extend beyond a magnet’s base plate.
“Wear depth has always been the hardest concept to convey to the scrap industry,” one magnet manufacturer says. “For whatever reason, it’s difficult for a scrap dealer to understand what we’re talking about and how important wear depth is.”
Processors who do understand wear depth—and who maintain their magnets’ poles—can realize the benefits of improved productivity and longer magnet life, which translate to reduced repair costs and higher profits. What’s more, magnet pole maintenance requires relatively little effort.
Pondering Poles
As a first step toward magnet pole maintenance, it’s helpful to understand magnet poles.
A circular lifting magnet has two pole surfaces—the center pole and the outer pole ring. The center pole is the core of the magnet case. (In the past, a bolt-on protective shoe covered this pole, but cost considerations led manufacturers to abandon this feature.) The outer pole forms a ring around the circumference of the magnet’s case.
The magnet’s magnetic field, which is generated by an energized coil inside the magnet case, flows from the center pole to the outer pole in a deep-reaching arc. As the magnetic power enters the outer pole, it travels through the top of the magnet case back to the center pole to complete the magnetic circuit.
Aside from creating the magnetic field, the poles add structural support to the magnet. They also protect the magnet’s bottom plate, in part by shielding the weld that secures the bottom plate to the case. The bottom plate, which supports and protects the magnet’s electric coil, is generally made of nonmagnetic manganese steel measuring ¾ to 1½ inches thick. (Years ago, bottom plates were made of stainless steel, and some of these are still in use today, a manufacturer notes.) The bottom plate sits on a machined ledge and is welded in place to provide a watertight seal for the electric coil.
Typically, the poles extend 2 to 3 inches beyond the bottom plate. That distance is the magnet’s wear depth. While such wear depth varies from manufacturer to manufacturer, you can make a rough estimate of what it should be by dividing the magnet’s diameter by 30, says Bob Bedard, general manager of NASCO-OP (New Philadelphia, Ohio), the purchasing cooperative for the scrap industry. A 66-inch magnet, therefore, would dictate a wear depth of about 2.2 inches (66 ÷ 30 = 2.2).
Why is wear depth important? As one manufacturer explains, “A magnet is designed to give optimum lift. To achieve that lift, you must maintain a certain amount of wear depth to get the maximum magnetic field down into the pile of scrap. As wear depth decreases, the magnetic field is reduced, which reduces lifting power.”
In addition to compromising lifting capacity, inadequate wear depth means less protection for the magnet’s bottom plate and welds. If the welds around the bottom plate crack, moisture can enter the magnet case and damage or destroy the electric coil. Thus, “the more wear depth you have, the better the chance that the pole shoes will hit the material first,” a vendor notes.
Preventive Medicine
There’s no denying that a lifting magnet’s wear depth is reduced during normal use. When a magnet is lowered onto a pile of scrap or dragged across the ground or a paved surface, the poles are subject to abrasion that wears them down. “Through use, the magnet’s bottom is subjected to severe wear,” Bedard says. “As more yards use pavement or concrete, that wear is accelerated. Dragging a magnet across a concrete sorting pad is no different than holding a grinder against the magnet’s surface.”
While it’s impossible to completely prevent wear on a magnet, it is possible to limit that wear through preventive maintenance procedures. The main purpose of a preventive maintenance program for magnet poles is to extend the life of the magnet. “Typically, a magnet’s life is seven years,” says one manufacturer. “If a scrap dealer follows all the preventive steps we recommend, that life span could be extended. You’d see more magnets living seven years rather than three or four years—and maybe that ‘typical’ seven-year life would sneak up to nine or 10 years.”
Aside from extending a magnet’s life, preventive maintenance can mean better magnet performance and reduced repair costs. “By maintaining wear depth,” one magnet veteran notes, “a three-year-old magnet should be able to operate at a 90-to-95-percent lifting efficiency as opposed to the 75-percent lifting efficiency that could result if no care is given.”
To measure a magnet’s lifting efficiency, the vendor says, you can keep track of the amount of scrap the magnet handles when it’s new and as it ages. One scrap processor also compared the performance of a new magnet he purchased against the production of his older one. In this case, the processor was able to make a direct comparison and could see “an obvious difference between performance,” the magnet vendor notes. “Watching just one magnet over a long period of time, the change is more subtle and difficult to notice—but there definitely is a change.”
So, what preventive maintenance steps should scrap processors take regarding their magnet poles? Here are two main suggestions from the experts:
• Measure and maintain the wear depth whenever you get a new or refurbished magnet. Keep in mind that wear depth generally doesn’t change quickly. “It’s a subtle wearing away that happens slowly over time,” one magnet producer says, “so you need to monitor that change on a regular basis.” How regular? Suggestions range from weekly to monthly to quarterly. Obviously, the timing is flexible and will depend on how you operate your magnet, the type of scrap your magnet handles, and your plant’s physical features (such as whether it’s paved or unpaved).
While checking the wear depth, be sure to also examine the physical condition of the poles and bottom plate welds. “You need to inspect for cracks, chunks, gouges, and uneven wear, particularly on the outer pole,” says one manufacturer. “Since magnets are often dragged, one side can be worn more than the other.”
• Make sure the poles have a hardface weld on their surfaces. “Though some scrap dealers still don’t do it, hardface welding is more of a common practice,” notes one magnet expert. The reason is simple: Over the years, scrap processors have “seen its value in extending a magnet’s life and maintaining a higher level of lifting performance.”
Some manufacturers provide a hardface weld as “standard equipment” on their magnets, while others treat it is an “option” that must be ordered by the customer. Some vendors also believe that hardfacing is best done in the factory by a magnet-welding expert, while others say it can be done just as effectively in scrap operations by a competent welder.
If you decide to do your own hardfacing, use an abrasion- and impact-resistant hardface-weld material that is magnetic, experts note. Also, the Rockwell hardness of the material should be 50 or above, says Bedard, who suggests telling your welding supplier what you want to do to ensure that you get the correct hardfacing material.
During welding, manufacturers suggest applying the weld in a wave, weave, or cross-stitch pattern in a single layer. Preheating the magnet can improve the adhesion of the weld, they note. If the magnet is cold, it can be preheated with a torch—as long as the operator knows what he’s doing.
As one magnet technician cautions, “There’s always the chance that excess heat will be applied to the magnet body, which could cause internal damage to the magnet.”
To avoid such problems, you can wait until the end of the workday, move the magnet to the maintenance shop, and do the hardface weld at that time. “The magnet heats up as it’s running through the day,” a vendor notes, “so it’s already preheated at the end of the shift and will remain hot for some 12 hours.”
It’s critical that hardface welding be applied only to the center and outer poles, not to the bottom plate. Hardfacing the bottom plate, which is not magnetic, will short circuit the magnetic field.
If a magnet has never been hardfaced or if the hardface has worn off, manufacturers recommend building up the poles using a mild steel rod. Since buildup welding concentrates heat in a specific area, a skip-welding technique should be used to avoid excessive heat concentration, experts advise. Bedard recommends that any weld buildup should be no more than ½ inch. “Areas requiring larger amounts of buildup are worn to the point where replacement poles must be used,” he says.
There are special welding considerations when working with the bottom plate, manufacturers and welding experts note. In general, stainless rod provides the best adhesion when welding mild steel (the magnet case) to manganese steel (the bottom plate). The main point, a vendor says, is to make the fillet weld between the case and the bottom plate “beefy enough” to withstand the impact that a magnet takes. “You need a substantial amount of weld with good penetration,” he notes. “In most cases, a crack in the bottom-plate weld results from impact and should be corrected immediately to prevent moisture from entering the magnet case.”
Adding another cautionary note, manufacturers point out that repetitive welding on a magnet can change the composition of the base material. Over time, repetitive heating can make the base material brittle, making it no longer feasible to weld on the magnet. When that happens, you’ll probably have to replace the poles, though you might be able to machine away the affected material. “You want to get to a nice clean surface again with good mild steel that hasn’t been altered greatly by heat,” a vendor says. “This has to be done on a large vertical boring mill, most likely by a magnet manufacturer.”
Replacing the outer pole is fairly expensive due to material, machining, and welding costs. Describing the work involved, one manufacturer notes, “you put the outer pole ring on a vertical boring mill to machine a step or ledge on the ring. Then you need to machine the magnet where it’s worn to create a mating step in the magnet case. You then put the two together and weld them inside and outside.”
Another magnet veteran says it’s quicker and cheaper if you don’t machine the step in the outer ring and the magnet case, but instead flat-weld the ring to the case. This approach isn’t perfect, however. “If you get a side blow to the magnet,” he says, “it has no support, and there’s a good chance the flat weld will crack.” As a result, he advises, “this work should be done the proper way by the manufacturing experts.”
Failure to maintain your magnet’s poles can be a costly mistake indeed in both time and money. It generally takes three to four weeks for a manufacturer to replace an outer ring and center pole, vendors note. As for cost, replacing the center and outer poles of a 66-inch magnet can cost $2,500 to $3,500—equivalent to 13 to 18 percent of the magnet’s original cost of about $20,000, Bedard estimates. Other estimates range from $3,000 to $8,000 depending on the magnet’s size.
The best way to avoid such costs—as well as to extend the life of your magnets and improve their performance—is to follow a preventive maintenance program regarding your magnet poles. Do you have such a program in place? If not, your profit picture probably isn’t as bright as it could be.
Jim Fowler is retired publisher and editorial director of Scrap.
Publisher’s Note: Special thanks to Bob Bedard of NASCO-OP for providing the background information and photos/diagrams for this article. To learn more about NASCO-OP, call 800/321-3396 or visit www.nascoop.com.
By maintaining the magnetic poles of your lifting magnets, you can extend their useful life, increase their productivity, reduce repair costs, and—ultimately—enhance your bottom line.