Permanent magnets offer a big boost: moving stock and materials around the shop, usually involves slinging steel from a hoist or using a forklift. Lifting with permanent magnets can significantly simplify many of these tasks.
Magnetic lifters get their holding power from permanent magnets and do not require electrical hookups, clamps, or special cables. Once they are hung from a hoist, crane, or other lifting device, they are ready to work. So-called permanent magnets are shaped or machined pieces of special materials, such as ceramic, Alnico alloy or rare earth materials, which have been magnetized through exposure to intense electrical fields. Once magnetized, these materials maintain their magnetic energy over a wide range of temperatures, physical shocks, and other environmental conditions--hence the name permanent magnets.
Magnetic lifters consist of permanent magnets arranged in a housing so that both north and south poles contact the surface of the types of ferrous objects that they are designed to lift. This concentrates their magnetic force within the contact area and gives them an outstanding ability to hold workpieces through air gaps. Most workloads within the weight range of a well-designed lifter can be handled safely, irrespective of surface finish.
In the "on" position, a lifter's magnets create a complete magnetic circuit when holding a ferrous workpiece. When the magnets are turned off, usually by rotating a lever, the magnetic field is reoriented and the magnetic circuit is broken, or "shorted out," so that there is no net holding force acting on the workpiece.
A very weak, temporary magnetic field can be induced in some of the ferrous materials magnetic lifters contact. The field is essentially limited to the immediate contact area. Once released, most ferrous objects quickly lose all measurable traces of this residual magnetism. The small physical shocks associated with releasing a part will usually demagnetize it instantly. Stock and parts made from higher alloy steels and some stainless steels tend to hold the induced magnetism longer. But even with these steels, the effects are normally short-lived and inconsequential.
Lifters differ in their ability to handle the wide range of workpieces commonly encountered in manufacturing, fabrication, and repair environments. Some lifting magnets are designed to lift both flat and curved objects, while others can only pick up one or the other. Shops should make sure that the configuration and pole design of the magnetic lifters they buy are suitable for their applications.
The choice was easy for a firm in Washington that manufactures hydraulic and pneumatic rotary actuators. The company uses magnetic lifters to load and unload parts from the CNC and manual machines in its machining department. A senior foreman says the lifters his company buys are built with a v-shaped groove that enables them to handle round loads equally well as flat plate steel, and that's important because most of the components he works with have rounded cylindrical surfaces.
Another factor in lifter design and in the selection process is the ratio of lift capacity to the physical size of the lifter.
"It pays to shop around. I first saw what works best for us at IMTS. The lifter magnets we buy now have more load capacity for their size than other units we've used in the past. The smaller lifters are easier to operate around our machining centers," advises the senior foreman. "We can maneuver the parts better and the ergonomic aspect of these lifters is superior."
Electro vs. Permanent
Magnetic lifters and electromagnets are obvious competitors.
They both use magnetic power, and electromagnets can theoretically be built to meet just about any demand for lifting capacity. However, their need for an external power supply can pose safety issues: Electrical hookups can become damaged or fail. A power failure might be no more than a temporary glitch in a scrap metal yard, but it could be catastrophic in a manufacturing shop. At the Kansas spring steel plant, management tried a basic electromagnetic setup, but it proved unreliable and was replaced by magnetic lifters.
Permanent magnetic lifters have no potentially hazardous electrical connections and no trailing cables to foul. Because they are not subject to power failures, they also eliminate the need to use inconvenient chains or straps, according to their manufacturers.
In most cases, their permanent magnets are adequately shielded from outside interference by the housings that encase them, and they cannot lose magnetism under normal operating conditions. Some manufacturers build in a fail-safe mechanism: Once the magnet has been "turned on" by rotating a control handle, the fail-safe mechanical locking mechanism is designed to ensure that it cannot be "turned off" accidentally. Some lifters are also completely insulated magnetically, preventing handling problems when working close to ferrous structures or parts.
The senior foreman with the Washington manufacturer of rotary actuators says, "With magnetic lifters we not only work faster--we also work safer. We don't have to move heavy, sharp-edged parts around trying to get straps around them, and we don't have to worry about the consequences of a greasy strap slipping out of position." Bunting Magnetics Co.
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|Title Annotation:||Material Handling|
|Publication:||Modern Applications News|
|Date:||Nov 1, 2004|
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