Lifting magnets can be the optimum load handling solution in many applications. Magnets can be the fastest and often the only way for a single operator to load, handle and unload ferrous material. They offer several advantages:
• Time savings can be achieved because there is no need for blocking and slinging.
• Effective floor space can be increased by eliminating floor aisle space and by using greater stack height.
• Product damage can be reduced by eliminating chains, hooks and other mechanical grabs.
• Operating cost is reduced by eliminating the cost of repairing and replacing slings and chains.
Close proximity magnets
Magnets operated at close proximity can be permanent, battery powered, electro- or electro-permanent types. These are available in capacities up to 5t (11,000lb).
Close proximity magnets are designed to be turned on or off by the operator. They are very flexible in that they can be moved from hoist to hoist without any special electrical installation. These magnets are used for various applications in steel service centres, for example, including the handling and storage of plate, structural products, and loading and unloading burning tables.
Permanent magnets use constantly active magnetic material to develop the magnetic field. The magnet is lever operated to turn the magnetic field on and off. Variants are available to handle flat and round parts up to a capacity of 1t (2,200lb).
Advantages of the close proximity type are that they are totally self-contained and can be transferred from location to location within the service centre. No external power connection is needed and little or no maintenance is required.
A disadvantage is the limited capacity, dictated by the properties of the permanent magnet material.
Battery powered electro-magnets are available in capacities up to 5t (11,000lb). They use a self-contained gel cell type battery to supply the power for the magnet. An automatic battery charger is built in to prevent overcharging. The electronic control on the magnet indicates, on a LED readout, the state of charge of the battery. It provides a visual indication and audible alarm to indicate when recharging is necessary.
Units are supplied with magnet mounted lift-released pushbuttons and an infra-red remote control to turn them on and off from up to 5m away. This eliminates the need for the operator to go to the magnet to operate it – an advantage when unloading a burning table for example.
Battery powered magnets are available in models to lift flat product and also for round and structural shapes. Advantages are that they can be moved from hoist to hoist, require no external power connection and are available with greater lifting capacities than permanent magnet types.
A disadvantage is that they require periodic battery recharges but a charge typically lasts longer than a shift.
Self-contained electro-magnets are available in capacities up to 4.75t (10,500lb). They operate on mains voltage AC power and are the least expensive magnets per kilogramme of lift.
A rectifier and control switch are mounted right on the top of the magnet.
Advantages of this type are their low cost per kilogram of lift and greater capacity under adverse conditions.
A disadvantage is that they required a source of AC power to operate.
The latest development in close proximity magnets is the electro-permanent magnet. This is a permanent magnet turned on and off using a pulse of electricity. Once the magnet is turned on or off, electric power is no longer required. They are available in capacities up to 5.4t (12,000lb).
An advantage of this type is that they are not dependent on external power. A disadvantage is that the magnetic field is produced by permanent material and is limited by the properties of that material used in manufacture.
Remotely operated magnets
Remotely operated lifting magnets are electro- or electro-permanent systems operated from either a crane cab, pendant switch or by radio control. These systems have basically an unlimited capacity and are used for handling single or multiple plate unloading or loading trucks, as well as storage areas and for loading and unloading cutting tables. Plate sizes can range to more than 3m wide and more than 12m long.
Systems are also used for handling single or bundles of rounds, flats, channels or angles. They eliminate the need for slings and chains, or dunnage in the storage area. They also eliminate the need for several people to fix slings or chains, as well as the need for increased storage capacity.
Systems are also used for handling up to 40t coils, both eye horizontal and eye vertical. Magnetic coil handling reduces coil damage from mechanical devices and reduces the storage area requirements for C-hooks.
The newest application is a total plate coverage system to load and unload plate onto a burning or cutting table. This system is designed to meet the special material handling needs required in flame, plasma, water jet and laser cutting table operations. The system can be used to load the plate onto the table and then remove the cut parts and the skeleton in one step. This increases the throughput of the table by using the table for cutting parts, not for sorting them out.
Such a magnet system is dedicated to a specific crane. It consists of the magnet/s suspended from the crane hook/s and a spreader beam if required. Magnets operate on DC power. A combination AC to DC rectifier and magnet controller is mounted on the crane. This device converts the AC crane power to DC and turns the magnets on an off. Various options are available depending on the application. These include: magnet selection to select particular magnets on a spreader beam; fanning to fan off plates if multiple thin plates are lifted; reduced initial pick feature for lifting multiples; and variable magnet power control for lifting thinner plate off of a stack.
A battery back-up system will instantly give 20 minutes of emergency power to the magnets in the event of an interruption in mains power to the crane. This system uses low maintenance sealed standby lead acid batteries. It is an online system that constantly floats the batteries across the DC bus so no switching is required to achieve the standby power.