Three kinds of magnet can lift steel. They differ in how they switch, what happens if the power fails, and how large they can be built. Choosing between them, and then sizing for the plate you actually lift, is what keeps a lift safe.
The three technologies
| Permanent | Electro | Electro-permanent | |
|---|---|---|---|
| Switching | By hand, on the magnet | Remote | Remote |
| Power | None | Continuous while holding | A pulse of about 2 seconds to switch on, and again to switch off |
| If power fails | Not affected | Load drops unless battery backup works | Load stays held |
| Size | e-LIFT range up to 5000 kg | Any size | Any size |
| Limits | Operator must reach the magnet | Heats in continuous use; coil failure drops the load | Not for high air gaps or scrap |
Electro magnets are the only choice for scrap and large air gaps (see MAGNALIFT - HD). For plates, blocks and billets on a crane, electro-permanent systems such as EPM-FB give remote control without depending on power to hold the load.
Derate for plate thickness
Rated capacity assumes a solid, flat plate more than 50 mm thick, of at most 2000 × 2000 mm. Thinner plate carries less flux, so the magnet lifts less:
| Plate thickness | Usable capacity |
|---|---|
| More than 50 mm | 100% |
| 25 mm | 60% |
Plate as supplied by a major steel plant is usually flat and smooth enough. Always hang the magnet from the centre of the load so it lifts in balance.
Long or thin plates: use more magnets
Keep overhang to 1000 mm or less, and only on plate over 25 mm thick that does not bend when lifted.
When plate is thin and longer than 2000 mm, use several lower-capacity magnets on a spreader beam rather than one large magnet, keeping overhang within 800 mm across the width and 1000 mm along the length.
Worked example: a plate of 6000 × 2000 × 12 mm needs at least four sets of magnets (two magnets per set, side by side) hung from a 4000 × 1000 mm spreader beam.
For yards handling many plate sizes, Sarda builds telescopic plate handling systems: a spreader beam with two fixed sets of magnets and two sets that move between 800 and 1100 mm.
