18.2 Close-Proximity and Remotely Operated Lifting Magnets

Magnetic attraction is conditional

A lifting magnet retains a suitable ferromagnetic load through magnetic attraction. The nameplate rating applies under defined material, thickness, contact and operating conditions. It is not a guaranteed capacity on every steel object, and it does not apply to nonmagnetic materials merely because they are metal.

The blueprint names both close-proximity operated magnets and remotely operated magnets. A close-proximity device requires a person near the magnet to operate its handle or control. A remotely operated device allows control from a separated station or transmitted command. This distinction affects access, release protection, communication and exposure; it does not identify whether the magnet is permanent or electrically energized.

A permanent magnet can retain a load without continuous electrical power, although a powered switching mechanism may be present in some designs. An electromagnet needs its designed electrical supply and may have a specified backup system. Do not claim that all magnets release immediately on loss of site power or that all retain indefinitely. Read the exact operating and emergency instructions.

Select for the actual contact conditions

Verify material suitability, load weight, dimensions, thickness, surface condition, flatness or curvature, temperature and CG. Thin material may have reduced capacity. Rust, paint, mill scale, dirt and uneven contact can introduce an air gap, reducing the magnetic circuit's effectiveness. A rating obtained on a thick clean test plate cannot automatically be transferred to a thin painted sheet.

Magnets for flat plate and round stock may have different rated capacities and contact geometry. A long flexible plate can peel away from the magnet even when its total weight is low. Use the specified minimum thickness, layout and derating chart; no single percentage accounts for all combinations of air gap and thickness.

Do not lift several stacked plates unless the device and procedure expressly permit that application. Attraction through the top plate does not establish reliable retention of every lower sheet. Likewise, loose attached pieces can fall independently of the main magnetic load.

Close-proximity operation

Plan safe access to position and activate the magnet while the load is supported. Follow the manufacturer's handle, latch or interlock procedure. A switching lever must be secured in the operating position as specified; a lever that cannot lock correctly is a stop condition.

After activation, move clear before weight is taken. Never ride with the magnet, use the handle to steady a suspended load or reach below the payload to check contact. The close-proximity operator needs a protected retreat path, particularly when the load may swing or rotate during take-up.

At the destination, land and support the load fully before approaching to release. Confirm the load cannot roll, tip or slide when magnetic attraction is removed. Follow the release sequence without forcing a jammed lever or bypassing a safety catch. Keep body parts away from pinch points between the magnet and steel.

Remote operation

Remote controls must be identifiable, functioning and protected from unintended release. Test the command and indications according to the instructions before a loaded operation. Verify the operator can distinguish the selected magnet, attachment state and release command. A wireless signal reaching the device does not prove that the load is securely attached.

Arrange communication with the crane operator and signalperson. Keep control of the release function assigned, and establish what happens on control loss, power loss or low reserve indication. Follow the manufacturer's emergency procedure rather than cycling the release control to diagnose a suspended magnet.

A remotely operated magnet can remove the need to stand next to the load during activation, but the route and destination still need exclusion areas. People must not pass beneath the load because the operator is distant. Use the intended alarms or backup indications and stop if the required monitoring is unavailable.

Rating example

A supplied product chart permits 1,000 lb on the proposed material and thickness after its stated surface reduction. The magnet itself weighs 180 lb. An 850-lb item is below that stated magnetic limit, but the crane must account for at least 1,030 lb from those two items. The comparison remains conditional on the load's geometry, contact and approved orientation. A 1,200-lb rating from a different thickness row cannot replace the applicable 1,000-lb value.

Inspection and handling

Inspect identification, structural members, lifting connection, magnet face, operating mechanism, locks, cables, connectors and power/backup system as applicable. Damage, overheating, loose connections, unreliable controls or a rough damaged contact face require assessment before use. Keep surfaces clean by the specified method; grinding a damaged face can alter its designed geometry.

Make the specified controlled trial lift in a protected area. Watch for incomplete contact, sliding or movement of attached pieces. Avoid impact and sudden acceleration. After landing, release deliberately and store the magnet so its face, cables and control mechanism are protected. Inspection frequency and records belong to the applicable ASME B30.20 program and manufacturer guidance, rather than an invented universal test before every lift.

Sources: CCO reference manual, lifting-magnet provisions, CMCO magnet application guidance.