The standards, and why they are strict
Lifting magnets fall under ASME B30.20, the safety standard for below-the-hook lifting devices, with design criteria in its companion ASME BTH-1. In the United States these are consensus standards rather than federal law, but OSHA treats them as accepted industry practice under the General Duty Clause — so following them is how due diligence is demonstrated after an incident.
The distinction that matters most
Rated capacity is not holding force. Rated capacity is the measured breakaway force divided by the design factor, established under ideal laboratory conditions — and then real-world derating applies on top of that, for thickness, surface, alloy and geometry. A magnet whose label says 800 lb may hold far less than 800 lb on a thin, painted, curved part.
The design factor is not spare capacity to be spent on a difficult load. It is the margin that keeps the load up when something is not ideal — and something usually is not.
What actually reduces holding force
Every factor below reduces the force available. They compound, and several can apply at once.
| Factor | Why it reduces hold | Practical effect |
|---|---|---|
| Material thickness | Thin material saturates before it can carry the magnet’s full flux — the circuit cannot close | The largest single derate. A thin sheet may hold a small fraction of the rated figure |
| Surface condition | Paint, scale, rust, oil or plating create an air gap, and field falls steeply with distance | Even a thin coating costs meaningful force; heavy scale is severe |
| Alloy | Rated on low-carbon steel. Higher carbon and alloy steels have lower permeability | Alloy and tool steels hold less; austenitic stainless holds essentially nothing |
| Surface geometry | A curved or irregular surface reduces contact area and introduces gaps | Round stock holds far less than plate; specialised pole geometry exists for it |
| Load flexure | A long thin plate sags, peeling the magnet away from one edge | Spreader beams with multiple magnets rather than a single lift point |
| Shear versus normal load | Resistance to sliding is far lower than resistance to a straight pull | Any off-vertical lift, swing or acceleration loads the magnet in its weak direction |
| Temperature | Remanence falls with temperature, and above the rated class the loss is permanent | Hot plate handling needs a temperature-rated device, not a standard one |
| Air gaps from debris | Swarf or grit trapped on the pole face is a distributed air gap | Pole face condition is part of the pre-use inspection for a reason |
Device types
| Type | How it holds and releases | Behaviour on power loss | Suits |
|---|---|---|---|
| Manually switchable permanent | A lever rotates an internal magnet assembly to shunt or direct the flux | Unaffected — no power involved | Fabrication, machine shops, plate handling. The workhorse |
| Electro-permanent | A current pulse switches the magnetic circuit; permanent magnets hold | Holds — no power needed to retain the load | Automated handling, robot end effectors, press feeding |
| Electromagnet | Coil current produces the field | Drops the load unless battery backup is fitted | Scrap handling, high duty cycle, where fast release matters |
| Battery-backed electromagnet | As above with a backup supply and alarm | Holds for a defined period on failure | Where electromagnet performance is needed but a drop is unacceptable |
The choice is mostly about the power-loss case
Permanent and electro-permanent devices hold a load through a power failure, an emergency stop or a cable fault. A plain electromagnet does not. That single difference determines suitability for most applications far more than capacity or duty cycle does — and it is why electro-permanent has displaced electromagnets across automated handling, where an unplanned release near equipment or people is unacceptable.
Electromagnets remain the right answer where load release must be instant and repeated at high frequency, and where the drop zone is controlled — scrap yards being the obvious case.
Non-lifting handling magnets
Not everything in material handling is a below-the-hook device, and the ones that are not fall outside B30.20 — though the derating physics is identical:
- Conveyor and transfer magnets holding parts to a moving surface
- Sheet fanners separating stacked sheet using repulsion between induced poles
- Magnetic sweepers recovering ferrous debris from floors and yards
- Workholding chucks for machining and grinding, where the same switchable circuits appear
- Rail and pallet magnets for tooling location and fixture retention
What we supply, and what we do not
Scope, stated plainly
Radial Magnets supplies magnet components and assemblies. We do not manufacture, test, rate or certify below-the-hook lifting devices.
A finished lifting magnet is a certified assembly. Its rated capacity comes from a measured breakaway test on that specific device, its marking and inspection regime follow B30.20, and its structural design follows BTH-1 — all of which belong to the equipment manufacturer. Buying magnets from us and assembling a lifter does not produce a compliant device; the design, test and rating obligations sit with whoever puts it into service.
Where we help is upstream of that: supplying the magnetic material with the right coercivity for a switchable circuit, matched consistently across an assembly, with certification traceable to the lot.
What we supply
- High-coercivity NdFeB for switchable and electro-permanent circuits, where the magnet must survive repeated exposure to a reversing field without losing strength.
- Alnico for the switchable circuits that depend on its particular low-coercivity behaviour.
- Ferrite where cost and volume dominate and the duty is modest.
- Pot and channel assemblies for holding and fixture applications.
- Matched sets with helmholtz moment consistency across the assembly.
- Per-lot material certification and full traceability.
The coercivity point is the one most often missed
In a switchable device the magnets are deliberately driven into opposing field conditions every time the lever is thrown or the coil is pulsed. That is a demagnetizing event, repeated for the life of the tool. A material selected on energy product alone — the highest grade available — has the lowest coercivity and degrades fastest under exactly that duty. Switchable circuits want coercivity margin, not maximum remanence, and getting that backwards produces a device that weakens gradually over its service life for no visible reason.
What to send us
- The circuit design, or the holding force required and the pole geometry available
- Whether the magnet is switched, and if so how many operations over its intended life
- The reversing field the magnet sees during switching
- Operating temperature range, including any hot-material handling
- Environment — outdoor, washdown, foundry, marine
- Envelope and the retention method planned
For estimating force during concept work, our pull force calculator gives a first approximation — but it assumes ideal contact with thick clean steel. Real capacity comes from testing on the actual assembly with the actual load, which for a lifting device is a regulatory requirement rather than good practice.