Magnets for Holding & Latching

Holding vs latching

• Holding — a static clamp: hold a door open, retain a panel, fixture a workpiece, keep a cover shut. The magnet stays engaged and the design question is how much force, in which direction.
• Latching — a make-and-break cycle: the magnet must grab reliably on approach and release cleanly on demand (a pull, a push-push, a cam). Now you care about the whole force-vs-distance curve, not just peak.

The two needs pull magnet choice in different directions: holding wants maximum captured force at contact; latching wants a controlled approach, a defined release, and self-alignment.

Pull force is a system property

The rated pull force assumes an ideal that your product almost never matches. Real holding force depends on:

• The steel target — low-carbon steel gives full spec; thin sheet saturates and gives less; austenitic (300-series) stainless is nearly non-magnetic and can drop force by 70–90%.
• Air gap — the steepest variable. A coat of paint, powder-coat, or a decorative laminate is an air gap. Pull typically falls 30–50% at ~1 mm and drops below 25% by ~3 mm.
• Contact area & flatness — force needs intimate metal-to-metal contact; a curved or rough mating face leaves a distributed gap.
• Target thickness — the steel must be thick enough to carry the flux without saturating (a rule of thumb: at least as thick as the magnet's pole region).

Pull vs shear (breakaway)

Two failure directions, two very different numbers:

• Pull (normal / tensile) — force straight off the face. This is the big datasheet number.
• Shear (sliding / breakaway) — force parallel to the face. It is resisted only by friction, so it is typically just 15–25% of the rated pull, depending on surface finish and coating.

Loads that push a held part sideways — a latch on a sliding drawer, a fixture resisting cutting forces — are shear loads. Sizing them off the pull rating overstates capacity by 4–6×.

Pot magnets vs bare magnets

A pot (channel/cup) magnet is a magnet set in a soft-steel cup that returns flux to the working face. That return path concentrates the field where the load is, so a pot assembly delivers roughly 2–3× the pull of the same bare magnet — and protects the brittle magnet mechanically.

• Use pots for holding almost always: more force per dollar, a threaded or countersunk mount, and a rugged package.
• Bare magnets suit thin profiles, dual-sided attraction (magnet-to-magnet latches), or where the mating structure itself is the return path. Stocked in pot, disc, and block forms.
• Two-magnet latches (magnet-to-magnet rather than magnet-to-steel) give a defined, symmetric closure and can encode polarity for keyed alignment — mind the magnetization direction on both halves.

Designing the latch

• Design the release, not just the grab. A latch that holds 15 lb but needs 15 lb to open is a bad user experience. Tune peak hold and release feel with a small air gap (a bumper, a thin non-magnetic shim) that flattens the force spike at contact.
• Use the distance curve for approach. Force rises steeply near contact — a latch snaps shut satisfyingly in the last millimeters. Set the geometry so the “catch” happens at your intended closing travel.
• Self-alignment — a tapered pole or a magnet-to-magnet pair pulls the closure into registration, forgiving mechanical tolerance in hinges and slides.
• Cushioning & noise — bare NdFeB snapping onto steel chips and clacks. A thin elastomer or coated contact face protects the magnet and quiets the latch.

Material & environment

• Neodymium — most force in the smallest package; the default for compact, high-hold latches. Needs a coating and a temperature check.
• Ferrite / ceramic — low cost, corrosion-proof bare, ideal for large-area or outdoor holding where size isn't critical (ceramic magnets).
• Alnico — where a legacy pull curve or high-temperature holding is required (alnico).
• Outdoor / wet: ferrite or fully encapsulated pots; on NdFeB, corrosion is a design requirement, not a finish (mounting guide). Temperature-sensitive latches near motors or heat should be checked against the temperature guide.

Mounting & retention

• Threaded pots (stud or tapped hole) — the cleanest mechanical mount; the load path is through steel, not the brittle magnet.
• Countersunk pots — a flush screw through the assembly; good for panels and doors.
• Adhesive — acceptable for bare magnets in low-shear holding; choose the adhesive and surface prep deliberately and capture the magnet mechanically where cycling or vibration is present (bonding & mounting).
• Never load a bare magnet in tension through the magnet body — NdFeB is strong in compression, weak in tension and shear. Route holding loads through steel or a pot.

Design & spec pitfalls

Specifying a holding magnet

Additions to the standard RFQ checklist for a holding or latch magnet:

• Load & direction: required hold in pull and/or shear, with margin, at the real gap.
• Target metal & finish: the actual steel grade, thickness, and any coating between magnet and steel.
• Worst-case gap: nominal plus tolerance and finish build-up.
• Form & mount: pot (threaded/countersunk), bare, or magnet-to-magnet; mounting method.
• Cycle life (latches): operate/release forces and expected cycles.
• Environment: temperature range and moisture/chemical exposure driving material and coating.