Magnetic Assemblies Guide
A bare magnet is rarely the best holding device. Put the same magnet in a steel cup and it pulls two to four times harder, gains a mounting feature, and stops chipping. Here's the family of pot, channel, countersunk, and rubber-coated assemblies — and how to pick the right one.
Why assemblies beat bare magnets
A magnet in open air wastes most of its flux — field leaves the back and sides and returns through space without doing work. An assembly wraps the magnet in a mild-steel circuit that collects that stray flux and delivers it to the working face. Three benefits arrive together:
- Force multiplication. A steel cup concentrates flux so effectively that a pot magnet typically delivers 2–4× the pull of the identical bare magnet at contact (the mechanics are in the pull force guide). Equivalently: the same force from a smaller, cheaper magnet.
- Mechanical protection. The steel body takes the impacts, prying, and torque that would chip a brittle bare magnet — and it provides threads, holes, and hooks, which sintered NdFeB itself cannot safely carry (see handling & safety on why you never drill a magnet).
- Self-shielding. Flux is steered to the face, so the back and sides run magnetically quiet — friendlier to nearby electronics, adjacent assemblies, and air-freight field limits.
An assembly's force is concentrated at the contact face — which makes it more sensitive to air gaps than a bare magnet. Pots dominate direct-contact clamping; for working across a gap (through a panel, across a housing), a bare magnet or magnet-on-plate arrangement often wins. Match the format to the gap.
Pot (cup) magnets
The round workhorse: a disc or ring magnet bonded into a plated mild-steel cup, working face ground flush. The cup wall becomes one pole and the exposed magnet face the other, closing the circuit through whatever the face touches.
- Shallow pot / flat-face: maximum force per diameter for flat-on-flat clamping — fixtures, latches, tool holders.
- Bi-pole / two-pole pots: steel poles on both sides of the magnet at the face; strong grip on thinner steel because flux path length is short.
- Ferrite vs. NdFeB pots: ferrite pots are larger and cheaper, tolerate 250 °C, and shrug off corrosion; NdFeB pots deliver several times the force per size. Same trade as the material comparison, in a cup.
- Force ratings: quoted at contact on thick ground steel, like all pull ratings — apply the full derating chain from the pull force guide, especially for painted surfaces, where a pot's gap sensitivity bites first.
Channel & sandwich assemblies
- Channel magnets: the rectangular counterpart to the pot — a block magnet in a U-shaped steel channel, often with countersunk holes along the centerline. The long working face suits mounting rails, sign and panel retention, door catches, and tool bars; force scales with length.
- Sandwich assemblies: a magnet between two steel plates, poles presented at the plate edges — the pattern inside lifter blocks, sweeper bars, and many holding fixtures. Multiple magnets alternate polarity between shared steel poles to multiply face force.
- Custom assemblies: beyond catalog shapes, the same circuit principles build sensor mounts, rotor sub-assemblies, magnetic chucks, and separator elements — a magnet-plus-steel design problem your supplier's engineers solve routinely. This is where a good RFQ pays off: specify the force, face size, and environment, and let the circuit be proposed.
Mounting hardware options
| Format | Attachment | Typical use |
|---|---|---|
| Countersunk hole pot | Flat-head machine screw | Flush face; latches, fixtures — the default |
| Through-hole pot | Bolt or shoulder screw | Heavier duty; blind-side access |
| Threaded stud pot | External male stud | Mounts into a tapped part or with a nut |
| Internal thread / boss pot | Female thread in the cup back | Stacked hardware: hooks, eyebolts, handles |
| Hook / eyebolt pots | Pre-fitted hook or eye | Hanging, rigging, cable management |
| Adhesive-mount pot | Plain back for bonding/VHB | No-fastener installs — pair with the bonding guide |
Tighten mounting screws to the assembly's specified torque, not gut feel — over-torquing a countersunk pot bows the cup and cracks the bonded magnet inside. If no spec is published, ask; it exists.
Rubber-coated assemblies
A magnet system fully jacketed in molded rubber trades some rated pull for three things bare steel can't offer:
- Shear grip. Rubber's friction coefficient is several times nickel-on-steel — so on vertical surfaces, a rubber-coated assembly often holds more real load than a stronger bare pot that slides (the shear math is in the pull force guide).
- Surface protection. No scratches on painted vehicles, machine enclosures, or architectural metal — the reason these dominate roof-light mounts, camera rigging, and sign attachment on vehicles.
- Weather sealing. The jacket seals the magnet and steel against rain and washdown for outdoor duty.
Ratings for rubber-coated products are usually given both as pull-away and as maximum shear/slide load — use the shear number for anything gravity-loaded on a vertical face.
Countersunk & through-hole magnets
Between bare magnets and full assemblies sit magnets manufactured with fastener features: countersunk rings and blocks that take a flat-head screw directly. The essentials:
- The hole is pressed and ground in at the factory — never field-drilled. Ordering the feature is the only safe path (the safety guide covers why).
- Polarity of the countersink side matters: the screw head sits on one face, so specify whether the countersink is on the north or south face — it determines which pole works. Standard catalog parts come in both.
- Screw to a steel bracket and you've built a pot: mounting a countersunk magnet against a steel plate adds the backing-circuit force bonus (~up to 2×) automatically.
- Torque-limit and washer rules apply as with pots — brittle material, compressive loads only, no bending.
Choosing by application
| Application | First choice | Why |
|---|---|---|
| Fixture / jig clamping, flat steel | Flat-face NdFeB pot | Max force, direct contact |
| Door catch, cabinet latch | Small pot or channel + strike plate | Cheap, flush-mountable, adjustable engagement |
| Hanging tools / cables / signage | Hook or eyebolt pot | Hardware included, load in shear handled by geometry |
| Vertical surface, painted (vehicle, enclosure) | Rubber-coated assembly | Shear grip + surface protection |
| Mounting rails, long parts, panels | Channel magnets | Distributed force along a line |
| Product component, screw-mounted | Countersunk magnet on steel bracket | Compact, serviceable, backing bonus |
| Working through a gap / non-contact | Bare magnet or magnet-on-plate | Pots lose their advantage across gaps |
| High temperature (to 250 °C) | Ferrite pot / SmCo custom | Material limit, not format — see grades & materials |
| Outdoor / washdown | Rubber-coated or stainless-housed assembly | Sealing beats coating upgrades |
Specifying an assembly
Alongside the standard RFQ items, an assembly inquiry should state:
- The load: magnitude, direction (pull / shear / peel / torque), static or dynamic — this drives format more than any other input.
- The mating surface: material, thickness, coating/paint, flat or curved. Thin sheet and paint change the answer (see derating).
- Envelope and mounting: available diameter/footprint, fastener preference, flush-face requirement.
- Environment: temperature, moisture/chemicals, outdoor exposure — selects magnet material, plating, and jacketing.
- Quantity and customization: catalog assemblies cover most needs at low MOQs; custom circuits (special sizes, integrated brackets, multi-magnet arrays) follow the custom process in the pricing guide.