Magnets for Maintenance & MRO

Where magnets hide in a plant

Knowing where the magnets are is half of managing them — and each location fails differently:

Identifying an unknown magnet

The magnet in your hand has no part number. Work the identification in this order:

• Material, from appearance and behavior. Bright plated metal and startling strength: sintered NdFeB. Dark gray ceramic, modest strength, often larger: ferrite. Metallic, unplated, high-temperature service: likely SmCo or alnico. The material comparison has the full field guide.
• Dimensions — measured with the coating question in mind. Plated NdFeB measures ~0.03–0.05 mm over nominal on a diameter; a corroded or chipped part measures under. Round to the obvious metric or imperial catalog size before assuming a custom.
• Magnetization direction. A compass or a second small magnet maps which faces are poles — axial (flat faces), diametric (across the diameter), or multipole. Getting this wrong is the most common replacement error; the directions guide shows each pattern.
• What NOT to trust: strength-by-feel or a bare surface-field reading from the old part. The old magnet may be partially demagnetized (next section) and a handheld gauss reading is technique-dependent — matching a replacement to a degraded reading specs the degraded state. Methods that do work are in how magnets are tested.
• Grade, last and honestly. Grade is not determinable by eye. Infer from the application's era and duty, or send the part in — identification from a physical sample is a routine supplier service, and the safe default for motors and couplings is matching temperature class, not just strength.

Why the old magnet “wore out”

Properly applied permanent magnets lose only fractions of a percent per decade — “worn out” is nearly always a specific event in disguise, and naming it prevents the replacement from repeating it. The full diagnostic is the demagnetization guide; the plant-floor short list:

• Overheating — the dominant cause. A seized bearing, a blocked fan, a slipping magnetic coupling, a separator next to an oven: one excursion above the magnet's real limit takes flux that doesn't come back on cooling, per the temperature guide. If the equipment overheated, budget the magnet as damaged even if it “still works.”
• Opposing fields — welding cables draped over equipment, magnetizers/demagnetizers nearby, another magnet installed backwards.
• Mechanical damage — chips and cracks from impact; corrosion under damaged plating quietly delaminating the magnet from inside. A rusty NdFeB magnet is a failing magnet.
• Ferrite in the cold — the counterintuitive one: ferrite loses coercivity as temperature drops, so outdoor ferrite equipment demagnetizes in winter, not summer.

Buying the replacement right

• Match strength by class and size, not by adjective. “Strongest available” in a sensor or switch application moves operate points and breaks calibration; in holding applications, more is usually fine. Know which kind of application you're in.
• Match the temperature class in hot service. An N-class magnet swapped into an H-class location works perfectly in the storeroom and fails at operating temperature — invisible until it isn't.
• Pull-force ratings are laboratory numbers. Catalog pull force assumes thick, flat, clean steel and zero gap; paint, air gaps, thin sheet, and shear loading cut real holding force dramatically — the derating logic is in the pull force guide. Size with margin for the real surface.
• Choose the coating for the environment: washdown, chemicals, and outdoor duty want epoxy-coated or ferrite; standard Ni-Cu-Ni is a dry-service coating. The coatings guide maps it.
• Prefer stock sizes — and say so upstream. Stock replacement magnets ship in days from U.S. inventory; a custom reorder inherits the tooling and MOQ math in the MOQ & lead-time guide. When a repair reveals a custom magnet in a critical machine, that's a spares-strategy flag (section 07), not just a purchase.

Shop handling & storage

Strong magnets are the storeroom item most likely to injure the person stocking them. The working rules — the assembly-side version is in the bonding guide's handling section:

• Pinch injuries are the #1 incident. Large magnets accelerate toward steel and each other from surprising distances. Gloves, spacers between stored magnets, and two-hand controlled handling for anything palm-sized or larger.
• Keep them off the bench grinder's world: never drill, grind, or machine a sintered magnet in the shop — it will crack, the plating is breached, and the dust is flammable.
• Slide, don't pull. Separating stacked magnets by sliding sideways off the stack beats prying; prying chips edges and launches parts.
• Storage discipline: original packaging or spaced compartments, away from magnetic media, access cards, instruments, and pacemaker wearers; keepers left on alnico and any magnet shipped with them. Mark the storeroom location for the strong ones.
• Heat is the storage hazard nobody posts a sign for: not on top of ovens, not next to steam lines, not in the truck dashboard in July.

Re-attachment that lasts

Most field “magnet failures” are attachment failures. The repair-bench version of the bonding & mounting guide:

• Prep both surfaces properly: degrease, lightly abrade the bond area (without breaking through plating), re-clean. The five-minute prep is the difference between a repair and a repeat.
• Toughened two-part epoxy rated above service temperature is the default answer; commodity super glue is a prototype-bench material, not a plant repair.
• Control the bondline. Magnetic attraction squeezes the joint to a starved film that fails early — a few glass beads, a wire shim, or a designed recess keeps a real adhesive layer in the joint.
• Add mechanical capture anywhere a release is a hazard: overhead, rotating, or near people, the adhesive positions and a lip, pocket, band, or cover plate retains. Never let glue be the only thing between a magnet and a projectile.
• Screw-mounting means buying countersunk or pot-mounted parts — pressed-in holes from the factory, never field-drilled. Pot (cup) magnets also multiply holding force on flat steel and take the abuse a bare magnet can't.

The spares strategy

• Walk the criticality list once. For each magnet-bearing machine that stops production: is the magnet a stock size (day-level replacement) or a custom (week-to-month replacement)? The customs on critical equipment are your spares candidates — magnets store indefinitely when kept cool, dry, and properly packed, so holding two is cheap insurance against the lead-time math in the MOQ & lead-time guide.
• Capture the identification while the machine is running. Dimensions, direction, coating, service temperature, and a photo, filed against the equipment record — identification is easy on a healthy machine and miserable during a breakdown.
• Watch OEM obsolescence. When an equipment OEM exits or a model sunsets, the custom magnets inside become unobtainium on a schedule; a drawing plus a qualified supplier converts that risk into a purchase order. That's the MRO edition of the continuity logic in the second-sourcing guide.
• Periodically verify the verifiable: separators and lifting gear get scheduled field checks against a baseline reading taken when new — strength loss is gradual, and the baseline turns “seems weaker” into a number.

Related: magnet demagnetization guide · pull force explained · magnet coatings guide