Why Permanent Magnets Lose Strength: Causes and Diagnostic Steps

First question: did the magnet change, or did the test change?

Field, force, torque and sensor readings are system measurements. A different test distance, steel plate, fixture, orientation, temperature, instrument, scan path or assembly state can produce a different result even when the magnet material is unchanged. Establish a repeatable baseline before investigating irreversible loss.

If the product uses a magnetic assembly, inspect the complete path: magnet, air gap, return steel, adhesive, sleeve, hub, housing, mating component and measurement position. A loose or shifted magnet can look like a weak magnet.

Permanent-magnet strength-loss diagnostic tree

Heat can cause reversible and irreversible changes

Permanent-magnet output changes with temperature. A reversible change can recover when the magnet returns to the reference temperature. Irreversible loss does not fully recover and can occur when the operating point moves beyond a safe region for the grade, geometry and magnetic circuit.

Do not diagnose from ambient temperature alone. Record the magnet’s actual continuous, peak and fault temperatures; heating during magnetization or assembly; exposure duration; cooling condition; external fields; and the measurement temperature. A thin magnet in an open circuit can have a different margin from a thicker magnet in a supportive steel circuit.

Opposing magnetic fields can demagnetize a magnet

Motor windings, coils, nearby magnets, magnetizing fixtures and fault currents can expose the magnet to an adverse field. The risk depends on direction, magnitude, duration, temperature, geometry, material coercivity and the operating point. A transient event can matter even if normal operation is safe.

For a motor or actuator investigation, collect current and fault history, rotor position, temperature and the magnetic-circuit model. Compare affected and unaffected poles using the same scan path and reference.

Cracks, chips and corrosion reduce effective magnet volume

Permanent magnets are not ductile steel parts. Impact, press fit, stress concentration, grinding damage, adhesive shrinkage, rotor loads or handling can create cracks or chips. Lost material changes the magnetic geometry and can reduce output even if the remaining material is not demagnetized.

Corrosion can also remove material or disrupt a coated edge. Inspect the finished state under adequate lighting and magnification, compare dimensions or mass where appropriate, and document the location of damage relative to the poles and functional air gap. Review the coating guide if environmental protection is involved.

Air gap and steel changes can look like magnet loss

A small increase in air gap can materially change the useful field or force. Adhesive thickness, coating build, wear, debris, a warped bracket, bearing movement, loose retention or an altered mating surface can all change the gap. Steel thickness, grade, saturation, contact area and surface condition also affect a holding-force test.

Measure the assembled condition and compare it with the released drawing. If the magnet was replaced, confirm that its dimensions, magnetization direction, pole reference and installed orientation match the original part.

Incomplete or changed magnetization

A part may be magnetized in the wrong direction, installed with reversed polarity, delivered in a different magnetization state or scanned from a different reference. Multipole products can show a weak waveform when the test path, pole pitch, measurement height or angular zero changes.

Use a controlled drawing definition for direction, N/S position, pole count, pitch, reference feature, viewing direction, delivery state and inspection. See magnetization direction and pole patterns for the required inputs.

Does a permanent magnet simply wear out?

Stable permanent magnets can retain useful magnetization for long periods when used within their designed conditions. Time alone is usually a weak diagnosis. Investigate thermal history, adverse fields, corrosion, damage, geometry, assembly movement and measurement changes before attributing the problem to aging.

If a long-term drift matters, define an accelerated or periodic verification method tied to the real product function. Avoid converting a generic aging percentage into a service-life guarantee.

A practical investigation sequence

1. Freeze the measurement method, instrument, fixture, distance, orientation, temperature and mating components.
2. Test an approved baseline, an unused control and the affected part under the same conditions.
3. Inspect geometry, chips, cracks, corrosion, coating, adhesive, retention and the assembled air gap.
4. Review temperature, current, fault, impact, storage, cleaning and chemical-exposure history.
5. Confirm material grade, coercivity family, dimensions, coating, magnetization and production lot.
6. Map the field or flux where a localized or multipole problem is suspected.
7. Use the evidence to separate measurement, mechanical, environmental and magnetic root causes before changing grade.

Source and evidence boundary

Dura Magnetics’ demagnetization guide supports the separation of geometry, volume loss, elevated temperature, external fields and coercivity in a strength-loss investigation. Its material numbers, examples and supplier recommendations are not Elite Magnets specifications.

The dated SERP sample also showed repeated coverage of heat, adverse fields, damage, corrosion and time, but fewer pages explicitly separated measurement and assembly changes from true material loss. EM-FAIL-01 adds that diagnostic boundary and requires comparable evidence before a conclusion.

What to send for a failure review

• Released drawing, material and magnetization specification, coating and assembly state.
• Original acceptance report and the current measurement method and raw readings.
• Photos of damage, corrosion, coating, adhesive, retention and mating surfaces.
• Temperature, current, fault, impact, storage and chemical-exposure history.
• Lot, date code, supplier change, process change and affected quantity.
• Good, failed and unused-control samples where available.

Connect the evidence to Quality, dimension inspection, Custom Capabilities or the RFQ form for a project-specific review.

Diagnostic note: A lower reading is an observation, not yet a root cause. Preserve the test conditions and compare the complete assembly before changing material or grade.