Why some gears must not be magnetic: the not-so-obvious case of residual magnetism
dicembre 03, 2025
In mechanical transmission, we often talk about hardness, microstructure, concentricity or noise. Much more rarely do we address a quieter, invisible, often underestimated phenomenon: residual magnetism. Yet for many manufacturers of high-end gearboxes, it’s considered a critical parameter. Some technical specifications for case-hardened gears even include a maximum allowable level of residual magnetic field, to be measured on every component and recorded for every batch.
But why can a gear or shaft with too much residual magnetism become a real problem?
Residual magnetism: a concrete risk for cleanliness and reliability
Residual magnetism isn’t abstract. It’s a physical property that can directly affect how a part behaves. A magnetized component attracts and retains fine chips, metallic dust, and abrasive residues. These contaminants can settle in sensitive areas like bearing seats, gear teeth, sealing surfaces or ground zones—causing micropitting, wear, and local surface damage.
And during assembly, magnetized parts complicate operations. They might attract small components unintentionally, interfere with sensitive dimensional or friction measurements, or disrupt flow on automated lines. In high-precision gearboxes, these aren’t minor issues—they’re critical.
In recirculating oil systems, the risks increase further. Magnetized areas can trap particles, disrupting lubricant flow, altering contact conditions, and reducing system life expectancy.
Residual magnetism: where it comes from and how it forms
Residual magnetism is the result of magnetic domains aligning within steel. Several common operations can cause it. One of the most frequent is lifting parts with magnetic devices—especially in large formats. These strong fields often leave a lasting magnetic footprint.
Another source is magnetic particle inspection. If proper demagnetization isn’t performed afterwards, the part retains magnetic charge. Even grinding, particularly with high contact pressure, can orient domains on the surface—precisely where cleanliness is most critical. Less frequently, vibrations, impacts or proximity to transformers and welders may also contribute.
There’s one phase that plays a key role: heat treatment. At high temperatures, any previous magnetization is effectively erased. It acts as a kind of natural reset. That means a part may be magnetized before treatment, become demagnetized during heat treatment, and then be magnetized again in later stages—by grinding, inspection, lifting, or handling.
So the real risk doesn’t lie at the beginning of the process, but after heat treatment, all the way to final shipment.
Residual magnetism: how to measure it and keep it under control
In strict technical specifications, residual magnetism is treated like any other measurable characteristic. It’s monitored using portable gaussmeters or Hall-effect probes, with measurements taken at multiple points on teeth, bores, or surfaces. A maximum value is usually set—often between 10 and 20 A/cm—and the supplier must record and archive the results for every batch.
To reduce or eliminate residual magnetism, there are proven techniques. The most effective is AC demagnetization, where a decreasing alternating magnetic field is used to gradually return the material to a neutral state. Best practice includes demagnetizing immediately after grinding or MT inspection, checking values before final washing, and avoiding magnetic lifting once the part is neutralized.
Handling also matters. Avoid magnets unless absolutely necessary, favor mechanical or hydraulic lifting, and monitor magnetism along the process to understand where it's generated.
Residual magnetism may be invisible, but it’s real. It can mean the difference between a component that stays clean and one that attracts contamination, a gearbox that runs reliably and one that wears out prematurely, an assembly line that flows and one that jams unexpectedly.
Understanding, measuring, and controlling it means delivering cleaner, more reliable, and more consistent components—especially for today’s high-performance transmissions.
Have you ever dealt with problems caused by residual magnetism? Do you measure it regularly, or only when requested by the customer? Let’s discuss it in the comments.
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