Why the magnet loses power and when it's a problem

 

Why a magnet loses strength in practice

Magnetic force is not one universal value. In technical operation, it depends on the magnet material, its shape, the working gap, the counterpart, temperature and the direction of load. That is why actual weakening of the magnet is often confused with deterioration of the whole application. The magnet may still have similar remanence, but the system holds more weakly because the air gap has increased, the contact surface has become contaminated or the character of the load has changed.

For this reason, it makes sense to first verify whether the magnetization has actually decreased, or only the working efficiency of the assembly. In lifting magnets, clamps, separators and simple mounting magnets, the second option is very common.

Temperature is the most common reason

If there is one cause that appears most often across applications, it is temperature. Every permanent magnet has a range of operating temperatures in which it retains its properties. If this range is exceeded, a temporary or permanent decrease in magnetic force may occur.

With neodymium magnets, particular attention must be paid to this. They are very strong at a small volume, but at the same time more sensitive to higher temperatures than, for example, SmCo magnets. If the magnet is operated above the recommended temperature for a long time, some of the domains are rearranged and the magnet may no longer return to its original level. This is different from a short-term change in performance, which may partially recover after cooling.

In industry, the problem usually occurs especially where the magnet is mounted near motors, furnaces, presses, moulds, welding workplaces or where heating caused by friction is underestimated. It is equally true that catalogue force measured under laboratory conditions does not automatically indicate how the magnet will behave at 60, 80 or 150 degrees Celsius.

Critical temperature and operating temperature are not the same

An important detail is the difference between the recommended operating temperature and the Curie temperature. The Curie temperature is the point at which the material fundamentally loses its ferromagnetic properties. In practice, however, problems appear much earlier. A magnet may become unusable for the application long before it reaches this physical limit.

For the designer or maintenance team, the maximum operating temperature of the specific magnet grade is therefore more important than the theoretical maximum of the material.

Mechanical shocks and vibrations

Another common answer to the question of why a magnet loses strength is mechanical shocks. A permanent magnet is not an indestructible block of energy. Repeated impacts, falls, blows and strong vibrations can change its internal arrangement or damage the magnet body itself.

This applies especially to more brittle materials such as neodymium or ferrite. If the magnet cracks, chips or the working surface becomes deformed, it does not mean only mechanical damage. The magnetic circuit also changes and the resulting holding force may decrease noticeably. In assemblies with a steel housing, thread or supporting structure, the mounting is sometimes damaged before the magnet itself, but the effect on function is the same.

In vibrating equipment, conveyors and handling systems, it is advisable to take this into account already at the design stage. A magnet that works well in a static test may not work equally reliably in long-term dynamic operation.

Corrosion and surface damage

Magnetic force is not lost only “from the inside”. The condition of the surface also has a major influence. Neodymium magnets are sensitive to corrosion and are commonly protected by a coating, for example nickel, epoxy or rubber sheathing. If the coating is damaged, moisture and an aggressive environment begin to act against the material.

Corrosion causes not only the gradual disintegration of the magnet, but also an increase in unevenness on the contact surface. This means a larger air gap between the magnet and the ferromagnetic counterpart, and this is exactly what can reduce efficiency very significantly. With magnets, a simple technical reality applies - even a small gap means a noticeable decrease in attractive force.

In operations with moisture, chemicals, salt or outdoor use, the choice of protection is therefore just as important as the choice of the magnetic material itself.

Incorrect storage and demagnetization in the assembly

A magnet can also weaken without being in active operation. Long-term storage in an unsuitable position, without keeper plates or near strong external fields can affect the resulting state, especially with more sensitive types or thinner shapes.

With some magnets, the way the magnetic circuit is closed also plays a role. If a magnet is designed to work with a counterpart and is used for a long time outside the designed assembly, the operating point may shift into a less favourable area of the demagnetization curve. This is a more technical topic, but for practice it leads to a simple conclusion - a magnet cannot be assessed separately from the application.

Influence of an external magnetic field

A strong external magnetic field can, under certain conditions, contribute to partial demagnetization. In a standard office or workshop this is usually not the main problem, but in industrial assemblies with electromagnets, pulsed fields or specific measuring devices, it must be taken into account.

If a permanent magnet is located in a field of the opposite direction and at the same time operates at an elevated temperature, the risk of a decrease in parameters increases.

The magnet does not always weaken. Often the contact weakens

In service practice, it very often turns out that the magnet is not “weak”, but contaminated. Dust, paint, grease, scale, rust or a thin non-magnetic layer appears between the magnet and the steel surface. This layer may be only a fraction of a millimetre thick, but from the point of view of the magnetic circuit it is a major obstacle.

The same problem occurs with an uneven or rough counterpart. Catalogue holding force is usually stated for a smooth, clean, sufficiently thick steel sheet and for pull perpendicular to the surface. If the magnet actually works on thin material, through a coating, in shear or with a gap, the result will be weaker even without any demagnetization.

That is why, during diagnostics, it makes sense to first check the contact conditions and only then draw a conclusion about a “discharged” magnet.

Which types of magnets lose strength most easily

There is no single material that is best for everything. Neodymium offers high force in compact dimensions, but it tolerates higher temperatures and corrosive environments worse if it is not properly protected. Ferrite is more stable in many common conditions and is resistant to corrosion, but it has a lower energy density. SmCo handles high temperatures very well, but it is more expensive and brittle. AlNiCo has good temperature stability, but in some geometries it is more prone to demagnetization.

This is why it is not possible to answer generally. If someone says that “magnets weaken over time”, it is only a half-truth. It is more accurate to say that magnets weaken at different rates depending on the material, design and operating conditions.

How to determine whether the decrease is still normal

A small natural decrease in magnetic parameters over time may exist, but with a high-quality and correctly used permanent magnet it is usually very low. If an application suddenly stops working, the cause is usually operational rather than “ageing” of the material.

Warning signs may include repeated falling off of parts, weaker separation of metal particles, reduced holding force on the same counterpart, the need for a smaller safety reserve or visible surface damage. In industrial applications, it is useful to compare the current state with a reference measurement using a gaussmeter, or at least with the original functional test on the same material and under the same conditions.

If safety-critical handling is involved, an estimate is not enough. In that case, condition inspection, measurement and, in case of doubt, replacement or reassessment of the design are required.

What helps prevent a magnet from losing strength prematurely

The basis is to choose the correct material for the specific temperature, humidity, mechanical load and type of contact. It then helps to protect the magnet from impacts, not underestimate surface protection, keep contact surfaces clean and not operate the assembly outside the designed range.

In technical solutions, it is also worth considering a reserve. If the application works “just enough” under laboratory conditions, it is not enough in real operation. A reserve in force, temperature and construction is cheaper than a subsequent production outage or unplanned service.

That is exactly why, when designing magnetic applications, it is not worth making decisions only according to dimensions and declared holding force. The decisive factor is the conditions in which the magnet will work every day. When you adapt both the material and the construction to them, the magnet does not lose strength “for no reason” - it only reacts to what the operation actually requires from it.