How Thermal Expansion of Permanent Magnets Affects Magnetic Assemblies

Permanent magnets change slightly in size as temperatures rise or fall. These dimensional changes are described by the thermal expansion coefficients of the magnet materials.

Different permanent magnet materials have different thermal expansion behaviors.

AlNiCo magnets generally have thermal expansion characteristics relatively close to those of common steels. Ferrite magnets usually show greater dimensional changes, while Neodymium and Samarium Cobalt magnets can behave differently depending on the directions being measured.

These differences are particularly important for sintered permanent magnets. Their internal crystal structures are oriented during manufacturing, so thermal expansion may not be the same in every direction.

The practical result is that single thermal expansion values cannot always describe the dimensional behavior of permanent magnets completely.

For many applications, these differences remain small. They become more relevant when magnets are large, tolerances are tight, operating temperature ranges are wide, or magnets are tightly constrained by surrounding components.

Neodymium, Samarium Cobalt, and ferrite magnets are commonly manufactured with preferred magnetic orientations. Their thermal expansion can also vary between magnetization directions and directions perpendicular to them.

For example, axially magnetized ring magnets may not change by exactly the same proportions in their axial and diametric directions.

These differences do not usually create significant problems for simple holding magnets or general magnetic products. They become more important in precision magnetic assemblies where magnet diameters, thicknesses, air gaps, or fitted positions need to remain within limited ranges as temperatures change.

The effects also increase with component size. Small differences in expansion behavior may be negligible in small magnets but become measurable across large rings, long magnetic assemblies, or wide operating temperature ranges.

In practical applications, however, magnets rarely expand freely. They are normally connected to other materials, which makes differences between the thermal expansion rates of magnets and surrounding components more important than the dimensional behavior of magnets alone.

Thermal expansion mismatch occurs when magnets and surrounding materials change dimensions at different rates.

This is common because magnetic assemblies often combine permanent magnets with steel, stainless steel, aluminum, plastics, adhesives, and other structural materials.

Whether these differences create practical problems depends on component sizes, temperature ranges, assembly structures, and how tightly magnets are fixed or constrained.

Magnets with Steel Components

Steel components are commonly used as back plates, magnetic circuit parts, housings, shafts, and structural components in magnetic assemblies.

Their thermal expansion behavior can be relatively close to that of some permanent magnet materials, so dimensional mismatch may remain limited in many ordinary applications.

However, differences can still matter in large magnetic assemblies or where magnets are strongly constrained. Repeated heating and cooling can also place additional loads on adhesive joints between magnets and steel components.

For this reason, bonded magnetic assemblies should be evaluated as complete structures rather than by considering magnet dimensions alone.

Magnets with Aluminum Components

Thermal expansion differences can become more noticeable when magnets are installed in aluminum components.

Aluminum materials generally expand more with temperature than Neodymium or Samarium Cobalt magnets. Aluminum housings, carriers, or rotor components can therefore change dimensions faster than the magnets installed within them.

These differences may increase or reduce clearances depending on the assembly structures. They can also affect interference fits, magnet positioning, retention conditions, and stresses transferred through adhesive layers.

Fit conditions that work correctly at room temperature may therefore behave differently at the highest or lowest operating temperatures.

These effects are particularly relevant for magnetic assemblies using aluminum housings, lightweight rotors, carriers, and other structures where dimensional control is important.

Bonded Magnetic Assemblies

Adhesives are widely used to fix permanent magnets to steel, aluminum, plastics, and other substrates.

When magnets and substrates expand at different rates, adhesive layers need to accommodate part of the relative movement. Repeated temperature cycling can therefore create shear or peel stresses within bonded interfaces.

Adhesive selection should not be based only on bonding strength. Operating temperature ranges, bond-line thicknesses, adhesive flexibility, surface preparation, and assembly geometries can all affect long-term performance.

These considerations become more important in outdoor equipment, motors, rotating magnetic assemblies, and industrial applications that repeatedly experience heating and cooling.

Retained and Press-Fit Magnets

Some magnets are retained mechanically rather than relying only on adhesive bonding.

Typical examples include press-fit magnets, sleeved rotors, magnets installed inside steel or aluminum housings, and magnetic assemblies using clamps or molded carriers.

In these designs, differences in thermal expansion can change contact pressures or fit conditions as temperatures change.

Tight fits may become tighter at some temperatures and looser at others, depending on material combinations and assembly geometries. For brittle sintered magnets such as Neodymium and Samarium Cobalt magnets, excessive mechanical constraint should also be avoided.

Temperature conditions therefore need to be considered together with assembly methods rather than treated only as magnetic performance issues.

Thermal expansion does not require the same level of attention in every magnetic product.

For small magnets used in simple holding or positioning applications, normal dimensional changes may have little practical influence.

More attention is usually required when magnetic assemblies involve large magnets, long components, tight air gaps, interference fits, aluminum housings, rigid adhesive joints, rotating structures, or wide temperature changes.

Outdoor and industrial environments can also expose magnetic assemblies to repeated daily or seasonal temperature cycling. Even when temperatures remain within the safe operating ranges of magnet materials, repeated dimensional movement can still affect surrounding mechanical structures.

For these applications, magnet materials, structural materials, assembly methods, and operating temperature ranges should be considered together.

WZ Magnetics supplies permanent magnets and magnetic assemblies for a range of industrial applications.

If your projects involve bonded magnets, fitted magnets, magnetic assemblies, or components exposed to changing temperatures, we can review magnet materials, surrounding structures, and assembly requirements together to help identify potential thermal expansion issues before production.