Permanent magnets in motors, actuators, and magnetic assemblies are among the most demanding adhesive bonding applications in precision engineering. The magnet must be positioned and retained with accuracy throughout the operational life of the device, under centrifugal forces from rotor rotation, thermal cycling between ambient and operating temperature, vibration from the motor and driven load, and in some cases chemical exposure from lubricants or coolants. The epoxy adhesive used to bond the magnet to the rotor hub, the stator assembly, or the actuator body carries all of these loads and must do so without creep at operating temperature — a single bond failure in a high-speed motor magnet causes catastrophic damage to the motor and downstream equipment. Selecting and applying the correct epoxy adhesive for magnet retention is a precision engineering decision.
The Loads on Bonded Magnets
Centrifugal force in motors. In surface-permanent-magnet (SPM) motors, magnets bonded to the rotor outer diameter experience centrifugal force proportional to the square of the rotational speed and the magnet mass. At 10,000 RPM, the centrifugal acceleration is approximately 550g for a magnet at 50 mm radius — the bond must carry a load fifty times the magnet weight continuously at operating speed. The shear and tensile stress on the bond varies by magnet geometry and mounting configuration.
Thermal cycling. Magnets are typically ceramic (sintered NdFeB or SmCo) or bonded magnet composite. These materials have low CTE — NdFeB has CTE of approximately 5 to 7 × 10⁻⁶/°C and thermal expansion anisotropy in the crystallographic directions. Steel rotors have CTE of approximately 12 × 10⁻⁶/°C; aluminium rotors are approximately 23 × 10⁻⁶/°C. The CTE mismatch between magnet and rotor generates shear stress at the bond interface on every thermal cycle from cold start to operating temperature.
Demagnetization temperature. NdFeB magnets begin to irreversibly demagnetize above 80°C to 120°C depending on grade. Operating the motor above this temperature causes both magnet performance loss and — from the adhesive perspective — elevated temperature service that reduces bond strength. The adhesive must be specified with adequate strength at the operating temperature, not just at ambient.
Electromagnetic forces. In actuators and voice coil motors, the magnet experiences rapidly alternating electromagnetic forces during operation. These forces load the bond in shear and tension at the actuation frequency, creating fatigue loading that accumulates over the device lifetime.
Epoxy Properties Required for Magnet Bonding
Rigid bond with no creep. Magnet positioning accuracy is critical for motor performance. An adhesive that creeps under centrifugal load at operating temperature allows the magnet to slowly migrate out of position, causing imbalance and performance degradation. Rigid, high-Tg epoxy with demonstrated low creep at the maximum operating temperature is required. The Tg must provide adequate margin above the maximum continuous operating temperature — if the motor operates to 100°C, the Tg should be 120°C or higher.
Controlled bond line thickness. Thin, uniform bond lines minimize the eccentricity and imbalance introduced by the adhesive between the magnet and rotor. Bond line thickness targets of 25 to 100 microns are common for precision motor magnets; glass bead or film adhesive spacers achieve this control.
Adequate shear and tensile strength at operating temperature. The bond must carry the centrifugal load with a defined safety factor at the maximum operating temperature. This requires specifying and testing the adhesive strength at temperature, not at ambient.
If you need magnet bonding adhesive selection, strength data at operating temperature, and application process guidance, Email Us — Incure provides magnet-specific epoxy formulation data and process support for motor and actuator assembly.
Surface Preparation for NdFeB and Ceramic Magnets
NdFeB magnets. Sintered NdFeB magnets are typically nickel-plated or epoxy-coated for corrosion protection. The bond surface for adhesive is the plating or coating — not the base magnet material. Nickel-plated NdFeB bonds well to epoxy with solvent degreasing and light abrasion of the nickel surface. Epoxy-coated magnets require abrasion and solvent cleaning; the epoxy coating surface energy is compatible with structural epoxy adhesive without additional primer.
SmCo magnets. Samarium cobalt magnets have better temperature stability than NdFeB and are used in high-temperature motor applications. The magnet surface is typically uncoated ceramic. Solvent degreasing followed by light abrasion provides adequate preparation for structural epoxy adhesion.
Steel or aluminium rotor surfaces. Standard degreasing and abrasion preparation as for structural metal bonding. For aluminium rotors, etch primer application before bonding improves adhesion durability under the CTE mismatch stress of thermal cycling.
Application Techniques for Precision Magnet Bonding
Film adhesive for flat magnet-to-rotor bonds. Epoxy film adhesive cut to the magnet footprint provides controlled, uniform bond line thickness and full coverage without air entrapment. Film adhesive is applied to the rotor bonding surface, the magnet is placed, and the assembly is cured in a fixturing jig that maintains position and applies controlled pressure. This method is used in high-precision motor manufacturing.
Paste adhesive with glass bead spacers. A small quantity of epoxy paste with glass bead spacers applied to the magnet or rotor surface, with the magnet placed and pressed to the target bond line thickness. Excess adhesive squeezed out at the perimeter confirms adequate coverage. Fixturing holds position during cure.
Anaerobic or UV-cure assist for fixturing. For production throughput, a small amount of UV-curing or anaerobic adhesive can be used to tack the magnet in position immediately after placement, while the structural epoxy cures over a longer time period. This eliminates the need for prolonged fixturing by providing immediate positional lock.
Contact Our Team to discuss magnet bonding adhesive selection, operating temperature strength requirements, and precision application processes for motors and actuators in your application.
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