How should you choose the surface-mounted magnetic steel adhesive for the inner rotor of a servo motor, drive motor, or high-speed turbine motor?

From power tools to several-kilowatt servo motors, new-energy vehicle drive motors, and high-speed turbine motors operating at over 100,000 rpm, surface-mount magnetic steel can be found in almost every application. This article introduces bonding techniques for internal-rotor surface-mount magnetic steel.

What exactly is magnetic adhesive actually subjected to?

Let us first do some calculations. Taking a typical brushless motor as an example: the rotor radius is 25 mm, and the mass of a single magnetic steel piece is 5 g.

5,000 rpm: centrifugal acceleration of approximately 6,850 m/s², equivalent overload of approximately 698g, and centrifugal pull force per magnetic steel piece of approximately 34 N.

10,000 rpm: centrifugal acceleration of approximately 27,416 m/s², equivalent overload of approximately 2,795g, and centrifugal pulling force per magnetic steel piece of approximately 137 N.

20,000 rpm: centrifugal acceleration of approximately 109,663 m/s², equivalent overload of approximately 11,179g, and centrifugal pulling force per magnetic steel piece of approximately 548 N.

This is analogous to the spin cycle of a washing machine: when the rotational speed doubles, the centrifugal force quadruples. At 20,000 rpm, the outward centrifugal force acting on a 5 g small magnetic steel piece is equivalent to the weight of a primary school student. Moreover, this is not an instantaneous impact; rather, it is a constant load resulting from the motor’s continuous operation.

In addition to centrifugal force, the magnetic steel also simultaneously withstands four types of loads:

• Thermal cycling shear: The coefficient of thermal expansion of neodymium–iron–boron is approximately (3–5) × 10⁻⁶/℃, while that of aluminum alloys is approximately 23 × 10⁻⁶/℃—a difference of 4 to 5 times. Each cycle of heating and cooling exerts tensile stress on the adhesive layer.

• Start-stop shock: Torque fluctuations generate tangential shocks, which are particularly pronounced in motors that start and stop frequently.

• Medium erosion: The adhesive may come into contact with cooling oil, ATF transmission fluid, salt spray, or moisture, all of which can gradually penetrate the adhesive layer.

• Risk of electromagnetic demagnetization: An excessively high curing temperature may cause irreversible demagnetization of neodymium-iron-boron magnets.

Therefore, magnetic steel bonding is not merely about “getting it to stick”; rather, it requires maintaining stability for more than ten years under the simultaneous action of these five types of loads.

Why are magnetic steel pieces bonded with adhesive?

Some may ask: If bonding is so cumbersome, why not use interference fits, screws, or snap-fit fasteners instead?

The answer is simple: neodymium iron boron is a ceramic, not a metal.

Sintered NdFeB is produced by powder metallurgy, and its microstructure consists of hard, brittle grains. This means that:

• Interference fit → circumferential tensile stress → magnetic steel cracking

• Screws passing through the magnetic steel will inevitably cause the drill hole to crack; additionally, NdFeB powder is flammable.

• Fastener/riveting → point load → microcrack initiation

To put it simply: magnetic steel is like ceramic tiles. You do not use nails to secure tiles; instead, you use adhesive. The same applies to magnetic steel—adhesion is the only method of fixation that can evenly distribute loads and prevent stress concentration.

How should you choose among the three major adhesive systems?

Three types of adhesives are commonly used for magnetic steel bonding, each with its own advantages and limitations. I will clearly explain the characteristics and suitable application scenarios for each type.

Epoxy adhesive (single-/two-component)

• Typical shear strength: 15~destructuring of the substrate

• Long-term temperature resistance: -55~180℃

• Curing method: thermal curing / room-temperature two-component

• Core advantages: highest strength, superior temperature resistance, and excellent medium resistance

• Key limitation: Slow curing (thermal curing requires 30–90 min); an oven is required

Acrylic structural adhesive

• Typical shear strength: 15–30 MPa

• Long-term temperature resistance: -40~150℃

• Curing method: activator + room temperature

• Key advantages: 1–3 minute positioning, clamp-free operation, and excellent impact resistance

• Key limitation: The upper temperature resistance is lower than that of epoxy, and the odor is more pronounced.

Anaerobic stabilizing gel

• Typical shear strength: 20–30 MPa

• Long-term temperature resistance: -55~150℃

• Curing method: oxygen-deficient curing in a metal gap

• Core advantages: specialized cylindrical surface matching and high shear strength

• Key limitation: Suitable only for gaps of 0.02–0.1 mm; relatively brittle

If we use a radar chart to assess the overall performance of these three types of adhesives, epoxy adhesives demonstrate balanced performance in shear strength, maximum temperature resistance, high-temperature retention, medium resistance, and gap filling; acrylic structural adhesives have the fastest curing speed and excellent impact resistance; while anaerobic adhesives exhibit extremely high shear strength but are suitable only for small gaps and have poor peel strength.

Six Common Failure Modes and Countermeasures

① Interface delamination

• Symptoms: Residual adhesive remains on the rotor; the magnetic steel surface is clean

• Root cause: surface oil contamination / lack of surface roughening / coating delamination

• Countermeasure: degreasing → roughening → re-cleaning; water-film rupture test for verification

② Adhesive layer cracking

• Symptoms: Cracks appear in the adhesive layer after thermal cycling

• Root cause: CTE mismatch / adhesive layer too thin or too thick / insufficient adhesive strength

• Countermeasures: increase toughening agent content + control adhesive layer thickness + reselect the adhesive

③ Desorption at high temperatures

• Symptoms: Magnetic steel displacement or ejection after the system has been operating for a period of time

• Root cause: Insufficient temperature resistance of the adhesive

• Countermeasure: Switch to a higher-temperature-resistant adhesive

④ Early failure due to adhesive deficiency

• Symptoms: Surface treatment is satisfactory, but the effect fails shortly thereafter

• Root cause: Magnetic attraction squeezes out the adhesive, leaving the adhesive layer at only the micrometer scale.

• Countermeasure: Select a highly thixotropic adhesive

⑤ Settlement during the curing period

• Symptoms: displacement of the magnetic steel position and asymmetry of the magnetic field

• Root cause: Magnetic side force drives the magnetic steel to slide during the curing process

• Countermeasures: hard positioning grooves / quick-setting adhesive / addition of fixtures

⑥ Humidity/Medium aging

• Symptoms: intensity gradually declines over several months

• Root cause: Water vapor/liquid permeates and degrades along the interface

• Countermeasure: Replace with chemically corrosion-resistant adhesive

Three Key Aspects of Process Control

Curing temperature vs. demagnetization risk

Thermosetting epoxy typically cures at 80–150 °C. If the magnetic steel has already been magnetized, the curing temperature may approach or exceed the maximum operating temperature of the magnetic steel (Level N: 80 °C; Level M: 100 °C; Level H: 120 °C; Level SH: 150 °C).

Either ensure that the solidification temperature is below the temperature rating of the magnetic steel, or apply the adhesive first and then magnetize the material. Do not rely on luck.

High-speed motors must be equipped with mechanical retention.

When the rotational speed exceeds 15,000 rpm or the surface linear velocity exceeds 50 m/s, the seal cannot independently withstand the centrifugal load. The standard practice is to use a carbon-fiber winding sleeve, an Inconel alloy sleeve, or a stainless steel sleeve: the seal provides positioning, while the sleeve provides the load-bearing capacity.

Verification should not be limited to testing only room-temperature shear behavior.

Comprehensive validation should include high-temperature shear testing, thermal cycling (-40 °C to 150 °C, 500 cycles), humidity aging (85 °C/85 % RH, 1,000 h), oil immersion (150 °C engine oil, 1,000 h), and vibration testing. Approving a product based solely on room-temperature shear testing would be irresponsible toward the product.

If you are working on a magnetic steel bonding project or have encountered issues with magnetic steel detachment, feel free to send me a private message or leave a comment below.

Verified JOME Engineering Grades & Specification Cross-Reference

To qualify an industrial-grade adhesive, potting compound, or thermal interface material for this application, compare verified Shanghai Jome Industrial Co., Ltd. (JOME) formulations below or explore our primary category hubs: Anaerobic Threadlockers & Retaining Adhesives and Industrial Structural Adhesives.

JOME Grade / SKU	Polymer Chemistry & Cure	Key Verified Specification	Primary Industrial Application
JOM 3107	Anaerobic Magnet Structural Bonding Adhesive	High impact & centrifugal shear (>20 MPa) \| +150°C rated	Servo motor, traction motor & humanoid robot rotor magnet bonding
JOME 1767 / JOME 2029A/B	1K Heat-Cure & 2K High-Temp Motor Epoxy	Continuous service up to +180–200°C \| High Tg & chemical resistance	High-speed turbine rotors, rare-earth & ferrite magnet bonding
JOM 638	High-Strength Cylindrical Bearing Retaining Compound	Fills 0.05–0.25 mm radial clearances \| Prevents fretting wear	Motor shafts, harmonic drive bearings, gears, bushings & sleeves
JOM 263/277 & JOM 204/93	Liquid Anaerobic & Microcapsule Pre-Coated Threadlockers	Breakaway torque 20–42 N·m \| Oil-tolerant & dry-to-touch film	Chassis bolts, robot joint fasteners, mass pre-coated screws

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Matched JOME Industrial Formulations: For production bonding, potting, and sealing qualification in this application, evaluate 2-part silicone bonding adhesive high elasticity, 2-part Acrylic Structural Bonding Adhesive（MMA）, One-part highly elastic SMP Hybrid Structural Adhesive (MS Adhesive), and 2K polyurethane structural adhesive (ISO 9001:2015 and CTI RoHS 2.0 verified; free engineering samples shipped via UPS freight collect).

Frequently Asked Engineering Questions

Q: Which JOME adhesive or potting grades are recommended for how should you choose the surface-mounted magnetic steel adhesive?
Depending on substrate pair, continuous operating temperature, and curing line constraints, Shanghai Jome Industrial Co., Ltd. (JOME) recommends evaluating JOM 3107, JOME 1767 / JOME 2029A/B, and JOM 638. Review our full specification table on this page or request a customized formulation sample.

Q: Can JOME customize viscosity, cure speed, or thermal conductivity for our automated production line?
Yes. JOME's Shanghai RandD laboratory customizes A/B mix viscosity, thixotropic index, pot life, Shore hardness, and thermal filler loading (0.75 to 25.0 W/m·K) to match your dispensing equipment and environmental test standards.