Why Adhesive Bonding Should Be Used in Electric Motor Assembly and How to Choose the Right Adhesive Type

Electric motors are ubiquitous in daily life. Consider Tesla automobiles: the electric motor provides the drive system. However, electric motors are not used solely to generate power in the drive system. They are used in many locations – window opening and closing, seat adjustments, sunroofs and similar applications. We can see motors in our kitchens, in electric bicycles, and everywhere. As technology advances, motors are becoming smaller and more efficient, which presents new challenges for assembly. Adhesive bonding offers numerous advantages in production and operational processes, providing design flexibility by giving engineers a wide range of adhesive options to choose from. All electric motor manufacturers share a common goal: to make motors smaller and more powerful while increasing efficiency. To achieve this goal, engineers must focus on many issues. For example: leaving the smallest possible gap between magnets and coils, and lamination.

Stronger Bonding with Adhesives

The joining methods currently used (mechanical compression or winding magnets) have reached their limits in terms of motor function and production processes. For example, reductions in motor dimensions require tighter manufacturing tolerances that increase costs. Efficient electric motor manufacturers use rare natural magnets. Since these materials are prone to corrosion, their surfaces are coated with nickel or epoxy resin. These coatings can be damaged during assembly and become exposed to external influences.

Figure 1. Typical bonding areas (Image: DELO)

Compared to known applications, adhesive bonding has several advantages: bonding magnets and laminations together, joining shafts and rotors. The adhesive not only tolerates high manufacturing tolerances and corrosion caused by contact and friction, but also provides high impact resistance against the high dynamic forces produced by the electric motor. Vibration damping characteristics reduce noise and provide acoustic improvement. Due to homogeneous stress distribution, the adhesive can tolerate thermal stresses resulting from different thermal coefficients of the housing and stator. Gap-filling capability prevents slippage and play in the shaft area.

Figure 2. Adhesive provides balanced stress distribution (Image: ebm-papst)

Adhesive bonding often helps reduce production costs. This is achieved by allowing manufacturers to work with wider tolerances, enabling automation, and eliminating the need to use heat sources.

Figure 3. Bonding magnets (f.l.t.r.) (Image: DELO)

Beyond this, adhesive bonding is also used to protect sensitive parts inside the electric motor from moisture, aggressive chemicals, and mechanical stress. For windings, it provides vibration protection and protection against abrasive substances; for solder joints, it provides corrosion protection.

Selecting the Right Adhesive

Due to the varying sizes of electric motors and the different environmental impacts to which they are exposed, there is no standard design or manufacturing process. Nevertheless, it is recommended to begin by evaluating the strengths and weaknesses of the main adhesive groups. Subsequently, tests should be organized based on the selected product group and the components to be bonded. While acrylic and polyurethane-based adhesives are common, they are not suitable for premium products due to their moderate durability performance. Apart from these, we can consider three groups: metal adhesives, one-component epoxy resins, and two-component epoxy resins. Although these adhesives have different properties, they are fundamentally classified according to the different processes and application types for which they are suitable.

One-Component Epoxy Resins

One-component epoxy resins have excellent properties. They exhibit very good adhesive strength up to 220°C. This means that these one-component epoxy resins can be used permanently, not temporarily, at high peak temperatures (Class C). Additionally, this class has high chemical resistance, gap-filling capability, and strong adhesion to nickel-coated surfaces. In this case, heat curing is mandatory and standard adhesives should be cured with heat for approximately 20-40 minutes. Considering energy consumption, convection ovens are used for small motors. For medium-sized motors and high-volume production, induction heating may be preferred. Induction heating is the fastest method for bonding parts with electrical conductivity and significantly reduces heating times. In accelerated processes, heating time can be reduced to as little as 1 minute.

Two-Component Epoxy Resins

For larger components, manufacturers typically prefer two-component epoxy resins. These adhesives combine excellent gap-filling properties, peel strength, and homogeneous stress distribution. Two-component epoxy resins generally cure at room temperature. This generally takes longer compared to other methods, but the process can be accelerated with additional heat. For example, 80°C for five minutes is quite sufficient to achieve initial handling strength, and an additional five minutes are needed for full curing. At 100°C achieved by induction, parts can reach adhesive strength of approximately 10 MPa in almost 1 minute. Nevertheless, final curing is completed at room temperature. We can think of additional heat sources like a catalyst. These relatively low curing temperatures compared to high-temperature cured one-component products result in lower energy consumption in production by reducing component heating time. Additionally, two-component epoxy adhesives are used for bonding heat-sensitive magnets and plastics due to their low curing temperature (Class H). If required, UV-curing resin can be supplied for faster curing processes, and hybrid products cured with heat and UV can be preferred.

Figure 4. Laboratory technicians conducting UV-hybrid adhesive tests (Image: DELO)

Metal Adhesives

Under operating conditions up to 200°C, methacrylate-based metal adhesives have high heat resistance despite curing at room temperature. They provide excellent adhesion to smooth surfaces along with high impact resistance. Metal adhesives can only be used up to a 250 μm adhesive film thickness. They are used in shaft bonding. Adhesive tolerances in this area are very tight. Low film thicknesses make selection of these adhesives necessary.

Complete Protection

"Dual curing" hybrid adhesives can be preferred. These adhesives begin to cure when exposed to UV light. However, they reach their actual strength values through heat or moisture curing. Thanks to faster processes, areas not exposed to UV light during process continuation cure at room conditions or under heat.

Figure 5. Adhesive as protective (Image: DELO)

For applications requiring performance at very high levels, such as excellent resistance to aggressive oils or low thermal expansion at high temperatures, stable adhesive selection is very important. As Delo, through special chemical formulations in our one-component or two-component adhesives, we can achieve a thermal expansion coefficient of 1 ppm/K.

Summary

As motors become smaller and more powerful, adhesive bonding becomes increasingly necessary. With its ability to work under diverse operating conditions and bring together very different materials, it will rank among the first choices of design and research and development engineers. Some materials show resistance to bonding with methacrylate-based adhesives. (For example, neodymium magnets used in high-performance motors). In these processes, an activator should be applied by spray. Activator use significantly increases adhesion and dramatically reduces curing time. If even faster cycle times are desired, hybrid adhesives called "dual curing" for rapid curing can be preferred. For initial fixation, a UV light source is held over the bonded area for approximately 10 seconds. After this time, initial bonding of the parts is achieved. As the assembly process continues, the adhesive completely cures during waiting times. This provides flexibility in production. Dr. Karl Bitzer Product Manager Delo Industrial Adhesives Official Distributor for Turkey Bond Teknik