Fueling the Future with Adhesives
Why Electric Motors Should Be Assembled by Bonding and What Type of Adhesive Is Best Suited
Electric motors are omnipresent in everyday life. Through technological advancements, they have become smaller and more efficient, creating new challenges for joining technology. Bonding provides numerous advantages in terms of production and operation, allowing engineers designing motors to choose from a wide range of adhesives.
Tesla has been instrumental in establishing electric cars as an ideal solution for efficient and sustainable mobility of the future. However, electric motors are not only used for emission-free driving, they are also found in window regulators and seat adjusters. In fact, they can be found everywhere – in electric bikes, in tools, even in our kitchens.
All manufacturers of electric motors have one common goal in mind: making them smaller and more powerful, while increasing their efficiency. In the effort to achieve this goal, engineers must consider many things: for example, the lamination design, an optimal embedding of the magnets into the lamination stack, and leaving the smallest gap possible between magnet and coil.
Better Joining with Adhesives
Established methods of joining, like mechanical clamping or bandaging of magnets, are reaching their limits in terms of both motor function and production process. For example, a progressive reduction in motor size leads to tightened manufacturing tolerances, which drives up costs. Manufacturers of efficient electric motors rely more and more on rare earth magnets. Since they are prone to corrosion, their surfaces are treated with a coating in the form of passivation, nickel plating, or epoxy resin plating. This coating may be damaged during assembly, openly exposing the magnets to direct environmental influences.
Compared to these conventional methods, bonding offers many advantages. It is a particularly suitable option for three steps in the assembly of electric motors: joining magnets and lamination stacks, joining shaft and rotor, and joining stator and housing.
Adhesives not only compensate for higher manufacturing tolerances and prevent fretting corrosion or contact corrosion, but also provide impact resistance, which is essential to withstand the high dynamic forces of electric motors. Their vibration-damping characteristics reduce noise and provide acoustic improvement. Thanks to a homogeneous stress distribution, adhesives are able to compensate for thermal stress that may be generated due to different coefficients of thermal expansion between stator and housing. Their gap-filling properties help prevent slippage and play in the area of the shaft.
Bonding often helps reduce production costs. It allows manufacturers to expand the tolerances of components, enables easy and efficient automation, and can be used without heat input.
Apart from these structural connections, the automotive industry additionally uses adhesives for casting sensitive components in electric motors to protect them from humidity, aggressive media, and mechanical stress. They provide vibration protection to the coil wire, corrosion protection to solder and welded contacts, and they protect the coil from contact with abrasive materials.
Selecting the Appropriate Adhesive
Given the variety of sizes of electric motors and the different environmental conditions they are exposed to, there is no single blueprint for a universal design or a standard production process. Nevertheless, it is advisable to start by examining the strong and weak points of the major adhesive groups, and then perform tests with individual products and components.
Although acrylates and polyurethanes do have their place, they are less suited for high-end applications because of their moderate reliability. This leaves three product groups to consider: metal adhesives, one-component epoxy resins, and two-component epoxy resins. They have partly different properties, but mainly differ with respect to the manufacturing process they are best suited for.
One-Component Epoxy Resins
One-component, heat-curing DELO MONOPOX epoxy resins feature excellent properties. They achieve good bond strength values even at temperatures of about 220 °C. This means they are able to withstand such peak temperatures not only temporarily, but can be used permanently at high temperatures, which makes them suitable even for motors of insulation class C. Furthermore, they exhibit high chemical resistance, good gap-filling capacity, and good adhesion to nickel-plated surfaces, which is particularly important for magnet bonding.
Heat curing is indispensable in this case, a process which takes between 20 and 40 minutes for typical adhesives and at typical temperatures. Considering the additional time required for heating the components to be joined and the energy costs involved, air convection ovens are rather used for small motors. For medium-sized motors and high-volume production, inductive heating is preferable. During this process, the metallic component is exposed, either partially or completely, to an alternating electromagnetic field created by a current-carrying coil. This field generates eddy currents within the material, flowing opposite of the original current and heating the material. Induction provides very fast heating of electrically conductive components and significantly reduces the heat-up time, thus accelerating the process and allowing curing times of up to one minute.
Two-Component Epoxy Resins
The more large and massive components are, the more will manufacturers tend to choose two-component DELO-DUOPOX products. These adhesives combine good gap-filling properties with peel resistance and equalization of tensions.
Two-component epoxy resins basically cure at room temperature, which definitely takes some time, but the crosslinking process can also be accelerated by heat. At a temperature of 80 °C, for example, five minutes are sufficient to achieve adequate handling strength and ten minutes are needed for full curing. With induction curing at 100 °C, the components can even be fixed within one minute, reaching a strength of 10 MPa. Final curing of the adhesive is then taking place at room temperature.
Compared to curing at higher temperatures as is done for one-component products, for example, these moderate conditions reduce the time needed for heating the components, leading to a lower energy demand in production. In addition, these adhesives can also be used for temperature-sensitive materials such as magnetized magnets or plastics. Since these products are by now also able to withstand higher temperatures, they are quite suitable for use in motors of insulation class H.
Constraints for using this type of adhesive may lie in the investment costs for a 2C mixing system and in the rather complex processing operation.
With a temperature range of use of up to 200 °C, methacrylate-based DELO-ML metal adhesives curing at room temperature are also very heat-resistant. Ranging from flexible to impact-resistant, they exhibit good adhesion to smooth surfaces. Since crosslinking takes place under exclusion of oxygen and in the presence of metal ions, these adhesives are suited only for low layer thicknesses of maximum 250 µm. Therefore, they are used in particular for shaft bonding, where the joining gap is only small due to very tight tolerances.
Some materials pose special challenges to methacrylates. For example, only few metal ions can be found in the coating of neodymium magnets used in high-performance motors. The same applies to the ferrite core of simpler motors. The application of a solvent-free activator, either by spraying or immersion, significantly improves adhesion to problematic surfaces. Additionally, an activator tremendously accelerates the curing process. Functional strength is typically achieved within 30 to 60 seconds.
If even shorter cycle times are required, dual-curing urethane acrylates will be used. In the area of the fillet, these adhesives cure within less than 10 seconds when exposed to UV/visible light, thus fixing the components and allowing them to be processed further. Final strength will then be reached as usual under exclusion of oxygen. One process step can even be skipped, since the components do not need to be fixed mechanically, as is often the case before heat curing.
Full Protection Through Encapsulation
There are two options for the already mentioned casting applications. Dual-curing products can be used for medium requirements. These adhesives begin to cure when exposed to light and reach full strength under the influence of air humidity or heat. They ensure fast further processing while curing takes place reliably and simultaneously in shadowed areas.
For applications with extremely high requirements, for example when excellent resistance to aggressive media such as gear oil or high thermal resistance with low thermal expansion is required, the choice should fall on highly reliable products. These are available in 1C or 2C versions for small and large volumes, respectively. Thanks to their special composition, thermal expansion coefficients of up to 11 ppm/K can be reached.
As motors become smaller and more powerful, adhesive bonding gains in popularity, and the variety of materials and surfaces it can be used on expands, more and more engineers are likely to consider using it for their applications.