Eamonn Redmond, Director of Inseto, explains where and how adhesives are being used to fix magnets in place… and for several other applications too.
When used in the construction of a motor, adhesives have several advantages over mechanical joining techniques. These include no disruption to the magnetic fields of the magnets, stresses are evenly distributed rather than being concentrated around a mechanical joint, and protection against vibration and noise reductions.
In addition, the use of adhesives is helping manufacturers achieve size, weight and power (SWaP) requirements that are, for example, being demanded in the automotive industry. Cost reductions are being sought there too and, again, the use of adhesives helps - not only as a material but also because the adhesive application process can be easily automated.
Securing Magnets
Within electric motors, adhesives are being used for many purposes, but one that has grown in popularity during recent years is securing permanent magnets in place. Figure 1 (pictured) gives examples of where magnets are being bonded.
The mechanical power needed from the motor will govern which type of magnetic material can be used. A DC motor for a fan (as found in a vehicle’s ventilation system, for instance) will get by with two poles made from ferrite. An EV’s drive train will use induction motors (for their high torque density), and they are typically rare earth neodymium iron born magnets.
Temperature affects magnets, and two limits are of importance. The first is the maximum operating temperature. As temperature increases, magnetic strength reduces. But provided the maximum operating temperature is not exceeded, the magnet’s full strength will return as temperature reduces.
The second, and higher, temperature is the Curie temperature (named after French physicist Pierre Curie). Between its maximum operating and Curie temperatures, a magnet will permanently lose some of its strength. Above the Curie temperature, the magnet is permanently demagnetised.
Ferrite magnets can be used at temperatures up to about 180oC (and it’s worth noting they are not particularly effective at subzero temperatures). High-temperature neodymium magnets on the other hand can operate up to 200oC.
Note: for any permanent magnet, its max operating and Curie temperatures are determined by not only its material, but also its size and shape.
Understandably, the maximum temperature the motor will be operated at and how its magnets are to be bonded govern which type of adhesive will produce the best results. For instance, three of Inseto’s UK-based customers are electric motor OEMs. Two are bonding magnets. Though we cannot name them, here are explanations of which adhesives they are using and what for:
- The first customer designs and manufactures motors for automotive and related applications. This company is using Delo-Duopox SJ8665, a thixotropic two-part epoxy mixed in a 2:1 ratio, to bond magnets in place. SJ8665 provides good resistance to chemicals, and Delo has conducted tests that expose the cured adhesive to ATF type III oil at 150oC for 1,000 hours. It lost little of its strength. SJ8665 also provides high temperature stability, up to 180oC, its cure time can be reduced to one hour at 80oC, it bonds well to nickel (which is notoriously difficult to bond to) and it has a shelf life of 12 months.
- The second company is using Delo-ML DB140, an anaerobic-curing adhesive, to bond unmagnetised ferrous structures in place. Once bonded, the structures are then magnetised. For this company, production speed is of the essence. Under ambient conditions, it take about three minutes for DB140 to achieve handling strength. However, the adhesive can be UV light-fixed in seconds. This enables the company to quicky move onto the next assembly stage. It too offers a high temperature stability of up to 180oC, and Delo has performed tests (1,000 hours) at this temperature to prove its bond strength remains good.
Adhesives can also be used to form segmented (a.k.a. stacked) magnets. These are typically neodymium and are popular in the automotive sector because of their high efficiency as they reduce eddy current losses and, as a result, heat build-up.
Other Applications
Stator-to-housing and shaft bonding are other examples of where adhesives are used. Stators are typically made of thin (less than 0.5mm) electrical sheet steel (with a 2 to 3% silicon content) that are bonded into their housings (made from cast aluminium or magnesium), as an alternative to other joining methods such as pressing or shrinking.
The use of adhesives is also supplanting pressing and shrinking methods for shaft bonding. Adhesives not only prevent play and slip but also protect against fretting or contact erosion. Anaerobic-curing, low-viscous adhesives tend to be used in this application as the joining gap can be quite small.
In addition, sensitive motor components must be protected against humidity, shock and vibration. Here, potting compounds with the following properties are used: good thermal conductivity (to help with heat dissipation), low thermal expansion (to minimise stress between the component and the compound), low viscosity (to ensure a good flow when applying), fast light fixation and heat curing.
Another application is gasketing, and that is what Inseto’s third customer making electric motors is doing. They are using SL4156, a highly thixotropic cure-in-place gasket (CIPG) adhesive that UV cures in just 8 seconds, as an alternative to traditional gaskets to seal motor housings. This adhesive contains a fluorescing agent, so automatic optical inspection (AOI) systems can be used to verify that a complete bead has been formed before joining the lid to the housing.
The many benefits of CIPGs were discussed in an article in FAST magazine issue 2 2022 but to recap the main points here they include: a) no need to stock gaskets of different sizes/shapes, b) complex/3D shapes can be formed (literally drawn) and c) they allow for some flexibility where machining/moulding surfaces are concerned (particularly with regards to surface smoothness). This customer is also benefitting from design guidelines – that include advice on surface preparation, groove design and bead size – that DELO has made available.
One last application to discuss is additive balancing. This process sees the addition of mass in precise locations on all rotating components to reduce vibration and extend the life of the motor. Compared to the conventional balancing methods of removing material by machining or using a balancing putty to add mass, applying light-cured adhesives is a quicker and more precise process.
Summary
The drive for SWaP efficiency and cost reductions in most industry sectors is seeing different types of adhesives being used as practical alternatives to mechanical joining techniques. Electric motors are relatively harsh environments though, particularly with regards to heat, dust and contaminants.
Thankfully, today’s adhesives are coping well with these harsh environment conditions, and their use means they can outperform mechanical joining techniques in terms of protecting against (and reducing) noise and vibration. Plus of course, cost is always an issue, and the use of adhesives is certainly paying off as a material and because of simplified manufacturing processes.
