The Magic of Magnets in Mechanical Design
Introduction
Despite the scientific understanding we’ve gained about magnets in modern times, there is still something magical about them. Whether it’s their ability to invisibly attract one another or their surprising strength-to-size ratio, magnets are fascinating and can greatly enhance our projects and designs. I’m a graduate research assistant in BYU’s Compliant Mechanisms and Robotics (CMR) lab. My colleagues* and I have conducted research on magnets and incorporated them in many designs, so keep reading as I share a bit of what we’ve discovered.
*A special thanks to Dr. Hunter Pruett and Fisk Lundgreen for their insights.
Neodymium magnets: Where did they come from?
The origin story of neodymium magnets epitomizes the expression, “Necessity is the mother of invention.” During the early 1970s, a controversial figure named Mobutu Sese Seko took over the country of Zaire (now known as the Democratic Republic of the Congo or DRC) in a political coup that would leave him in power for the next 26 years. At that time, and even currently, over half of the world’s cobalt was mined in the DRC. Mobutu’s attempt to nationalize several massive mines in the country led to a dramatic spike in cobalt prices during the late 1970s [1-2].
At the time, samarium-cobalt magnets were prevalent in many industries. The cobalt price increase left many companies scrambling for alternatives. General Electric created a team led by John Croat, a chemical engineer, to develop a cheaper alternative magnet. On the other side of the planet, Sumitomo Special Metals – a large steel manufacturer in Japan – hired a man named Masato Sagawa to work on a similar project. Unbeknownst to both, these two teams developed the Neodymium-Iron-Boron magnets we use today within weeks of each other in 1982. Their coinciding work was discovered at a conference in Pittsburg, PA, in 1983 where they publicly presented their findings. Despite the decades of patent law battles and manufacturing agreements that have followed, Neodymium magnets are nearly ubiquitous and used in everything from your phone’s autofocus lens to Tesla’s electric motors [3-5].
What are neodymium magnets?
Neodymium magnets are one of several types of magnets made from rare earth metals. Rare earth metals are a group of elements that have special numbers of valence electrons. This gives them unique properties that are harnessed in many products we use every day. The term “rare earth metals” is a bit of a misnomer as these elements are not necessarily rare [6], though historically some of them have been difficult to mine. We’ve since found that some of them are as common in the earth’s crust as copper or tin. Neodymium is a rare earth metal and neodymium magnets are made from a compound of three elements: neodymium, iron, and boron. Neodymium magnets are by far the most common and cheapest magnets available, so we will focus on them, though other magnets may be appropriate for some applications.
How can I use magnets in my designs?
Magnets can greatly enhance many aspects of your design. This section will explore various applications of magnetic hinge technology. This application is particularly useful in stabilizing parts that might otherwise be wobbly once unfolded. Dr. Hunter Pruett has done much pioneering work in designing various magnet-incorporated hinges, and the following hinges come directly from one of his papers [7].
Simple Bistable Hinges
Figure 1: Diagram and prototype of a simple bistable magnetic hinge. (a) and (c) show the open and closed stable positions, respectively. (b) shows the unstable position. See [7] for more details.
This hinge (Figure 1) is perhaps one of the simplest conceivable hinge designs and consists of laying two magnets together and rotating one about the other, around one of their common edges. As shown in the figure above, magnetic poles are aligned in both the open and closed positions. This creates two stable positions, making this a “bistable” hinge. Figure 1(b) shows the unstable position of this hinge, and some sort of energy input would be required to fold or unfold the hinge past this point. One variation on this design includes flipping the polarity of one of the magnets to reverse the stable and unstable equilibrium positions. Another variation is achieved by adding a third magnet to eliminate the unstable equilibrium position altogether (see Figure 2).
Figure 2 shows a variation on the hinge demonstrated in Figure 1. This hinge is monostable due to the additional magnet that pulls the hinge through the unstable position. See [7] for more details.
Monostable Single Pair Hinges
If you want magnets to help your design unfold or deploy in some way, you probably want something that has a single stable position (known as “monostability”). Pruett’s paper [7] explores this type of hinge much more, including two variations of the design: MaLO and MaLDO hinges (shown in Figure 3). MaLO stands for “Magnetic Longitudinal Offset,” which is a fancy way of describing the magnets as being side by side along the hinge axis. MaLDO is short for “Magnetic Lateral/Depth Offset” and describes the magnets as layered on top of each other in the hinge. Depending on the values of xoff and zoff shown in Figure 3, many different magnet behaviors can be achieved, so review this paper [7] for some interesting variations..
Figure 3 shows isometric, side, and top schematic views of the MaLO (a-c) and MaLDO (d-f) hinges. Note the zoff and xoff dimensions shown in the figure. These variables can be manipulated to achieve various behaviors. See [7] for more details.
Double-pair Magnet Hinge
The hinge in Figure 4 illustrates another way to achieve monostability in a design. By placing attractive magnets in the desired position and repulsive ones in the undesired position, a designer can guarantee that a design takes the shape they want.
Figure 4 shows monostable magnet hinges that use multiple magnets on each side of the hinge to achieve their behavior. See [7] for more details.
Diametrically Magnetized Gears
Housing circular magnets in 3D-printed gears produce interesting results. Figure 5 shows the stable vertical positions as well as an unstable horizontal position. Rigid links with gears like these on their ends could be made to rotate 180° relative to each other. Such a system would require negligible energy input to create this rotation, assuming the initial position was the unstable. Additionally, the centers of the gears would not need to be fixed since the magnetic attraction would keep their teeth in contact. Finally, the gear teeth would not require lubrication since they are not transmitting dynamic forces, and hard stops could be designed into the system to prevent overshoot.
Figure 5 shows diagrams and prototypes of diametrically-magnetized gears in stable (a, c) and unstable (b) positions. See [7] for more details.
What are some practical design and assembly tips for using magnets?
Prevent Shattering
Neodymium magnets create large forces on other magnets and many metallic objects. In our experience, if they are allowed to freely accelerate toward one another from more than a few centimeters distance, they will likely chip or shatter. Similar damage can occur when magnets freely accelerate toward metallic objects (such as the leg of a table or a screwdriver). We found that the best way to prevent accidents was to keep an organized workspace. Magnets should never be placed on a table unattached to anything else. Firmly attach them to the table leg, a cabinet drawer, or some other fixed, magnetic surface. Tools and other hardware should be kept at least an arm’s length away from your main workspace to prevent them from being pulled unexpectedly toward your project. You may find it easiest to attach all your screws to one of your fixed magnets to prevent the screws from being picked up every time your project or a magnet in your hand passes near them.
Wrapping your magnets in tape can also help prevent accidental shattering, especially during assembly. However, we do not recommend taping magnets to your project as a permanent solution. They can tear through or fall off as the tape peels off or gets old.
Shattering can also be avoided by leaving small gaps between attractive faces in your design. Allowing the housing to bear the brunt of the deployment impact will prevent your magnets from shattering when they snap together. Be cautious not to leave too large of a gap, though. Manufacturers (like K&J Magnetics, Inc.) have graphs showing how quickly their magnets lose attractive strength as the gap between them increases.
Fastening
Speaking of strength, you should be careful how tightly you secure magnets to your project. Due to their brittle nature, we found that magnets shatter quite easily as their securing hardware is tightened. Also, consider using brass hardware. It’s not magnetic and will make assembly much easier!
Gluing magnets can also be a great option for your project. Make sure you check the orientation of the magnetic field before you glue them! And, if your magnets are very strong, hardware is a safer option than adhesives.
Neodymium magnets are often shipped in stacks like this with plastic separators to facilitate pulling them apart from each other. Photo credit: Adobe Stock Images - Educational License
Assembly
Another difficulty in dealing with magnets is removing them from the stacks in which they are shipped. Pulling magnets directly apart from each other can be difficult and they are often in the orientation in which their attraction is strongest. We found that twisting them to right angles from each other before levering them apart was the easiest (and sometimes the only) way) to separate them.
Environment
When deciding which magnets you should add to your project, you should consider the environment in which it will be operating. Neodymium magnets, like all magnets, are only effective below certain temperatures. A magnet’s Curie temperature is the temperature at which a magnet’s crystalline structure will become unaligned, and it will become totally demagnetized (about 310°C/590°F for Neodymium magnets). One of the reasons Neodymium magnets have Iron and Boron in them is to raise this temperature, which is relatively low. Magnets should not be used above their maximum recommended operating temperature (around 150° C/300° F for Neodymium) [8].
Neodymium magnets are susceptible to corrosion in harsh or damp conditions. Make sure you check for a corrosion-resistant coating such as nickel or tin when buying your magnets!
Machining
Machining magnets can be difficult because they are so brittle, and it could affect their magnetic properties. Manufacturers sell a wide range of magnet geometries, including magnets with chamfered holes to admit hardware, so double check that what you want is unavailable before trying to machine your own.
Do you have any examples of magnets being used in engineering design?
In the CMR lab, we have learned many of the lessons mentioned above through trial and error. One opportunity for trial and error came in the form of a project we worked on for the Air Force Research Laboratory. Collaborating with engineers at the University of Florida, we designed, among other things, a deployable reflectarray antenna.
One important deliverable of this project was a scaled-down prototype of the antenna to demonstrate the functionality of the design. An enormous amount of work went into finding an origami-inspired design that could accommodate the thickness of the antenna panels (read more about thickness-accommodation origami techniques here). Once the pattern had been defined, the team was still missing the hinges that would hold the antenna together.
These hinges had a tough set of requirements. First off, they had to be able to function in space. One tricky part of designing space-worthy hinges is that normal lubricants outgas in the low-pressure environment of space [9]. Another requirement was that the hinges provided the antenna’s deployment force. Finally, the hinges had to accurately hold the whole antenna array with enough stiffness to dampen and eliminate the deployment-produced vibrations. Now, for the demonstrator, we didn’t need to perfectly meet all of these requirements, however, it was important to demonstrate that it would be possible to meet them by using space-grade materials.
The above images show initial prototypes using (left) a simple bistable configuration with very strong magnets, (middle), a simple bistable configuration using smaller magnets, and (right) a combination of MaLDO and simple bistable magnet pairs. All three hinges are designed with compact compliant joints on the bottom face (not visible) such that they form a mountain fold when opened (or an upside-down “V” shape). The black panels were added to the figure on the right to increase the user’s mechanical advantage; the hinge was too strong to unfold otherwise.
In connection with these options, the team started exploring compliant hinges. These could work well in space since they don’t need lubrication and thus avoid the outgassing problem. Additionally, many compliant hinges have an inherent torsional springiness due to their deflection in a folded state. This helps with producing a deployment force. Unfortunately, compliant hinges’ flexibility often makes them wobbly once deployed, and sometimes the hinges allow motion in undesirable directions (“parasitic motion”).
A more advanced prototype is shown that uses a combination of MaLDO and MaLO hinges
A section of the final prototype shows both the carbon fiber hinges and the stabilizing magnet configurations
Drawing on many of the principles and designs mentioned previously, the team started exploring magnets as a method to mitigate the weaknesses of the compliant hinges. We built prototypes using MaLDO, MaLO, simple bistable, and other magnetic hinge configurations. First, we built simple two-panel prototypes, but soon we moved on to building more complex sections of the reflectarray antenna. Through a series of trial and error, prototyping, and testing, the design evolved to include a combination of several magnet configurations that were guided by robust carbon fiber-compliant hinges.
The final demonstrator is shown above with the deployment and then stabilization performed using the prototype’s own energy and damping.
Conclusion
Whether you are a design hobbyist or are pushing the limits in your field, magnets can be a convenient and even invaluable addition to your projects. When used in combination with compliant mechanisms or origami-inspired designs, magnets can be crucial to obtaining the performance you need. Regardless of your background or design, we recommend considering the magic of magnets as an avenue toward greater innovation.
Where can I go to learn more?
Some of our favorite magnet suppliers are totalElement® and K&J Magnetics, Inc. K&J Magnetics specifically has lots of great design and technical information about their magnets. If you are interested in more of the technical side of things, Dr. Hunter Pruett’s papers below on the subject are a great starting point. Here are some other links where you can go to get more information.
Magnetship.com’s Design Guide - Contains more technical information about many of the topics presented in this article
MatWeb’s Neodymium Page - Contains mechanical and magnetic properties for N-45 Neodymium magnets
Stanford Magnets’ Knowledge Base - Magnet supplier with many articles on designing with magnets
References
[1] Wikipedia contributors. (2024b, September 20). Mobutu sese seko. Wikipedia. https://en.wikipedia.org/wiki/Mobutu_Sese_Seko
[2] Al Barazi, Siyamend & Naeher, Uwe & Vetter, Sebastian & Schütte, Philip & Liedtke, Maren & Baier, Matthias & Franken, Gudrun. (2017). COBALT FROM THE DR CONGO – POTENTIAL, RISKS AND SIGNIFICANCE FOR THE GLOBAL COBALT MARKET 1. Commodity Top News #53. https://www.researchgate.net/publication/326060301_COBALT_FROM_THE_DR_CONGO_POTENTIAL_RISKS_AND_SIGNIFICANCE_FOR_THE_GLOBAL_COBALT_MARKET_1_Commodity_Top_News_53/figures
[3] Zorpette, G. (2023, March 29). The magnet that made the modern world. IEEE Spectrum. https://spectrum.ieee.org/the-men-who-made-the-magnet-that-made-the-modern-world
[4] Wikipedia contributors. (2024, August 30). Neodymium magnet. Wikipedia. https://en.wikipedia.org/wiki/Neodymium_magnet
[5] Stanford Magnets. (n.d.). Use of NDFEB magnets in mobile phones | Stanford Magnets. https://www.stanfordmagnets.com/use-of-ndfeb-magnets-in-mobile-phones.html#:~:text=Vibration%20Motors,motors%20only%20need%20one%20magnet
[6]Science History Institute. (2023, May 11). History and Future of Rare Earth Elements | Science History Institute. https://www.sciencehistory.org/education/classroom-activities/role-playing-games/case-of-rare-earth-elements/history-future/#:~:text=Rare%20earths%20are%20not%20rare,in%20our%20technologies%20around%20us.
[7] Pruett, H, Coleman, N, & Magleby, S. (2023, August) "Preliminary Concepts for Magnetic Actuation and Stabilization of Origami-Based Arrays." Proceedings of the ASME 2023 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference. Volume 8: 47th Mechanisms and Robotics Conference (MR). https://doi.org/10.1115/DETC2023-116615
[8] Magnet Design Guide. (n.d.). https://www.magnetshop.com/magnet-design-guide
[9] Fusaro, R. L. “Lubrication of Space Systems” (1994). NASA technical memorandum 106392. Lewis Research Center, Cleveland, Ohio. https://ntrs.nasa.gov/api/citations/19940024896/downloads/19940024896.pdf
