By: Michael Brand, President, SM Magnetics
Whether you use a magnet to hold up a piece of paper on a whiteboard, hang a picture, or hold up heavier loads in an industrial application, Cap Magnets may be just the solution. Also known as Pot Magnets, these magnets are predominately used in holding or hanging applications.
What is a Cap Magnet?
Cap magnets, also known as Pot Magnets or Mounting Magnets, utilize a strong neodymium magnet inside of a non-enclosed steel housing that enhances the magnetic field on one side of the steel / magnet assembly. This balance of steel and magnet allows these smaller assemblies to produce a stronger holding force on the non-enclosed side than the magnet would alone.
Cap Magnets come in many sizes and configurations of magnets and steel. Choosing the correct configuration for your application is important so you get the hold and result that you desire. Do you need a threaded screw on top of the pot magnet? Or a will you need a threaded rod to fit a screw or bolt? Or how about a through hole? There are even cap magnets with counter-sunk holes to fit a screw with a tapered head.
So which cap magnet or pot magnet is correct for your requirement? Let’s explore some considerations and quick facts before choosing a cap or pot magnet.
How much weight or mass do you need to hold?
Cap magnets may appear small, but because of their design they have the ability to hold more weight in a smaller area. Determine the weight requirement and then choose a cap magnet that would hold about 25%-50% more. So, for example, one of the cap magnets on the SuperMagnetMan website is part number CAP_C25. This pot magnet is rated to hold up to 48.2 lbs, however let’s look at factors that determine that rating.
Holding Force Calculation: A cap magnet is given a rating based on the amount of weight it should be able to hold under ideal situations. When a calculation is determined it is done using pull force, not shear force (see below for more details on pull vs. shear testing) using simulations and pull force calculations. These simulations and calculations use a piece of steel greater than ½” thick with no gap between the steel and cap magnet. This creates “an ideal” holding situation for the cap magnet but may not necessarily mimic your actual situation.
Pull Force vs Shear Force: It will be important to know the surface and materials to which the cap magnet will be attached. For example, a magnetic white board surface is very slick so even a cap magnet with a 10 lb holding force can slide (shear force) after holding only 6-7 lbs due to the slippery surface. However, if this same cap magnet is attached to a steel beam with a rougher surface it should easily hold the 10 lbs in both the pulling and shearing directions. For magnets that are going to be on a slick surface, it is recommended to utilize a cap magnet that has a higher holding force rating, as mentioned above.
What to ask yourself before selecting a cap magnet
Sometimes several options, through trial and error, are required until you get to the correct option for your cap magnet. Below is a series of 9 questions you can ask yourself before selection to narrow your choices and your expenses.
- What is the “approximate” weight you need to hold? Our suggestion is to consider over-estimating the weight by about 10-20%. This may also allow the cap magnets to have additional uses in the future to hold heavier items.
- What is the surface the cap magnets will be used against? Is it smooth or rough? Is it painted or bare steel? Will the cap magnet be hanging horizontally or vertically against the surface? Remember, a smooth surface with a horizontal application can lead to shear force issues.
- Do you need to easily remove the cap magnet after use? Now this may seem like a contradiction to point #1 above, but don’t get a cap magnet that is too strong and difficult to detach. Most cap magnets have multiple uses and surfaces they need to stick to, so pick something that can be removed as easily as possible but still give the holding force required.
- Is there going to be a gap between the cap magnet and steel? This is one of the most important questions to ask yourself. Why? Because a cap magnet is only as strong as the “direct contact” that it has to the surface it is holding against. For example, if a holding magnet has a holding force rating of 50 lbs but will have 25 pieces of paper between it and the steel, then the holding force may quickly drop to less than ½, or more, of the original rating due to the gap between the cap magnet and steel produced by the 25 pieces of paper.
- Do you need the cap magnet to have a rubber or PTFE coating? Cap magnets can be made with rubber or PTFE coatings as a way to protect the metal surface it is holding against. But there are a couple of considerations when determining which coating to use. First, the rubber and PTFE coating put an air gap between the cap magnet and steel, so the holding force will be less than rated. Second, the PTFE coating is slippery, so putting this coating on a cap magnet and then adhering it to another slippery surface increase shear force issues. Third, the rubber coating is fantastic to avoid shear force and can come in a variety of colors, but tooling and longer lead times must be considered. Finally, a protective coating generally adds to cost; however, it does prolong the life of the cap magnet, which may offset the added cost.
- What is your budget? Cap magnets are a great example of lower cost with higher volumes. If your application requires a “standard size” cap magnet then tooling will not be a factor and cost should come down as volume goes up. But, if you don’t need large volumes, consider ordering online from a vendor, hopefully SuperMagnetMan, that already has purchased cap magnets in larger volumes and is able to pass the savings to customers even at lower quantities.
- If your application requires a custom cap magnet will it be expensive? In general, cap magnets are reasonably priced, even for custom designs. To get a custom quote the best method is to contact us and give us your specs to quote. We can then determine if the cap magnet should be made with neodymium or ceramic, or if there will be tooling. We may even suggest an alternative that can be considered to eliminate tooling. Cap magnets are usually very easy to produce without much machining or production issues, so cost is usually within budget.
- Can I get the “exact size” of cap magnet for my application? The answer is …. you will be close. Since production of cap magnets is done with stamped steel, the tolerances are usually larger than normal. Design your cap magnet requirements with larger tolerances to handle the tolerances from stamping steel. As a general rule, use a tolerance of +/- 5% - 10%.
- Will the magnetic field from a cap magnet affect ferrous material or electrical wires around it? In general, the answer is “no”. Cap magnets have a shunted magnetic field on the backside of the assembly due to the steel housing. On the side of the cap magnet that is not shunted the field is stronger but once the cap magnet is placed upon the metal surface that it will be holding, the magnetic field will be in a closed circuit and should not affect any ferrous surface nearby.
About SM Magnetics: SM Magnetics is privately owned company, providing assistance with permanent magnets, magnetic design & optimization, engineering support, and production. SuperMagnetMan is the online retail division of SM Magnetics offering neodymium magnets and other materials.