How Much Can A Magnet Hold?
I see the Pull Forces listed on your website, but what do they really mean?
We get a lot of questions about how much a magnet will pull in a given situation. The answer usually amounts to: it depends! Learn more about our testing methods, what it all means, and how all this information might apply to your specific application.
Our Testing & Specifications
The Pull Force data listed on our product pages and that determined by our Magnet Calculator come from tireless experimental testing. We test our magnets in very specific loading conditions — if your situation is different, you should expect different results. First, let’s look at how we test and what applications this closely corresponds to.
Pull Force Case 1
The most basic case is a single magnet stuck to a steel plate. We use a thick, solid steel plate that has been machined to be nearly perfectly flat. It is larger than the magnet being tested. A pull force gauge measures the force required to pull the magnet away from the steel plate.
If you stick your magnet to an overhead I-beam, you should see results just like our Pull Force Case 1 numbers. If you stick a D88 disc magnet to a steel ceiling, and hang weights from it until it lets go, you should see it hold right up to about 14 lbs, as listed.
Loading in shear – Friction
Friction Force Diagram
Another typical use is the refrigerator magnet. How much weight will it hold stuck to the door of my refrigerator? Magnets don’t pull with any lateral force when stuck to a steel plate. The magnetic force pulls the magnet straight towards the steel door. As gravity tries to pull the magnet down, the friction between the magnet and the door prevents it from sliding.
Only this friction force keeps the magnet from sliding down the face of the refrigerator door.
Why don’t we publish numbers for this? Because the friction coefficient varies greatly depending on the two materials and any lubrication that might be there. The paint on your refrigerator might be different than mine. Or perhaps my fingerprints are greasier — small changes can make a big difference! From the Wikipedia article on Coefficient of Friction:
The coefficient of friction is an empirical measurement it has to be measured experimentally, and cannot be found through calculations.
Check out this reference for a list of friction coefficients for a number of various materials. Note the disclaimers all over it — you really have to try it out in your specific application. Being cautiously pessimistic, try starting out with an assumption of 0.1 – 0.25 for your coefficient of friction. That is, you might only get 10% – 25% of the listed pull force when loaded this way.
Steel all around — Pull Force Case 2
Pull Force, Case 2
On all our product pages, we also provide Pull Force data for a second type of test called Case 2. That’s where we sandwich a magnet between two steel plates and pull them apart. For very thin magnets, this can have a big amplifying effect on the pull force. The thinner the magnet, the more pronounced the effect. Taller magnets that are over one inch tall are not any stronger in this configuration.
For example, a single D82 (1/2″ diameter x 1/8″ thick) disc magnet has a Pull Force, Case 1 of 6.44 lb. When tested between two steel plates, however, the force required to separate them jumps up to 16 lb! That’s a huge difference.
Now, you might not see quite this much of an increase in your own application. We’re using very flat, raw steel plates, held quite parallel by our test fixture. It’s an ideal situation. Still, these numbers serve as a good example of how any surrounding ferromagnetic material (such as steel or iron) might dramatically affect your results.
Pull Force vs. Distance
The distance between your magnet and what it’s sticking to can make a very big difference in the pull force you’ll see. Our Magnet Calculator is a handy tool for playing with these numbers.
For example, that D82 magnet pulls with 6.44 lb against a steel plate. Insert 10 pieces of paper between the magnet and the steel plate, and you’ve now distanced them by about 0.03″. The force drops to 3 lb.
All of our product pages now include a link to a Force vs. Distance graph, such as this one for the D82.
Prototype, prototype, prototype!
While we’re available by email to help you choose the right size magnet for your application, at some point you to have to get your hands dirty and try it out yourself. While some email pointers can be helpful, you’ll ultimately have to try actual magnets in your specific application. Order a few magnets of different sizes and experiment with them. There’s no minimum order!
If you’re ordering only a few magnets for prototyping purposes, make your shipping dollars count and order a few different sizes. Nothing beats having these magnets in your hands to see what they’re capable of. Our Sample Packages are a great value, and a good way to have a number of different sizes and shapes on-hand.