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  pull test equipment in action

Question: "Scott the PAINTER" from California asks: I recently purchased several round base style magnets from the hardware store and they don’t seem to work properly.  I spoke with the folks at the store and they told me there is nothing wrong with them.  They attached them to those orange beams in the store and they worked great.  I am using them on the roof of my car to hold a small sign for my business.  The packaging stated that they would hold 80 pounds.  In the store they felt really strong but when I applied them to the roof of my car it felt like I could pull them off with less then 80 pounds of effort.  Can you explain to me why this is happening?  My car is made of steel.  I have already had one sign blow off and I can’t afford to loose another one, please help.

Round Base Magnets

Answer: Hello Scott,  Thank you for your post.  This is a common issue we have experienced with magnetic assemblies.  When Adams Magnetic Products or any other magnet manufacturer test their assemblies we base the results on a .375" thick piece of steel

that is perfectly flat and very clean.  When you reduce the thickness of the mating material it has a drastic affect on your results.  Here is a link to a graph that I hope will illustrate what you are experiencing: RB80 and effect of surface thickness.   The large orange beams commonly found at Home Depot are almost ¼”(.250”) thick so your results should be very good, as much as 110 pounds.  The steel on the roof of your car I would guess is closer to .050” thick.  Then when you add the air gap from the primer and paint I would think this magnet would be capable of holding approximately 50 pounds.  Your results may vary but I feel confident that your magnet is working properly.  Another factor with these types of magnets is leverage.  If you are trying to remove your sign from several inches above the magnet this could also give you a feeling of less strength.  You can find more helpful information about magnetic assemblies in our helpful tools section.  I hope that this information will help.

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