FAQ - Frequently Asked Questions (and their answers!)
Why does my magnet not carry the maximum weight on the wall?
The maximum weight that a magnet can hold on a certain surface varies depending on the direction of the application of force. Our specified adhesive force applies to a holding strength vertical to the contact surface. If the holding strength acts parallel to the contact surface, its maximum value is much smaller. Our measurements with a thick and polished iron plate indicated 15-20% of the specified adhesive force. Our rubber tape as well as experimenting with different surfaces can improve those values.
Maybe you witnessed this phenomenon yourself: If you try to pull a magnet off a contact surface at a right angle, you need much more strength than if you slide the magnet sideways. Analogy: It is also much harder to pick a heavy box off the floor than pushing it on a smooth floor.
This sideways force is called shear or moving force.
Holding strength vertical to contact surface
The maximum theoretical adhesive force that is specified for each magnet applies, among other factors, when the holding strength is at a right angle to the contact surface (see FAQ for other factors "How strong is this magnet?").
a = Magnetic adhesive force
b = Holding strength
c = Frictional force
b = Holding strength
c = Frictional force
Holding strength diagonal to the contact surface
If the holding strength is not at a right angle to the contact surface, its maximum value also depends on the frictional force between the object and the magnet. If the maximum value is exceeded, the magnet starts slipping. The maximum frictional force depends on the following factors:
- About the roughness of both contact surfaces. The rougher both surfaces, the higher the maximum frictional force. The uneven contact surfaces interlock (see picture).
- About the contact pressure. The higher the force that presses both contact surfaces together, the higher the maximum frictional force.
Roughness vs. contact pressure
The rougher both contact surfaces are, the better the interlocking. On the other hand, it decreases the contact pressure since the roughness increases the middle distance between the contact surfaces. Hence, roughness causes two things at the same time:
- an increase of the maximum frictional force due to stronger interlocking
- a decrease of the maximum frictional force due to declining contact pressure.
a = Magnetic adhesive force
b = maximum holding strength
b = maximum holding strength
Due to varying surface features of the object that we don't know about, we can only offer a rough rule of thumb regarding the maximum holding strength parallel to the contact surface:
Approximate holding strength parallel to contact surface = 15-20% of the magnetic adhesive force
Example: If you mount a FTN-40 pot magnet with hook with a specified adhesive force of 50 kg on a wall, you can only hang a weight of maximum 8-10 kg on it.
Optimisation possibilities
The above stated value of 15-20% originates from a measurement on a thick and polished iron plate. A different contact surface could push this value up to 50% of the magnetic adhesive force: Our rubber tape between magnet and surface can significantly increase the maximum adhesive force parallel to the surface.Experiment with rougher or smoother metal surfaces to find the optimum.
