Difference adhesive force and displacement force

Or: Why won’t my magnet on the wall hold the maximum weight?

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 (shear direction), its maximum value is much smaller (approx. 15-25% of the specified adhesive force).

These values may be improved with the use of special products from our shop and by experimenting with different surfaces.

Table of Contents

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 force 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?").

Illustration – Holding strength perpendicular to the contact surface

a = Magnetic adhesive force
b = Holding strength

Holding strength diagonal to the contact surface

a = Magnetic adhesive force
b = Holding strength
c = Frictional force

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:

Roughness of both contact surfaces: The rougher both surfaces, the higher the maximum frictional force. The uneven contact surfaces interlock (see picture below).
Contact pressure: The higher the force that presses both contact surfaces together, the higher the maximum frictional force.

Illustration – Roughness of magnet and contact surface

Holding strength parallel to the contact surface: shear force / moving force

Illustration – shear force/displacement force

a = Magnetic adhesive force
b = Maximum holding strength

In many applications the holding strength acts parallel to the contact surface, for instance, when you put a knife on a magnetic knife rack. In these cases the magnet can be loaded much less than the indicated maximum adhesive force.

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.

Rules of thumb for shear force / shifting force

Magnet material	Material combination	Magnetic adhesive force (a)	Holding strength (b)
Neodymium magnets	Iron - Iron	100%	approx. 15%
Ferrite magnets	Iron - Iron	100%	approx. 15%
Magnetic tapes and sheets	Plastic - Iron	100%	approx. 25%

Example: On a wall-hanging hook magnet BD-FTN-40 (material: neodymium) with an max. adhesive force of 50 kg you can hang approx. 7,5 kg weights before the magnet starts sliding down the wall.

Optimisation possibilities

The above stated values originate from a measurement on a thick and polished iron plate. Through a careful selection of materials and the resulting friction, this value could be increased to up to 50 % of the magnetic adhesive force. Our suggestions:

Glue our rubber tape to the backside of the magnets.
Use magnets with rubber coating or silicone discs on the bottoms side of the magnets.
Use appropriate rubber caps or metal discs with rim on the bottom side of the pot magnets.
Experiment with rough or smooth metal surfaces to figure out the optimum.

All these measures can increase the maximum adhesive force in shear force significantly.