Learn more about magnetizing forces

What is the reference magnetising force (pull-out force)?

All the magnets and magnetic products you will find on our site have a reference magnetising force, known as the pull-out force, indicated in kg. This force is a theoretical maximum value obtained under laboratory conditions. Depending on the conditions in which your magnet is used, its actual power may vary.

This magnetising force is different from the classification code for neodymium magnets, which you will find in the magnets' technical specifications and which take into account data such as remanence or coercive field.

Below are detailed explanations of how magnetic force works, to help you estimate the power you need.

How is the reference magnetising force calculated?

The reference magnetising force is calculated under conditions that ensure optimum magnetisation:

The magnet must be in direct contact with its ferrous support. There is no object or material between the magnet and the support. The further apart the magnet and the support are, the lower the magnetising force will be.

When calculating power, the magnet support is made of pure soft iron, flat and with no rough edges, so that the contact surface between the magnet and its support is as large as possible.

The magnet is placed on a support parallel to the ground so that other forces such as gravity do not interfere with the magnetising power.

When a magnet is magnetised on a wallboard or fridge door, the actual magnetising power observed is much lower. Gravitation pulls the magnet down and causes it to slip, reducing the magnetising force. This is known as the shear force.

The reference magnetising force that we indicate on our website does not take into account the constraints mentioned above, which may alter the actual magnetising power. 

In practical terms, what does a reference magnetising force of 1kg correspond to?

Let's take a hook-shaped neodymium magnet that is magnetised on a pure iron ceiling. In our example, if the hook has a magnetising force of 1kg, this means that you could theoretically hang an object weighing up to 1kg from the end of the hook without it becoming detached from its support.

In practice, you need to take into account the quality of the ferrous support, its coating, the object's centre of gravity, its parallelism with the floor, etc.

Why doesn't the reference magnetising force always correspond to the weight a magnet is capable of supporting?

The magnetising force indicated on our site is a maximum theoretical value obtained under laboratory conditions. In practice, it will vary considerably depending on the conditions of use.

The magnetising power of a magnet is reduced by factors such as :

 - The shearing force (most often this is gravity),

 - The contact surface between the magnet and its support. The smoother the surface, the lower the magnetising power,

 - The composition and thickness of the ferrous backing. A backing that is too thin will quickly reach saturation, leaving part of the magnetic field unused,

 - The distance between the magnet and the substrate (paint, paper, etc.). The magnetising power will be much lower if the magnet is not in direct contact with its support.

What does pull-out force mean? 

A pull-out force occurs when a force is applied perpendicular to the magnet support. This is the case, for example, when gravity exerts a vertical force on a magnet fixed to the ceiling.

The pull-out force is the force required to separate the magnet from its support when the force is exerted perpendicular to the magnet's support. This force is measured in kg or Newton: a mass of one kg corresponds to a force of 9.81 Newton in the Earth's gravitational field.

What is shear force?

Shear force occurs when a force is applied parallel or obliquely to the magnet support. This is the case, for example, when gravity exerts a vertical force on a magnet fixed to a wall. In this case, it is the friction between the magnet and its support that prevents the magnet from sliding towards the ground.

Try it out at home: it's much easier to move a magnet by sliding it rather than by pulling it from its support (pulling force).

How much is the reference magnetising force of a magnet subjected to a shearing force reduced?

A magnet subjected to a shearing force will have 5 to 10 times less magnetising power than the same magnet subjected to a pulling force.

To determine the influence of the shearing force, several factors come into play:

 - The centre of gravity of the magnetised object.

 - The force of friction between the magnet and the support. 

 

Friction limits this sliding effect and enhances the real magnetising power. The rougher the contact surfaces, the greater the friction force. The force of pressure - i.e. the force with which the surfaces are pressed together - also influences the strength of the friction force.

How can I reduce the impact of the shearing force?

Do you need a magnet that will be subjected to shear forces? There are several ways of guaranteeing sufficient magnetic power.

Firstly, you can choose the strength of the magnet. The stronger the magnet, the greater the force of pressure between the magnet and its support, which increases friction and counteracts the slippage of the shearing force.

To further increase frictional forces, we have designed magnetic products with an extremely rough rubber coating.

The rubber creates a high level of friction with the ferrous substrate, helping to maintain a good grip despite the shearing forces.

Finally, to further reduce the impact of shear force, we also offer rough ferrous supports.

These rough magnets and supports will slightly reduce the magnetization strength in the case of pull-off forces but will significantly increase it for shear forces.

Each use case is different, and it is challenging to predict in advance the magnetization strength you will need.

Not sure if the magnetization strength will be sufficient for your application?

You can explain your project to one of our advisors. We will recommend a magnetic product that is suitable and powerful enough to withstand potential shear forces.