Adhesive Force vs. Shear Force in magnets: Differences, Influencing Factors, and Physical Principles

Adhesive force and shear force are two central concepts in the field of magnets. Particularly with strong neodymium magnets, it is crucial to understand how these forces differ, the direction in which they act, and the factors that influence them.

What is meant by adhesive force and shear force?

Adhesive force is the force with which a magnet adheres to a ferromagnetic surface and resists a perpendicular pulling force. It therefore acts directly along the magnetic field lines. It is the force that manufacturers typically specify as a reference value.

Physically, it arises from the interaction of the magnetic field with the opposing material; in this process, the magnetic flux density and the size of the contact surface determine the result.

Shear force (also referred to as sliding force), on the other hand, describes the force required to slide a magnet laterally. It acts parallel to the contact surface and is not a purely magnetic force. It results from the combination of adhesive force and friction between the surfaces.

In short:

This fundamental distinction is crucial for every application. Pay particular attention to this when attaching objects to walls, ceilings, or sloped surfaces.

How significant is the difference between adhesive force and shear force in practice?

In practice, the difference between adhesive force and shear force is distinct. The adhesive force is typically many times greater than the shear force. This is because magnetic attraction acts most effectively along the field lines - that are perpendicular to the surface contact.

However, as soon as a lateral load is applied, the force acts against the friction between the magnet and the surface. Consequently, the effective adhesive force is significantly reduced.

To provide a better frame of reference, the following table illustrates typical differences:

Consequently, a magnet with an adhesive force of 20 kg may, under certain circumstances, slip when subjected to a lateral load of just a few kilograms. This difference is crucial for safe applications.

Which magnet shape offers the best adhesive force?

The shape of a magnet significantly influences its adhesive force. Particularly effective are so-called pot magnets - or magnetic systems - in which a neodymium core is embedded within a steel housing.

This design concentrates on the magnetic field and increases the usable flux density at the contact surface.

The following magnet shapes offer particularly high adhesive forces:

• Pot magnets: maximum concentration of the magnetic field; ideal for high adhesive power
• Flat gripper magnets: large contact surface for optimal force transmission
• Disc magnets: a good balance between size and adhesive force
• Ring magnets: for specialized applications requiring central mounting

Crucially, it is not only the shape that matters, but also the contact surface. The larger and smoother this surface is, the greater the resulting adhesive force.

Which factors influence the adhesive force of a magnet?

The adhesive force of a magnet depends on several physical and material-related factors. These determine how strongly the magnet actually "adheres.".

Among the most important influencing factors are:

• Material and alloy: neodymium magnets offer the highest adhesive force due to their strong magnetic properties
• Contact surface: a larger and smoother contact surface significantly increases the force
• Surface finish: roughness, paint, or dirt reduce the effective adhesive force
• Thickness of the counter material: thin metal plates cannot fully absorb the magnetic field
• Air gap: even small gaps drastically reduce the adhesive force
• Magnetic field and flux density: the higher the magnetic flux density, the greater the force

The adhesive force is measured under ideal conditions. This is done on a perfectly smooth, thick steel plate and with direct contact. In real-world applications, these conditions are rarely met.

What factors influence the shear force of a magnet?

Other factors influence shear force or sliding force much more strongly than adhesive force. Friction plays a crucial role here.

The key influencing factors include:

• Coefficient of friction of the surface: rubber or rough materials increase shear force
• Weight and load distribution: uneven loads reduce stability
• Coating of the magnet: rubber-coated magnets significantly increase sliding force
• Material of the contact surface: steel provides better conditions than painted or coated surfaces
• Moisture or oil: both reduce friction and therefore decrease shear force

A rubber-coated magnet exhibits a significantly higher sliding force than an uncoated magnet.

How are adhesive force, shear force, and friction related?

Physics clearly describes the relationship between adhesive force, shear force, and friction.
Shear force is essentially the product of adhesive force and the coefficient of friction.
This means:

The higher the adhesive force and the greater the friction between the magnet and the surface, the greater the shear force.

Friction acts as an "amplifier" of the adhesive force in the lateral direction. Without sufficient friction, the magnet would easily slip despite a high adhesive force.

In practice, this means:

• Smooth metal surfaces → low friction → low shear force
• Rubber-coated surfaces → high friction → high shear force

Some magnets in our shop therefore have a rubber coating. This not only protects sensitive surfaces but also significantly increases the sliding force.

Conclusion