From theory to practice
If you plan to work with magnets, it pays to know what is behind the numbers that describe their strength. Because the strength of a magnet is not just one number – it depends on several factors. This guide will give you practical information to make it easier to understand the theory and to put it into practice.
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In catalogues and specifications of magnets, we often encounter terms such as
strength in kilograms, but also physical units such as
Tesla (T) or
Gauss (Gs). Tesla and Gauss describe how intense the magnetic strength is at a certain point in a magnet. However, since the field has a different value at different points around the magnet, these units cannot be used to determine the total strength of the magnet expressed as a single number. Therefore, Tesla and Gaus are important primarily for scientists and engineers in research and precision calculations, but they are hardly of any use to the ordinary user. So in our e-shop, we indicate the magnet strength in kilograms. This figure tells how heavy a weight a magnet would hold against gravity until it broke away from the steel surface. You can imagine that when you grab a magnet with your hand and pull, the resistance you feel is similar to lifting a weight of equal mass.
The magnet strength value given in the catalogue reflects the maximum strength measured under ideal laboratory conditions. In practice, however, the result is often lower. This depends on a number of factors – mainly the quality and thickness of the steel surface, the direction in which the magnet is pulled off, and how well the magnet adheres to the surface.
For the magnet to hold its full strength, the steel surface should be made of mild, unalloyed steel (common structural steel without admixtures), which conducts magnetic flux excellently. Conversely, stainless steel is less suitable for a magnet – depending on its type, it can be almost non-magnetic or stick with only noticeably weaker strength.
Equally important is the thickness of the steel surface. Ideally it should be at least half the height of the magnet itself. If the steel surface is too thin, the magnetic field passes through it and the resulting magnet strength is much lower than you might expect.
In other words, a magnet is only as strong as the steel on which it sticks.
The magnet has the most strength when you pull it perpendicularly upwards from the steel surface. This is when the so-called breakaway strength acts. Conversely, when the magnet is attached to the wall and the load is pulling it down, the shear force – which is much smaller – is applied. Therefore, a magnet that would hold several kilograms on the ceiling will easily slide off the wall with less weight.
For the magnet to hold as strongly as possible, it must be in full contact with the steel surface. Even a thin layer of paint or rust will create a small gap (distance) that will noticeably reduce its strength.
But beware, it’s not only the size of the gap that matters. The rate of strength drop also depends on the type of magnet. With conventional neodymium magnets, a gap of just 1 mm can cause the attractive force to drop by up to half. With potted magnets, this drop is even more pronounced. Although they are stronger than separate neodymium magnets of the same size when in full contact with the steel surface, their strength drops off at a faster percentage at the same distance.
In order to confirm the theory from the previous section in practice, we prepared a series of simple measurements. We have included in the test two types of magnets that behave differently:
All measurements were made on a digital hanging scale. The steel base from which we pulled the magnet was 19 mm thick.
Measurement results
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Breakaway strength
- Declared strength: 2.8 kg – Measured: 2.88 kg (corresponds to the catalogue value).
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Effect of distance
- 1 mm gap → 1.6 kg (which is 56% of the original strength)
- 2 mm gap → 0.69 kg (which is only 24% of the original strength)
- 3 mm gap → 0.35 kg (i.e. only 12% of the original strength)
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When two magnets get close to each other
When in direct contact, two magnets hold with similar strength to a magnet on a steel surface. However, the difference becomes apparent when there is a gap: while a single magnet quickly loses strength with even a small gap to the steel, two magnets are able to retain a larger percentage of their original attractive force. For example, with a 1 mm gap, the strength of the magnet pair still remains at approximately 70%.
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Breakaway strength and the effect of distance
- Declared strength: 10 kg – Measured: 12.5 kg (slightly more than the catalogue figure).
- At 1 mm distance, the strength dropped to about 11% of the original value.
A 20×6 potted magnet with a screw hole is designed to concentrate its strength in the front side, which is in contact with the steel surface. This makes it significantly stronger on direct contact than a stand-alone magnet of similar size – in our test it achieved a measured strength of 12.5 kg, which even slightly exceeded the catalogue figure of 12 kg.
The disadvantage becomes apparent when there is a gap between the magnet and the steel surface. Although it is extremely strong on direct contact, at a distance of 1 mm, its strength dropped to about 11% of its original value. This means that although these magnets are ideal for attaching directly to a surface, even a small gap will weaken them faster than conventional neodymium magnets.
Conclusion
Magnet strength is not just a number in a catalogue. It is affected by the steel surface material, the direction of pull, the quality of the contact and also whether a gap is created between the magnet and the steel surface.
We hope that this practical information has helped you to better understand magnetic strength and will make it easier for you to carry out your own projects.
