There are several terms used to describe the strength of a magnet, these include:
Pull – This is how much force is needed to pull the magnet off a steel surface, and is usually referenced in kilograms.
Gauss reading (flux density) - If a Gauss meter or flux meter hall probe is placed on the pole of a magnet, a reading can be taken showing the number of lines of magnetism in every cm2 (1 Gauss = 1 line of magnetism in 1cm2), also known as flux density. This reading is an 'open circuit' value which will be substantially lower than the Br value and will be directly related to the material and the length to diameter ratio of the magnet. Long magnets with small diameters will have a much higher open circuit flux density than short magnets with relatively large diameters, even when they are manufactured from the same grade of magnetic material. If you had a rod magnet measuring 5,000 Gauss on the poles and you cut it in half, you would not expect the two smaller length magnets to have the same Gauss reading in open circuit.
Hysteresis graph testing - This is a thorough test where the magnet is magnetised and demagnetised within a closed circuit situation and a value for Br, Hc and (BH)max are obtained. These relate to maximum amount of magnetism in the closed circuit magnet, the resistance to being demagnetised and the total energy within the magnet.
What factors can reduce the performance of a magnet?
All magnets have a 'pull' rating measured in kilograms and this relates to how much force acting perpendicular to the magnet is required to pull the magnet from a steel plate or equal thickness when in direct, flush contact.
The 'pull' rating is obtained under the following ideal conditions:
- the test bed steel plate is thick enough to absorb all the magnetism (typically 10mm thick)
- it is clean and ground perfectly flat
- the pulling force is slowly and steadily increased and is absolutely perpendicular to the magnet face.
In actual applications, perfect conditions are unlikely and the following factors will reduce the given pull:
Steel thickness
If a magnet needs the contact steel to be 10mm thick to absorb all the magnetism and deliver maximum pull, then fixing the magnet to a 1mm thick sheet steel surface will result in 90% of the magnetism being wasted and the actual pull delivering only 10% of its capability. To test if the contact steel is thick enough to absorb all the magnetism from a given magnet, simply fix the magnet in place and then offer a small steel plate behind the contact steel, directly behind the magnet and if it sticks, then it is being held in place by stray magnetism which is breaking out from insufficiently thick steel. If it falls away, then the contact steel is absorbing and conducting all the magnetism and increasing the thickness of the steel will not increase the 'pull' from the magnet.
Air gap
If the contact steel is rusty, painted or uneven, then the resulting gap between the magnet and the contact steel will lead to a reduced 'pull' from the magnet. As this gap increases, the pull decreases using an inverse square law relationship.
Material
All pull tests use mild steel as a contact steel. Alloy steels and cast irons have a reduced ability to conduct magnetism and the pull of a magnet will be less. In the case of cast iron, the pull will reduce by as much as 40% because cast iron is much less permeable than mild steel.
Temperature
Subjecting a magnet to temperatures above its maximum operating temperature will cause it to lose performance that won’t be recovered on cooling. Repeatedly heating beyond the maximum operating temperature will result in a significant decrease in performance.
Sheer force
It is five times easier to slide a magnet than to pull it vertically away from the surface it is attracting to. This is entirely down to the coefficient of friction which is typically 0.2 for steel on steel faces. Magnets with a rated pull of 10kg will only support 2kg if they are being used on a vertical steel wall and the load is causing the magnets to slide down the wall.
How long will a neodymium magnet last?
Neodymium magnets are permanent magnets, and lose a fraction of their performance every 100 years if maintained within their optimum working conditions.
There are two factors which can shorten a magnet’s lifespan.
Heat
If the temperature of a magnet exceeds the
maximum operating temperature (e.g. 80oC for N42 grade neodymium magnets), then the magnet will lose magnetism that will not be recovered on cooling. Samarium cobalt magnets are not quite as strong as neodymium magnets but they do have a much higher operating temperature of up to 350 degrees Celsius.
Corrosion
If the plating on a magnet is damaged and water can get inside, the magnet will rust and again this will result in a deterioration in magnetic performance. Samarium cobalt magnets and ferrite magnets are both resistant to corrosion but aren’t as strong as neodymium magnets.