How to calculate the pull force of a magnet?

Friends often ask me, how much can a magnet pick up? It is said on the Internet that NdFeB magnets can absorb objects 600 times their own weight. Is this accurate? Is there a formula for calculating the attraction force of a magnet? Today, I understand the magnetic emperor and tell you about the "attraction" of magnets.

In the application of magnets, magnetic flux or magnetic flux density is an important indicator to measure its performance (especially in motors), but in some application fields, such as magnetic sorting, salvage, etc., magnetic flux is not an effective measure of sorting or The amount of adsorption effect, magnetic attraction is a more effective indicator.

Magnetic attraction refers to the weight of ferromagnetic substances that a magnet can absorb. It is affected by factors such as the performance, shape, size, and adsorption distance of the magnet. There is no mathematical formula that can calculate the attraction of a certain magnet. The magnetic attraction measuring device is used to measure the magnetic attraction value (usually measuring the pull force of the magnet, and then converting it into weight), as shown in the figure below. The attraction force of the magnet will gradually decrease with the distance of the object being adsorbed.

If you search for the calculation of magnet attraction force on the Internet, many websites will write "According to experience, the magnetic force of NdFeB magnet itself is 600 times its own weight (also written 640 times)". Is this experience correct? Let's verify it through experiments. Just know.

In the experiment, sintered NdFeB N42 magnets of different shapes and sizes were selected. The surface coatings were all nickel-copper-nickel magnets, which were magnetized in the height direction. The maximum pulling force (N pole) of each magnet was measured and converted into adsorption weight. The result is as follows:

It is not difficult to find from the measurement results:

The ratio of the weight that magnets of different shapes and sizes can pick up and their own weight varies greatly, some are less than 200 times, some are more than 500 times, and some can reach more than 3000 times, so the 600 times written on the Internet is not completely correct

Cylinders or round cakes of the same diameter, the greater the height, the greater the weight that can be sucked up, and the suction force is basically proportional to the height

Cylinders or round cakes (blue cells) of the same height, the larger the diameter, the greater the weight can be sucked up, and the suction force is basically proportional to the diameter

Cylinders or round cakes (yellow cells) of the same volume and weight have different diameters and heights, and the weight that can be picked up varies greatly. Generally, the longer the magnet orientation direction, the greater the suction force.

Magnets with the same volume may not necessarily have the same suction force. Depending on the shape, the suction force may vary greatly. Conversely, the same is true. Magnets that absorb the same weight of ferromagnetic substances may have different shapes, volumes and weights.

Regardless of the shape, the length of the magnetic orientation direction has the greatest effect on the determination of the suction force

So what is the difference in suction between different brands of magnets, we will do tests to compare them later.