Magnetic holders

1Understand what the steel shell does

A magnetic holder is a magnet seated in a steel pot. The shell channels the magnetic field to the working face instead of letting it spread in all directions, which is why a pot magnet holds substantially more than the same magnet on its own.

The consequence matters when you design the part: the force acts on the open face only. The sides and the back of the shell hold almost nothing, and the holder needs a flat steel counter surface to reach its rated figure. You will also find these products called magnetic lenses, holding magnets or pot magnets.

2Start from how it will be fixed, not from the force

Mounting decides the shape, and the range covers every common case:

• Flat lens: plain, for gluing or seating into a recess.
• Countersunk hole: screwed on with a countersunk screw sitting flush.
• Internal thread: you screw a bolt or rod into the holder.
• External thread or stem: the holder is screwed into your part or fixed with a nut.
• Hook: for hanging without any drilling at all.
• Rubberised: where the counter surface is painted or must not be scratched.
• Brass cylindrical with h6 tolerance: for jigs and fixtures that need a defined fit in a bore.

3Pick the diameter for the force you need

Diameters run from 4 mm up to 125 mm, and the holding force follows the diameter far more closely than it follows the height. Work with a reserve of 2 to 3 times the weight you need to hold.

Remember what the stated figure means: direct contact with thick, flat, clean steel, pulled straight off. Paint, thin sheet metal, a rough surface or a screw head standing proud will all reduce it, in some cases by more than half.

4Match the thread to the hardware you already have

Threaded versions in this range use common metric sizes such as M3, M4 and M6, and each product states the thread length as well as the size, for example M4 with a thread length of 7 mm.

Check that figure against your screw. A bolt that bottoms out in the thread pushes against the magnet inside the shell, and neodymium is brittle enough that this cracks it. Tighten by hand until the holder seats, and use a thread locking compound rather than more torque on anything that vibrates.

5Check the temperature at the holder

The category can be filtered by temperature resistance and covers three bands: 41 to 80 °C for ordinary interior use, 151 to 200 °C for warm machinery, and 351 to 550 °C for genuinely hot applications.

What counts is the temperature at the holder under continuous operation, not the temperature of the room. Below the stated maximum the holder gives up a little force while hot and recovers on cooling; above it the loss is permanent.

6Think about the direction of the load

A holder resists a straight pull many times better than a sideways slide. A holder rated at 20 kg can start to creep under a few kilograms of shear load, which is what happens on a vertical wall when a sign gradually works its way down.

Tip: If the load acts sideways, a rubberised holder usually solves it better than a bigger one. The rubber adds friction against the surface, spreads the contact and protects the paint underneath, which is exactly why it is the standard choice on vehicles and painted panels.