What is the difference between adhesive force and displacement force-shear force? Or: Why won't my magnet on the wall hold the maximum weight?

? Magnet Adhesion Force vs. Displacement Force – Why Your Magnet Can't Hold Maximum Weight
Many people expect their magnet to hold its advertised weight under any conditions. However, adhesion force and displacement force are two different factors that affect how much weight a magnet can truly hold.

? Adhesion Force (Pull Force)
? Definition: The maximum force required to detach the magnet perpendicularly (90°) from a surface.
? Measurement Conditions:
✔ The magnet must be fully in contact with a thick, smooth, and clean steel plate.
✔ Force is measured when pulling directly away from the surface.
✔ Usually measured in Newtons (N) or kilograms (kg).
? Example: If a magnet’s rated adhesion force is 100 kg, it means you need 100 kg of pulling force to lift it straight off an ideal steel surface.

? Displacement Force (Shear Force)
? Definition: The force needed to slide the magnet sideways across a surface.
? Why is it much lower than adhesion force?
✔ Friction between the magnet and surface is limited.
✔ Surface imperfections, dust, or oil reduce friction.
✔ Load distribution: If the weight is uneven, the magnet slides off more easily.
? Typically, displacement force is only about 15–30% of the adhesion force!
? Example: If a magnet has 100 kg adhesion force, its displacement force may be only 15–30 kg.

? Why Can't Your Magnet Hold the Maximum Advertised Weight?
If your magnet fails to support its listed weight, consider these factors:

1️⃣ Surface Material
✔ Magnets work best on thick, smooth, uncoated steel.
✔ Thin metal reduces holding power significantly.
✔ Stainless steel, aluminum, copper, or plastic – little to no magnetism!

2️⃣ Surface Condition
✔ If the surface is painted, rusty, oily, or uneven, the magnet’s actual force drops.
✔ Any air gap between the magnet and metal reduces its strength drastically.

3️⃣ Adhesion Force vs. Displacement Force
✔ If the magnet is attached horizontally (e.g., on a ceiling or wall), displacement force comes into play.
✔ Even small sideways forces can cause the magnet to slip off.

4️⃣ Load Positioning
✔ If an object hangs from the magnet and swings, it creates a sideways force, making it detach more easily.
✔ Direct downward force = strongest holding power.

5️⃣ Temperature Sensitivity
✔ Neodymium magnets lose strength above 80°C (some up to 200°C).
✔ High temperatures weaken the magnetic field permanently in standard neodymium magnets.

6️⃣ Protective Coating
✔ Magnets often have a nickel, zinc, or epoxy coating that can slightly increase the distance between the magnet and metal, weakening the adhesion.

? How to Increase Your Magnet’s Holding Strength?
✅ Use a thicker steel surface – thin metal does not create a strong enough magnetic field.
✅ Ensure the surface is clean, smooth, and rust-free.
✅ Increase the contact area between the magnet and metal (if possible, use multiple magnets).
✅ Avoid sideways loads – hang objects directly under the magnet.
✅ Use rubber-coated magnetic holders – they increase friction and reduce slipping.

? Conclusion
If your magnet isn’t holding its advertised weight, it’s likely due to displacement force, surface quality, or how the load is attached. To achieve maximum adhesion, choose the right magnet for the right conditions. ?