What Is Shear Force, and Why Does a Magnet Hold Less Weight on a Vertical Surface?
Shear force is a load that pushes a magnet sideways, along the steel surface, instead of pulling it straight off. On a wall, door or other vertical surface, an object’s weight acts as shear force, and only friction between the magnet and the steel keeps it from sliding. That friction is typically about 15 to 25% of the magnet’s rated pull force, so on a vertical surface a magnet holds roughly a fifth of its rating, and less on thin or painted steel.
- Shear force
- A load along the steel surface
- What resists it
- Friction between magnet and steel
- Typical shear holding force
- About 15 to 25% of the pull force
- Sizing rule for vertical surfaces
- Total pull force of 7 to 10× the weight
What Is Shear Force?
In mechanics, shear force is a force that acts parallel to a surface, trying to slide one surface across another. For magnets, it means any load that pushes the magnet along the steel rather than away from it. It’s also called sliding force or slide resistance.
The opposite is tension, or direct pull: a load that pulls the magnet straight away from the steel. A magnet’s listed pull force measures tension, so its shear holding force is much lower. Some products list both: our 66 mm Rubber Hooks are rated 80 lb hanging below steel and 16 lb sideways.
Common shear loads:
- A tool, hook or sign on a steel wall, cabinet side or fridge door
- A magnetic knife holder or tool bar on a vertical surface
- A panel or cover held against a steel frame
- Anything pushed sideways, such as a door catch or a sliding part
Why Does a Magnet Hold Less Weight on a Vertical Surface?
On a vertical surface, three forces are at work:
- Magnetic pull presses the magnet against the steel. On thick steel with no gap, it’s up to the magnet’s rated pull force.
- The load pulls straight down, along the steel. That’s the shear force.
- Friction between the magnet and the steel resists the sliding.
The magnet’s attraction doesn’t stop the slide directly. On a large, flat steel surface, it pulls the magnet toward the steel, not up or down. Only friction holds the load, and friction is a fraction of the force pressing the surfaces together. For a nickel-plated magnet on clean, smooth steel, that fraction is roughly 0.15 to 0.25.
So a magnet with 50 lb of pull force can resist only about 7.5 to 12.5 lb before it slides. For a worked example with a 20 lb magnet, see does a 20 lb magnet actually hold 20 lb?
Once It Starts Sliding, It Keeps Going
Friction is strongest just before something starts to move. Once a magnet begins to slip, the friction drops and it usually keeps sliding. A bump, vibration or a door slamming can start that slide at a load the magnet held a moment earlier, which is one reason to leave a generous margin.
Tension vs. Shear vs. Peel
| Load type | Direction | What resists it | Typical holding force |
|---|---|---|---|
| Tension (direct pull) | Straight away from the steel | The full magnetic attraction | Up to the rated pull force |
| Shear (sliding) | Along the steel | Friction | About 15 to 25% of the rating |
| Peel (leverage) | Pries one edge away from the steel | Attraction, against the lever | Depends on how far the load sticks out; can fail before it slides |
Peel is common on vertical surfaces: a hook, shelf bracket or anything that sticks out from the wall acts as a lever, lifting the top edge of the magnet away from the steel. The farther the load hangs from the wall, the stronger the lever.
What Affects a Magnet’s Shear Holding Force?
| Factor | Effect on shear holding |
|---|---|
| Pull force | Friction is a share of the pull, so anything that lowers pull lowers shear holding just as much |
| Air gaps, paint and coatings | Reduce the pull, and with it the friction. See air gaps |
| Thin sheet steel | Saturates and roughly halves what a magnet holds |
| Surface finish | Smooth, polished, oily or wet steel is slippery; clean, dry steel grips better |
| Magnet coating | Rubber grips much better than nickel plating, though it adds a small gap |
| Leverage | Loads that stick out from the wall peel the magnet off sooner |
| Vibration and impacts | Can start a slide below the normal limit |
| Heat | Weakens the magnet; standard grades are rated to about 80 °C (176 °F) |
How to Make a Magnet Hold More on a Vertical Surface
Add a Ledge or Stop
Let a shelf lip, screw or bracket carry the weight. The magnet then only has to hold the item against the wall, which uses its full pull.
Use Rubber-Coated Magnets
Rubber grips the steel far better than bare nickel and protects painted surfaces. Compare ratings, since the coating adds a small gap. Shop Rubber Coated Magnets.
Add More Total Pull Force
Use several magnets, spread across the item, until their combined pull force is 7 to 10× the weight.
Hang It Below Steel
If you can mount under a beam or shelf, the load pulls straight off and you get the most from the rating.
Use Thicker Steel
A Steel Strike Plate at least as wide as the magnet beats a thin panel.
Keep the Load Close
Keep heavy items tight to the wall and avoid long hooks or brackets that peel the magnet off.
How Much Pull Force Do I Need for a Vertical Surface?
- Weigh the item, including anything that will hang from it.
- Multiply by 7 to 10. Use the high end, 10, on thin sheet steel such as a fridge door, filing cabinet or cabinet side.
- Add more for any gap, such as paint, fabric or a cover, and for vibration or bumps.
- Add up the listed pull force of all your magnets until you reach that total.
- Test on the real surface before trusting the load.
Example: to hang a 5 lb tool on the side of a steel cabinet (thin sheet steel), aim for the high end: 5 × 10 = 50 lb of total pull force. Seven 1/2 × 1/8 Inch N52 Discs (57 lb together) spread across the tool cover it; small magnets lose less on thin steel than large ones. Hanging the same tool below a thick steel beam needs only about 7.5 lb, such as one 1/2 × 1/8 Inch N52 Disc at 8.15 lb.
Our magnet calculator does this math for our most popular discs. For more fixes, see how to keep a magnet from sliding down a wall.
Overhead and Safety-Critical Loads
Don’t rely on magnets alone where a sliding or falling item could hurt someone or damage property. Add a ledge, mechanical fastener or safety cable.
Takeaway
A magnet’s pull force is measured in tension, straight off thick steel. On a wall, the load is shear, and friction holds only about 15 to 25% of that rating. Size for 7 to 10× the weight, using the high end on thin steel, keep loads close to the wall, and use a ledge or rubber-coated magnets when you need more grip.
- Rubber Coated MagnetsScratch-Free Grip
- Pot (Mounting) MagnetsSteel Cup Focuses the Pull
- Magnetic HooksRated from 9 to 55 lb
More Questions About Magnet Holding Force
Does a magnet with 20 lbs of pull force actually hold 20 lbs?
Only when pulled straight off thick, flat steel. On a wall it slides at about 3 to 5 lb. See does a 20 lb magnet hold 20 lb?
How many magnets do I need to hold my item?
Aim for total pull force of at least 1.5× the weight for straight pulls and 7 to 10× for sliding loads. See how many magnets you need.
What does pull force mean?
The force needed to pull a magnet straight off thick, flat steel with no gap. See what pull force means.
Why won't my magnet stick?
Usually the surface isn’t magnetic steel, the steel is thin, or a gap or sliding load is to blame. See why a magnet won’t stick.