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Continue ShoppingSSKC-013 - Pull Force vs Shear Force: Why Magnets Slide Before They Pull Off
Pull Force vs Shear Force: Why Magnets Slide Before They Pull Off
Document ID: SSKC-013
Series: Engineering Series
Difficulty: Intermediate
Reading Time: 10–12 minutes
Last Updated: September 2026
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Since 1969, Simple Signman has supplied magnetic materials to Canadian manufacturers, printers, sign professionals, distributors, and industrial businesses.
One of the most common misunderstandings in magnetic mounting is assuming that a magnet's published pull force also represents how much weight it can safely hold on a vertical surface.
It does not.
A magnet can be extremely difficult to pull straight away from steel and still slide relatively easily across the same surface.
This guide explains why.
Introduction
Imagine a magnet rated at 100 lb of pull force.
It is tempting to assume that the same magnet can safely support 100 lb on a vertical steel wall.
But pull force and shear force are different loading conditions.
Published pull force usually refers to the force required to separate the magnet from steel in a direction perpendicular to the surface.
A vertical load tends to make the magnet slide parallel to the surface instead.
This difference changes the physics of the application.
A magnet can have impressive pull force and still slide before it pulls off.
Understanding the difference between pull, shear, friction, and peeling is essential for designing reliable magnetic mounting systems.
Contents
- What Is Pull Force?
- What Is Shear Force?
- Why Magnets Slide Before They Pull Off
- How Surface Material Changes Sliding Performance
- Why Rubber-Coated Magnets Perform Differently
- Pull Force vs Shear Force vs Peeling Force
- Why Leverage Makes the Problem Worse
- Air Gap + Shear Force
- Steel Thickness + Shear Force
- Will a Higher Magnet Grade Solve Sliding?
- Real-World Applications
- How to Improve Resistance to Sliding
- Engineering Recommendations
- Common Mistakes
- Frequently Asked Questions
What Is Pull Force?
Pull force is the force required to separate a magnet from a steel surface when the load is applied perpendicular to the contact surface.
This is the condition most commonly used for published magnet ratings.
Typical laboratory pull-force testing assumes:
- thick steel;
- clean steel;
- flat contact surfaces;
- little or no air gap;
- room temperature;
- direct perpendicular pull.
Under these conditions, the magnet can achieve its maximum or near-maximum holding potential.
Engineering Insight
Published pull force is a useful comparison value, but it does not automatically represent safe load capacity in every mounting direction.
For a deeper explanation, see SSKC-001 — How Strong Are Neodymium Magnets? Pull Force Explained.
What Is Shear Force?
Shear force acts parallel to the mounting surface.
Instead of pulling the magnet away from the steel, the load tries to make the magnet slide across it.
Simple Example
Magnet mounted on a ceiling: the load tends to pull the magnet directly away from the steel.
Magnet mounted on a vertical wall: the load tends to make the magnet slide downward.
The magnet is the same, but the load direction is different.
Why Magnets Slide Before They Pull Off
When a magnet is attached to steel, the magnetic attraction pulls the two surfaces together.
That attraction creates a normal force between the magnet and the steel.
Resistance to sliding depends strongly on friction between the two surfaces.
In simplified form:
Friction force ≈ coefficient of friction × normal force
This relationship explains why surface material matters so much in vertical magnetic mounting.
A smooth metal-to-metal interface may provide relatively low friction.
A rubber-coated interface may provide much more friction, even if the coating slightly reduces direct magnetic attraction.
Engineering Insight
Magnetic attraction helps create the contact force, but friction determines how strongly the assembly resists sliding across the surface.
How Surface Material Changes Sliding Performance
| Interface | Typical Sliding Behaviour |
|---|---|
| Nickel-plated magnet / smooth steel | Relatively low friction |
| Painted steel | Depends on paint finish and surface texture |
| Powder-coated steel | Depends on coating texture and thickness |
| Rubber-coated magnet | Higher friction is often possible |
| Oily or contaminated surface | Potentially much lower friction |
Two installations using the same magnet can therefore have very different vertical holding performance.
Why Rubber-Coated Magnets Perform Differently
Rubber-coated magnets provide one of the best examples of the difference between pull force and real-world holding performance.
The rubber coating creates a small air gap between the magnetic element and the steel.
This may reduce direct magnetic pull.
However, the rubber also:
- increases friction;
- helps resist sliding;
- protects painted surfaces;
- reduces scratching;
- helps absorb vibration;
- can improve outdoor durability in suitable designs.
This creates an important trade-off.
Rubber can reduce direct magnetic pull while improving resistance to sliding.
This is why published pull force alone should not determine the best magnet for a vertical application.
For more detail, see SSKC-002 — Rubber-Coated Magnets vs Pot Magnets.
Pull Force vs Shear Force vs Peeling Force
| Load Direction | Description | Typical Risk |
|---|---|---|
| Pull | Force applied perpendicular to the steel | Magnet detaches |
| Shear | Force applied parallel to the steel | Magnet slides |
| Peeling | Load begins separating one edge first | Progressive separation |
Peeling can be especially important when mounting:
- sign panels;
- plates;
- brackets;
- fixtures;
- assemblies that project outward from the mounting surface.
Why Leverage Makes the Problem Worse
Load weight alone does not tell the complete story.
The distance between the load and the mounting surface also matters.
For example, a 10 lb sign mounted directly against a steel wall does not apply the same load to the magnets as a 10 lb sign that projects 12 inches away from the wall.
The projecting load creates leverage.
This can introduce rotation, peeling, and uneven loading across the magnetic assembly.
The weight may be the same, but the load on the magnetic attachment is not.
Engineering Insight
When a load extends away from the mounting surface, evaluate leverage and peeling forces—not just the total weight.
Air Gap + Shear Force
An air gap reduces magnetic attraction between the magnet and steel.
Lower magnetic attraction means lower normal force.
Lower normal force can also reduce the friction available to resist sliding.
This creates a direct connection between air gap and shear performance.
Paint, powder coating, adhesive, protective film, rubber, plastic, and surface irregularities can all influence the result.
For a complete explanation of this effect, see SSKC-012 — The Air Gap Effect: Why Small Gaps Dramatically Reduce Magnet Holding Force.
Steel Thickness + Shear Force
Steel thickness also influences shear performance indirectly.
If the steel is too thin, the magnetic circuit may not develop the full attraction available from the magnet.
Reduced magnetic attraction means lower contact force between the magnet and steel.
That can reduce the friction available to resist sliding.
This is why steel thickness, air gap, and shear force should be considered together.
For more information, see SSKC-011 — How Steel Thickness Affects Magnet Holding Force.
Will a Higher Magnet Grade Solve Sliding?
Not necessarily.
Upgrading from N35 to N52 may increase magnetic attraction when dimensions and geometry remain comparable.
However, sliding may still be controlled by:
- surface friction;
- air gap;
- steel thickness;
- surface contamination;
- load direction;
- leverage;
- vibration.
Before automatically selecting a higher grade, determine what is actually limiting the assembly.
| Observed Problem | First Factor to Investigate |
|---|---|
| Magnet slides vertically | Surface friction |
| Magnet feels weak overall | Air gap and steel thickness |
| Load projects away from wall | Leverage and peeling |
| Assembly moves under vibration | Dynamic load and safety factor |
| Everything else is optimized | Consider magnet size, geometry, or grade |
Real-World Applications
Signage
A magnetic sign mounted vertically may slowly slide downward even though it is difficult to pull directly away from the steel.
Industrial Equipment
Magnetic brackets on painted machinery may be affected by both reduced magnetic coupling and surface friction.
Vehicle Mounting
Thin painted steel, vibration, curvature, and motion all influence real-world holding performance.
Retail Displays
Accessories attached to vertical steel panels are often governed more by shear resistance than direct pull force.
Sensors, Cameras and Fixtures
Loads that project outward from the mounting surface can create leverage and peeling forces.
Temporary Installations and Events
Surface conditions can vary from one installation to another, making testing especially important.
How to Improve Resistance to Sliding
| Problem | Possible Improvement |
|---|---|
| Magnet slides | Increase surface friction |
| Smooth metal interface | Consider a rubber-coated magnetic assembly |
| Air gap too large | Reduce unnecessary material layers |
| Steel too thin | Improve the target steel if possible |
| Load extends outward | Reduce leverage or change mounting geometry |
| Dynamic or vibration load | Increase safety factor and test dynamically |
| Magnet too small | Increase magnetic contact area or redesign the assembly |
Engineering Recommendations
Best Practice
- Determine the actual load direction.
- Do not convert published pull force directly into vertical load capacity.
- Evaluate surface friction.
- Measure the complete air gap.
- Verify steel thickness.
- Consider leverage and peeling forces.
- Test the finished assembly in the final orientation.
- Include vibration, shock, and movement where relevant.
- Use an appropriate safety factor.
- Prototype critical installations before production.
Expert Tip
If the application is vertical, test it vertically. Horizontal pull-force tests may not reproduce the actual failure mode.
Common Mistakes
| Common Mistake | Why It Matters |
|---|---|
| Assuming 100 lb pull = 100 lb vertical load | The load direction is different |
| Ignoring friction | Sliding resistance depends heavily on the interface |
| Testing horizontally | The test may not reproduce the installed orientation |
| Ignoring leverage | Leverage can create peeling and rotation |
| Automatically specifying N52 | Higher grade may not address the root problem |
| Ignoring coatings | Coatings affect both air gap and friction |
| Using no safety factor | Real-world conditions vary |
Frequently Asked Questions
What is the difference between magnet pull force and shear force?
Pull force describes resistance to separation perpendicular to the steel surface. Shear force describes loading parallel to the surface, where the magnet tends to slide.
Why does my magnet slide down steel?
The magnetic attraction may be strong, but the available friction between the magnet and steel may not be sufficient to resist the vertical load.
How much weight can a magnet hold vertically?
There is no universal conversion from published pull force to vertical holding capacity. The result depends on friction, steel thickness, air gap, magnet geometry, surface condition, vibration, leverage, and other factors.
Does rubber increase magnet holding force?
Rubber does not necessarily increase direct magnetic pull. However, it can increase friction and improve resistance to sliding, which may improve practical performance on vertical surfaces.
Does N52 improve shear force?
It may increase magnetic attraction in some assemblies, but shear performance also depends heavily on friction, surface condition, air gap, and geometry.
How do I stop a magnet from sliding?
Possible solutions include increasing friction, reducing the air gap, improving steel thickness, reducing leverage, changing magnet geometry, or using a rubber-coated magnetic assembly.
Why is published pull force higher than my real-world holding force?
Published pull force is typically measured under ideal conditions with direct pull. Real applications may include shear loads, coatings, thin steel, air gaps, vibration, and surface contamination.
What is peeling force?
Peeling occurs when one edge of the magnetic assembly begins to separate before the rest. This can dramatically reduce the force needed for progressive detachment.
Canadian Perspective
Canadian installations may be exposed to vibration, moisture, road salt, ice, dirt, painted surfaces, and large temperature swings.
These factors can change both magnetic coupling and surface friction.
For outdoor or mobile applications, test the complete assembly under conditions that closely represent actual use.
Safety Reminder
Never assume that a magnet's published pull-force rating represents the safe working load for an object mounted vertically or overhead.
Where failure could cause injury or property damage, use an appropriate engineering safety factor, secondary retention where required, and real-world testing.
Conclusion
Published pull force tells you how strongly a magnet can resist separation under specific test conditions.
It does not automatically tell you how much weight the magnet can safely support on a vertical surface.
Real-world holding performance depends on the complete system:
- magnetic attraction;
- steel thickness;
- air gap;
- surface friction;
- load direction;
- leverage;
- vibration;
- environment.
Magnet + steel + air gap + friction + load direction + leverage + environment = actual holding performance.
Before choosing a larger or stronger magnet, first determine how the load is applied and what failure mode is most likely.
Need Help Evaluating Your Magnetic Application?
The Simple Signman team helps Canadian manufacturers, designers, distributors, sign professionals, and industrial businesses evaluate magnetic mounting systems for real-world use.
Tell us:
- what you are trying to hold;
- how much it weighs;
- the mounting orientation;
- the steel surface;
- any coatings or material layers;
- whether the load is static, vibrating, or moving.
We can help identify the right magnetic solution for the complete application—not just the published pull force.
Related Products
Related Guides
- SSKC-001 — How Strong Are Neodymium Magnets? Pull Force Explained
- SSKC-002 — Rubber-Coated Magnets vs Pot Magnets
- SSKC-005 — Neodymium Magnet Grades Explained
- SSKC-010 — How to Mount Neodymium Magnets Safely
- SSKC-011 — How Steel Thickness Affects Magnet Holding Force
- SSKC-012 — The Air Gap Effect: Why Small Gaps Dramatically Reduce Magnet Holding Force
Continue Learning
Next: SSKC-014 — Magnetic Saturation Explained: When Stronger Magnets Stop Helping
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Since 1969, Simple Signman has been Canada's leading source for flexible magnetic materials and neodymium magnets. We help manufacturers, printers, distributors, sign professionals, and industrial businesses find magnetic solutions that perform reliably in real-world applications.
Sharing Magnetic Knowledge Since 1969.