Three Concepts Routinely Treated as One
Perpendicular Pull Force
The force required to pull the magnetic head directly away from a flat steel surface, measured under controlled laboratory conditions: thick, clean, flat steel plate, force applied perpendicular to the magnetic face. This is the number on the specification sheet. It is the correct international method for rating and comparing permanent magnets — for that purpose.
Lateral Shear Force
The resistance the magnetic interface offers to force applied sideways — across the face of the magnet, not straight away from it. Mechanically different from perpendicular pull force. Depends on friction between the magnetic face and the steel surface, which varies with surface texture, coating, rust, and contact area.
Guidance Force
The practical, usable force available to an operator during real guidance work. During load positioning, the operator is not pulling the magnetic head straight away from the surface — they are pushing, pulling, and nudging the load sideways. Guidance force is overwhelmingly a function of lateral shear, not perpendicular pull.
What Field Conditions Do to Magnetic Performance
| Surface Condition | Effect on Attachment Quality |
|---|---|
| Clean, bare steel | Best. Closest to the laboratory rating. |
| Light paint or primer | Reduced. A thin coating creates a small but measurable air gap. |
| Thick coating, mill scale, or light rust | Significant reduction. Contact area and coupling both affected. |
| Heavy corrosion, curvature, oil, or debris | Field verification mandatory before relying on the tool. |
Why Higher Rated Force Is Not Always Better
If the magnetic interface resisted its full perpendicular pull rating during every lateral guidance movement, the operator would be working against excessive resistance with every small correction — making fine positioning slow, tiring, and impractical. The engineering objective is not maximum holding force. It is force calibrated to the tool's intended use: strong enough to maintain positive directional control, workable enough that frequent small adjustments remain comfortable throughout a shift.
This is why PSC's telescopic fibreglass pole uses a 60 mm, 275 lb head rather than the 75 mm, 550 lb head used on fixed-length tools — the lower rating is deliberately matched to the greater lever arm at full extension.
"How does the worker need to control the component?" — not "How strong is the magnet?"
What to Select On Instead
Start with the task, not the specification. Before consulting a pull-force rating, establish: the actual surface condition of the work surface; the standoff distance the task requires; whether the operator's body position changes during the task; and what type of control is required — sustained or momentary. The pull-force rating is relevant only after those questions are answered — and even then, it is a laboratory baseline that field conditions will reduce, not a guarantee of available guidance force.
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