Effect of Material Surface on Permanent Magnetic clamping
There is still quite a bit of misconception in the community on what constitutes ideal conditions for lifting magnet clamping. Generally, when it comes to magnetic lifting of any nature, a minimum adherence to safety standards such ASME B30.20-2003 and minimum other precautions are usually followed. However, due to an inconsistent standard of understanding of how external factors can influence or depreciate magnetic clamping power, it is never too much to overemphasize certain factors to watch out for; over and over again - especially as humans are usually exposed to what may be potentially risky work.
Today lifting magnet equipment manufacturers build equipment with an inherent safety factor of 3x (three times) or 4x (four times) specially to cater to unforeseen real-life conditions that may temporarily depreciate magnetic holding power quite significantly.
One such real life field condition is the surface quality or condition of the steel material. The magnetic holding power or pull force (magnetic flux density of a simple dipole) is governed by
F ∝1/d3
Or the “Inverse Cube Law”.
When there is no space at the contact surface between the magnet and steel material, the magnet can realize its full holding power. However, as the distance between the magnet and steel increases, the magnetic holding force depreciates exponentially to the inverse cube of the distance. What this means is that at 2 times the unit of distance, the holding force will decline to 1/8th of its value and at 3 times the unit of distance, the holding force will decline to 1/27th of its value. Of course, in real world magnetic lifting applications, the flux densities and field shapes and orientations are far more complex than those of the simple dipole and its "Inverse Cube" law effect above but essentially the principle is still valid and applicable. The effective unit air gap distances of 2 to 5mm and above can cause significant loss of magnetic holding power and therefore both the magnet pole face and the steel load material must be as reasonably smooth and clean as possible to ensure even contact.
Therefore, it may well be imagined that if the permanent lifting magnet equipment is being made to lift an unusually rough surface, the activity is fraught with danger and should be stopped immediately, until a safety test is carried out. Some simple guides are presented below.
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| Grinded Finish: Best Condition for Magnetic Lifting |
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| Milled Finish: Good Condition for Magnetic Lifting. |
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| Painted Surface: Acceptable to Lift Magnetically |
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| Profiled surface: If the profiles cause uneven contact, lifting should be avoided |
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| Scaly/Rusty Surface: Danger! The scales or rust introduce unwanted air gaps which must be removed |
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| Rough Surface: Careful handling required including some prior test lifts with magnetic equipment. |
