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) especially 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 œ 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 a 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;


Grinded Finish : Best condition for
magnetic lifting

Milled Finish : Good condition for
magnetic lifting.

Painted Surface : Acceptable to
lift magnetically
 
Profiled surface : 1mm profiles do
not pose problems but if the profiles
cause uneven contact surface then
lifting should be avoided.
 
Scaly/Rusty Surface : Danger
The scales or rust introduce unwanted
air gaps which must be removed

Rough Surface : Careful handling
required including some prior test
lifts with magnetic equipment.