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Impact of Air Gap on Lifting Capacity of Lifting Magnets

Posted on June 20, 2016

The lifting capacity of any permanent lifting magnets decreases with increases in air gap, but the decreasing rate can vary among different brand products. Allow us to explain the secret to you.

First, let's take a look at two brands of permanent lifting magnets and compare their decreasing rates of lifting capacity with increasesing air gap.

1) NL-2200B (Rated Lifting Capacity of 2200 lb) Permanent Lifting Magnets

Armstrong's NL-2200B lifting magnets
The Lifting Capacity of NL-2200B Derated by Air Gap
0 1/128" 1/64" 1/32" 1/16"
100% 81% 73% 54% 35%
2) Brand X (Rated Lifting Capacity of 2200 lb) Permanent Lifting Magnets of this shape, similar to our discontinued products - NL lifting magnets.

BrandX lifting magnets BrandX lifting magnets
When you compare two charts above carefully, you can find that the lifting capacities of NL-2200B and Brand X are almost the same when the air gap is less than 1/64”(0.4mm). However, the decreases in NL-2200B lifting capacity would be less with increases in air gap than Brand X when the air gap becomes greater than 1/64”. In conclusion, NL-2200B is better for a load with dirt or uneven surface.

Why?

The secret is that the magnets installed in NL-2200B are thicker than that installed in Brand X!

Please look at the following sketch of Neodymium magnet demagnetization curve.
Neodymium magnet demagnetization Curve
When a load to be lifted is clean and flat, the air gap between the permanent lifting magnets and the load is almost nil. Both NL-2200B and Brand X would have same magnetic flux density B1 at their poles which contact the load and therefore have equal attractive force.

If the load is not clean or not flat there should be an air gap, 1/32” long for example, between the load and the permanent lifting magnet. In such case, NL-2200B could have larger magnetic flux density B2 in the air gap than B3 of Brand X and consequently NL-2200B produces greater attractive force to the load because thicker magnets has greater magnetomotive force and has its work point higher than that of thinner magnet at the demagnetization curve.

Should there be any other factors which influence the decreasing rate of the lifting capacity with increases in air gap except the thickness of the Neodymium magnets (assuming they are the same magnetic grade)? Of course yes. But the thickness of the Neodymium magnets is the key reason.