What is Air Gap in Magnetics?
In magnetics, “air gap” refers to the gap between the contacting surface of a magnet or magnetic assembly and the load surface. The gap in between is non-magnetic. Examples include air, plastic, painting, wood, etc.
Besides being used in electric motor, permanent magnets are massively used in holding applications. In many cases, however, magnets are fitted into steel housing. Why is that? For one thing, the steel housing can protect the brittle magnets from chipping and cracking. For another, the steel housing is ferromagnetic, which redirects the magnetic flux line. This will significantly increase the flux density on the magnet surface and increase the holding strength. Pot magnet is one of the most popular magnetic assemblies in holding applications.
As illustrated in the chart below. The holding or pull force of the disc neodymium magnet with steel housing is multiple times stronger than it alone. As mentioned before, however, the magnetic field line is redirected to travel through the steel housing, the range of influence of the magnetic flux becomes way smaller. This also causes the holding force of a pot magnet decay so sharply (red line) with the increase of air gap.
Factors that increase Air Gap
Ideally, if a contacting surface is flat and clean, air gap between a pot magnet and contacting surface is not considered. In reality, any dust, scraps, paint, rust, and not-so-flat surface can create the air gap that takes a toll on holding strength.
These real-life factors also explain why the claimed holding force of a lot of magnet manufacturers and distributors is far beyond above its practical holding force value. When you decide to use pot magnets or other similar magnetic assemblies for your holding purpose, it’s important to consider the air gap factors.
