HANGZHOU TENGYE MAGNETIC MATERIALS CO., LTD.
In magnetics, “air gap” refers to the room between the contacting surface of a magnet or magnetic assembly and the load surface. The room in between should be non-magnetic, such as air, painting, wood, plastic, etc.
Besides being used in electric motor, permanent magnets are massively used in holding applications. However, in many cases, especially in industrial scenarios, stand-alone magnets are rarely seen. Instead, they are oft fitted in a 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 a ferromagnetic material and it redirects the magnetic flux line. This will significantly increase the magnetic strength and thus holding force of the magnet. Pot magnet design is the most popular magnetic assembly in holding applications.
As illustrated in the chart below. The holding 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.
Ideally, if a contacting surface is flat and clean, the air gap between the pot magnet and the contacting surface is almost zero. Any dust, scraps, paint, rust, and not-so-flat surface can create the air gap that takes a toll on holding force.
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.
About Tengye: Tengye supplies finest permanent magnet materials and magnetic assemblies to world industry. We work closely with our customer from designing stage to mass production.