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Useful information

How to choose a magnet?

  • Ferromagnetic materials can only be clamped using magnetism. Most steels are ferromagnetic and have good magnetic properties. Wood, glass, aluminium, and brass are not magnetic.
  • A component is part of the magnetic circuit between the north and south poles of the magnet. The component must close the circuit as best as possible.
  • Magnetic chucks have various types of pole orientation; see the figures below with the correct position of the workpiece.

SQUARE CHUCKS

Location of workpieces on square chucks

Poles - square chucks

CIRCULAR CHUCKS

Location of workpieces on circular chucks

Poles - circular chucks

HOLDING FORCE

Fp is the holding force of magnetic chucks. This force is often specified as the “nominal” clamping force or the clamping force in dan/cm2 or kgf/cm2.

Holding force

Nominal holding force is based on the standard testing method:

  • Workpiece 50 x 50 x 20 mm in size
  • Soft, low-carbon steel, e.g. S235JR
  • Bottom contact surface
  • Ambient temperature 20-25 °C

JIS standard: Contact area of the test piece is 1 cm2. This leads to a very high holding force per cm2.

FACTORS AFFECTING THE HOLDING FORCE

1. Contact area and surface condition – air gap

The closer contact between the element and the clamping area, the stronger the force. Weak contact, roughness, irregularities, abrasions, dirt, etc., create an air gap, which reduces the efficiency of the magnetic field.

2. Element thickness

The maximum holding force is only reached if the element is capable of conducting all available magnetic flux from the magnet pole. If the workpiece thickness is smaller than 50 % of the pole width, the holding force will be weaker.

3. Composition and material condition

Soft, low-carbon steel like S235JR has very good magnetic properties and it is an ideal material in terms of magnetism. Alloys and heat treatment reduce the magnetic properties and the maximum possible holding force. See Table.

4. Temperature

The nominal holding force is measured at room temperature, ranging from 20 °C to 25 °C. Higher temperatures, such as 60, 80 °C or more, affect the holding force due to the increased resistance in the magnetic circuit. The reduction of the holding force for permanent magnetic chucks is approximately 3% per every temperature increase by one degree Celsius.

DIN marking DIN marking Max. content of nonmag. alloy Heat treatment Magnetic Force
Pure iron - 0,00% Moderate 105%
Structural steel
St37-2 1,0037 - Moderate 100%
St52-3 N 1,0570 - Moderate 98%
St50-2 1,0050 - Moderate 79%
Hardened steel
C10 1,0301 1,22% Moderate 98%
Hardened steel 50%
C15 1,0501 1,27% Moderate 98%
Hardened steel 50%
17CrNiMo6 1,87 5,43% Moderate 76%
Hardened steel 40%
16MnCr5 1,31 3,06% Moderate 87%
Hardened steel 45%
Moderate 86%
Hardened steel 44%
Nitrided steel
34CrAl6 1,04 4,29% Untreated 81%
Nitrided 53%
31CrMoV9 1,19 4,65% Untreated 80%
Nitrided 51%
34CrAlNi7 1,50 5,93% Untreated 74%
Nitrided 48%
39CrMoV13-9 1,23 6.44% Untreated 71%
Nitrided 46%
Free-cutting steel
15S10 1,0710 1,77% Untreated 95%
9SMn28 1,0715 1,92% Untreated 94%
45S20 1,0727 2,21% Untreated 93%
60Pb20 1,0758 2,71% Untreated 89%
Heat-treated steel
C22 1,0402 2,96% Moderate 88%
Tempered 51%
C45 1,0503 3,20% Moderate 85%
Tempered 50%
Ck45 1,1191 3,50% Moderate 85%
Tempered 50%
C60 1,0601 3,57% Moderate 85%
Tempered 49%
Ck60 1,1221 3,65% Moderate 84%
Tempered 49%
43CrMo4 1,63 3,62% Moderated 84%
Tempered 49%
36CrNiMo4 1,11 4,37% Moderate 81%
Tempered 47%
Roller bearing steel
100Cr6 1,01 3,11% Moderate 87%
Hardened steel 45%
100CrMn6 1,20 5,26% Moderate 77%
Hardened steel 40%
C102CrMo17 1,43 22,72% Moderate 27%
Hardened steel 14%
X8WMoCrV6-5-4 1,53 11,40% Moderate 46%
Hardened steel 25%
Spring steel
Ck67 1,1231 2,04% Moderate 93%
Hardened steel 48%
60SiMn5 1,42 3,15% Moderate 87%
Hardened steel 45%
51MnV7 1,25 2,87% Moderate 88%
Hardened steel 46%
Cold extrusion steel
Cp15 1,1132 1,10% Moderate 99%
41Cr4 1,35 3,55% Moderate 85%

OPTIMISATION OF THE HOLDING FORCE

  • Air gap reduction: Remove burrs, lumps, rust, and scale. If necessary, clean the element. If the element comes loose, use magnetic adapters.
  • If the workpiece has small dimensions, extend or combine the magnetic circuit by creating groups of small elements, thereby allowing the magnetic flux to rest against a larger area.
  • For large elements, use extensions to avoid overturning or tilting during machining.
  • Use stop bars to secure the workpiece against displacement.
  • Use pole extensions for components with irregular shapes.