Measuring Residual Magnetism of Ferromagnetic Parts
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Maurer Magnetic AG – White paper Measuring Residual Magnetism of Ferromagnetic Parts Dipl. Ing. Marek Rohner Head of Technology Maurer Magnetic AG 8627 Grüningen Switzerland

Measurement of residual magnetism Maurer Magnetic AG, your specialist for: • Industrial demagnetizing devices and systems • Instruments for measuring magnetic fields • Degaussing services • Troubleshooting in residual magnetism • Permanent magnets and magnetic systems

Measurement of residual magnetism Abstract Modern manufacturing processes use increasingly complex and sensitive manufacturing processes, which are dependent of a very high quality of the intermediate products. Manufacturing processes, e.g. coating, welding, shaping or stamping processes depend on a minimum possible residual magnetism on materials, parts, tool, and machinery. The assembly of modern internal combustion engines, precise ball bearings, gear parts, electronic components, and medical injection systems, affords a strictly clean manufacturing environment. To meet the more and more...

Measurement of residual magnetism Configuration and strength of magnetic fields The density of field lines decreases disproportionately with increasing distance from the magnetic pole. The rate of this decrease depends essentially on the size and configuration of these poles. Between these poles, we find magnetically neutral zones acting as pole separations. In the most simple case, two opposite poles may be theoretically treated as a dipole. The field strength H of a magnetic dipole decreases with the distance r according to the law H~1/r3. The field strength close to the surface of a part...

Measurement of residual magnetism Residual magnetism of ferromagnetic parts The residual magnetism of a part is largely dependent on the previous production and handling processes. The first magnetic influence of the part is already happening in the steel mill. Under the influence of the magnetic field of earth, annealing, rolling, and transport processes may magnetize the steel with great depth up to values of 10...40A/cm on the surface. After these processes, the residual magnetism of the crude steel tends to a dipole magnetization. The same kind of dipole magnetization is created by...

Measurement of residual magnetism Field strength at increasing distance from the part surface Inappropriate probe: - Distance of the Hall effect zone to the part surface unknown - Probe with flux collector - Inaccurate positioning Suitable probe: - distance of the Hall effect zone to the part surface ~0.5mm - Probe without flux collector - Accurate positioning Comparison between both probes: Residual magnetism measurement at increasing distance from the part surface. The stray field of the part corresponds to a dipole field. Dipole field Comparison chart dipole field appropriate probe...

Measurement of residual magnetism Field strength at increasing distance from the part surface On parts with wide range stray field (dipole magnetization), probes of different design show little difference in readout of residual magnetism. Small pole fields are detected and correctly measured only with probes of which the Hall effect zone is closer to the tip than 1 mm. Hall probes equipped with a flux collector on the Hall element smoothen and suppress the stray fields of fine pole residual magnetism, which leads to erroneous readings. The use of magnetic field measuring instruments with...

Measurement of residual magnetism Induced magnetic fields The magnetic field of earth has an average value of ~ 0.03 ... 0.06mT. The direction of the field lines running in the open air is substantially parallel to the N-S axis, with an inclination of about 45° to the Earth's surface. Inside buildings, the magnetic field of earth is further distorted in direction and strength by surrounding ferromagnetic structures, e.g. beams of steel or reinforced concrete. Ferromagnetic materials collect and conduct magnetic fields due to their high permeability. A ferromagnetic part attracts the lines...

Measurement of residual magnetism Measurement inside magnetic field shielding Residual magnetism measurement is not affected anymore by induced magnetic fields when the measurement is conducted inside a magnetically shielded environment. The effect of the magnetic field of earth can be avoided in two different ways. The passive shielding chamber consists of walls made of highly permeable material. The magnetic field of earth is diverted into the wall of the shield, and the influence of the magnetic field inside the chamber decreases approximately by a factor of 5…10. This is sufficient for...

Measurement of residual magnetism Typical values of induced magnetic fields -Hall probe attached on contact at the end of the part -Magnetic field of earth ~0.05mT Part Magnetic field of earth shielded; Orientation of the part inessential Part oriented along the magnetic field of earth Part perpendicular to the magnetic field of earth Maximum induced field Ball bearing steel Cylinder roll LxD = 40x50mm L/D = 0.8 Free cutting steel Rod LxD = 1500x15mm L/D = 100 The influence of the measuring method The detection of the technically critical maximum value of residual magnetism on ferromagnetic...

Measurement of residual magnetism Searching for maximum values on surfaces with fine pole magnetism Because of the limited range of fine pole stray fields, the interesting surfaces of the part must be scanned with the probe extensively to detect the maximum values. For this task, a "peak hold“ function on the meter is recommended, which automatically retains the highest reading. The operator can concentrate on the accurate guidance of the probe on the part during the measurement. This ensures that maximum values can be reliably detected even for fine multi-pole residual magnetism. The...