Do magnets knock down the instant they are repelled?
Magnetic flux lines can not cross each other, so magnets in repulsion develop radial vectors whose intensity increases as the magnets approach each other. To the extent that the amplitude of the radial component of flux density exceeds coercivity of magnetic domains, there will be changes to the properties of the magnet. These changes are due to reorientation of these domains.
Materials such as Nd-Fe-B, Sm-Co, Ceramic, and bonded Nd-Fe-B will typically see very small permanent changes, if any. This will be noticeable as a change in the external flux density, and the total flux available to the magnetic circuit. Magnets with a ‘knee’ in their second quadrant normal curve, such as Alnico 5 or true Ceramic 5 can see experience significant demagnetization.
What amount of energy is required to magnetize each of the magnet materials?
To fully saturate a magnet, it must be exposed to a magnetizing field of sufficient amplitude for a time long enough to orient all of the mass of magnet.
Alnico requires 3000+ Oersteds – longer pulse times may be needed to overcome eddy currents in large sections. Ceramic requires 10,000+ Oersteds. Sm-Co typically requires 20,000+ Oersteds but may require over 40,000+ Oersteds on some grades. Nd-Fe-B typically requires 30,000+ Oersteds but may require over 40,000+ Oersteds on some grades. Bonded Nd-Fe-B or NeoForm requires 35,000+ Oersteds.
What are the limits of pole density on a ring magnet? For example, on a ring, 1.0″ OD by 0.5″ ID, what is the maximum number of poles?
The pole density on any magnet is limited by the energy needed to magnetize it, its anisotropy, and its geometry. If a magnet is anisotropic, it needs to be radially oriented to accept the magnetization pattern. If it is isotropic, it can be oriented in almost any configuration.
Ceramic magnets are one of the easiest materials to magnetize in a multipole fashion. Rings of 1.0″ diameter can have upwards of 100 poles, but usually require less than 24. It is important to remember that as the pole density increases, the depth of saturation and reach of the external field decreases. Consequently, the wall thickness between the OD and ID of the magnet should decrease to conserve material costs. Further, as the pole density increases, so will the energy requirements. As a direct result, materials such as bonded and fully dense Nd-Fe-B, requiring higher magnetizing forces, are usually more limited than Ceramics on available pole densities.
Specialty magnetizers and fixtures have been designed which are capable of magnetizing a 1.0″ diameter fully dense NdFeB magnet with over 240 poles. Unfortunately, the magnetization process is very laborious and time consuming, making high volumes of such products impractical. Further, as stated above, such high pole densities limit the reach of the magnetic field. Without extremely tight tolerances and concentricities, standard sensing equipment may not be able to detect a field change in such a magnet.
What is the magnetizing process?
High energy materials are generally magnetized by discharging a bank of capacitors into an air core solenoid surrounding the part, or a stack of parts. The current pulse supplies energy to overcome the self demagnetizing effect, due to part geometry, as well the energy required to align magnetic domains. One way to reduce the self demagnetizing effect is to increase the permeance coefficient (load line) of the mass being magnetized. This is easily done by stacking parts to create a longer magnetic length. Limiting factors are the potential for personnel injury and physical damage to parts when separating the magnetized stack of magnets.
In the magnetizing process, a magnetic field develops around the solenoid windings first, and then expands and decays with time. To be fully effective, the pulse reaching the center of the magnet must have sufficient amplitude to align domains there. When magnetizing parts with low resistivity and/or a large area normal to the direction of orientation, eddy currents slow propagation of the magnetizing field into the part, so pulse width is an important consideration.
Pulse width is determined by the reactance of the magnetizing system, including the capacitor bank, and the resistance and inductance of the solenoid. A wide pulse insures that all domains are exposed to an adequate magnetizing field strength. However, a pulse wider than necessary results in losses due to heat, so production magnetizing systems must be well planned.
What is the proper way to remove a magnet when they are shipped in rows (slugs for example)?
Each row should first be separated from the adjacent row. To do this, the row should be pulled away directly, (do not slide one row down another row). In order to isolate individual magnets, the magnet should be pulled directly away from the next magnet. Do not slide the magnet in any way. It may be necessary to “break” the magnet stack over your finger. This is acceptable as long as the faces are not allowed to slide across each other.
These rules are good for all magnets, though they are crucial for handling of Alnico magnets. Sliding Alnico magnets over each other will likely result in skewing of the poles.
Why are my magnets reading low magnetically?
Since most magnetic measurements are performed with a gaussmeter, it is usually necessary to first question measurement techniques. Readings acquired with gaussmeters and probes are prone to repeatability issues if a test fixture is not manufactured and a probe is not fully dedicated to testing a single product. Gauss probes are subject to manufacturing limitations, which can lead to anti-symmetry in the probe itself. If not fixed in position, variations from one side of the probe to the next can materialize as low field readings in a magnetic sample. It is also necessary to assure that no magnetic materials are in the vicinity of the testing. If a piece of steel (or a steel table) is near the test apparatus, some flux may be shunted from the magnet leading to an anomaly.
If the measurement technique is sound, material limitations must be considered. Alnico magnets are notorious for producing magnetic irregularities. Their low coercive forces allow them to easily demagnetize. Simply placing two Alnico magnets in repulsion (N-N or S-S) is often enough to reduce an Alnico magnet’s performance. Consequently, when handling these magnets, it is important to limit their exposure to demagnetizing fields. Failure to do so may produce magnetically low readings.
Today’s advanced materials have higher coercive forces than Alnico and are not prone to the same performance degradation. If these materials (Ceramic, Sm-Co, Nd-Fe-B) are reading low, then the material may not have been fully saturated. This rarely occurs, but is usually caused by the degradation of a magnetizing fixture over time. Since there are no physical signs of degradation in magnetizing fixtures until catastrophic failure, higher coercive force magnets may be shipped unsaturated.
Why is it so difficult to develop multi-pole OD/ID magnetizing patterns in high energy materials?
Multipole magnetization on a continuous surface is achieved by injecting a high intensity magnetic field into the surface, as there are no salient poles to work with. While the magnetizing vectors may be parallel to the orientation direction in the polar areas, all flux must transition between poles beneath the magnet surface. Here, the magnetizing vector is normal to orientation, the resistance to magnetization is about double, and induction is less.
Because it is not possible to surround individual poles with conductors, a steel fixture with wound polar extensions must be used to direct the magnetizing field to the part. This places the magnetizing windings in a less favorable, more distant position, and drives the steel fixture well into saturation were it creates losses that must be overcome with additional energy input. The magnetizing field in the fixture must also transition between adjacent poles. This takes place in an unwound section of the steel fixture, where flux loss to leakage is unrestricted, and these losses must also be compensated for with more energy input.
The space available between poles limits the conductor size and number of turns in the coil so, although the energy required is greater, the coil design inherently delivers less, and the coil heats up more quickly. Heating reduces the mechanical strength of the conductor while the intense magnetic field exerts great tensile stress in the conductor, so at some point the conductor will be torn apart. These considerations make multi-pole ID fixtures even more difficult to make than multi-pole OD fixtures.
Magnetizing individual high energy magnets within a solenoid requires a massive energy pulse; multipole magnetization is akin to attempting to saturate a magnet with the reduced axial field density on axis outside of the coil.