Page 291 - 世霸SuperToolVol.6

Basic knowledge about magnets

[Adsorption force] ■ Magnet types and properties
Samarium- Neodymium
Alnico Ferrite
The force needed to separate a magnet from the piece it cobalt magnet magnet
magnet magnet
is attached to is called the adsorption force. (rare earth) (rare earth)
Residual magnetic
The unit used to describe the adsorption force is the 11.5 4.4 11 13
flux density Br (kG)
kilogram-force (kgf).
Coercivity HCB 1.6 3.2 6.8 10
(kOe)
[Attraction force] Coercivity HCJ 1.6 3.3 7 11 APPENDIX
(kOe)
When a piece is attracted to a magnet placed at
Maximum energy
distance, the applied force is called the attraction product (MGOe) 11 4.6 30 40
force. If the attraction force is strong, it doesn’t mean (BH) max.
necessarily that the adsorption force is strong too.
Temperature
The unit used to describe the attraction force is the coefficient Br (%/℃) –0.02 –0.18 –0.03 –0.12
millimeter (mm).
Curie point ℃ 850 460 800 310
[Residual magnetic flux density] Specific gravity 7.3 5 8.4 7.5
Also called remanence. The higher this value is, the
Table for
stronger is the magnetic force. However, for applied ● Alnico magnet width across
equipment, the residual magnetic flux density cannot be This magnetic material has a high magnetic flux density. Its flats and
magnetic properties are little dependent to temperature change bolt / nut
considered as strictly equal to the performance. screw size
and it doesn’t get demagnetized easily on low temperature or
when heated. Its hardness is strong (very hard to cut or to drill)
and it is a highly stable magnet material. Alnico magnet has been Conversion
[Coercivity] a major material for a long time, but sourcing issues of cobalt and table for
nickel, its main components, as well as unstable costs led to its inch / mm
The higher this value is, the less magnetic force is replacement by less expensive ferrite magnet. But with its high
lost. The lower it is, the more easily the magnet gets magnetic resistance to temperature change, there is still a strong Table for hex
demagnetized. demand mainly for measuring instruments. key wrench /
hex lobe key
wrench
compliant
● Ferrite magnet screw
[Maximum energy product] This magnet material is currently used in every kind of fields. Its main
component is a metallurgical powder obtained from iron oxide and Reference
The maximum energy product (BH) is the product of the table for
while its residual magnetic flux density is rather low, it possesses the tightening
residual magnetic flux density (Br) and the coercivity (Hc). big advantage of having a high coercivity. There are two types of torque value
The higher Br, Hc and BH max are, the better and more ferrite, isotropic and anisotropic. Compared to isotropic ferrite, the
stable is the magnet. magnetic force of anisotropic ferrite is approximatively twice stronger.
The low raw material costs allow relatively affordable product Conversion
table for
prices. However, as a metallurgic product, it is extremely fragile international
and processing like cutting or drilling are very difficult. The magnet torque value
[Curie point] properties variability under temperature change is relatively highy.
The Curie point is the temperature at which the residual Chart for
pipe size
magnetic flux density of a magnet gets to 0. ● Samarium-cobalt magnet (rare earth) and diameter
For ferrite magnet, the Curie point is at approximately This material is composed of two series. The series 1:5 has an per type
excellent magnetic anisotropy and a high coercivity, and the series
460℃.
2:17 has a high magnetic energy. Compared to neodymium magnet, Lifting angle
the temperature coefficient of residual magnetic flux density and and rated
coercivity is lower for both series, which have excellent stability load
correlation
[Magnetic flux density] against heat. Their corrosion resistance is also better than table
neodymium, so they can be used without any surface treatment like
The magnetic flux density is the amount of magnetic flux nickel plating applied for neodymium. The high cost of samarium
Table for
through a unit area. It is not directly visible to the eye, but and cobalt raw materials makes this magnet own cost relatively main steel
for instance when a sheet of paper is placed on top of high, and it also has low mechanical strength, so it breaks easily material
and it is hard to handle. weight
a magnet and some iron powder is sprinkled over it, the
shape of the iron powder distribution on the sheet can be
seen. The unit of the magnetic flux is called Wb (Weber). ● Neodymium magnet (rare earth) Comparison
table for
The magnetic flux density is described as the amount of This material has high magnetic properties, and its relatively hardness
abundant main components, neodymium and iron make its cost
magnetic flux through a cross-sectional area of 1m2. A
relatively lower than samarium-cobalt. Then, its specific gravity
magnetic flux density of 1W/m2 is called 1T (Tesla).
being more than 10% lower than samarium-cobalt, it is really
［Originally, the unit G (Gauss) was commonly used, but it considered as a magnet with high energy density. Moreover, its Table for
changed to T (Tesla) under the SI system. (1T=10,000G, high mechanical strength makes it hard to break or crack and bearing size
standard
1mT=10G)］ easy to handle. On the other hand, as its temperature coefficient
of residual magnetic flux density and coercivity is higher than
samarium-cobalt, attention must be paid to the temperature of use
and the working points. The variability of magnetic flux amount Basic
knowledge
due to the environment temperature for neodymium is lower than about
for ferrite, but relatively high compared to samarium-cobalt, and magnets
as it gets largely demagnetized especially under high temperature
environment, special attention is required.
Additionally, its corrosion resistance is low so it rusts easily, and for
application under high temperature and high humidity conditions,
surface treatment is required.
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