Permanent magnets :
17 products
ALNICO MAGNETS
For over 50 years an acknowledged leader in the field of permanent magnets, Arnold continues its tradition of superior quality and value with the full line of Alnico products.
One of the oldest
members of our magnet family, Alnico remains the workhorse of the permanent magnet industry and can be relied upon to deliver impressive flux density at an economical price. Alnico also boasts the lowest temperature coefficient of any commercial magnet material (.02% per degree centigrade) allowing for excellent stability over a wide temperature range.
Alnico has experienced renewed interest in recent years in such temperature-sensitive applications as hall-effect and MR-based automotive and electronic sensors, in addition to the traditional magnetron, TWT amplifiers, actuators, motors and instruments applications that have long relied upon Alnico.
DESIGN CONSIDERATIONS
Because Alnico magnets are coarse-grained, hard and brittle, they cannot be drilled or conventionally machined. Given these mechanical properties, Alnico parts are not normally used as structural members. Where a choice exists, select simple shapes. Slots are preferred over holes. Cross sections of less than .125" (3.18 mm) should be avoided. Finished surfaces, when required, may be produced by grinding; however, for many applications, the "as cast" or "as sintered" surface yields a satisfactory result at a significantly lower cost. To facilitate mounting and give adequate protection to the magnet, special assemblies such as rotors and magnetron magnets may be supplied with an aluminum jacket. Additionally, low carbon steel pole pieces may be attached to magnet pole faces by adhesive bonding or with studs or bolts. Plastisol or paint can be applied for improved appearance.
MAGNETIZATION
The most efficient use of Alnico requires that it be magnetized after the magnet has been assembled with its pole pieces into the final magnetic circuit. (Shipping the product non-magnetized also eliminates the need for special packaging, and prevents contamination by stray iron-chips, etc.) Magnetic saturation at the end-use location requires the application of a magnetizing force 4 to 5 times greater than the coercive force of the material. For Alnico 5, a magnetizing force of 3000 oersteds (240 KA/m) is recommended. For Alnico 8, the magnetizing force should be at least 7000 oersteds (560 KA/m). The magnetizing force need be applied only momentarily. Thus, impulse magnetizers employing a capacitor discharge are commonly used. Direct current magnetizers are also effective.
STABILITY AND TEMPERATURE EFFECTS
Alnico magnets offer excellent stability with respect to temperature changes: reversible change is 0.02% per degree Centigrade. Heating may produce an irreversible loss of magnet strength. The magnitude of loss depends upon the dimensions of the magnet and its composition, but is usually less than 5%, and may be recovered by remagnetization. At temperatures exceeding 1000oF (538oC), a metallurgical change takes place which causes magnetic strength to reduce rapidly. This change will not be recovered by remagnetization.
External magnetic fields can also induce partial demagnetization. In some critical applications, it is desirable to stabilize the magnet by intentionally reducing magnetic output by 5% to 10%. Such stabilization may reduce, or even eliminate, the effect of stray external fields.
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Arnold Magnetic Technologies Groupe - SPS
HARD FERRITE MAGNETS
Hard ferrite (ceramic) magnets were developed in the 1960's as a low cost alternative to metallic magnets. Even though they exhibit low energy (compared with other permanent magnet materials) and
are relatively brittle and hard, ferrite magnets have won wide acceptance due to their good resistance to demagnetization, excellent corrosion resistance and low price per pound. In fact, measured by weight, ferrite represents more than 75 percent of the world magnet consumption. It is the first choice for most types of DC motors, magnetic separators, magnetic resonance imaging and automotive sensors.
To better serve our customers of hard ferrite magnets, Arnold sources material from qualified manufacturers throughout the world.
GENERAL CHARACTERISTICS
Hard ferrite magnets are manufactured to rigid magnetic and physical standards which normally exceed Magnetic Materials Producers Association (MMPA) standards.
The chemical composition is SrO-6(Fe2O3), strontium hexaferrite. The raw materials used to produce ferrite magnets are strontium carbonate and iron oxide both of which are readily available and low in cost. As a result, the use of ferrite magnets in most applications is more economical than other materials.
Ferrite magnets are formed by compaction in dedicated, multi-cavity dies followed by sintering in high temperature furnaces. This produces a hard, brittle part that requires diamond wheels for grinding to close tolerances. While physically quite strong, these magnets should not be considered a structural member in an assembly. And like most ceramics, they are brittle and should be handled so as to avoid chipping and cracking.
TEMPERATURE EFFECTS
Temperature variation can result in both reversible and irreversible changes in magnetization. A reversible change occurs at the rate of approximately -0.2% per degree centigrade. That is, as temperature rises above ambient, induction (Br) will decrease. Coercivity, a measure of resistance to demagnetization, changes at a rate of about 0.27% per degree centigrade. As temperature rises, a ferrite magnet will increase in coercivity!
Irreversible changes can result from exposure to very low temperatures, and the magnetic quality is restored only by remagnetization. Irreversible changes can be avoided by providing an adequate permeance coefficient.
APPLICATIONS
Hard ferrite must be magnetized in the direction of orientation, which is the same as the direction of pressing.
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Arnold Magnetic Technologies Groupe - SPS
