Arnold Magnetic Technologies Groupe - SPS

United Kingdom

See product on arnoldmagnetics.com

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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