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Permanent Magnets
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 October 2006 Feature
Reinhold M.W. Strnat is the secretary of Committee A06 on Magnetic Properties and the chairman of Subcommittee A06.01 on Test Methods. He is the technical manager of KJS Associates, Inc. and the quality manager of the Magnetic Instrumentation, Inc. Calibration Laboratory, providers of magnetic testing and calibration services, where he has specialized in magnetic testing technology.
Committee A06 to Meet in Atlanta

The fall 2006 meeting of Committee A06 on Magnetics will occur Nov. 13-14 at the Hyatt Regency Atlanta, Ga.

For more information contact Nancy Morrissey, ASTM International (phone: 610/832-9736; nmorriss@astm.org).

Permanent Magnets

Magnets have applications in an incredible variety of products, ranging from toys and audio loudspeakers to life- and mission-critical aerospace and military devices. Computer disk drives, magnetic separation and holding machinery, ear buds for portable audio players, magnetic bearings, electric motors and generators, variable reluctance sensors and other electromagnetic transducers, and many medical devices are possible thanks only to permanent magnets. Even electric guitar pickups rely on magnets.

Until about a decade ago, the manufacture of permanent magnets was largely distributed among Brazil, Europe, Japan and the United States; now the bulk of magnet manufacture has migrated to China. Permanent magnet standardization was somewhat problematic. Today, the proliferation of Asian magnet suppliers has rapidly expanded the array of magnet offerings, which frequently do not fit neatly into previously established categories.

The Permanent Magnet, a Complicated Beast

By their nature, permanent magnets can exhibit an almost infinite combination of flux density (B) vs. applied field (H) characteristics. Each product design requires that one or more key magnetic properties falls within specified limits; sometimes a property must simply meet or exceed a minimum value, but the requirement could specify a maximum or an acceptable range. Typical manufacturing tolerances allow for about +/-5 percent variation on any one magnetic parameter. To further complicate matters, the operating performance of a device may depend on two or more of the key magnetic properties falling within specified limits. This is often the case in electric motors, where the magnetic flux density determines the torque and speed characteristics, while the resistance to demagnetization must be adequate over the operating temperature range to assure that, under adverse conditions, armature reaction does not permanently demagnetize the magnets.

The control of multiple properties simultaneously to close tolerances can be quite difficult and expensive. It often involves the need to measure, sort and select a subset of magnets from a larger production run. In some applications, such as linear transducers, the entire continuum of B vs. H values within an operating region of the magnet may be critical to proper device performance. In addition, all magnetic properties vary as a function of temperature, with short-term and long-term exposure having different effects.

The list of magnetic characteristics of which both producer and user must be aware includes: Br (remanence), Hc (coercive field strength), Hci (intrinsic coercive field strength), BHmax (maximum energy product), recoil permeability, and reversible temperature coefficients of magnetic induction and coercive field strength.

(See sidebar, "How Strong Is This Magnet?" here.)

Committee A06 and Magnetically Soft Materials

Until recently, the test method and material specification documents prepared by ASTM Committee A06 on Magnetic Properties focused on magnetically soft materials. Through the late 1950s, the available permanent magnets were mostly steel-based, with the exception of the very expensive platinum cobalt magnets, which were used primarily by the military. With the advent of sintered ceramic ferrite, and later the rare earth-based high-energy magnets, the landscape of the magnetic materials world was drastically altered.

Although the physics is the same, the specific procedures and equipment required for testing permanent magnets are quite different from those for magnetically soft (low Hc) materials. Test methods that were suitable for even platinum-cobalt magnets did not lend themselves to accurate testing of modern, high-energy magnets such as the rare earth-based SmCo and Nd-Fe-B (“Neo”) magnets.

(See the sidebar, "Primary Classes of Permanent Magnets and Their Energy Storage Capacity" here.)

Due to increasingly high flux density values and large resistance to demagnetization, new permanent magnet test methods were developed in the 1960s and 70s by research groups such as the U.S. Air Force Materials Laboratory at Wright-Patterson Air Force Base and the University of Dayton (Ohio) Magnetics Laboratory, and then documented by ASTM International, the International Electrotechnical Commission and other standards developing organizations. This test technology is now well established and widely used. Common permanent magnet test methods involve the use of magnetic hysteresigraphs and vibrating sample magnetometers. Material standards pose their own set of difficulties.

The Challenges of Developing Permanent Magnet Material Specifications

Permanent magnets are in a constant state of evolution. Their manufacture involves complex thermal, metallurgical and chemical processes in which every variable must be tightly controlled to achieve a particular result. Manufacturers continually introduce new magnet grades that offer some targeted, often incremental, improvement over existing grades.

It is not always clear which offering would best satisfy the needs of a particular user. Some manufacturers of lower quality magnets may have trouble duplicating their own recipes from batch to batch. The constant tradeoff between quality, consistency and cost means that there is a large envelope of possible magnetic properties, even within a particular type or “grade” of magnet.

To identify each possible magnet grade within a sufficiently narrow window to be of use to a design engineer is a daunting challenge. The general solution has been to try to identify the most commonly specified property sets for the various chemical classes of magnets and break them down into categories that roughly cover the expected performance envelope. Invariably this process leaves gaps that will be unsatisfactory to some magnet users.

Without at least some well-defined starting point, however, chaos will reign and companies will have difficulty documenting and obtaining the magnets they need for their products. Reference to well-written material standards as a base guideline, followed by additional application-specific details, will remain a necessary part of documenting magnetic material requirements.

A06 Is Developing Permanent Magnet Standards

Committee A06 is in the process of developing a series of material standards that attempt to address the standardization issue from a practical, user-friendly standpoint. The goal is to keep the standards relatively simple and concise, and not try to codify each and every detail (which would make the documents so long and dense that no one will want to use them). When used in combination with ASTM test standards such as A 977/A 977M, Test Method for Magnetic Properties of High-Coercivity Permanent Magnet Materials Using Hysteresigraphs, it is hoped that the new magnetic material specifications will help streamline the design engineer’s statement of his requirements and allow for reliable verification of the actual performance of materials obtained as a result of those requirements. Interested parties are strongly encouraged to join A06; input from a broader base of magnetic material users will always result in more useful and comprehensive standards. //

 
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