SECTION I
1.0 SCOPE & OBJECTIVE
1.1 Scope: This standard defines magnetic, thermal,
physical and mechanical characteristics and properties of
commercially available permanent magnet materials as
listed in Table 1.
There are a large number of permanent magnet materials
in use which are not described in this document.
These materials generally fall into one of the following
categories:
(a) Older materials that have been largely replaced by
new materials.
(b) Materials made by only one company with a specialized
and limited use.
(c) Materials evolving. from development status to
production which at this time are not mature from a
commercial viewpoint.
For reference purposes, the principal magnetic properties
of the materials in the above categories are listed in
Appendix A.
1.2 Objective: The objective of this standard is to establish
criteria by which users of permanent magnet
materials may be assured of magnets manufactured to
present commercial standards.
2.0 DEFINITIONS & TERMS
2.1 Definitions: The following definitions characterize
materials covered in this standard:
2.1.1 Permanent Magnet (Magnetically Hard)
Material: A permanent magnet material, also designated
as a magnetically hard material, has a coercive force
gener-ally greater than 120 Oe.
2.1.2 Individual Magnet: The term individual magnet
denotes a magnet purchased in a size and shape to be
ready for direct incorporation into a magnetic circuit.
2.1.3 Bulk Magnet Material: The term bulk magnet material
designates bar, rod, slab, strip, sheet, etc., from
which the purchaser cuts, stamps or forms individual
magnets.
2.1.4 Polarity of a Magnetized Magnet: The North
Pole of a magnet is that pole which is attracted to the
geographic North Pole. Therefore, the North Pole of a
magnet will repel the north seeking pole of a magnetic
compass.
2.1.5 Demagnetized Magnet: For the purposes of this
standard, a magnet shall be considered demagnetized if,
when any of its poles is dipped in soft iron powder (of
-5, + 10 mesh), not more than 3 particles of powder
adhere to it anywhere upon withdrawal.
2.2 Terms: A glossary of terms commonly used with
permanent magnetic materials is given in Appendix B.
3.0 CONDITION
Unless otherwise specified, bulk magnet materials shall
be furnished in the non heat-treated condition, as rolled,
as forged, or as-cast condition as applicable. Individual
magnets shall be furnished in a fully heat-treated and
demagnetized condition.
4.0 CLASSIFICATION & DESIGNATION
4.1 Classification: The classification of permanent
magnet materials covered by this standard is given in
Table 1. Section numbers for the material classes covered
in this standard as well as reference to the International
Electrochemical Commission (IEC) material code
numbers are also given in the table.
TABLE 1
MATERIAL CLASSIFICATION
Material MMPA
Section
IEC Code
Alnico
Ceramic
Rare-Earth
Iron-Chromium-Cobalt
II
III
IV
V
R1
S1
R4, R5
R2
4.2 Designation: Permanent magnet materials in this
specification will be divided into separate sections by the
MMPA Class. Each standard section will address the
relevant properties, characteristics and specifications of
each class of materials and the established sub-grades. In
general, reference will be made to historically recognized
subgrade descriptions (such as Alnico 1,2, etc., or
Ceramic 5, 8, etc.) and to a system, referred to as the
Brief Designation, that classifies each subgrade by typical
normal energy product and typical intrinsic coercive
force. In this system, for example, a material having
maximum normal energy product of 5.0 megagaussoersteds
(MGO) and an intrinsic coercive force of 2000
oersteds (2.0 kOe) would be assigned a Brief Designa –
tion of 5.01/2.0. When similar grades exist the nearest
IEC Grade Code Number will also be listed for cross
reference.
5.0 MAGNETIC PROPERTIES, THERMAL
PROPERTIES & OTHER CHARACTERISTICS
The magnetic, thermal, surface and internal structure,
and other physical characteristics are set forth in tables in
each section for the different classes of magnetic
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materials. The figures in these tables are intended to be
descriptions of each of the materials. The properties of
the materials produced by individual manufacturers may
differ somewhat from those shown. For information
concerning properties of actual grades produced, refer to
individual manufacturer’s literature. The properties
shown in the tables with each class shall not be used as
inspection criteria of either individual magnets or bulk
magnet materials.
5.1 Principal Magnetic Properties: Permanent
magnet materials are identified by the following
principal magnetic properties:
Maximum value of energy product (BH)max MGO
Residual induction Br gauss
Coercive force Hc oersteds
The measurement of the principal magnetic properties
are made in a in closed magnetic circuit permeameter by
commonly accepted procedures such as given in IEC
Standard Publication 404-5 “Methods of Measurement of
Magnetic Properties of Magnetically Hard (Permanent
Magnet) Materials or the “MMPA Permanent Magnet
Guidelines” (current edition).They are accurate only for
magnets having a straight magnet axis and produced
with a constant cross section along the axis of
magnetization. The minimum magnet volume of a
sample used to measure these magnet properties shall be
one cubic centimeter and the smallest dimension shall be
at least 5mm. The’ performance of a permanent magnet
circuit is de-pendent on the dimensions of all
components and the properties of the other components
of the circuit, as well as the properties of the permanent
magnet. It is recommended not to use unit properties of a
material as the specification. These are generally
recommended to be only used on prints or drawings to
show a subgrade within a material group. Section 8.0 of
the specification describes the proper means of
specifying the acceptable properties of a permanent
magnet component part.
5.2 Thermal Properties: Predicting magnet
performance as a function of the magnet’s temperature
requires knowledge of the following thermal properties:
Reversible temperature coefficient of the residual
induction TC (Br) %/ºC
Reversible temperature coefficient
of the intrinsic coercive force TC (Hci) %/ºC
Curie temperature Tc ºC
Maximum service temperature T max ºC
The values listed for each class of materials for thermal
properties are typical values intended as design guidelines
only and are not to be used as a basis for acceptance
or rejection. Values for irreversible temperature
characteristics are not listed because they depend on the
magnet material, geometry and circuit in which the
magnet is used.
5.3 Surface and Internal Structure Characteristics:
Permanent magnet materials have been developed
primarily for their magnetic properties. The magnetic
properties of some materials are produced using
manufacturing techniques which are not consistent with
pro-ducking perfect physical specimens. Minor physical
imperfections rarely impair the magnetic capabilities of a
magnet or compromise its stability or ability to resist
demagnetization. Imperfections commonly found in
permanent magnet materials shall be judged acceptable if
the following conditions are met:
(I) The magnet meets the magnetic performance criteria
agreed upon between the magnet manufacturer and
customer.
(2) The imperfections do not create loose particles that
would interfere with proper assembly or functioning in
the end use device. Unless otherwise agreed, visual
imperfection guidelines listed in the individual material
sections apply.
5.4 Other Physical Properties: Typical values for
other physical properties important to a magnet user are
listed in the tables in the sections for each class of
permanent magnet material and are intended to be descrimptions
of the material, not criteria for acceptance or
rejection.
6.0 MECHANICAL CHARACTERISTICS
Most permanent magnet materials lack ductility and are
inherently brittle. Such materials should not be utilized
as structural components in a circuit. Measurement of
properties such as hardness and tensile strength are not
feasible on commercial materials with these inherent
characteristics. Therefore, specifications of these
properties are not acceptable. Measurements of
mechanical properties shown in the tables were
performed under very carefully controlled laboratory
conditions. The values are shown only for reference and
comparison to other classes of materials.
7.0 DIMENSIONS AND TOLERANCES
Dimensions and tolerances shall be as specified on the
magnet drawing and must be agreed upon between the
magnet manufacturer and user before an order is accepted.
Normally the magnet user furnishes a drawing to
the manufacturer showing all dimensions and tolerances.
When no drawing is available from the user, the
manufacturer may furnish a drawing to the user for his
approval before manufacturing parts. The standard for
drawing, drawing notation and tolerancing is that
established in ANSI Y 14.5. Although individual
manufacturers will each have their own capability to
hold a given tolerance, standard tolerance tables
applying to specific classes of these materials are listed
in the individual sections.
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8.0 PROCESS CONTROL
Most manufacturers use statistical process control to
monitor key parameters at each process step.
9.0 INSPECTION &TESTING
Unless otherwise agreed upon, magnets will be inspected for
all specified characteristics by the use of a statistically valid
sampling plan. Such plans may be derived from, Quality
Planning And Analysis: From Product Development
Through Use, J.M. Juran and F. M. Gryna, 3rd Edition,
McGraw Hill (1993), Chapter 19. ISBN 0-07-033183-9
9.1 Performance Testing Approach--Magnetic
Characteristics: The principal characteristics--Br, Hc,
Hci, and (BH)max of a magnetic material are used to
identify a specific subgrade within a material class.
Generally, individual manufacturers can hold unit
magnetic property tolerances of ± 5% for residual flux
density, Br and ± 8% for coercive force, Hc. The range
for the energy product, (BH)max is ± 10%. Intrinsic
coercive force, Hci, is generally specified as minimum
value only.
The size and/or shape of the actual magnet to be
produced may cause magnets to have propertie
considerably different from these characteristics.
Therefore, use of these characteristics in specifying
acceptable properties for a given magnet shape is not
recommended.The recommended means is to specify the
minimum magnetic lines of flux at one or more load
lines on the major or minor hysteresis loop. A magnet
producer can assist in magnetic circuit analysis which
will determine this actual operating flux. From the
analysis, a method of test shall be chosen which will
cause the magnet being tested to operate at levels which
duplicate the performance in the final circuit. The
magnet user and supplier shall agree upon a reference
magnet to be used to calibrate the test equipment. The
acceptance limits shall be agreeable to both manufacturer
and user. The acceptability of a magnet shall be judged
solely by a comparison with the reference magnet tested
in an identical manner.
9.2 Visual Characteristics: The recommended
procedure for establishing acceptable levels for visual
characteristics is for manufacturer and user to prepare a
mutually agreed upon set of go/no-go standards or
sample boards. In the absence in such a set of standards
or other descriptions of acceptable criteria, the
guidelines set forth in each individual section apply.