Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2002-03-11
2003-12-16
Jenkins, Daniel J. (Department: 1792)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
Reexamination Certificate
active
06663827
ABSTRACT:
REFERENCES CITED
U.S. Patent Documents
2,488,729
November 1949
Kooyman
171/209
3,943,698
March 1976
Ono
58/23
3,953,752
April 1976
Bannon
310/156
4,035,677
July 1977
Kusayama, et al.
310/42
4,067,101
January 1978
Ono
29/598
4,095,129
June 1978
Tanai, et al.
310/49 R
4,206,379
June 1980
Onada
310/156
4,340,560
July 1982
Migeon
264/249
4,700,091
October 1987
Wuthrich
310/49
4,888,507
December 1989
Plancon, et al.
310/40 MM
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable.
BACKGROUND
1. Field of Invention
The present invention relates to electrical stepping motors, and more particularly, to micro-miniature stepping motors used to drive the gear works for turning the time-indicating hands in quartz analog timepieces.
2. Description of Prior Art
A stepping motor is one that rotates by way of short, essentially uniform angular movements rather than continuously. An important application of stepping motors is in electronic watches having an analog display formed by rotating hands. In a watch of this type, low-frequency timing pulses, derived by frequency division from a high frequency crystal-controlled time base, serve to actuate a stepping motor which drives the gear works rotating the hands of the watch.
Various types of stepping motors are known which make use of a rotor incorporating a drive member such as a gear pinion, a permanent magnet, usually having two poles, and a pinion shaft with journals rotatably mounted in the watch movement. The permanent magnet is often in the shape of a ring with a central hole through it for the pinion shaft. A typical shape would be a toroid measuring about 1.50 mm O.D.×0.35 mm I.D.×0.50 mm thick.
In the context of an electronic watch, the stepping motor must not only be in micro-miniature form to minimize space requirements, in order to make same particularly suitable for use in a small sized wristwatch, but its power consumption must be extremely low. In order to energize the watch with a miniature power cell which will last at least a year, the allowable power consumption is usually less than 8 microwatts. Another desirable characteristic of a stepping motor rotor is that it have a low moment of inertia about its axis of rotation.
Permanent magnets used for watch rotors, typically anisotropic rare earths such as samarium-cobalt, are extremely brittle and difficult to machine. Because of the small size of the rotor, close tolerances are required in the stepping motor. Various improvements have been suggested to reduce breakage of the magnets and to reduce the cost of manufacturing a stepping motor rotor.
The possibility of breakage increases when designs require a press fit of the magnet material to the rotor pinion shaft, or when compressive forces on the outer diameter are necessary to hold it in place on the rotor. Protective bushings have been used to prevent breakage but, since the moment of inertia varies as the square of the radius of rotation, the use of protective bushings between the inner diameter of the magnet and the rotor shaft or the use of metallic shells encasing the outer diameter of the magnet is to be avoided.
Examples of prior art rotor assemblies with plastic internal bushings disposed between the shaft and the magnet are shown in Onda, U.S. Pat. No. 4,206,379; Bannon, U.S. Pat. No. 3,953,752; Kooyman, U.S. Pat. No. 2,488,729 and Migeon, U.S. Pat. No. 4,340,560.
An example of a stepping motor rotor with permanent magnet clamped axially between metal bushings press fit with an interference fit to the rotor shaft is seen in Kusayama, U.S. Pat. No. 4,035,677.
Constructions where the inner hole diameter of the permanent magnet is finished and pressed directly onto the rotor shaft are shown in Ono, U.S. Pat. No. 3,943,698 which uses a reinforcing plate of stainless steel or other non-magnetizable material bonded to the magnet to prevent cracking the brittle material.
A rotor assembly which employs a radial wall and outer protective metal sheath into which the magnet is press fit on its outer diameter is disclosed in Tanai, U.S. Pat. No. 4,095,129. Although this permits forming the inner diameter of the rare earth magnet by a rough drilling process, the outer metallic protecting sheath both increases the moment of inertia and offer the possibility of breakage by compressive stresses due to the press fit on the outer diameter of the magnet.
The magnet rotor must hold its strength under adverse conditions, otherwise it will change in its performance. Hence, high coercivity and temperature stability are essential. Finally, in order to reduce the magnet volume and weight, it is important that the magnet have a high energy product (BHmax). Typically, the specification for the energy product of rare earth watch rotors is at least 22 MGOe (170 kJ/m3).
Only the rare earth permanent magnet materials samarium-cobalt, Sm
2
Co
17
, and neodymium-iron-boron, Nd
2
Fe
14
B, usually shortened to just NdFeB, have energy products greater than 22 MGOe (170 kJ/m3). Sm
2
Co
17
has a high energy product at about 30 MGOe (238 kJ/m3) and high coercivity at about 10 KOe. Stable at temperatures up to 350° C. it is also very resistant to corrosion. Its disadvantages are its high cost and difficulty to machine.
NdFeB has an exceptionally high energy product at about 40 MGOe (318 kJ/m3) and exceptionally high coercivity at about 15 KOe. Relatively easy to machine and relatively inexpensive, the major disadvantage of NdFeB magnets is their poor corrosion resistance and instability above 150° C. These shortcomings severely limit their application.
Studies aimed at improving the corrosion resistance of NdFeB magnets have mostly emphasized protective coatings or the addition of alloying elements, however, the effectiveness of coatings is not always assured and alloying to increase the corrosion resistance usually reduces magnetic properties. While new corrosion resistant NdFeB alloys may change the situation in future, the current use of Sm
2
Co
17
for quality watch rotors is almost universal.
Sm
2
Co
17
magnets are anisotropic and must be magnetized in the orientation direction. Hence, the provision of a prealloyed powder is a prerequisite. The process starts by vacuum induction melting a carefully optimized blend of alloying ingredients and casting a Sm
2
Co
17
ingot. The ingot is then crushed under protective atmosphere to a coarse, typically minus 50 mesh (297 micron) prealloyed powder. The resulting powder is further coarse ground, usually by autogenous grinding under hydrogen atmosphere in a heated cylindrical mill (hydrogen decrepitation method). Finally, the powder is jet milled under high pressure (about 120 psi) N
2
to a critical size depending on the size of individual crystallites. Following screening to remove undesirable undersize and oversize particles the highly pyrophoric powder is stored under argon atmosphere until ready for pressing.
Pressing starts by blending a powder mixture based on chromatography results. Depending on the type of magnet being produced, the powder is either isostatically pressed into a block—the method used in the fabrication of watch rotors—or die pressed to a particular component shape. In either case the operation is conducted in a pulsed magnetic field (typically 10 kOe). The effectiveness of the pulses in magnetically aligning the crystallites diminishes as the powder is being compacted. During the latter part of the pressing step, stresses introduced as a result of plastic deformation as well as density gradients may lead to a less than perfect grain alignment.
Although the magnetic properties of isostatically pressed parts are usually higher than those of pressed parts, the uniformity of the magnetic characteristics of the former is usually lower than that of the latter. After pressing, the block or shapes are demagnetized with a decaying 60 Hz field.
Sintering is performed in high vacuum. The partial formation of liquid phase during sintering affects the angularity tolerance of the magnetization. This, combined wi
Billiet Romain L.
Nguyen Hanh Thi
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