Magnetic drive

A magnetic drive comprises a plurality of identically shaped and sized permanent magnets for transmitting torque to a shaft through a nonmagnetic cylindrical barrier wherein the magnets have inner and outer cylindrical surfaces with the outer cylindrical surfaces having a radius substantially the same as the inner cylindrical surfaces.

BACKGROUND OF THE INVENTION 
This invention relates to an improved magnetic drive for use in transfer of 
torque to corrosive or pressurized environments. Magnetic drives are known 
for transferring torque through nonmagnetic barriers, especially for 
pumping or stirring liquids on the interior of a sealed enclosure. 
Most commercial pump suppliers are required to offer a line of products to 
customers having a range of maximum torque transfer capability. In the 
past, this has meant that the overall size of the products had to be 
increased as the maximum torque transfer capability was increased, 
resulting in a different set of parts for each product or pump in the line 
of products. Thus, manufacturers of magnetically driven pumps have found 
it necessary to purchase or manufacture magnets of many sizes. Typically, 
the magnets must have increased axial length as the need for increased 
torque was required. Also, the driving magnets and the driven magnets for 
each size pump had different shapes or configurations. 
This patent application is based upon a unique application of the more 
powerful permanent magnets that have become available in the last several 
years. The strength of permanent magnets (as measured by energy products 
(BH).sub.max) has rapidly increased in recent years. Approximate strengths 
for each type of permanent magnet is set forth in the following table. 
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ENERGY PRODUCT (BH).sub.max 
MATERIAL MGOe(kJ/m.sup.3) 
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Ceramic (Ferrite) 4 (32) 
Alnico 12 (95) 
Samarium Cobalt (SmCo.sub.5) 
18 (143) 
Samarium Cobalt (Sm.sub.2 Co.sub.17) 
27 (215) 
Neodymium Iron Boron (Nd--Fe--B) 
35 (280) 
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In this patent application, we will refer to the samarium cobalt (either) 
and the neodymium iron boron magnets as rare earth magnets. The rare earth 
magnets offer the possibility of a radical new approach to design of 
magnetic couplings that transfer torque. Unless high temperatures are 
likely, the neodymium iron boron magnets are the preferred rare earth 
magnets for the practice of this invention. 
It is an advantage, according to this invention, to construct a magnetic 
drive with magnets of the rare earth type having a single size and shape 
used on both the driving and driven magnet assemblies. 
It is a further advantage of this invention to construct a series of 
magnetic drives having different maximum torque carrying capacities using 
magnets of the rare earth type having a single size and shape in both the 
driving and driven magnet assemblies of all magnetic drives in the series. 
It is yet a further advantage, according to this invention, that the need 
for thrust bearings can be eliminated in the magnetic drives in many 
applications. 
It is a further advantage, according to this invention, to provide a 
magnetically driven pump or series of magnetically driven pumps wherein 
the magnets have a single size and shape on both the driving and driven 
magnet assemblies for all pumps in the series. 
SUMMARY OF THE INVENTION 
Briefly, according to this invention, there is provided a magnetic drive 
comprising a plurality of identically shaped and sized permanent magnets 
for transmitting torque to a shaft through a nonmagnetic cylindrical 
barrier. The magnetic drive comprises a first assembly positioned to 
rotate outwardly of the cylindrical barrier, said assembly having a 
ferromagnetic outer ring with an inner radius RI. The magnetic drive 
further comprises a second assembly positioned to rotate inwardly of the 
cylindrical barrier having a ferromagnetic inner ring with an outer radius 
RO. The first and second assemblies have an identical number of 
circumferentially spaced permanent magnets spaced around the rings. The 
magnets have an inner and outer cylindrical surface. The outer cylindrical 
surface has a radius substantially the same as the inner radius RI of the 
outer ring and the inner cylindrical surface has a radius substantially 
the same as the outer radius RO of the inner ring. Preferably, the axial 
dimension of the cylindrical faces of the magnets and the circumferential 
dimension are substantially equal. Preferably, the magnets are radially 
magnetized and an even number of magnets are spaced about each ring with 
alternating polarities. Preferably, the magnets are of the rare earth type 
and particularly are of the samarium cobalt or the neodymium iron boron 
type. 
In a preferred embodiment, a magnetically driven pump is provided which 
comprises an impeller chamber and an impeller positioned to rotate in the 
chamber mounted on a shaft. The magnetic drive comprises a first 
ferromagnetic ring positioned to rotate outwardly of a cylindrical barrier 
and a second ferromagnetic ring positioned to rotate inwardly of the 
cylindrical barrier. The second ring is connected to the impeller. The 
first and second rings can be transposed and still achieve the same 
function. The first and second rings have an even number of 
circumferentially positioned permanent magnets as above described. 
There is also provided, according to this invention, a method of making a 
series of magnetic drives with different maximum torque capacities from 
parts having identical dimensions. The method comprises assembling a 
plurality of identically shaped permanent magnets, a nonmagnetic 
cylindrical barrier, an outer ring positioned to rotate outwardly of the 
cylindrical barrier and an inner ring positioned to rotate inwardly of the 
cylindrical barrier. The identically sized and shaped magnets are 
circumferentially spaced about the first and second rings in pairs with 
opposite magnetic polarity. The only difference between magnetic drives of 
different maximum torque capacity is the number of pairs of permanent 
magnets spaced around the inner and outer rings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 and 5, there is illustrated in section a 
magnetically driven pump. A pump casing 10, nonmagnetic barrier 11 and 
standoff 12 are assembled together to define two chambers sealed from each 
other. The pump casing 10 and nonmagnetic barrier 11 define the impeller 
chamber and a chamber for accommodating a driven magnet assembly attached 
to the impeller. The standoff and nonmagnetic barrier define a chamber for 
a driving magnetic assembly. The standoff 12 is typically attached to a 
motor (not shown). 
A driving magnet assembly 13 is positioned within the standoff 12 and is 
secured to the drive shaft 14 of the motor. The body of the driving magnet 
assembly has an inverted (as shown in the drawing) cup shape with a 
ferromagnetic (for example, steel) ring 15 around the rim. Secured to the 
inside of the ring are a plurality of permanent magnets 16 of the rare 
earth type. 
The nonmagnetic barrier has radial flanges 17 which are captured between a 
radial flange 18 on the standoff 12 and a radial flange 19 on the pump 
casing. The three radial flanges are clamped by bolts (not shown) passing 
through holes provided in the flanges 17 and 19 and engaging threads 20 
provided in flange 18. An O-ring 21 squeezed between the flanges seals the 
impeller chamber. 
The nonmagnetic barrier has an inverted cup portion 22 which nests inside 
of the driving magnet assembly. The inverted cup has a cylindrical wall 23 
with an axis that substantially coincides with the axis of the shaft 14. A 
cylindrical pin 24 is fixed to the nonmagnetic barrier. The axis of the 
pin 24 also substantially coincides with the axis of the motor shaft 14. 
A driven magnet assembly 25 has a bushing 29 journaled on the pin 24. 
Attached to the front of the driven magnet assembly is the impeller 26. 
The driven magnet assembly 25 has a ferromagnetic ring 27 mounted therein. 
Secured on the outer cylindrical face of the ring 27 are a plurality of 
permanent magnets 28 of the rare earth type. The ring 27 and magnets 28 
are encapsulated in a nonmagnetic resin to protect them from attack by 
corrosive liquids in the impeller chamber. The driven magnet assembly 25 
slides axially along the pin 24 as well as rotates on the pin. The inner 
and outer magnetic ring assemblies can be transposed without affecting the 
function or embodiments of this invention. The magnets in the driven 
magnet assembly are positioned so that with a slight axial movement of the 
assembly, they can align with the magnets in the driving magnet assembly. 
No thrust bearings are required as the attraction between the two sets of 
rare earth magnets will hold the axial position of the driven magnet 
assembly and impeller. 
The ferromagnetic ring 15 in the driving magnet assembly 13 has an inner 
cylindrical surface having a radius of curvature R.sub.I. The 
ferromagnetic ring 27 in the driven magnet assembly has an outer 
cylindrical wall having a radius of curvature R.sub.O. Referring now to 
FIG. 2, the permanent magnets 16, 28 all have an identical shape and size. 
The magnets have two cylindrical faces, an outer face having a radius 
R.sub.I to match the inner cylindrical surface of the ring 15 in the 
driving magnet assembly and an inner face having a radius of curvature 
R.sub.O to match the outer cylindrical surface of the ring 27 in the 
driven magnet assembly. Preferably, the center of curvature of both 
cylindrical surfaces lies on the same line extending through an axial line 
bisecting the circumferential width of the inner face 30 and outer face 33 
of the magnets. The axial length L.sub.A of the magnet faces and the 
circumferential width W.sub.C of the inner magnet face 30 are in a ratio 
from about L.sub.A /W.sub.C =1.5:1 to L.sub.A /W.sub.C =1:1.5. 
The thickness of the magnets in the radial direction varies. The magnets 
are thickest near the circumferential end walls 31 and 32. Preferably, the 
edge of the circumferential end walls are rounded. This minimizes chipping 
and, in the case of the edges along the outer face 33, reduces the 
possibility that the encapsulating coating on the driven magnet assembly 
will be cut by the edges and come apart from the assembly exposing the 
magnets. 
As should now be apparent, the inner face 30 of the magnets can lie flush 
against the ring 27 and the outer face 33 of the magnets can lie flush 
against the ring 15. This has been achieved by permitting the gap between 
the magnets on the ring 27 and the magnets on the ring 15 to be variable. 
While the shape and size of all magnets are identical, the magnets are made 
in two sets, one magnetized north pole toward the radius of curvature of 
the faces (inward) and the other set magnetized north pole away from the 
radius of curvature of the faces (outward). Each ring has an even number 
of magnets equally spaced around the circumference thereof with magnets 
having opposite polarity alternating. The magnets may be installed using a 
jig that establishes the correct spacing. The magnetic attraction holds 
the magnets temporarily in place until an adhesive permanently secures the 
magnets to the rings. 
One of the advantages of the magnetic coupling described above is the 
torque transfer capability can be increased or decreased with no need to 
increase the number of different parts. Referring now to FIGS. 3A, 3B and 
3C, there is shown the arrangement of the rings 15 and 27 and the magnets 
16, 28 for three different maximum torque levels. In FIG. 3A, six pairs of 
magnets are arranged around the rings, in FIG. 3B eight pairs and in FIG. 
3C ten pairs. The same identically sized and shaped magnets are used in 
all three arrangements. Going from the arrangement shown in FIG. 3A to 
that shown in FIG. 3B, maximum torque is increased about 35% and going 
from the arrangement 3B to the arrangement of FIG. 3C, the maximum torque 
is increased about 25%. These changes are possible without the need to 
make magnets of different sizes. 
FIG. 4 illustrates an embodiment of this invention similar to that 
illustrated and described with reference to FIG. 1 except that the driven 
magnet assembly is fixed to the pin 35 and the pin 35 slides axially in a 
bushing 36 mounted in the nonmagnetic barrier 11. The end 37 of the pin 35 
may have a cone shape. The bushing 36 may have a reduced radius section 38 
that the apex of the cone-shaped end of the pin can enter. If the axial 
forces on the driven magnetic assembly overcome the axial restraining 
forces of the magnets, the cone-shaped end will contact the bushing along 
a ring of contact minimizing the heat that would be generated due to 
friction. 
The driving and driven magnet assemblies preferably are molded from a 
strong and tough plastic. In this way, the assemblies channel the magnetic 
flux through the magnets, the ferromagnetic rings and the gap between the 
aligned magnets. The magnetic barrier should be strong and tough plastic, 
brass or nonmagnetic stainless steel, for example. 
Having thus described our invention with the detail and particularity 
required by the Patent Laws, what is desired protected by Letters Patent 
is set forth in the following claims.