Method for making an encapsulated magnet carrier

A method is provided for manufacturing an encapsulated magnet carrier by forming a wax pattern of the magnet carrier with an annular conducting ring annular chamber section, placing the conducting ring over the wax pattern in the annular chamber section, and forming an investment casting shell mold around the wax pattern and conducting ring. The magnet carrier is then cast with the conductor ring in place by pouring molten casting material into the mold, solidifying the casting material, and then separating the cast magnet carrier with the cast in place conductor ring from the surrounding mold.

BACKGROUND OF THE INVENTION 
This invention relates generally to encapsulated magnet carriers and, more 
particularly, to encapsulated magnet carriers used in magnetically driven 
sealless type pumps. 
A magnetically driven sealless pump is typically a centrifugal pump that 
has its impeller and bearing system isolated from the impeller driving 
mechanism by an isolating wall of a casing that seals the pumping 
mechanism from the surrounding environment and eliminates the necessity to 
use rotary seals to seal the pumped fluid against leaking along the shaft. 
This type of pump is particularly desirable when pumping corrosive or 
toxic fluids which are dangerous when allowed to leak. The driving 
mechanism is coupled to the pump impeller by an arrangement of magnets 
located on the opposite sides of the isolating wall which magnetically 
connects the torque of the driving mechanism to the impeller. 
A magnetically driven sealless centrifugal pump typically includes an inner 
magnet carrier mounted on the shaft. The inner magnet carrier must be 
sealed against leakage and be corrosion resistant. Inner magnets are 
disposed in individual chambers disposed around the carrier and in contact 
with a conducting ring and in the arrangement of magnets located on the 
opposite must include an impeller bearing system which is independent of 
the motor driving bearings and, therefore, necessitates that the impeller 
bearing system carry the full load on the impeller including both radial 
and thrust forces. 
In the past, a designer of this type of pump generally used a carrier made 
with an "L" cross-sectionally shaped inner piece typically made from a 316 
stainless steel or an alloy casting on wrought bar stock. After the 
initial machining of the carrier, a circumferential row of magnets having 
a ferrous conducting ring in contact with block magnets are pressed onto 
the carrier. The conducting ring is usually machined with a three decimal 
place tolerance ID (inner diameter) and a flat for each block magnet on 
the OD (outer diameter). The flat retains the block magnet in its 
peripheral position. After the row or rows of block magnets are pressed in 
place, an "L" cross-sectionally shaped outer shield is placed over the 
magnets. The outer shield is made from solid wrought bar or heavy wall 
tubing. Investment castings were experimented with but the castings proved 
to be too porous. After the shield is in place, it is welded to the "L" 
cross-sectionally shaped inner piece at both ends of the "L" shaped 
shield, thus, forming a waterproof encapsulated chamber containing the 
magnetic blocks. When energized magnets are used electron beam welding is 
used for the welding. When un-energized magnets are used Gas Tungsten Arc 
Welding (GTAW) may be used. After the shield is welded in place, the 
carrier is given final welding and then balanced. This process involves 
many steps and includes a difficult machining of the flats on the 
conducting ring which is due to the small tolerances that are desired. 
Furthermore, welding of the "L" shaped shield at two ends of the L involve 
two different radii of those ends and makes the assembly more difficult to 
weld. Differential thermal growth can produce a shortened life span for 
the carrier. The L shaped shield is also costly to manufacture and weld 
because of its shape and required tolerances. 
The foregoing illustrates limitations known to exist in present methods of 
manufacturing encapsulated magnet carriers. Thus, it would be advantageous 
to provide an alternative directed to overcoming one or more of the 
limitations set forth above. Accordingly, a suitable alternative is 
provided including features more fully disclosed hereinafter. 
SUMMARY OF THE INVENTION 
The present invention includes a method of manufacturing the annular magnet 
carrier by forming a wax pattern of the magnet carrier. The pattern has a 
cage section with an annular axially extending base wall section and 
annular forward and aft end wall sections extending radially from the base 
wall section, an annular conducting ring annular chamber section that is 
bound by the base wall section and the end wall sections, and a plurality 
of circumferentially located compartment sections that are bound by the 
annular chamber and the end wall sections. Next, a conducting ring is 
placed over the wax pattern and positioned in the conducting ring annular 
chamber section and an investment casting shell mold is formed around the 
wax pattern and conducting ring. The forming of the wax pattern may 
include forming the wax pattern with slot sections in the end wall 
sections. The cast magnet carrier is then cast with the conductor ring in 
place by pouring molten casting material into mold, solidifying the 
casting material, and then separating the cast magnet carrier with the 
cast-in-place conductor ring from the surrounding mold. 
The foregoing and other aspects will become apparent from the following 
detailed description of the invention when considered in conjunction with 
the accompanying drawings.

DETAILED DESCRIPTION 
Referring now to the drawings in detail, wherein identical numerals 
indicate the same elements throughout the figures, FIG. 1 illustrates a 
sealless magnetically driven centrifugal pump 10 including a pump casing 1 
containing an axial inlet 2, a pumping chamber 3 and an outlet 4, all of 
which are interconnected by passages extending through the casing. The 
casing 1 also contains a mounting foot 5 and an annular flange 6 
surrounding the pumping chamber 3. An axially extending impeller shaft 11 
carries a semi-open pump impeller 12 rotating in the pumping chamber 3 
during pump operation, all of which is covered by a casing cover 30 
attached to the annular flange 6. The semi-open impeller 12 includes a 
shroud 13 and a series of vanes 14 having one edge integral with the 
shroud 13. Mounted within the casing 30 are rear journal bearing bushings 
33 and 34, respectively. The impeller shaft 11 extends through bushings 33 
and 34 and carries respective bearing sleeves 36 and 37 rotating in the 
bushings 33 and 34. The bearing sleeves 36 and 37 are slipped on the shaft 
11 with a spacing sleeve 39 located therebetween. 
A power frame 16 fits over the casing cover 30 and is attached to the 
flange 6 by a series of bolts 17 circling the outside of the flange. The 
power frame 16 further includes a mounting foot 18 adapted to support the 
pump 10 in conjunction with the mounting foot 5 on the casing 1. A drive 
shaft 19 is rotatively mounted in the power frame 16 by a pair of axially 
spaced bearings 20 and 21 fixed in the frame 16 on the opposite sides of a 
bearing chamber 22 adapted to contain lubricant for the bearings 20 and 
21. The outer end of the drive shaft 19 is adapted to be coupled to a 
driving motor (not shown) using a conventional coupling means. 
The rear end of the impeller shaft 11 carries an inner magnet carrier 50 in 
accordance with an exemplary embodiment of the present invention, which is 
rotationally secured on the impeller shaft 11 by a key 51 in a position 
engaging the rear face of an aligning ring 48, and is held in place by a 
nut 52 threaded on the rear end of the impeller shaft 11. The nut 52 locks 
all of the rotating components mounted in place on the impeller shaft 11 
of pump 10. The periphery of the inner magnet carrier 50 carries a series 
of magnets 58 which rotate closely about the interior of a relatively thin 
can-shaped shell 59 which fits over the inner magnet carrier 50, thus, 
providing a leak proof seal between the cartridge and the power frame 16. 
The power frame 16 contains an outer magnet holder 61 attached to and 
rotating with the drive shaft 19 around the can-shaped shell 59 in close 
proximity thereto. The outer magnet holder 61 carries a series of magnets 
62 spaced around its interior which are magnetically linked to the magnets 
58 on the inner magnet carrier 50 for transmitting torque from the outer 
magnet holder 61 to the pump impeller shaft 11. Further details of this 
type of pump are disclosed in U.S. Pat. No. 4,871,301, issued Oct. 3, 
1989, titled "Centrifugal Pump Bearing Arrangement", invented by the 
present inventor Frederic W. Buse. Driving a pump impeller using magnets 
in this manner is well known in the art of sealless pumps. The present 
invention provides a new, unique and unobvious construction and method of 
manufacture of the inner magnet carrier 50. The inner magnet carrier 50 of 
the present invention preferably includes a can-annular web 88 extending 
from the cage 68 to an inner rim 87 about a shaft bore 89 where the web is 
integrally cast with the cage to form a single cast piece inner magnet 
holder 91 of the inner magnet carrier as shown in more detail in FIGS. 
2-5b. Note that, although shown with forward and aft annular end walls, 
the carrier can also be formed with only the aft end wall. (Not shown) The 
choice of design depends on size and fabricability preferences of the 
manufacturer. 
Referring now to FIGS. 2-5b, the inner magnet carrier 50 has a single piece 
integrally cast cage 68 circumferentially extending about a carrier axis 
70, which of course coincides with the axis of the impeller shaft 11 
(shown in FIG. 1), and preferably includes an axially extending annular 
base wall 72, annular forward and aft end walls 74 and 76, respectively, 
extending radially from axially opposite ends 80 of the base wall, an 
annular chamber 82 that is bound by the base wall 72 and the end walls, 
and a plurality of circumferentially located compartments 84 that are 
bound by the annular chamber and the end walls 74 and 76. A ferrous 
conducting ring 86 is disposed in the annular chamber 82 and about which 
the cage 68 was cast. 
A more particular embodiment provides one magnet means for providing a 
magnetic field in the form of either energized or un-energized bar magnets 
90 in each of the compartments 84. An annular sheathing 92 is positioned 
over the compartments 84 and magnets 90 and is bonded, preferably, by 
welding to the inner magnet carrier 50 such that the annular chamber 82 
and the compartments are hermetically sealed. The magnets 90 may be 
adhesively bonded to the conducting ring 86. The magnets 90, preferably, 
have a substantially rectangular block shape with a circular surface 93 
opposite the sheathing 92 and circumscribed about the carrier axis 70 
concentric with the sheathing, as shown in FIGS. 4 and 5b. 
The cage 68, preferably, further includes magnet spacing fingers 96 axially 
extending from the annular forward and aft end walls 74 and 76, 
respectively, between the chambers. The fingers may be formed by shoulder 
slots 98 formed at ends 100 of the chambers 82 in the end walls and 
undercut annular grooves 102 formed in the forward and aft end walls 74 
and 76, respectively, beneath the slots. The cage 68 and the rest of the 
magnet holder 91 is preferably made of stainless steel. The conducting 
ring 86 is preferably made of a ferrous material chosen from a group of 
ferrous materials comprising cast iron and carbon steel 1010, carbon steel 
1001, and carbon steel. The sheathing 92 is preferably thin 300 stainless 
steel tubing. 
The present invention includes a method of manufacturing the annular inner 
magnet carrier, illustrated by the flow chart in FIG. 6, by first forming 
a wax pattern 110 illustrated in FIG. 5a of the inner magnet holder 91 
including the inner magnet carrier 50, web 88, and rim 87 as illustrated 
in FIGS. 1-5. Referring to FIGS. 4 and 5a, the pattern 110 has a cage 
section 112 with an annular axially extending base wall section 114 and 
annular forward and aft end wall sections 116 and 118, respectively, or 
aft end wall section 118, only, extending radially from the base wall 
section, an annular conducting ring annular chamber section 120 that is 
bound by the base wall section and the end wall sections, and an annular 
compartment section 124 bound by the annular chamber and the end wall 
sections. The wax pattern 110 is formed in at least first and second 
portion 130 and 132, respectively, so that the conducting ring 86 can be 
placed over the second portion 132 of base wall section 114 of the wax 
pattern 110 and positioned in the conducting ring annular chamber section 
120. Next, an investment casting shell mold is formed around the wax 
pattern 110 upon which is mounted in place the conducting ring 86. The 
forming of the wax pattern may include forming the wax pattern with slot 
sections 140 in the forward and aft end wall sections 116 and 118, 
respectively. The cast inner magnet carrier 50 is then cast with the 
conductor ring 86 in place around the wax pattern 110 by pouring molten 
casting material into mold. The molten casting material is then solidified 
and the cast inner magnet carrier 50 with the cast in place conductor ring 
86 is then separated from the surrounding mold. Preferably, the method 
further also includes forming circumferentially disposed magnet spacing 
fingers by machining undercut annular grooves in end walls of the cage of 
the cast inner magnet carrier beneath the slots in the end walls. The 
method further includes placing one magnet 58 in each of a plurality of 
circumferentially located compartments that are bound by the ring and the 
end walls of the cage, positioning a sheathing 92 over the cage 68 and 
welding the sheathing to the forward and aft end walls 74 and 76, 
respectively. When only the aft end wall is cast on the magnet carrier, 
the sheathing must either have a complimentary L-shaped cross-section to 
that of the carrier, or it must be made in two separate sections to form 
the L-section. Preferably, each of the magnets has a substantially 
rectangular block shape opposite which the sheathing is to be positioned 
and are machined to form a circular surface circumscribed about the 
carrier axis and concentric to the sheathing which is preferably made from 
thin stainless steel tubing. 
The magnet carrier and method of production of the present invention is 
advantageous as compared to that of the prior art which requires more 
extensive machining and more parts. The present invention is less 
expensive and easier to manufacture and has improved structural and wear 
capabilities because it eliminates the difficult steps of machining flats 
on the conducting ring while maintaining a proper outside diameter of the 
ring. It also eliminates pre-machining of the cage before insertion of the 
ring.