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BACKGROUND OF THE DISCLOSURE
In the design of vacuum furnace systems commonplace to employ a hot zone
structure, which is made up of a plurality of heating elements strips,
fabricated from molybdenum which is commonly called moly and will be
called moly throughout this description. In one prior art design of a moly
hot zone, each moly heating strip is formed into an incomplete circle and
those circular elements are stacked beside one another to form a
cylinder-like structure. In another design of a moly hot zone, each
heating ring is formed of a plurality of heating strips. Together the moly
heating strips make a chain of heating elements which when viewed together
form an open ended ring-like structure with a gap between the starting
heating strip and the ending heating strip. The one piece moly ring and
the ring of the moly elements, are incomplete because, at one of the two
open ends, electrical energy must be applied and at the other of the two
open ends, the electrical energy completes the circuit excursion and
passes on to another open ring or to an electrical terminal. While the
foregoing moly hot zones have been quite acceptable, there are occasions
when it is preferable to use a graphite hot zone. For instance, in a
brazing operation the filler material, or the brazing material often drips
toward the bottom of the furnace and lands on the moly heating elements
and does damage to those moly heating elements. If the vacuum furnace is
equipped with a graphite hot zone, there is virtually no damage from the
dripping filler material because the graphite is rugged and does not
either mechanically or chemically respond damage-wise to dripping hot
filler material. Accordingly, in the event of such prospective use, the
furnace user often wants to employ a graphite hot zone design. It has been
the practice heretofore to design a graphite hot zone by having a
plurality of graphite heating strips formed in an open ended circular
chain-like fashion. The ends of each of the strips are held by support
members which are secured to some secure structure in the furnace. In the
prior art, the end of the graphite heating strip chain (that is connected
to the electrical energy terminal) is held by a graphite support member
which was fashioned to go to the top of the furnace chamber. Near the top
of the furnace chamber the graphite support member makes a direct
connection to the copper input leads which come from the electrical energy
source. Such an arrangement in the past gave rise to difficulty when the
user wanted to convert the furnace from a graphite hot zone into a moly
heating ring mode of operation. By having the graphite support member, in
the prior art, fashioned to pass through the insulating section
surrounding the hot zone, the conversion is made quite difficult. The
present arrangement permits the terminal support assembly to be readily
clamped to a moly terminal rod and readily removed therefrom if the user
decides to convert the furnace from a graphite hot zone into a moly hot
zone arrangement. In addition, the present system includes graphfoil
(laminated graphite) washers located between the graphite bolts and the
graphite heating elements as well as between the graphite heating elements
and the support ears, or protrusions, of the support member so that the
resiliency of the graphfoil permits the expanding graphite to expand while
at the same time does not permit the graphite bolt to work its way into a
"loose" condition.
SUMMARY OF THE DISCLOSURE
The present arrangement is directed to employing graphite members instead
of molybdenum members as hot zone structure. Graphite material is rugged
and not bendable so in order to construct a circular type hot zone, the
ring must be made up of a plurality of straight graphite heating elements
in the form of a polygon. In a preferred embodiment the graphite heating
elements for a vacuum furnace are each 10 1/16" long except for the first
heating element and some twelve such graphite heating elements are
employed. The first heating element of the ringlike chain, of the
foregoing preferred embodiment, which is connected to the electrical
terminal, is 9" long. The graphite heating elements are not overlapped but
are held in pairs along the ring-like path by graphite support members.
The graphite support assembly members are made up of three types. The
first type is the electrical terminal support assembly, the second is the
ring periphery support assembly and the third type is the bridge support
assembly. The electrical terminal support member, in the present
arrangement, is very different from the prior art terminal support member.
In the prior art the terminal support member was formed and disposed to
pass through the heat insulating ring, which normally surrounds the hot
zone. The terminal support member is extended to a position which is very
close to the outside wall of the furnace chamber. At that position the
terminal support member is connected to the copper leads from the
electrical power source. The present power terminal support assembly
comprises a block which has a channel cut therein. The channel measurement
is the width of the moly power rod which is normally connected to the
copper leads. However, the channel is slightly less in measurement along
its depth dimension than is the moly power rod. In addition to the block
there is a keeper member which bridges the width of the channel. When the
keeper member is secured to the block, the assembly of the keeper and the
block is secured to the moly rod by friction. In the present arrangement
there is a laminated graphite washer located between the block and the
keeper on each side of the channel and the depth of the washer is equal to
the difference between the depth of the moly rod and the depth of the
channel. It should also be understood that there is a first and a second
threaded aperture formed in the block and a first and a second aperture
formed in the keeper. Accordingly, when the keeper is properly located,
with respect to the block and the molybdenum rod, then the first aperture
in the keeper is axially in alignment with the first threaded aperture in
the block while the second aperture in the keeper is axially aligned with
the second threaded aperture in the block. In addition, there are two
molybdenum spacers and two graphite bolts involved. The first graphite
bolt along with the first molybdenum spacer (which has an aperture
therein) are lined up on one side of the block so that the bolt can be
passed through the aperture in the molybdenum spacer, through the first
aperture in the keeper, and threaded into the first threaded aperture in
the block. The second graphite bolt along with the second molybdenum
spacer (which has aperture therein) are aligned up opposite the second
aperture in the keeper and the second threaded aperture in the block. When
the second bolt is passed through the aperture in the second molybdenum
spacer and through the second aperture in the keeper, it can be threaded
into the second threaded aperture in the block and thus the keeper is
"tightened up" against the molybdenum rod which is located in the channel.
It should be noted that there is a graphfoil washer (laminated graphite)
located between the first molybdenum spacer and the keeper. Note also that
there is also a second graphfoil washer located between second molybdenum
spacer and the keeper. Thus, when the spacer and the keeper and the
graphite bolts are assembled as described above, the graphfoil washers
provide resilience to the assembly to compensate for expansion and
contraction of the materials. In this particular assembly, when the
terminal support moly terminal rod gets warm or hot, it expands. At the
same time, the moly spacers expand. Accordingly the molybdenum power
terminal would tend to expand and try to push the keeper away from the
block while the two molybdenum spacers would expand and tend to push the
keeper toward the block as well as toward the molybdenum rod. In this way
the expansion efforts are counterbalancing and the assembly stays in a
fixed position, firmly secured to the molybdenum power rod and without
causing the graphite bolts to loosen up.
The second type support assembly (identified as ring periphery support
members) including a block member which has first and second wing-like
protrusions extending therefrom. The block member of the second type
support assembly has an aperture therethrough and through that aperture
there is located a support rod assembly. In each of the wing-like
protrusions, there is formed a threaded aperture and on each of those
wing-like protrusions there is located in abutment the end of an
associated graphite heating element. Each of the heating elements has an
aperture therein so that a graphite bolt can be passed therethrough and
threaded into the threaded aperture of the associated wing-like
protrusions. Accordingly, each of the graphite ring periphery support
members supports a pair or at least the ends of a pair of graphite heating
elements. It should be noted that between the head of the graphite bolt
and the upper end of the heating element there is located a graphfoil
(laminated graphite) washer and between the lower side of the heating
element and the wing protrusion there is located a second graphfoil
washer. The graphfoil washers just mentioned serve as described before to
provide resiliency (as well as a good electrical connection) between the
various graphite members so that whatever expansion may take place (even
though graphite has a relatively low coefficient of expansion) that
expansion can be compensated for by the graphfoil washers. This
arrangement serves to keep the graphite bolts in place without loosening
up.
The objectives and features of the present invention will be better
understood in view of the following description taken in conjunction with
the drawings wherein:
FIG. 1 is a pictorial schematic of a graphite hot zone located a vacuum
furnace;
FIG. 2 is a front view of a power terminal support assembly;
FIG. 3 is a top view of the assembly shown in FIG. 2; and
FIG. 4 is a side view of a ring support assembly.
Consider FIG. 1. In FIG. 1 there is shown a vacuum furnace 11. The vacuum
furnace 11 is composed of an outside chamber 13, a plenum 15, a heat
insulating enclosure 17 and a plurality of plenum outlet nozzles only one
of which is shown at 19. Inside of the vacuum furnace, as just described,
there is located a hot zone. The hot zone is made up of the graphite
heating elements 21 as well as the graphite periphery ring support members
23. In addition to the graphite periphery ring support members 23, there
are two special support members, namely the bridging support member 25 and
the power terminal support member 27. The graphite heating elements, of
course, heat up when electrical energy is passed therethrough. The
electrical energies applied from the copper terminal 29, through the
terminal support member 27 along each of the heating elements 21, through
the associated periphery ring support assemblies 23, back to the bridging
terminal 25. Bridging terminal 25 has a bridge member 31 connected thereto
which provides a circuit path over to a similar ring which lies (with
respect to the FIG. 1) in back of the ring just described. In other words,
the current coming in from terminal 29 at some point in time would be
going counter clockwise around the heating elements 21 and the periphery
ring support assemblies 23, through the bridge member 31 to a second ring
of heating elements and support members and clockwise through those last
mentioned heating elements and support members back to a terminal similar
to the terminal 29 to complete the circuit.
Let us consider the power support assembly 27 which is shown in greater
detail in FIG. 2. In FIG. 2 there is shown the copper electrical terminal
29 which is bolted to the moly terminal rod 33. The moly terminal rod 33
is located inside of the ceramic sleeve 35 and both the ceramic sleeve 35
and the moly terminal rod 33 pass through the plenum 15 and through the
heat insulation package 17. If we examine FIG. 3 which is a bottom view of
the power terminal support assembly, we see that the moly power rod 33
fits into the channel 37. The channel 37 is cut, or formed, into the block
39. It will be noted that the depth dimension 41 of the channel 37 is not
as long as the depth dimension 43 of the moly power rod 33. Accordingly
when the keeper 45 is pulled up against the block 39 in response to the
graphite bolts 47 and 49 being threaded into the threaded apertures 51 and
53, there is a friction contact between the keeper 45 and the moly power
rod 33. As can be detected or determined in FIG. 3, there is a washer 55
located between the block 39 and the keeper 45 on the right hand side as
viewed in FIG. 3 and there is a washer 57 located between the block 39 and
the keeper 45 on the left hand side of FIG. 3. The washers 55 and 57 are
laminated graphite commercially known as graphfoil. The laminated graphite
has a certain resiliency so that when the bolts 47 and 49 push the keeper
45 toward the block 39, there is a good electrical contact between the
keeper 45 and the block 39 through the washers and the washers provide
some resiliency or flexibility with respect to expansion (particularly
expansion of the moly power rod 33 and the moly spacers 59 and 61) due to
heat.
As also can be determined by examining FIG. 3, there is a molybdenum spacer
59 located between the graphite bolt 47 and the keeper 45 on the right
hand side of the figure. On the left hand side of the figure it can be
gleaned that there is a molybdenum spacer 61 located between the graphite
bolt 49 and the keeper 45. In between the molybdenum spacer 59 and the
keeper 45 on the right hand side, there is located a graphfoil washer 63
while on the left hand side of FIG. 3 it can be determined that there is a
graphfoil washer 65 located between the molybdenum spacer 61 and the
graphite keeper 45. The washers 63 and 65 also provide resiliency to
accommodate for the expansion of the molybdenum spacers 59 and 61 when the
structure is heated. By having the molybdenum spacers 59 and 61 located as
shown in FIG. 3, we have determined that if the power terminal becomes
heated up, as it does in fact become, the molybdenum rod 33 tends to push
the keeper 45, in response to its expansion, away from the block 39. On
the other hand the molybdenum spacers 59 and 61 tend to push the keeper
toward the block 39 and hence there is a set off of the expansion forces
and the power terminal assembly remains firmly locked onto the molybdenum
power rod 33. We have found empirically that if the washers 55, 63, 65 and
57 are not employed, and the power terminal assembly 27 goes through
cyclical expansions and contractions, the bolts 47 and 49 tend to "loosen
up". On the other hand, we have found that if the structure is arranged as
shown in FIG. 3 and the power terminal assembly 27 is subjected to cycles
of hot and cold, the bolts 47 and 49 do not loosen up.
Note in FIG. 3 that there is an ear-like protrusion 69 extending from the
block 39 and it is to that protrusion 69 that the first of the heating
elements is secured. This can be better seen in FIG. 2. In FIG. 2 there is
shown a heating element 71 which is secured to the ear-like protrusion 69
by the graphite bolt 73. Further, in FIG. 2 it can be noted that there is
a ceramic sleeve 75 which surrounds the ceramic sleeve 35 and the ceramic
sleeve 75 is capped by a washer 77. The sleeve 75 and washer 77 prevent
material, which is floating within the vacuum furnace, from building up
between the wall of the ceramic sleeve 35, the insulation package 17 and
the terminal support member 27.
If we look at FIG. 1 again, and in particular look at the periphery support
member 23A we find that it is supporting the graphite heating elements 21A
and 21B. Let us look at FIG. 4 which depicts in detail the periphery
support member 23A. The periphery support member 23A is made up of a
center block 79 from whence there is formed two earlike protrusions 81 and
83. In the center of the block 79, there is formed an aperture 85. Located
through the aperture is a ceramic sleeve 87 and within that ceramic sleeve
87 there is located a stud 89. It can be gleaned from FIG. 4 that located
between the center block 79 and a graphite washer 91, there is located a
spacer 93. On the back end of the block 79 is located a second spacer 95
which is located between the block 79 and a bar 97. The bar 97 is made of
graphite and has a plurality of apertures therein. The stud 89 passes
through an associated aperture in the bar 97, through the insulating
package 17, and through a bolt 99. Bolt 99 is threaded externally and as
can be seen in FIG. 4, a washer 101 and a take up nut 103 are secured onto
the bolt 99 to provide the support for the stud 89 and therefore the
physical support for the periphery support member 23A. Located through the
stud 89 is a cotter pin 105 and threaded up against the cotter pin there
is a second takeup nut 107. At the front end of the stud 89 there is
located a second cotter pin 109.
As can be seen in FIG. 4, the ear-like protrusions 81 and 83 provide a
basis for supporting the two graphite heating elements 21B and 21A which
we discussed in connection with FIG. 1. Note in FIG. 4 that the graphite
heating element 21B is secured against the ear-like protrusion 83 by
virtue of the graphite bolt 111. Graphite bolt 111 passes through an
aperture 113 in the heating element 21B and is threaded into the ear-like
protrusion 83. Located between the graphite bolt 111 and the heating
element 21B is a laminated graphite washer 115. Also located between the
graphite heating element 21B and the ear-like protrusion 83 is a laminated
graphite washer 117. The structure on the other protrusion side of the
block 79 is virtually identical as that just described, with a graphite
bolt 119 passing through an aperture 121 to be threaded into the ear-like
structure 81. Located between the graphite bolt 119 and the heating
elements 21A is a laminated graphite washer 123 while a second laminated
graphite washer 125 is located between the heating element 21A and the
ear-like protrusion 81. The laminated graphite washers 115, 117, 121 and
125 provide resilience and good electrical connections to accommodate the
take up procedure when the heating elements 21A and 21B are fastened to
the terminal assembly 23A. We have found empirically that if the washers
115, 117, 123 and 125 are not employed, then the bolts 119 and 111 tend to
loosen with repeated cycles of hot and cold. Note also in FIG. 4 that the
ear-like protrusions 81 and 83 are bevelled at the inside corner as
depicted by the bevels 82 and 84. We have found that if the corners are so
bevelled it eliminates the arcing phenomenon between the block and the
insulation package 17.
The foregoing described graphite hot zone arrangement enables the system to
be readily changed from a graphite system to a molybdenum system and vice
versa. By virtue of having the power terminal assembly readily clamped
onto the molybdenum power rod, as described above, enables the whole
assembly to be disassembled without the difficulty experienced in the
prior art. By having the periphery support graphite assemblies readily
removable from the support studs such as support stud 89 in FIG. 4, the
remainder of the hot zone can be readily disassembled because the heating
elements simply come apart by removing the graphite bolts and that
removable feature includes removing the bridging support device 25.
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