Note: Descriptions are shown in the official language in which they were submitted.
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STOPPER ROD
This invention relates to a stopper rod and to a means of making an improved
stopper rod.
Stopper rods, or stoppers, are well known for use in controlling molten metal
flow
from a vessel, e.g. a tundish, into a mould. A stopper rod is raised and
lowered by
a suitable mechanism positioned adjacent the vessel and can thereby be used to
close fully or partially or to open fully the outlet from the vessel, thus
controlling
flow of the metal from the vessel.
Conventionally, a stopper rod is formed as an elongate one-piece bonded
alumina-graphite composition by isostatic pressing in a suitable mould. The
stopper rod usually has a hollow structure in that it is formed to have a
longitudinal, central bore by use of a suitable removable core in the mould.
The
lower end of the stopper rod bore may be closed or it may be vented to allow
gas,
e.g. argon, to be passed through the rod.
Thus in the conventional manufacture of a stopper rod, the desired
alumina-graphite composition is placed around a removable core in a deformable
mould, i.e. the composition fills the mould space between the core and the
deformable mould. The deformable mould is then placed in a tank of liquid,
e.g.
water, and the pressure of the water is increased by known means to the
pressure
required to convert the alumina~raphite composition to the desired
self supporting structure. After removal of the isostatically pressed product
from
the mould, the core is removed to provide the hollow stopper rod.
The upper end of the stopper rod must be provided with means by which it can
be
attached to the lifting and lowering mechanism. Various prior proposals have
been made, including co-pressing a threaded bush in the upper end of the
stopper
rod to receive a threaded shaft; forming the upper end of the bore of the rod
with
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an internal thread to receive a ceramic or metallic insert, and placing a
retaining
pin through coterminous holes drilled through the stopper rod and an
attachment
rod.
In the particular embodiment in which the upper end of the bore of the stopper
rod
is formed with an internal thread to receive an insert, this is conventionally
achieved by positioning an externally-threaded annular insert in the mould at
a
location to form the internally-threaded portion of the stopper rod bore so
that it
extends along a portion of the bore from the commencement of the bore at the
upper end of the rod. It has been found, however, that a number of problems
can
arise from such an arrangement.
For example, cracks may be formed in the pressed stopper rod body. These may
be discovered after stripping the tooling from the pressed product but some
cracks
may only be visible after the pressed product has been fired. Other cracks
that
are formed internally of the product can only be detected by X-ray
examination. In
all these cases the presence of cracks leads to the scrapping of the
expensively-
formed product. Moreover, the stopper rods may have low strength as measured
by the torque used to tighten the stopper to the fitting mechanism, they may
show
inconsistent performance due, for example, to breakage at a relatively low
torque
or during transport after assembly, and they are liable to failure during use.
The present invention aims to provide an improved means of making a stopper
rod having a body with an internally-threaded bore portion and, hence, also
aims
to provide an improved stopper rod.
Accordingly, in one aspect, the present invention provides a method of making
a
stopper rod in which a composition to form the body of the stopper rod is
charged
into a mould, a removable core being positioned in the mould to form a
longitudinally-extending bore in the stopper rod body, an externally-threaded
removable insert being positioned in the mould in correspondence with the
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removable core to form an internal thread on a portion of the bore and a plate
being positioned in the mould to define a flat upper end of the stopper rod
body,
wherein a spacer is positioned between the externally-threaded insert and the
plate whereby the internal thread in the bore of the stopper rod body is
formed
spaced from the upper end of the body when the composition is pressed in the
mould.
It will be appreciated that by "upper end" of the stopper rod body is herein
intended to refer to the stopper rod in its use position.
In another aspect the invention provides a mould for the manufacture of a
stopper
rod body, the mould comprising a removable core corresponding to a desired
bore in the body, an outer shell to surround the removable core to define a
cavity
in which the body is to be formed, an externally-threaded removable insert to
be
fitted in the mould in correspondence with the core to form an internal thread
on a
portion of the to-be-formed bore, a plate to be positioned in the mould to
define a
flat upper end to the body and a spacer to be positioned between the
externally-threaded insert and the plate, whereby the intemai thread in the
bore
will be formed spaced from the upper end of the body.
Thus the amount by which the internal thread is recessed into the bore is
equal to
the thickness of the spacer.
In a yet further aspect the invention provides a stopper rod having a body
formed
by the method or by using the mould of the immediately preceding paragraphs.
Thus the invention also provides a stopper rod having a body containing a
longitudinally-extending bore, the bore opening out at a flat upper end of the
body
and having an internally-threaded portion spaced from the upper end of the
bore.
When attaching the stopper rod of the invention to a raising and lowering
mechanism, the annular recess teft at the upper end of the stopper rod by the
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removal of the spacer can be filled with refractory cement around the part of
the
raising and lowering mechanism that passes through the recess when the end of
the mechanism is threaded into the threaded internal bore portion.
Alternatively, in
a preferred embodiment, a ceramic ring may be fitted into this recess. The
attachment of the raising and lowering mechanism may then be facilitated with
easier and more accurate centreing of the attachment rod. Moreover, a better
sealing effect may be achieved. The ceramic ring may conveniently be made of
the same composition as the stopper rod body, although this may not be found
to
be essential.
The basic means used to mould the stopper rod body of the invention may be any
convenient means and may, therefore, be the conventional method using
isostatic
pressing in a deformable mould Thus the mould shell may be deformable as
described above.
The lower end of the stopper rod bore may be closed or it may be vented to
allow
gas, e.g. argon, to be passed through the rod.
The threaded insert may be a solid cylinder or a cylindrical annulus. It may
be
attachable to one end of the removable core or be formed as an extension of
the
core so that it can be inserted into the mould in one piece with the core.
Clearly, the spacer will be of a thickness determined by the amount that it is
desired to space the commencement of the threaded portion of the bore from the
upper end of the rod body. For example, the spacer may be from 10 to 50 mm
thick, although it will be appreciated that thicknesses outside this range may
well
prove suitable in certain circumstances. It may be attached to the threaded
insert
by any convenient means.
The spacer is preferably of metal. e.g. steel, but may be of any other
suitable
material, e.g. a ceramic material.
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The externally-threaded insert may be the same as has been conventionally
used.
Thus, by way of example only, it may be of steel and of external diameter from
50
to 150 mm and of length from 25 to 150 mm.
We have found that stopper rod bodies formed according to the invention are
surprisingly and significantly less prone to cracking than prior stopper rod
bodies
in which the threaded portion of the bore extends to the upper end of the
stopper.
Although not wishing to be limited to any particular theory, it is believed
that a
summary of some of the advantages obtained and of the reasons for some
advantages is as follows.
1. Cracking at the end of the bore during pressing is greatly reduced or
eliminated
due to the absence of thread in this critical region.
2. Higher torque values can be tolerated because the thread area is
consistently
under compression during assembly.
3. The thread area is better protected from oxidation during firing of the
pressed
body.
4. Better gas tightness can be obtained.
Moreover, further advantages include the fact that conversion to the design of
the
invention from conventional designs is very simple and requires a minimum of
tooling changes. It may also be found possible to eliminate the need to use a
ceramic or other insert threaded onto the threaded portion of the bore, i.e.
the
stopper fittings to the lifting and lowering mechanism may be directly
threaded
onto the threaded portion of the bore.
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Embodiments of the invention will now be described by way of example only with
reference to the accompanying drawings in which:
Figure 1 is a diagrammatic representation of a portion of a conventional mould
for
the manufacture of a stopper cod body;
Figure 2 is a similar view of a portion of a mould according to the invention;
Figure 3 is a longitudinal section through a stopper rod of the invention;
Figure 4 is a similar view to Figure 3 but showing a stopper rod with a
ceramic ring
insert at its upper end; and
Figure 5 is a longitudinal section through the upper end of a further stopper
rod of
the invention.
In Figure 1, a mould cavity 10 is defined between a removable core 12 and a
deformable shell 14. At one end of core 12 is positioned an externally-
threaded
insert 16, which at its end remote from core 12 is in contact with a plate 18.
Insert
16 has an externally-threaded portion 16A, a central stepped portion 16B and a
tail portion 16C. Plate 18 defines the end of mould cavity 10 that corresponds
to
the upper end of the stopper rod body that is to be formed in the cavity. It
wiH be
appreciated that the thread 20 on portion 16A of insert 16 will be moulded
into the
stopper rod body that will be formed when cavity 10 is filled with a suitable
composition and pressed between shelf 14 and the core 12 and insert 16. This
thread wil! be on the wall defining the bore that is formed in the body and
will
extend along the bore from its end corresponding to plate 18.
The use of stepped insert 16 enables a number of cores 12 of different lengths
to
be used depending on the overall length of the stopper required.
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In Figure 2 like parts are given the same reference numerals as in Figure 1.
In this embodiment of the invention mould cavity 10 is again defined between
core
12 and deformable shell 14. An externally-threaded insert 16 is again attached
at
one end of core 12. However, according to the invention, insert 16 is
separated
from plate 18, which again defines the end of mould cavity 10, by a spacer 22.
The threaded end portion 16A of the insert is attached to spacer 22 by
screw-threaded means 22A.
Thus it will be appreciated that the pressed body to be formed in this mould
will
also have a threaded portion on its bore wall but this portion will be spaced
from
the end of the body by an amount equal to the thickness Nt" of spacer 22.
In Figure 3, a stopper made in the manner described with reference to Figure 2
is
shown. The stopper 30 has a longitudinally-extending bore 31 which emerges as
a narrower bore 32 at its lower (in use) end 33. As is conventional, a porous
plug
34 has been inserted in bore 31 near end 33.
At its upper (in use) end 35 the bore emerges as an unthreaded recess 36,
wider
than bore 31 and corresponding to spacer 22. A threaded portion 37,
corresponding to threaded portion 16A, lies between recess 36 and main bore
31.
In Figure 4 is shown a similar stopper rod to that of Figure 3 and in which
the parts
are numbered identically except that recess 36 of Figure 3 has had a ceramic
ring
38 inserted into it to enable fitting to a raising and lowering mechanism to
be
facilitated.
The ring 38 need not be adhered into recess 36 but may advantageously be a
relatively tight flt.
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In Figure 5 the stopper rod 40 has a longitudinally extending bore 41 having
an
unthreaded recess 42 at its upper end and a threaded bore portion 43
immediately below recess 42.
A threaded ceramic insert 44 is threadingly engaged in the threaded bore
portion
43 and a ceramic ring 45 lies in recess 42 in contact with the upper surface
of
threaded inserk 44. Threaded ceramic insert 44 ensures a tight seat in the
stopper
body for the connecting rod 46, the lower end 47 of which lies within bore 41.
Rod
46 has an annular collar 48 which seats on a ledge 49 in bore 41 immediately
below threaded portion 43 of the bore. The rod 46 is inserted in the bore
before
the threaded insert 44, the lower end of the latter locating on the upper
surface of
the collar 48 and thereby locking the rod firmly inside the bore 41.
A fibre gasket 50 is positioned on rod 46 and lies in contact with the upper
surface
40A of stopper rod 40, although it is shown spaced from that surface for
clarity in
Figure 5. A screw nut 51 on the rod 46 can be screwed downwardly onto a
washer 52 above fibre gasket 50 to lock the components tightly together.
This arrangement provides a good air-tight assembly at the upper end of the
stopper rod, which is highly desirable as there may be a vacuum inside the
bore
41 during use.
