Note: Descriptions are shown in the official language in which they were submitted.
7~3
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Electrode Holder for Electric Arc Furnaces
FIELD OF THE INVENTION
The invention relates to electrode holders for electric arc
furnaces comprising a water-cooled metal shaft and a working
part of consumable material, with the metal sha~t being,
at least partly, surrounded by moldings of high-temperature
resis-tant material.
BACKGROUND OF THE INVENTION
Electrodes with electrode holders of the type mentioned are
available in two basic designs. According to the first design
the electrode consists of two axially aligned sections, i.e.
the electrode holder, which constitutes the upper section,
comprising a cooled metal shaft, a,nd, at its lower tip,
the active part of consumable material where the electric
arc is produced. This type of electxode is generally known
as combination electrode. With the second type of design,
the active part of consumable material is axially movable
within the electrode holder which basically comprises
a cooled metal shaft. The active part of consumable material
consumed at its lower tip may, therefore, be compensated for
by axial movement. This type of electrode is generally known
as conventional feed through electrode. A common criterion
of both designs is that the electrode holder, i.e. the
li~uid-cooled metal shaft, during operation projects,
at least partly, into the interior of the furnace.
However~ electrodes for electric arc furnaces are exposed
to high thermal and mechanical stress. Thermal stress
conditions result from the hiqh working temperatures reached
particularly in the production of electric steel. Mechanical
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stress ls caused e.g. when the el~ctrode, upon its insertion
inko the furnace, hit~ scrap material, or it may also result
from movements of ~he moL~en metal or scrap material, or from
vibrations caused by the electric arc.
In order to ensure th~ usefulness of these electrode~ it is,
therefore, imperative to effectively protect the cooled metal
shaft of the electrode holder, which during operation is in
the interior of the furnace, against th~ thermal and
mechanical stress me~tioned. Numerous solutions have been
offered to this proble~.
The electrode holder of the combinakion electrodes described
in BE-PS 867 876 takes the for~ of a metal shaft which contains
the cooling syst~m and which is covered with a high-temperature
resistant coating on the outside. In order to improve the
adhesive capacity of this coating on the sl~eath area, the
metal ~haft has hooks holding the coating in place.
Similar combination electrodes are described in GB-PS 1 223 162.
Their electrode holder is completely covered with a protective,
ceramic coating. With this type of solution, attention has to
be paid to the thickness of the ceramic coating, which should
be as small as possible, and to the extent of lts application,
which should include the electrode holder proper to ensure
the insulation of its pipes. These pipes serve not o~ly as
cooling water ducts but also aR current supply to the active
part of graphlte~
~heEuropean patent application 0 010 305 published April 30,
1980 (U.S. Patent 4,291,190 is equivalent~ describes a combin-
ation electrode with an electrode holder comprising a mekal
shaft ~hich is electrically insulated against the current-
carrying cooling system and which can be sufficiently cooled by
a high-temperature resistant material between the cooling sys-
tem and the metal shaft. The lower section of the metal shaft,
~3~'7~
which constitutes the electrode holder, is covered with
a ceramic coat that is also secured by hooks.
The combination electrode of DE-AS 27 25 537 has a metallic~
liquid-cooled upper section constituting the elec-trode
holder, which is insulated by a high-temperature resistant
material covering thermally conductive pro~ections. The
purpose of these projections is to prevent a direct
mechanical contact with the line system if, as a result
of high local stress~ the high-temperature resistant material
is locally damaged by rigid scrap material. At the same time
these projections act as a kind of fusep thereby preventing
excessive currents from passing.
Finally, DE-AS 27 30 884 describes a conventional feed-through
electrode whose cooled metal shaft, which constitutes the
electrode holder and serves as passage through which the
active part of graphite is fed, is covered with high-temperature
resistant ma-terial. At the same time the metal shat has
projections directed radially towards the outside which fasten
the high-temperature resistant material. Thlese projections,
which are distributed along the periphexy and in axial
direction as evenly as possible, are designed to ensure
a more constant cooling and better adhesive capacity of the
high-temperature resistant material. This solution corresponds
to the protec-tive coat designs mentioned in connection with
combination electrodes. According to the most recent state of
technology, the same solutions are offered for the electrode
holders of combination electrodes and conventional elec-trodes.
All these electrode holders have one disadvantage in common,
i.e. that the protective jacket, even if it is only slightly
locally damaged, has to be removed from the metal shaft of
the electrode holder and a new protective jacket has to be
applied,which causes lengthy interruptions and high costs.
A further disadvantage of conventional electrode holders is
the formation of slag and mekal layers on the protective
jacket of ceramic material, which leads to disorders in
furnace operation.
It was, therefore, suggested to create an electrode holder
whose cooled metal shaft is protected by rings of a material
containing carbon, preferably graphite. Electrode holders of
this type have been employed and the protective jacket
mentioned has proved extremely useful. The graphite rings
act as an excellent protective coat from the mechanical
as well as the thermal viewpoint. One advantage of such a
protective jacket is that, i it is partly damaged, the
graphite ring in question may be exchanged, while complete
removal of the jacket is only required if continuous
protective coatings are used. A further advantage is that
the formation of slag or metal layers is avoided, for due
to the destruction of the graphite surface by oxidation
they keep falling off the protective jacket. One disadvantage
is, however, that in some cases the rings show a certain
tendency towards cracking which is caused by the different
thermal expansion of the protective jacket and the metal
shaft constituting the electrode holder, and, consequently,
by the resulting tensions within the protective ring.
Furthermore, there is one problem that all electrode holders
described have in common~, namely, how the metal parts can
be fastened above the furnace lid. In general, this is
achieved by means of clamping devices. Considering such
factors as easy handling or ~uality of electric contact
it is advantageous to use the mechanical clamp also as
a means of current transferO As a consequence, graphite
or carbon moldings are usually used between the metallic
part of the electrode and the clamping jaws of a bearer
arm, for these moldings combine favourable current transfer
properties with good mechanical and therma:L properties.
However, the way how these moldings are to be fastened to
the electrodes poses problems, as the moldings may break
due to the clamping forces required. This may, in turn,
lead to the loss of the moldings when, in varying the
place of clamping e.g., t~e clamps have to be re~loved.
OBJECT OF THE INVENTION
The object of the presen-t invention is to create a protective
jacket for the cooled metal shaft of electrode holders of the
type mentioned which fully meets all thermal, mechanical,
and electrical requirements, which has a design as simple
as possible, which can be easily mounted and repaired, and
which guarantees a good heat transfer to the cooled metal
shaft of the electrode holder in order to improve the service
life of the pro-tective jacket.
With regard to the electrode holder in question, this problem
is solved by the connection of moldings to the metal shaft
and/or among each other in a removable manner by means of
form locking and/or resilie~tconnection elements.
The solution in accordance with the invention provides a
protective jacket that meets all electrical, thermal, and
mechanical requirements. I~ a ~resilient connection i5
used, the prestress of the individual moldings of the
protective jacket makes them rest snugly on -the metal shaft
of the electroda holder, which results in a good heat transfer
between protective jacket and metal shaft over the entire
area. This good heat transfer is achieved without inserting
any filling material between the protective jacket and the
metal shaft of the electrode holder~ In addition, on account
of the~ resilient connection of the sectors, the individual
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moldings are capable of balancing tensions caused by the
different thermal expansion of the material of the protective
jacket on the one hand and the material of the metal shaft
of the electrode holder on the other. Thus r there is no danger
of the protective jacket belng damaged by this thermal
expansion. The protective jacket is, therefore, in a position
to meet all thermal requirements.
The s~me holds true for mechanical stress. By connecting
the sectors in accordance with the inventionl i.e.among
each other or to the metal part, it is possible to balance
the production tolerances of the moldings so that their
inner sheath area is always snuyly pressed against the
sheath area of the metal shaft of the electrode holder.
In this way, compressive and bending ~orces are transferred
from the protective jacket to the metal shaft of the electrode
holder without any excessive strain on the material of the
protective jacket as a result of insufficient contact between
protective jacket and metal shat. At the same time, the
metal shaft of the electrode holder is also protected by
the moldings. Finally, depending on the requir~ments,
the moldings are easy to mount or dismount. For this
purpose, individual moldings or groups of r~oldings may be
axially moved along the metal part. It is e.g. possible
to connect several moldings or sectors which then form a
partial ring, or to connect several partia:L rings which
then form one complete ring. This means that a protective
ring may be directly mounted on the metal shaft of the
electrode holder. If one or several sectors of the protective
ring are damaged, the damaged sector may simply be exchanged.
If the protective iacket consists of several rings each of
which is made up of sectors, the ring at the lower tip of the
metal shaft of the electrode holder which :is, of course,
exposed to the highest strain within the furnace and, therefore,
more likely to be damaged or worn than the rings arranged
further to the top, should be removed first. It should be
replaced by either a new ring or a ring used in the upper
section of the metal shaft that is still suitable for the
lower section. In this way it is possible to replace the
rings successively, to reduce assembly time, and to cut
the main~enance costs for the protective jacket of the
electrode holder.
Advantageous designs of the electrode holder in accordance
with the invention result Erom other claims as wellD
According to one version the sectors may consist of
non-graphitic or partly graphitic materials containing caxbon.
This results in economical service times of the protective
rings, while the properties of the carbon material with
regard to slag or metal splashes are also favourable. If the
dimensions of the protective rings are correct, the desired
slow oxidation of carbon will be achieved especially on the
hotter exterior peripheral areas of the rings, which prevents
the accumulation of slag or metal parts, a disadvantage
frequently observed when ceramic coatings are used.
Since the solution in accordance with the invention offers
an ex~ellent heat transfer from the protective jacket to
the metal shaft of the electrode holder, it is recommended
that the protective jacket be made of materials that have
a low thermal conductivity. Among the materials con~; n; ng
carbon, the so-called non-graphitic or partly graphitic
materials are, therefore, especially suitable.
If the user can do without the advantage of self-purification
of the surface of the protective ring, céramic materials may
also be used.
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It is useful to use ceramic materials for the sectors
in the upper section of the metal shaft of the electrode
holder, and materials conta-ning carbon for the sectors
in the lower section. Othex solutions, such as a mixed
arrangement of rings or sectors of different materials,
are also possible.
It is certainly an advantage that the non-positive
connection of the sectors among each other and the
formation of prestress,which enables the ring consisting
of sectors to snugly rest on the metal shaft of the
electrode holder, are achieved by spring power.
As far as the arrangement of the springs producing the
spring power is concerned, there are a number of possibilities.
Each protective ring may have on~ or several spring rings,
with each sp~ing ring being formed either by one spring or
by several springs connected in series.
The springs are located in sector bores or recesses which
are concentric to the ring. In this way the springs are
incorporated in the sectors, which is a great advantage
because they are protected against excessive thermal and
mechanical stress.
In order to further reduce the ~herm~l strain on the springs,
the bores or recesses are located near the inner sheath area
of the sectors, which means that the springs and the cooling
system of the metal shalt are as near as possible to each
other so that the temperature in the spring zone is kept
as low as possible.
The springs used may be spiral springs or leaf springs.
It is of special importance that the springs consist of
a non-magnetic material in order to avoid the heating
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of the springs as a result of hysteresis losses.
Basically r the springs should be characterized by a high
heat resistance. For this reason the springs may be made
of austenitic chrome-nickel-molybdenum steels or of
material containing berylliumO
In accordance with another preferred embodiment of the
invention, the abutting surfaces of neighbouring sectors,
which lie in peripheral and/or axial direction, have
at least one complementary radial graduation. Even i the
abutting surfaces of neighbouring sectors do not snugly fit
due to tolerances, these interlocking graduations guarantee
that the sectors are well sealed, which, in turn, results
in a safe protection of the cooled metal shaft of the
electrode holder.
According to another embodiment the width of the sectors
measured in peripheral direction is relatively small,with
the abutting surfaces and the radial beam of the hollow
cylinder forming an angle. This means that in relation to
the respective radii the relatively thin sectors of a ring
rest in an oblique manner on the metal shaft. Thus tolerances
are balanced as a result of the so-called l'effect of self-
adjustment", with the sectors adjusting thems~lves in a
more vertical or a more horizontal position depending on
the diameter of the metal shaft and/or the inside diameter
of the sector ring.
This "e~fect of self-adj~stment" results from the fact that
the inclined sectors of the protective riny are arranged by
and in accordance with the tangential force component of the
spring tension. The tangential force component of the spring
tension is achieved if in the individual sectors - which
are aligned along the periphery - the one end of the bore
~31 9~ 76~
or recess for the spring has a greater distance from t~e
sheath area of the metal shaft than the other and of the bore
or recess in question.
This sector adjustment can be observed particularly if the
inner sheath areas of the sectors are smaller than the outer
sheath~areas resulting theoreticall~ from the circular
division. The result is a protective ring which, if properly
mounted t has splits between the sectors. These splits become
wider towards the inside. ~he sectors are arranged in such
a way that wedge-like splits are formed between the sectors.
As mentioned before, these splits always open up towards
the inside and are closQd on the outside, even if the diameter
decreases.
For the adjustment process described above it is useful
that the sectors have a plane inner sheath area so that
they can move and align themselves accordingly on the
sheath area of the metal shaft. The outer sheath areas
of the sectors may also be plane and need not have a
cylindrical shape. In addition, the imler as well as the
outer sheath area of the sectors may have suitable
profilesO
In order to avoid the heating of sprinys, particularly
of springs closed in peripheral direction, on account of
potential parasitic currents, it may be useEul to build
at least one electrically insulating connection element
into the springu Such a connection element may e.g. consist
of highly sintered aluminium oxide.
The same consideration may be the reason for the incorporation
of electrically insulating elements between the abuttillg
surfaces of the sectors. This applies above all to abutting
surfaces in peripheral direction. A~utting surfaces in
~ ~2 ~ ~
axial direction may also be kept at a distance by mean~ of
electrically insulating elements.
If form locking connections are used~ it will be advantageous
to design the connection elements as -sliding
connections the sliding direction of which is parallel to
the axis of the metal shat of the electrode holder. In
this way it is possible to move the moldings or individua~
sectors either up or down in the simple way described
so that partly damaged rings or sectors can be exchanged
without causing extensive assembly work or the use of
an excessive number of spare or replacement rings or
sectors.
In the concrete case the positive sliding connection
mentioned is designed as dovetail guide. This dovetail
guide is not only mechanically solid but also enables
the user to slide the sectors in a simple manner against
the sheath area of the metal shaft of the electrode holder.
The grooves of the dovetail guides are located on the inner
sheath areas of the sectors, while the contact strips are
mounted on the sheath area of the metal shaft. This location
of grooves on the inner sheath area of the sectors is
advantageous because the loss of the relatively expensive
sector material, which is extremely resistant to thermal
and mechanical stress, is kept to a min;ml1m.
For reasons of expediency, the contact strips are separate
components which are fastened to the sheath area of the metal
shaft by riveting, bolting, welding or a similar method.
This is helpful not only in saving material for the metal
shaft of the electrode holder but also in mounting the
contact strips on relatively thin~wall~d metal shafts.
The cooled metal shaft of the electrode holder usually
consists of copper, which is very expensive so that material
13
savings are really decisive. Moreover, the metal shaft or
the pipes making up the metal shaft which are intended
for the cooling agent and the current supply, have to
have relatively thin walls in order to obtain a cooling
efficiency that is optimal for the entire unit.
If the protective jacket consists of several rings that
are made up of sectors, it may be practical to place one
one-piece ring between any two rings consisting o sectors.
As a result, the carrying capacity of the protective jacket
may be further enhanced.
In accordanc~ with another embodiment of the invention
the contact strips are interrupted in axial direckion
of the metal shaft, with the distance between two aligned
contact strip sections being not greater than the twofold
axial height of a sector in this region. In this way it
is possible to remove damaged or worn sectors and replace
them by new ones even in the middle 20ne of the protectivP
jacket without having to remo~e the sectors above or below.
This also helps to reduce assembly time.
Furthermore~ it is advantageous if the contact strip sections
are grouped in rings and if the contact strips of one group
and the contact strips of the axially neighbouring yroup
are staggered in peripheral direction. This results in a
sector arrangement that is staggered ring by ring, which
leads to a further increase in the mechanical stability of
the protective jacket.
If that embodiment of the invention is used where at least
one one-piece ring is part of the protective jacket, it is
recommended that at the place where the ring is to be
mounted the axial distance between two neighbouring groups
of contact strips be somewhat greater than the axial height
of the one~piece ring. The ring may then be turned at this
place so that its grooves and the contact strips of the
neighbouring groups are offset, which results in the ring
being arrested in axial direction. Thus the ring serves as
a support for all sectors or rings consisting of sectors
that are located above the ring. If, therefore, the sectors
below the ring break off partly or completely, the sectors
above the ring are prevented from slipping along. This means
that a large part of the metal shaft of the electrode holder
r~ma; n.~ protected by undamaged sectors, even if the protec-tive
jacket is damaged considerably. In this way it is possible
to keep the damage to a minimum.
Sectors and/or one-piece rings should consist of non-graphitic
or partly graphitic materials containing carbon. As a
consequence, the life time of protective rings will be
satisfactory from the economic point of view. A further
advantage of the carbon cont~ining material relates to
its favourable properties as far as slag or metal splashes
are concerned. If the protective rings have the proper
dimensions, the oxidation o~ carbon will proceed as slowly
as desired~ especially on the hotter peripheral areas of
the rings, thus preventing the troublesome accumulation of
slag or metal parts that is requently observed on ceramic
coatings.
The solution in accordance with the invention guarantees
an excellent heat transfer from the protective jacket to
the metal shaft. It is, thexe~ore, recommended that materials
of low thermal conductivity be used for the pro-tective jacket.
Among the materials cont~; ni ng carbon the so-called
non-graphitic or partly graphi~ic materials are especially
suitable for this purpose.
I the user intends to do without the advan-tage of self-
purification of the protective ring surface, ceramic
materials may also be employed.
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Depending on the respective requirements the material of the
protective jacket may vary, i.e. materials con~Ain;ng carbon
as well as ceramic materials may be used for both sectors
and one-piece rings. It is favourable to use ceramic materials
where the sectors of the upper section o-f the metal shaft
are con~erned, and materials cont~;ning carbon where the lower
section is concerned. Other solutions such as a mixed
arrangement of rings or sectors of different materials are
also possible.
When connecting the moldings located between metal shaft and
clamping jaws in accordance with the invention, it is preferable
to use a form loc~ing connection which is supported by resilient
clamping.
For this purpose axially directed pairs of rails are affixed
to the metal shaft the shape of which renders it possible
to keep the axially directed rims or edges of the moldings
between the rail parts projecting from the metal shaft and
the metal shaft proper.
Therefore, the rails are characteri~ed by a section adjacent
to the metal shaft, a section leading away from it~ and a
section that is parallel to the metal shaft.
BRIEF DESCRIPTION OF THE DRAWINGS
Some embodiments of the electrode holder are illustrated in
the accompanying figures in which
Figure 1 is a schematic full-view illustration of an electrode
with an electrode holder in accordance with the invention,
Figure 2 ls an illustration of a hollow cylinder sector
several of which make up the protective jacket of the
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16
electrode holder in accordance with the invention,
Figure 3 is a sec~ional view of a par~ial ring consisting
o~ several sectors,
Figure 4 is a projection of such a partial ring,
Figure 5 is an illustration of the assembly of the protective
jacket of the electrode holde.r in accordance with the invention,
with the ring being made up of sectors that con~ist of several
partial rings,
Figure 6 is an illustration of a possible type of connection
of the springs connecting the sectors which then form a ring
or a partial ring,
Figure 7 is an illustration of another embodLment of the
se tors,
Figure 8 is a perspe~tive view of the sectors as lllustrated
in Figure 7,
Figures 9 to 11 are illustrations of possible axial
connections of rings consisting of sector~,
Figures 12, and 13 are illustrations of further ~bodiment~
of sectors,
Flgure 14 is an illustration of another possibility of
joining springs which then form a spring ring,
Figure 15 ~s an axial sectional view o~ an electrode with an
electrode holder in accordance with the invention,
Figure 16 is a radial secti.onal view of the electrode a~ shown
in Figure 15 along the intersection line XVI~XVI,
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Figure 17 is an illustration of the metal shaft of the
electrode holder with paxtly omitted protective jacket
to show the distribution of contact strips on the sheath
area of the metal sha~t,
Figure 18 is a radial sectional view of the electrode
as illustrated in Figure 17 along the intersection line
XVIII--XVIII,
Figure 19 is a perspective view of a sector several of which
form the protective jacket of the electrode. holder in
accordance with the invention,
Figure 20 is a perspective view of a one-piece ring intended
for use in a protective jacket of an electrode holder in
accordance with the invention,
Figure 21 is a modified embodiment of a sector for the
protective jacket of an electrode holder in accordan~e with
the invention,
Figure 22 is a further embodiment of a sector for the
protective jacket of an electrode holder iIl accordance
with the invention,
Figure 23 is an illustration of the metal shaft with three
moldings which are arranged between metal shaft and
clamping jaws and which are fastened by~means of form
locking rails.
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DETAILED DESCRIPTION OF THE DE~AWINGS
Figure 1 is a schematic illustration of the basic structure
of a combination electrode for electric arc furnaces. This
electrode comprises an electrode holder that is formed by
a cooled metal shaft 1. An active part 2 of consumable
material, e.g. graphite, is attached by means of a threaded
nipple 3 to the lower tip of the metal shaft 1 constituting
the electrode holder. The electrode is held by a supporting
structure 4 affixed to the upper section oE the metal
shaft I of the electrode holder. Figure 1 is a schematic
illustration only, the electrical components and cooling
elements are not shown, as they may be of the conventional
type. The only part that is important in connection with
the invention is the hollow cylindrical protective jacket 5
of temperature-resistant material which surrounds that section
of the metal shaft 1 which remains within the furnace,
thus protecting it from excessive thermal and mechanica
stres~.
The protective jack~t 5 is made up of hollow cylinder
sectors 10 as shown in Figure 2. The hollow cylinder
sector has an inner 11 and an outer sheath area 12,
two abutting surfaces in peripheral direction 13l and two
faces in axial direction 14. In addition, the sector has
two bores 15 which are located on one chord.
Figures 3 and 4 clearly show ho~ several s~ctors 10 form a
partial ring by joining their abutting surfaces 13. The
sectors 10 of this partial ring are connected by springs
which pass through the bores 15~ In the figure the springs
shown are spiral springs 20. Fork-like clamping elements 21
guarantee that the spiral springs 20 are secured at the
end, thus prestressing them. These elements 21 hook on
counter stops located at the end of the springs 20 or
~l76V
on their windings, thus keeping the springs 20 in a prestressed
position.
Figure 4 illustrates on the left how the clamping elements 21
are fastened, while on the right of the illustration they
are already arrested.
Figure 5 shows how partial rings, which consist of sectors lo,
are joined,thus forming a complete ring. The partial rings
are successively joined by connecting the respective tips
of the springs 20 which are kept in a prestressed position
by clamping elements 21. Upon the removal of the clamping
elements 21 the abutting surfaces of the sectors rest snugly
on each other also at the places where the partial rings
join.
Such a ring, which is made up of sectors 10, may either be
slid on the metal shaft 1 from one end or may be radially
mounted ~n the metal shaf~-t in the w~y described
by joining partial rings.
Th~ decisive criterion is that the rings, which consist
of sectors 10, rest directly and with prestress on the
sheath area of the metal shaft 1, as can be seen in
Figure 1. This results in the advantages alxeady described,
namely A good heat transfer between the protective jacket 5
and the metal shaft 1, less wear and tear due to oxidation,
and the complete lack o~ detrimental tensions within the
protective ring which-may result from a di:Eferent ~hPr~l
expansion of the protective ring and the metal shaft or
from radial temperature gradients within the protective ring.
Figure 6 illustrates a further possible type of connection
of two springs connected in series 20 which, on the one
hand, help to connect the sectors 10 among each other in
the way described and which, on the other, are helpful
_ 20
in mounting a ring consisting of sectors 20 with a certain
prestress on the metal shaft 1. As shown in Figure 6, the
tips of the springs 20 are equipped with stops 22 for the
respective recesses 16 at the ends of the sector bores 15,
with the respective spring 20 bracing the respective
sectors 10 and at the same tirne pressing the entire ring,
which consists of sectors 10, in assembly position with
prestress against the sheath area of the metal shaft 1.
Figures 7 and 8 show a ~urther embodiment of thP sectors 10.
According to this embodiment, the abutting surfaces 13 of
the respective sectors 10 which lie in periPheral direction,
have at least one complementarY qraduation 17 in radial
direction which engages in the way illustrated. In this case
the metal shaft 1 is always safely protected, even if the
abutting surfaces 13 of neighbouring sectors 10 do not snugly
rest on each other, thus forming a split between the individual
sectors 10 that is covered by the graduations 17~ Small splits
between the individual sectors may be observed in the assembly
position whenever the outside diameter of the metal shaft 1
is greater than specified and/or the inside diameter of the
ring, consisting of sectors 10, of the protective jacket 5
is smaller than specified.
Figure. 9 illustrates a possible way in which the rings which
consist of sectors 10 can be axially joined, if the protective
jacket consists of several rings made up of sectors 10. In
this case the faces 14 of the individual sectors 10 have
grooves 18 in peripheral direction which are intended for
the joining rings 19. As a result, there is also a tight
connection between the faces 14 of the sectors 10 of
neighbouring rings.
Figure 10 shows a further possibility of how the springs 20
may be fastened in the sectors 10. According to this
embodiment the faces 14 of the sectors 10 have recesses 15a
in peripheral direction which accommodate the springs 20
6~D
21
in a similar way as the bores 15 do. The recesses 15a
may also act as grooves 18 required in the embodiment
according to Figure 9.
Figure 11 is an illustration of a sector 10 whose axially
directed faces and/or abutting surfaces 14 have a
complementary radial graduation 17a. This graduation
enables neighbouring sectors 10 to be positively joined
in axial direction. If the faces and/or abutting surfaces 14
of neighbouring sectors 10 do not snugly fit over the
entire area, the resulting split is covered by these
graduations 17a. As a result, the metal shaft 1 will
always be safely protected. Furthermore, by joining the
sectors 10 in a positive manner, the protective jacket 5
will be even more resistant from the mechanical point of
view.
It is, of course, possible to graduate the peripheral
abutting surfaces 13 as well as the axially directed
abutting surfaces 14 of each se~tor in order to brace
the entire structure of sectors, which constitutes the
protective jacket, not only in a form locking bu~ a1so
in a resilient manner.
Figure 12 and Figure 13 show further embodiments of a
protective ring for an electrode in accordance with the
invention. According to these embodimentsl the width of
the individual sectors 10 measured in peripheral direction
is relatively small so that a large number of sectors is
required for one protective ring. The sides 11 and 12 may
be plane. Furthermore, the shape of the sectcrs 10 may
be such that their abutting surfaces 13 and the radial beam
30a and/or 30b of the hollow cylinder form an angle or two
angles oC and/or ~ of different sizes. The inner sheath
areas 11 of the sectors 10 may be smaller than the outer
sheath areas 12 calculated on the basis of a circular division.
3'7~
- 2~ -
As a result, wedge-like splits 40, which become wider towards
the interior, will show between the abuttiny surfaces 13
of the assembled protective ring that rests with prestress
on the metal shaft 1. The tangential force component of
the spring tension presses the sectors 10, which join in
an oblique manner and which have smaller inner sheath areas 11J
in such a manner against the metal shaft 1 that the wedge-
shaped splits 40 are always closed on the outside. This
effect makes it possible to balance tolerances on the
outside diameter of the metal shaft 1 and/or on the inside
diameter of the ring made up of sectors 10. But even if the
sector areas 12 are attacked by oxidation, the wedge-like
splits will basically remain closed on the outside.
As is shown in Figure 12, the bores 15 are not located on
the chord of an ideal cylinder sector but they are rather
at an angle to this chord. This means for each sector
that the distance between the sheath area of the metal
shaft 1 and one tip of the bore 15 is greater than the
distance between the sheath area of the metal shaft 1 and
the other tip of the bore, with the respective tips
having the same peripheral direction. In this way the
tangential force component results from the spring tension
which causes the "effect of self-adjustment" described
earlier.
In order to prevent the spring which is closed in peripheral
direction from being heated by possible parasitic currents,
it may be useful to build at least one electrically
insulating connection element into the spring. This
embodiment is shown in Figure 14, where the electrically
insulating connection element 50 is used for the connection
of two springs 20. The electrically insulating connection
element 50 may e.g. be made of highly sintered aluminium
oxide.
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23
The same consideration m~y play a role if electrically
insulating elements, e.y. of asbestos, are inserted between
the abutting surfaces of the ~ectors. This type of
embodiment is not illustrated. This solution is recommended
especially for abutting surfaces 13 in peripheral direction,
it may, however, also be used for faces and/or abutting
surfaces 14 in axial direction.
Figure 1S is again a schematic illustration of ~he basic
structure of a combination electrode for electric arc
furnaces. This electrode comprises a cooled metal shat 51
constituting the electrode holder. An active part 52 of
consumable material, e.g. graphite, is attached to the
lower tip of the metal shaft 51 by means of a threaded
nipple 53. The electrode is held by a supporting structure 54
located in the upper section of the metal shaft 51 of the
electrode holder. Since Figure 15 is a schematic illustration
only, the electrical components and cooling elements of the
electrode holder are not included, as these components may
have a conventional design. What is important in connection
with the invention is a protective jacket 55 in the form of
a hollow cylinder that consists of one of the temperature-
resistant rnaterials mentioned. This jacket surrounds the
metal shaft 51 of the electrode holder along the section
located within the furnace, thus protecting it in the way
described against detrimental thermal and mechanical stressO
The protective jacket 55 is made up of hollow cylinder sectors.
One of them is shown in the perspective illustration of
Figure 19. The hollow cylinder sector 60 has an inner sheath
area 61 and an outer sheath area 62, two abutting surfaces 63
in peripheral direction, and two faces 64 in axial direction~
A number of such sectors 60 make up a ring. Several rings
consistiny of such sectors 60 form the protective jacket 65.
7~
The sectors 60 and ~he sheath area of the cooled metal shaft 51
are joined by means of positive connection elements. In the
concrete case these positive connection elements are dovetail
guides. Basically, there are two possible designs.
According to Figures 15 and 16 the grooves 65a run in axial
direction over the sheath area of the metal shaft 51~ i.e.
they are cut into the sheath area, while the corresponding
contact strips 60a run over the inner sheath area 61 of
the respective sectors 60. If this embodiment is used, the
grooves 51a run continuously over the entire length of the
metal shaft 51, which simplifies the manufacture of the
metal shaf-t 51. However, in this case the sectors can be
slid on the metal shaft 51 only from one end.
If the embodiment according to Figures 17 and 18 is used,
the contact strips of the dovetail guides are located on
the sheath area o the metal shaft 51. They are divided
into contact strip sections 72 which are grouped in rings 73
taking up at least one ring, preferably,however, two or
more rings consisting of sectors 60.
The individual contact strip sections 72 are riveted or
bolted 74 to the sheath area of the metal sha~t 51 and
are thus removable, if required.
The axial distance 75 between two groups 73 and 73' of
contact strip sections 72 is not greater than the twofold
axial height of the sectors 60 to be arranged in this region.
As a rule, it is rec~mm~n~d to have an axial distance 75
that is somewhat greater than the axial height of the
sectors 60 to be arranged in this region. In the area where
the contact strips are interrupted it is thus possible to
slip the sectors 60 on the individual contact strip sections 72
so that damaged or worn sectors can be exchanged in the
middle 20ne of the protective jacket 55 without having to
remove all sectors above and below.
- 25
It has proved useful to place a one-piece ring at certain
intervals between the ring-shaped groups. Such a ona-piece
ring 80 is shown in Figure 20. The inner sheath area 81
of such a ring 80 has grooves 82 which correspond or are
complementary to the contact strip sections 72. Such a ring
will, of course, be placed between two groups 73' and 73"
of contact strips 72. For this purpose the axial distance 76
between these two groups 73' and 73" of contact strip
sections 72 is somewhat greater than the axial height of
the ring 80. In this way the ring 80 may be moved from one
end of the metal shaft 51 to the zone of interruption 76
and turned in such a way that the grooves 82 and the contact
strip sections 72 of group 73~i and, if necessary, also those
of group 73' are staggered. In this way the one-piece rlng 80
will be firmly secured in axial direction elther against the
bottom or against both sides. If the sectors arranged below
the ring 80 break under certain extreme conditions, the
sectors arranged above the ring 80 are safely held by the
ring 80. As a consequence, any damage of the protective
jacket 55 or of the metal shaft 51 is kept to a minimum.
The group-wise staggering OL the contact strips 72, which
was described earlier, serves the same purpose, since it
prevents the sectors above from slipping down in case of
a complete braakdown of the sectors below. An additional
advantage of the groupwise staggering of the contact strip
sections 72 is that the abutting surfaces 6C of the sectors 63
are also staggered groupwise, which further increases the
solidity of the protective jacket 55.
Although the measures described help not only to obtain
a snug connection between the inner sheath area 60 of the
sectors 61 and the sheath area of the metal shaft but also
guarantee that the abutting surfaces of neighbouring sectors
rest snugly on each other, the latter may be improved even
more. Such an improvement is shown in Figure 21. The abutting
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- 26
surfaces of two neighbouring-sectors 60, wh:ich lie in peripheral
direction 63, have a complementary radial graduation. As shown
in Figure l5, these complementary graduations 66 of two
neighbouring sectors 60 engage. This mea~s that eve~ if
there is a split between two neighbouring sectors 60, this
split will be covered by the graduations 660 ~s a result,
the metal shaft 51 will nevertheless be safely protected.
Figure 22 shows that the axially directed abutting surfaces
and/or faces 64 of the sectors 60 of two axially neighbouring
rings may have radial complementary graduations that guarantee
the safe cover of these areas even if greater tolerances are
involved.
Another possibility are ring-shaped coverings between the
faces of axially neiqhbourinq sectors which rest on the
correspondinq periPheral qrooves in the sector faces, thus
guaranteeing the desired safe cover of possible splits.
Figure 23 illustrates a cross section of tha upper part
of an electrode holderD On the outside wall of the metal
shaft 91 there are three graphite moldings 92 which are
held by rails 94. Clamping jaws 93 are pressed against
the electrode holder which is thus secured in a certain
axial position.
The graphite moldings 92 may be distributed along the
periphery in an even or uneven manner, they may have,
but need not have, the same size as far as their length
on the periphery is concerned, but they should have the
same thickness. The rails ~4 are affixed to the metal part 91
by suitable bolts. As the graphite moldings occasionally
have to be exchanged, bolting is more practical than a fixed
connection such as welding.
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-- 27
The rails are shaped in such a way that they encompass the
graphite moldings at their axial edges in such a m~nnPr
that they are held against the metal part 91 without tension,
for in view of ~he strong clamping forces of the clamping
jaws 93 an excessive prestress would be highly unfavourable.
In order to be able to fulfill this holding function, the
rails 94 are designed in such a way that one part 96 rests
directly on the metal part 91, one part 97 leads away from
the metal part, and one part 98 runs at a certain distance
parallel to the metal part. This distance 99 is basically
identical with the thickness of the graphite moldings.
