Note: Descriptions are shown in the official language in which they were submitted.
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~IELD OF THE INVENTIQ~
The present in~ention reLates to contact assemblies and
seeks to provide a contact assembly which will give an adequate
electrical contact between two conducting elements disposed in
face to face relationship and which will permit satisfactory
passage of electricity and heat without demanding impracticably
fine tolerances in manufacture of the various parts.
BACKGROUND OF THE INVENTION
There are various applications in which electrical
currents must be passed between two opposed faces which are
clamped together or biassed towards each other by spring force
or gravity or the like, a particular and important example being
the feeding of electrical current to an electrode. ;~
Clamps for cylindrical graphite electrodes, such as are
used on an electric arc furnace, are normally made from cast or
wrought copper and often include passages for water cooling.
In recent years, and in order to reduce the consumption
of the graphite electrode, a techni~ue has been developed of
coating the graphite with aluminum and a large number of electric
arc furnaces have been converted to use the aluminum coated type
of electrodes. This has produced the result that, whereas prev-
iously a direct copper contact was satisfactory between the
clamp and the electrode, a different system had to be devised for
the aluminum coated electrode, because it is impracticable to
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make contact ~etweçn c~ppex and aluminium, partiçula~ly in hot
conditions.
For this reason the present practice is to provide the
electrode clamp with two graphite pads to give the required
electrical contact between the clamp and the coating of the
electrode.
This has given rise to disadvantages in that the large
surface axeas of bolh the cur~ed and the flat faces demand a
high degree of dimensional and angular accuracy which is vital
in order to achieve good electrical and thermal transfers. In
practice it is found that the tolerances of the various parts
cannot be kept within such strict limits as to ensure good
contact surfaces. Furthermore, it is difficult to ensure dirt
is not trapped between the copper and the graphite insert. This
in turn results in a poor transfer of heat and in arcing, and
this in turn damages the copper electrode clamp.
Another type of connection is to be found in the electro
slag refining process, where the electrode is constituted by a
steel billet. Electric current is fed to this billet through a
stainless steel pad which is welded onto the top of a stainless
steel bar which in turn is welded to the top of the billet. The
stainless steel pad is supported on a fork which carries the
weight of the assembly and through which the vertical height can
be controlled so that an arc is maintained on the bottom of the
electrode.
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~ t t~e p~e$ent time~ in o~de~ to connect one phase of
the electrical supp~ to eac~ o~ t~e t~xee billets representing
the three electrodes, a copper stu~ clamp is used which is
shaped to form two-line contacts with the top face of the stain-
less steel pad and which is mechanically loaded with approxima-
tely nine tons to maintain the necessary electricl contact
between the copper stub clamp and the steel pad and also to
retain the pad securely on the fork. Owing to the relatively
low conductivity of stainless steel, it is often found that when
a large current is applied between the line-contacts of the stub
clamp and the surface of the stainless steel pad, there is local
over-heating and subsequent damage to the contact faces.
SUMMARY OF THE INVENTION
The present invention, inter alia, enables a more
efficient system for electrical suppy to both these different
types of electrode.
According to the present invention we provide a contact
assembly capable of passing large electrical currents, the
assembly comprising two conducting elements disposed face to
face, a plurality of insert-receiving grooves in one of the
faces, each groove having opposed side areas or walls which
converge away from the face, a plurality of wedge-shaped graphite
inserts in the grooves, each insert being shaped to fit between
the convergent walls of a groove, leaving one end of the insert
spaced from the end of the groove and the other end of the
insert proud of the said one face, the said other end of the insert
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being of a ~ha~e to make elect~ical c~ntact with the other of
the faces, and ~eans ~hexe~y the t~ conduçting elements press
towards each other giving good eLectrical contact between the
graphite inserts and the two conducting elements and thereby a
path for the electrical current from one conducting element to
the other through the graphite inserts.
It is preferred that at least four carbon inserts be
used. The wedge angle between the side walls of the grooves and
the graphite inserts should be such as to give sufficiently firm
pressure between the graphite inserts and the sides of the groove
on the one hand and the said other of the faces on the other
hand to provide adequate electrical contact between these parts,
whilst permitting slight movement of the inserts under pressure
to take up any inaccuracies in the said other of the faces.
BRIEF DESCRIPTION OF DRAWINGS
The invention will now be described in further detail
with reference to the accompanying drawings, wherein:
Fig. 1 is a plan view, the top half in section, of one
form of clamp for a cylindrical graphite electrode incorporating
a contact assembly according to the present invention;
Fig. 2 is a side view, partly sectioned on the line
II-II of Fig. l;
Fig. 3 and Fig. 4 are a side view and end view
respectively of one of the graphite inserts of the clamp;
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Fig 5 i~ a side View of the upper end of the support
and cont~ct for an electrode billet ~or the eIectro sla~
refining process, s~o~ing the eIectrical connection thereto,
being a contact assem~ly accroding to the present invention;
and
Fig. 6 is a plan view thereof.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figs. 1 and 2 show an electrode clamp according to the
present invention. The body 10 of the clamp is made from cast
copper with the usual bus tube connection 16 and is provided
with water passages 18, the construction of which will be well-
known to those skilled in the art.
The body 10 is formed with ten longitudinal, vertical
grooves 22 in its cylindrical electrode clamping face 20. The
side walls 24 of each groove are planar and form between them
a small included angle so that each groove is a wedge-shaped
recess. In each recess is placed a graphite insert 26, as
shown in the upper part of Fig. 1. Each insert, seen in detail
in Figs. 3 and 4, is similarly wedge-shaped to the grooves 22
with side faces 28 having the same included angle as the side
walls 24 of the groove. The rear face 30 can be planar and the
front face 32 given a curved surface of the same radius as the
outer surface of an electrode which is to be clamped. An out-
line of part of such an electrode is shown in broken lines at
34 in Fig. 1. The ends 36 of the inserts are chamfered, so
that the inserts can be held in position by a bottom retainer ;:`
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38 ,made of, non-~,a,~netic steel and upper aluminium bronze
retainers ~0.
When the 'clamp is to be used ~or clam~ing an electrode,
the inserts 26 are comparatively loosely wedged in the wedge-
shaped grooves 22. When clamping pressure is applied betweenthe clamp shown in Fig. 1 and a clamping band on the other side
which is not illustrated, the clamping pressure between the
clamp and the electrode forces the inserts 26 into their res-
pective grooves and creates good electrical contact on the one
hand between the electrode and the inserts and between the
inserts and the clamp.
The function of the bottom and top retainers 38 and
40 is of course to hold the inserts in position before clamping
pressure is applied. As seen in Fig. 1 there are two top clamps
40, each comprising an aluminium bronze member which is cast
with four inwardly directed fingers 42 and two inwardly directed
flanges 44,46 at the ends, these flanges having slots 48 which
enable the retainer to be held in position under spring loading
from headed bolts 50 screwed into the clamp body 10.
Of the five graphite inserts 26 which are retained by
each retainer 40, four of the inserts are held by the fingers
42 and one by the end flange 46. When it is desired to remove
one of the inserts, it is only necessary to slide the upper
retainer so that the fingers 42 on the one hand and the flange
46 on the other hand move away from the top of the respective
inserts, after which the insert can be removed by inserting a
suitable instrument behind it, i.e. between the rear of the
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insext and the end of the gXoo~e, and wedgin~ the inseXt out-
wards.
The included wedge angle between the side walls 24 of
the grooves 22 and the side faces 2~ of the inserts 26 should
S be sufficiently high to ensure the adequate contact which is required and
sufficiently low to p~t the slight movement which is necessary when the
clamping is effected. In theory the optimum angle might well differ as
between the grooves which are nearer the centre of the clamp
and those which are nearer the two ends, since any mcvement of the clamp
towards the electrode will cause greater movement of the central
insert than those at the ends. On the other hand, it would be
a disadvantage of having different wedge angles that the inserts
would not be interchangeable from one groove to another. At
present we prefer to have the same angle for each of the grooves
and an included angle of 14 has been found to be satisfactory
with one particular grade of graphite.
As can be seen from Fig. 1, the effect of the wedge-
shaped inserts is to provide a large contact surface with the
electrode where the larger front faces 32 of the inserts contact
the out periphery of the electrode. It can be readily seen
that the clamp which is shown in the drawings could be made
with a smaller number of larger inserts or a larger number of
smaller inserts. Where there are a large number of inserts it
may even be found unnecessary to form the front surfaces with a ;
cylindrical surface.
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~ig~ 5 shows the upper end 52 of a stainless steel
~ar which is weIded to the top of the steel billet ~not shown)
which becomes an electrode in the eIectro slag refining
process. To the upper end of the stainless steel bar 52 is
welded a stainless steeI pad 54 which is 5Q mm in height, 356
mm in length and 240 mm in breadth. Four wedge-shaped grooves
58 are located lengthwise through the upper face 56 of the
bar 54 and in each of these grooves is a wedge-shaped insert
60. As with the arrangement of Figs. 1 to 4, the inserts 60
have their lower end spaced from the end of the groove and
the upper end proud of the face 56 as seen in Fig. 5. In this
case the electric supply is from a stub clamp the underface
of which is flat (this stub clamp being indicated as 62 in
chain lines in the drawings) and in consequence the upper end
of each insert 60 presents a flat face to register with the
flat bottom face of the stub clamp. When pressure is applied
to the stub clamp in order to weight it down onto the pad 54,
good electrical contact is made with the inserts 60; if any of
these are initially standing proud of the others, the effect
of the clamping pressure is to push it down into its groove
so as to level out the top surfaces to the level of the lower
face of the stub clamp whilst maintaining excellent electrical
contact both with the stub clamp and with the pad 54.
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In ~oth ~orm~ o~ the inVent~on illustrated, the
inserts are substantially trapezoidal in cross-section,
providing side faces which are angled in relation to each
other, a rear face, and a larger front face which is to make
5 . contact with a conducting element.
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