Note: Descriptions are shown in the official language in which they were submitted.
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A DIFFERENTIAL HYDRAULIC JACK FOR THE CONTROL
OF ELECTRIC CIRCUIT-BREAKERS
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a differential
hydraulic jack for the control of electric circuit~
breakers, of the type in which the annular chamber of
the jack as defined by the internal surface of the jack
cylinder and by the external surface of the emergent
piston~rod is continuously connected to a high-pressure
hydraulic fluid source.
Description of the Prior Art
The emergent rod of the hydraulic jack is
coupled to the mo~ing contact of the circuit-breaker and
a supply/drain orifice formed in the end of the main
chamber of the jack can be selectively connected to said
~: 15 high-pressure source ("supply" position) so as to thrust
back the piston or else to a low-pressure tank ("drain"
posltion) in order to allow the piston to return to its
initial position under the action of the high pressure P
maintained within the annular chamber.
Thé first operation causes outward displace-
~: ment of the piston-rod and moves the circuit-breaker to
: the engaged or closed position whilst the second oper-
ation causes inward displacement of the piston-rod into
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the cylinder and moves the circuit-breaker to the dis-
engaged or open position.
Hydraulic circuit-breaker control systems of
the differential jack type considered in the foregoing
are well-known and have been described for example in
French patent No. 2,317,532 (or in U.S. patent
No. 4,026,523).
The design of differential jacks for this
application gives rise to constructional difficulties,
in particular by reason of the fact that they have to
guarantee permanent and absolute leak-tightness over
very long periods of time despite very high hydraulic-
fluid service pressures P of the order of 300 to
400 bar and high velocities of the order of lO to
lS 15 meters per second. As shown in the prior patent
cited earlier, these jacks must be provided with a
first packing seal for the passage of the piston-rod
which emerges through the end of the cylinder and with
a second packing seal on the piston.
The first of these packing seals is relatively
easy to construct with a view to obtaining absolute
leak-tightness~ This packing seal is in fact stationary
and it is an easy matter to obtain a ground and polished
piston-rod which does not wear the packing. Furthermore,
~5 the solid piston-rod does not vary in diameter under the
action of pressure and the packing is practically not
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subjected to pressure variations since the annular
chamber is always subjected to the permanent pressure P
- delivered by an oleopneumatic accumulator which constit-
utes the source of hydraulic fluid under pressure.
The second packing seal considered, or in
other words the packing ring which forms a leak-tight
seal between piston and cylinder, has to withstand
operating conditions which are much more difficult.
The aforesaid packing ring, which is mounted
on the piston, suffers from the following drawbacks :
- the packing ring is movable and subjected to high
accelerations as well as impacts by reason of the
fact that the operations take place during very
short time intervals of less than a few hundredths
of a second ;
- said packing ring is sub~ected to substantial
pressure variations between the closed and tripped
positions ;
- the cylinder expands under the influence of pressure
(of the order of 300 to 400 bar) ;
- the internal surface of the cylinder must be perfectly
ground and~polished in order to maintain integrity of
the packing ring ;
- finally, at the end of a trip displacement, the
packing ring may be subjected to considerable over-
pressures (several thousand bar) which are liable to
impair the strength of the packing ring if certain
precautions are not taken, such precautions being
attended by structural complications.
Heretofore, sealing rings for pistons have
S been satisfactorily constructed only in forms which are
relatively complicated as well as costly (and for which
a large number of patents have been filed). Preference
is given to the use of the so-called "spring-loaded
packings" which are formed of one or a number of rings
of elastic material maintained in the compressed state
by a spring. One example of a spring-loaded packing
of this type is illustrated in FIG. 2 of the prior
patent cited earlier. In any case, piston packings of
this type are capable of cooperating only with a
cylindrical surface which is perfectly polished.
Apart from the difficulty involved in the con-
struction of piston seals or packing rings and in achieve-
ment of the requisite state of surface of the cylinder,
differential jacks are subject to a further disadvantage
in that provision has to be made for a large-section duct
or passage which permits rapid transfer at a high flow
rate between the main chamber and the annular chamber
of the jack. In fact, the operations of closing or of
~;tripping of the circuit-breaker (or in other words the
; 25 outwardortheretum stroke of the jack piston) must take
~ ~place in a very short period of time of the order of a
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few hundredths of a second, which makes it necessary toavoid any braking action by a circulation of oil at a
reduced flow rate.
Up to the present time, this large-section
transfer passage has in the majority of instances been
provided by means of a jacket which surrounds the jack
cylinder, said passage being formed by the annular gap
between the jack and the jacket. One example of a
design of this type which complicates the construction
of the differential jack even further is shown in
FIG. 3 of the prior patent cited earlier.
The aim of the present invention is to over-
come the disadvantages of the differential hydraulic
jacks which have been known up to the present time and
to permit a much more simple and economical construction
while ensuring even greater reliability by reducing the
number of components.
In the description which follows hereafter,
the term "closiny stroke" will designate the displace-
ment of the piston from the end of the main chamber to
the end of the annular chamber and the term "tripping
stroke" will designate the return displacement of the
piston in the opposite direction.
SUMMARY OF THE INVENTION
This invention is directed to an improved
differential hydraulic jack of the type defined in the
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foregoing in which the piston is not provided with anyform of packing ring in cooperating relation with the
internal surface of the jack cylinder and in which the
piston is associated mechanically with a valve for
ensuring continuous leak-tight closure of the supply/
drain orifice of the main chamber of the jack when the
piston is in the tripping end-of-travel position.
In accordance with this arrangement, the
external cylindrical surface of the piston is in sub-
lG stantially direct metai-to-metal slidlng contact wlth
the internal surface of the cylinder. Since no pro-
vision is made for any packing ring, there clearly
takes place an oil leakage between the piston and the
cylinder but the present Applicant has discovered that
this leakage was negligible during the movements of the
piston in one direction or in the other, particularly
as these movements last only a few hundredths of a
second.
In the "closed" position, the pressures (P)
are identical on both faces of the piston and no
leakage therefore occurs on each side of the piston.
In the "open" position, the pressure P
;~ maintained on the annular face of the piston reaches
the opposite face of the piston as a result of the
leakage existing between the piston and the cylinder
but is then checked by the leak-tight closure of the
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valve which has been actuated by the piston at the end
of its tripping stroke. This accordingly has the effect
of preventing any continuous flow of oil in the open or
"trip" position, which would be unacceptable in a
circuit-breaker control system.
In accordance with a simple solution, the
valve is rigidly fixed to the aforesaid opposite face of
the piston and the valve-seat is formed at the periphery
of the supply/drain orifice of the main chamber.
As will be apparent hereinafter, this arrange-
ment of a piston without any packing ring achieves
further substantial advantages such as, in particular,
the possibility of designing the cylinder in the form of
a casting which calls only for simple machining whereas
this would be incompatible with a piston ring or packing.
Moreover, the large-section transfer duct between the
main chamber and the annular chamber of the jack can be
formed directly by casting, thus considerably reducing
the number of jack components.
sRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a
differential jack in accordance with the prior art and
of a known hydraulic circuit-breaker control system in
which said jack is incorporated.
FIG. 2 is a sectional view of a differential
jack in accordance with the present invention.
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FIGS. 3 and 4 are fragmentary views showing
two other forms of construction of the closure valve
which is actuated by the piston.
FIG. 5 is an axial sectional view of a
differential jack in accordance with the invention
with a cast cylinder block.
FIG. 6 is a sectional view taken along line
VI-VI of FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMÆNTS
In order to gain a clear idea of the prior
art, there is illustrated in FIG. 1 the known hydraulic
diagram of a circuit-breaker control system of the
differential-jack type.
This control system includes a differential
jack 2 having a cylinder 4, a piston 6 r an emergent
piston-rod 8 which is coupled with the moving contact
10 of a circuit-breaker in order to make or break the
circuit between the stationary contacts 12-12' of the
circuit-breaker. The position shown in FIG. 1 is the
"tripped" or open position (open contacts~ of the
circuit-breaker in which the piston 6 is close to its
bottom position within the cylinder.
The annular jack chamber 14 which is located
above the piston 6 and surrounds the piston rod 8 is
continuously connected via a pipe 15 to a high-pressure
(300 to 400 bar, for example) oleopneumatic accumulator
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18. The pressure within said chamber 14 tends to
continuously restore the circuit-breaker to the open
position.
Within the main chamber 20 of the jack is
formed a passage or orifice 28 which can be selectively
connected, by means of a supply/drain pipe 22 in which
is interposed a three-way valve 24, either to the
accumulator 18 via a pipe 22' and a transfer duct 16
or to a low-pressure drain tank 26 (position shown in
FIG. 1) via a pipe 25.
The operation of this system is sufficiently
well-known to require no comment beyond the fact that
positioning of the valve 24 in the supply position
establishes the high pressure within the chamber 20 on
the large surface of the piston 6. The piston is there-
fore displaced upwards in opposition to the lower force
exerted by the same high pressure on its annular surface,
thus moving the circuit-breaker to its closed position.
Tripping of the circuit-breaker is obtained by
placing the valve 24 in the drain position (shown in
FIG. 1). The high pressure which is continuously
~ maintained within the annular chamber 14 has the effect
; of displacing the piston 6 in the downward direction,
the oil contained within the main chamber 20 of the jack
being drained-off rapidly at a high flow rate by virtue
of the fact that the supply/drain orifice 28 and the
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associated hydraulic elements 24-25 have large cross-
sectional areas, thus making it possible to carry out a
circuit-breaker trip in a very short time of the order
of a few hundredths of a second.
A differential-jack hydraulic control system
for circuit-breakers of the type discussed in the fore-
going has been described for example in French patent
No. 2,317,532 or in U.S. patent No. 4,026,523, and
illustrated in FIG. 3 of these documents~
As in the design shown in FIG. 1, known
differential jacks include a packing seal 30 provided
for the passage of the piston-rod 8 which emerges
through the end-wall 32 of the jack as well as a packing
ring 34 on the piston 6 for forming a fluid-tight seal
between the piston and the internal surface of the cylin-
der 4. This packing ring is illustrated in the known
form of a spring-loaded packing of the type shown in
FIG. 2 of the prior patent cited earlier and is designed
to ensure perfect fluid-tightness of the chamber 14 under
20 a pressure of approximately 300 to 400 bar in the tripped
position shown.
The accompanying FIG. 2 is a sectional view of
a differential jack in accordance with the invention, in
which all the elements described earlier are designated
by the same reference numerals.
It is apparent from this figure that a packing
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seal 30 is provided for the passage of the piston-rod 8
through the screwed packing gland or end-piece 32 of
the cylinder 4. The jack piston 6, however, is not
provided with any packing ring for forming a fluid-
tight seal between the metal piston 6 and the metalcylinder 4. In consequence, the external cylindrical
surface 36 of the piston 6 is capable of sliding in
substantially direct metal-to-metal contact with the
internal surface 38 of the cylinder 4 but without pro-
viding a ~luid-tight seal between these two surfaces
and without entailing any need for particularly accu~
rate machining of the surface 38.
Beneath its bottom face 40, the piston 6 is
adapted to carry a projecting portion forming a frusto-
conical valve 42. At the end of the tripping stroke
(position shown in FIG. 2), said valve 42 is adapted to
close the supply/drain orifice 28 which is at low
pressure in the tripping end-of-travel position shown
in full lines in FIG. 2.
2Q The valve 42 is adapted to fit in leak-tight
manner on a frusto-conical valve-seat 44 formed on the
edge of the orifice 28 in the screwed end-piece 46 of
the cylinder. As can readily be understood, the valve-
seat 44 can be constituted by an added part of material
- 25 having a degree of hardness which is suited to that of
the valve and placed in the cylinder end-piece 46.
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In the closed position of the circuit-
breaker, the supply/drain passage 28 of the jack is
connected to the accumulator 18 via the ducts 22-22'-16
and via the valve 24 in the supply position. The piston
6 is therefore upwardly displaced within the cylinder
and occupies the pcsition shown in dashed lines in FIG.
2 in which the piston is designated by the reference 6'
: and the valve carried by this latter is designated by
the reference 42'. Since the same pressure P prevails
on both faces of the piston,.the force F1 applied on the
cylinder in the upward direction in order to maintain
the circuit-breaker in the closed position is :
Fl = P Sl - P (Sl )
where S1 is the surface area of the piston and s is the
cross-sectional area of the rod 8, as is the usual
practice in a differential jack.
In this position, which is the normal service
position of the circuit-breaker and which may last for
several weeks, the absence of any packing ring on the
piston is not attended by any disadvantage since the
same pressure P prevails beneath the piston (within the
main chamber 20) and above the piston (within the
annular chamber 14).
In order to trip the circuit-breaker, the
supply/drain passage is connected to the drain tank by
means of the valve 24 and the piston is thrust downwards
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under the action of the force F2 = P(Sl-s), the drain
pressure PO beneath the piston being substantially
atmospheric pressure at least during the beginning of
acceleration of the piston. In the position shown in
full lines in FIG. 2 corresponding to the end of the
tripping stroke, the valve 42 is applied against its
seat 44 and closes the opening 28 in leak-tight manner,
thus determining at the same time an end-of-travel stop
for the piston. The pressure P which is continuously
maintained within the annular chamber 14 is therefore
prevented from escaping -through the drain passage and
there is consequently no permanent flow of oil which
should be compensated by a pump (not shown) for re-
charching the accumula-tor 18.
The pressure P which prevails within the
annular chamber 14 may pass by leakage flow between the
piston 6 and the cylinder and may thus, at the very
most, reach the bottom face of the piston 6 or in other
: words the internal space of the main chamber 20 (of
reduced volume at that moment). The piston 6 is there-
fore subjected to the pressure P both on its top annu-
lar surface "Sl-s" and on its bottom annular surface
"Sl-S2", where S2 is the cross-sectional area of the
valve 42.
In order to obtain an application force which
is sufficient to ensure a high standard of leak-tightness
between the valve 42 and its seat, the cross-sectional
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area S2 of the valve must be appreciably larger than the
cross-sectional area s of the piston-rod, namely of the
order of 50 ~, for example. Good results have been
obtained with values within the range of 25 % to 100 %.
During the circuit-breaker closing or tripping
displacements of the piston which last only a few
hundredths of a second, the oil leakages which occur
between the piston and the cylinder represent only a
negligible volume which does not have the effect of
slowing-down the operations of the jack.
It is therefore apparent that the combination
of a piston without any packing ring and of a valve
actuated mechanically by the piston for closing the
supply/drain orifice at the end of a trip displacement
makes it possible to satisfy all conditions of oper-
ation and safety of a hydraulic circuit-breaker control
system.
The effect of this arrangement is therefore
not only to dispense with any further need for a piston
ring which is difficult to construct but also (as men-
tioned earlier with reference to the cylinder 4 of
FIG. 2 and as will be seen in greater detail with
reference to FIGS. 5 and 6) to remove any need for
accurate machining (grinding) of the internal surface
38 of the cylinder.
Although FIG. 2 shows a frusto-conical valve
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42 associated with the piston, it remains possible to
choose any other type of valve such as, for example, a
ball-valve in which the closure member is a sphere or a
portion of sphere 48 (FIG. 3) or else a flat valve
(FIG. 4) formed by the top face 50 of the end-piece 46
of the cylinder against which is applied a circular lip
52 machined in the bottom surface 54 of the piston 6.
As can readily be understood, the reverse arrangement in
which the sealing lip would be carried by the top face
50 of the cylinder end-piece could also be adopted.
Provision may also be made for an annular seal 56 of
suitable material which is adapted to cooperate with
the lip 52.
It should be noted that, even with a flat
valve, tightness of valve closure can readily be
achieved by virtue of the fact that, in the bottom trip
position of the piston 6, guiding of said piston is
effected at two remote points (on the one hand the
packing seal 30 of the piston-rod 8 and on the other
hand the contact between the piston and the cylinder).
Thus the parallel alignment of the bearing face of the
valve is accurately reproduced at each operation.
The valve can be rigidly fixed to the piston
6 or form an integral part of this latter as shown in
FIGS. 2 and 4. Alternatively, said valve may be en-
dowed with a certain degree of freedom as shown in
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FIG. 3 in which the hemispherical valve 48 is engaged
in a recess 58 formed in the piston 6 and is retained
within said recess by a resilient snap-ring 60. Final-
ly, as shown in FIG. 3, provision can be made for a
spring 62 interposed between the valve 48 and the pis-
ton 6 so as to ensure that the valve closes on its seat
44 shortly before the end of travel of the piston in
order -to limit the end-of-travel impact on the valve.
Finally, in accordance with another variant
1~ (not illustra-ted), the valve (42 or 48) is no longer
carried directly by the piston but can be actuated by
this latter at the end of travel by a member such as,
for example, a control rod carried by said piston.
It should be recalled at this point that, in
FIG. 2, the duct 16 connected to the orifice 64 which
opens into the annular chamber 14 of the cylinder 4
must have a large cross-sectional area in order to
permit rapid transfer of oil between the two chambers
14 and 20.
FIG. 5 is a sectional view of a preferred
embodiment of the invention in which the jack cylinder
4 is a casting and not a steel tube.
In this figure, the piston 6 i5 shown in the
left-hand portion in the top or closed position and is
shown in the right-hand portion of the figure in the
.~ bottom or open position. Said piston 6 is adapted to
~; carry a frusto-coni.cal valve 42 on its underface. Said
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valve is adapted to cooperate with a frusto-conical
valve-seat 44 cut in the edge of the supply/drain
orifice 28 in the plug 46 which forms the end of the
cylinder.
The large-section transfer duct 16' is cast in
one piece with the part which forms the cylinder 8 and
consequently no longer requires to be connected to the
orifice 64 of the annular chamber 14 as a separate
component. FIG. 6 shows the transfer duct 16' within
the casting 4 in a cross-section taken along line VI-VI
of FIG. 5.
The internal cylindrical surface 38 of the
cylinder block need undergo only a simple and economical
machining operation without either grinding or polishing
since no packing ring on the piston 6 is liable to be
damaged by imperfect machining of the cylinder.
The foregoing arrangements accordingly achieve
a constructional design which is considerably less cost-
ly than that of the differential jacks known heretofore
while at the same time maintaining requisite standards
of safety and reliability for control of electric
circuit-breakers.
