CA1319728C - Differential hydraulic jack for the control of electric circuit-breakers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The piston of a differential jack for the control of electric circuit-breakers is not provided with any form of packing ring in cooperating relation with the internal surface of the jack cylinder. The jack piston is associated mechanically with a valve for ensuring continuous leak-tight closure of the supply/drain orifice of the main jack chamber when the piston is in the end-of-travel position corresponding to a circuit-breaker trip. The jack cylinder is constructed in the form of a casting having an internal cylindrical surface which does not require any accurate machining operation such as grinding or lapping.

Description

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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 ~3~7~

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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~3~ ~7~8 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 :

' , ~3~7~8 g 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 ~3~728 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 , ~` ~3~ ~728 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 ,j.,.,~,~

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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.

Claims (10)

1. A differential hydraulic jack for oleopneumatic control of electric circuit-breakers, comprising a cylinder, a piston and an emergent piston-rod which defines within the cylinder an annular chamber on one side of the piston and a main chamber on the other side of the piston, said emergent piston-rod being coupled with the moving contact of the circuit-breaker, said annular chamber being continuously connected to a source of hydraulic fluid under high pressure and said main chamber being provided in the corresponding end of the cylinder with a supply/drain orifice for said chamber, wherein the external cylindrical surface of the piston is substantially in direct metal-to-metal sliding and non-leaktight contact with the internal surface of the cylinder, no provision being made on said piston for any packing seal forming a tight seal with the cylinder, wherein said piston is associated mechanically with a valve for effecting leak-tight closure of the orifice aforesaid at the end of the return travel of the piston towards the aforesaid cylinder end, and wherein the body of the cylinder is a casting.

2. A jack according to claim 1, wherein the casting which constitutes the cylinder body is cast in one piece with a large-section transfer duct which opens into the annular chamber of said jack.

3. A jack according to claims 1 or 2 wherein the inter nal surface of the cylinder body made of a casting undergoes only a simplified machining operation, without any grinding or lapping.

4. A jack according to claims 1 or 2 wherein the surface area S2 of said valve is approximately 50% larger than the cross-sectional area s of said piston-rod.

5. A jack according to claim 1, wherein the valve is carried by the piston and projects beneath the principal piston surface which is directed towards the main chamber.

6. A jack according to claim 1, wherein the valve is rigidly fixed to the piston.

7. A jack according to claim 1, wherein the valve is adapted to cooperate with a seat formed in the cylinder end around the edge of the supply/drain orifice, said valve-seat forming an end-of-travel stop for the piston.

8. A jack according to claim 7, wherein the valve is a frusto-conical valve which is adapted to cooperate with a frusto-conical valve-seat.

9. A jack according to claim 1, wherein the valve is a part which is separate from the piston and retained within a recess formed in said piston.

10. A jack according to claim 9, wherein resilient means are interposed between the valve and the piston.