EP0060181B1 - Prestressed hydropneumatic accumulator with pressure relief - Google Patents

Description

The present invention relates to improvements in oleopneumatic accumulators of the type comprising two enclosures separated by a deformable wall, made of rubber, one of the enclosures being filled with gas under pressure and the other receiving hydraulic fluid under pressure; the parts constituting said accumulator being kept assembled by a prestressed device.

In patent application EP-A-35 427 filed on February 18 by the same applicant (publication date 9.9.1981), an oleopneumatic accumulator has been described consisting of two substantially hemispherical shells, assembled together with interposition of a flexible membrane and a shim of cylindrical thickness, the two shells and the shim being held tight against each other by an external belt subjected before assembly to a preliminary prestressing generating a clamping force greater than the maximum separation force of the same parts when the accumulator thus formed is subjected to the maximum operating pressure.

The prestressed accumulators according to patent application EP-A-35 427 give excellent results, but they have several drawbacks. First of all, their mass production is delicate, because a fairly complex mechanism is required, the implementation of which requires time to arrive at prior tensioning of the cylindrical belt surrounding the parts. Then, it turns out that when the prior tensioning force is released, part of the prestress is absorbed by a relative crushing of the wedge and by a deformation, even very slight of the threads. As a result, the value of the prestress which remains after assembly of the parts is quite variable.

In French patent FR-A-1150 762 of April 5, 1956, a hydraulic accumulator has been described in three parts: a cylindrical central part placed between two hemispherical caps, these three parts being held tight against each other by a central tie rod crossing them. However, according to this patent, the tightening of the three elements is obtained by simply screwing a nut on one of the ends of the central tie rod. One thus obtains, by the mere fact of the inherent elasticity of the metal of which the central tie is made, a certain prestress: but this prestress is arbitrary, indeterminate and can in no case induce a tightening force which is equal to the force of separation caused by the maximum hydraulic pressure for which the accumulator must work.

The object of the present invention is to combine the advantages of the two manufacturing methods described above, which makes it possible to obtain a prestressed hydraulic accumulator which in the event of overpressure produces a leak of liquid and which can therefore withstand very high pressures while being of a much lower manufacturing price than that of the hydraulic accumulators currently used. This combination also has other advantages relating to the precision and reliability of the construction which will be explained below.

The oleopneumatic accumulator according to the present invention is of the type comprising two chambers separated by a flexible membrane, one of the two chambers being filled with a pressurized gas, the other being in communication with the hydraulic circuit in which is placed the accumulator, the latter being constituted by a cylindrical body disposed between two upper and lower plugs after interposition of the rim of a flexible membrane between one of the plugs and said cylindrical body, the two plugs being held tight on the cylindrical body by at at least one tie rod, one end of which is integral in traction with one of the two plugs and the other end of which is provided with a thread on which is screwed a nut bearing on the other plug, characterized in that each tie rod is put, before screwing its nut under a prior tension equal to the separation force caused by the maximum hydraulic pressure to be admitted into the accumulator so as to ensuring an assembly of the two plugs on the central body under a determined prestress; so that when the pressure admitted into said oleopneumatic accumulator exceeds said predetermined maximum pressure, the cylindrical body separates slightly from one or the other of the lower and / or upper plugs, which causes a hydraulic leak which causes the hydraulic pressure, which can therefore never exceed said predetermined value.

According to a first embodiment, the two plugs are held tight on the cylindrical body by a plurality of tie rods arranged outside and around said cylindrical body.

According to a second embodiment, the two plugs are connected to each other by a single central tie passing through them at their centers.

According to the present invention, the tie (s) is tensioned by hydraulic means before the nut is screwed, which makes it possible to precisely determine the prestress and to determine it as a function of the maximum hydraulic pressure at which must work the accumulator.

Preferably, the central cylindrical body is beveled at at least one of its ends in order on the one hand to have a self-centering of the parts and on the other hand, to introduce into the assembly a radial component which, at the Longitudinal prestress superimposes a radial prestress.

• Figure 1, une vue en coupe longitudinale d'un accumulateur oléopneumatique selon la présente invention;
• Figure 2, une vue en coupe longitudinale du montage de l'accumulateur de la figure 1;
• Figure 3, une vue en coupe longitudinale d'un second mode de réalisation d'un accumulateur selon la présente invention;
• Figures 4 et 5, deux vues illustrant deux variantes de réalisation de la membrane séparatrice;
• Figure 6, une vue en coupe longitudinale d'une première variante de réalisation de l'accumulateur représenté à la figure 1;
• Figure 7, une vue en coupe longitudinale d'une autre variante de réalisation de la membrane de séparation;
• Figure 8, une vue en coupe longitudinale illustrant la mise en précontrainte;
• Figure 9, une vue en coupe longitudinale d'une variante de réalisation de l'accumulateur représenté aux figures 1 ou 6.

By way of nonlimiting example and to facilitate understanding of the invention, the accompanying drawings show:

• Figure 1, a longitudinal sectional view of an oleopneumatic accumulator according to the present invention;
• Figure 2, a longitudinal sectional view of the assembly of the accumulator of Figure 1;
• Figure 3, a longitudinal sectional view of a second embodiment of an accumulator according to the present invention;
• Figures 4 and 5, two views illustrating two alternative embodiments of the separating membrane;
• Figure 6, a longitudinal sectional view of a first alternative embodiment of the accumulator shown in Figure 1;
• Figure 7, a longitudinal sectional view of another alternative embodiment of the separation membrane;
• Figure 8, a longitudinal sectional view illustrating the prestressing;
• FIG. 9, a view in longitudinal section of an alternative embodiment of the accumulator shown in FIGS. 1 or 6.

Referring to these figures, we see that the accumulator is constituted by a tubular body 1 closed at its two ends by plugs 2 and 3, the flexible membrane 4 being anchored by blocking its periphery 5, provided with a heel , between the upper rim of the cylindrical body 1 and the inner face of the plug 2. In the case of FIGS. 1 and 2, the two plugs 2 and 3 are of a diameter greater than that of the cylindrical body 1 and are connected one to the other by a plurality of tie rods 6 so that the body 1 is compressed by the traction exerted by these tie rods.

Preferably, as shown in Figures 1 and 2, these tie rods 6 are constituted by threaded rods, one end 6a being screwed into a thread formed in the mass of the plug 2, the other end 6b being provided with a thread on which a nut 7 engages which bears against the external wall of the plug 3. As shown, the bolts 7 can engage in housings 8 hollowed out in the wall of the plug 3.

Also preferably, one of the ends of the central body 1 has a conical surface 1a which comes to bear against a conical surface of the same slope 3a formed in the internal wall of the corresponding plug.

As is known, the plug 2 is provided with a gas filling valve 9 and the plug 3 with a conduit 10 intended to be connected to a hydraulic circuit.

According to the present invention, the prestressing is carried out by means of a hydraulic jack whose body 11 is coupled by means of tie rods 12 to the tie rods 6 previously described; the piston 13 of the jack is supported on the external face of the plug 3; while by the tie rods 12 and 6, the cylinder body exerts traction on the plug 2.

Preferably, as shown, the internal bore of the jack body 11 is strictly equal to the internal bore of the central body 1.

The hydraulic pressure is introduced into the chamber 14 of the jack and, when the desired value "p" of prestressing is reached, the bolts 7 are screwed until they come into contact with the outer wall of the plug 3 and the hydraulic pressure is released.

When, in service, the hydraulic fluid pressure reaches and slightly exceeds the preload pressure “p”, the surfaces 1a and 3a slightly deviate and the hydraulic fluid can leak out of the accumulator as through a pressure relief valve. The value of the maximum pressure from which the accumulator will leak corresponds almost exactly to the value of the pressure “p” of prestressing, which therefore makes it possible to determine with great accuracy the characteristics of the accumulator.

On the other hand, as the various mechanical parts are subjected to the prestressing force before it is released, there is not, as in the case of the accumulator described in the patent application EP- A-35 427 decrease in the value of the prestress, by compaction and elastic deformations of the parts since these deformations take place before the pre-stress pressure “p” is released.

In addition, the fact that the surface 1a of the end of the body 1 is conical and bears against a surface 3a which is also conical ensures both excellent sealing, self-centering of the parts and reveals a radial component of the force. prestressing which in Figure 1 is illustrated by the arrows f which converge towards the center.

Figures 3 to 9 show a second embodiment of the invention consisting in replacing the tie rods 6 placed around the tube 1 outside thereof by a single tie rod axially passing through the two plugs 2 and 3, the cylindrical body 1 and the membrane 4.

It is known per se, in particular from French patents 1,150,762 and 1,378,955, to assemble the different parts of a hydraulic accumulator by a central rod axially passing through the various constituents of the accumulator, including the deformable membrane. However, these patents do not make known the essential provision according to which an assembly is carried out under a prestress, which is determined very precisely as a function of the maximum operating pressure of the accumulator so that the hydraulic fluid automatically starts to leak as soon as this maximum use value is reached.

As seen in Figures 3 to 9, the accumulator is constituted by a hollow cylindrical central body 1, between two upper and lower plugs 2 and 3. The upper plug 2 is placed flat on the upper end of the cylindrical body 1; while the lower plug 3 comprises a skirt 3b connecting with the lower end of said cylindrical body 1. The upper plug 2 has an opening 15, formed in its center and the lower plug 3 an opening 16, also formed in its center.

The three parts 1, 2, 3 constituting the accumulator are assembled by a central tie-rod 17. This tie-rod 17 comprises a head 18 bearing against the external wall of the plug 3 and a threaded end 19 on which a nut 20 is screwed bearing on the outer wall of the plug 2.

In the example shown in Figure 3, the membrane 4 is fixed by anchoring and pinching its heel 5, formed over its entire periphery, between the plug 2 and the upper end of the central cylindrical body. However, the membrane additionally comprises a sort of central chimney or sleeve 21 which forms a hollow tube in which the tie rod 17 is housed. The upper edge of the sleeve 21 comprises a bead 22 which engages in a correspondingly shaped groove 23 formed in the wall of the central opening 15 of the plug 2.

The base 4a of the membrane has an annular shape practically matching the shape of the annular space 24 formed by the internal wall of the skirt 3b of the plug 3 and the tie rod 17. The plug 3 has a bore 25 which communicates with the space 24 by a plurality of orifices 26. The bore 25 is provided with a thread 27 which allows the accumulator to be screwed onto a socket (not shown) putting said bore 25 in communication with any suitable hydraulic circuit (also not shown) . Preferably, the lower part 4a of the membrane 4 is provided with pads 28 opposite the openings 26.

The plug 2 is provided with a lateral opening 29 which by means of a bent pipe 30, provided with a non-return valve 31, communicates with the enclosure to be filled with pressurized gas.

Figures 4 and 5 show two alternative embodiments according to which the central sleeve of the membrane does not rise over the entire height of the interior volume of the accumulator but only over a portion of this height. In these two figures, it can be seen that the central sleeve 21 of the membrane 4 only rises over about a quarter of the height of the internal volume and is simply fixed by a rod 32 which blocks it in a groove 33 formed on the tie rod 17. To prevent the membrane 4 from being pushed back inside the orifices 26 when the accumulator is completely empty of hydraulic fluid, it is possible, as shown in FIG. 4, to rest its annular bottom 4a on a movable plate constituted by a washer 34 constrained by a spring 35, or else provide it with pads 36, as shown in FIG. 5; these pads 36 being in two parts riveted one on the other instead of being included in the mass of the rubber like the pads 28 of FIG. 3.

The central tie-rod keeps the parts 1, 2 and 3 assembled under prestressing so that when the pressure of the hydraulic fluid exceeds a predetermined value, the elongation of the tie-rod 17 is such that a leak appears between the connection between the cylindrical body 1 and plug 2 or plug 3; this has the result that the hydraulic pressure cannot in any case exceed a predetermined maximum value, a value which is a function of the elastic characteristics of the tie rod 17 and of the prestressing to which the parts are subjected.

Figure 6 shows an alternative embodiment of the accumulator described in Figures 3 to 5 which has the advantage of improving the operation of said accumulator in the event of overpressure.

It has indeed been observed that if the internal section of the central cylindrical body 1 is constant, as is the case in FIGS. 3 to 5, the leak in the event of overpressure can occur both in the upper part and in part low, that is to say both between the plug 2 and the top of the body 1, as between the plug 3 and the bottom of the body 1. When the leak occurred between the plug 2 and the top of the body 1 , the membrane is likely to be entrained, to be pinched between the plug-2 and the upper body 1 and to be torn, which puts the accumulator out of use.

To overcome this drawback, at the base of the body 1, at the junction between the body 1 and the skirt 3b of the plug 3, a chamfer 1a is formed. Preferably, the skirt 3b is also machined so that the bearing surfaces of the base of the cylindrical body 1 and of the skirt 3b are equal. The section D2 of the base of the cylindrical body 1 is then greater than the sectin D1 subjected, at the top of the body 1 to the hydraulic pressure located in the space 24: this will have the result that in the event of overpressure the leak will occur always between the body 1 and the skirt 3b and never between the body 1 and the plug 2.

Figure 7 shows another alternative embodiment.

The method of anchoring the membrane 4 by means of a heel 5 as shown in FIGS. 3 to 6, has the drawback that the surface of the section D1, mentioned above, is not rigorously determined . Indeed, a greater or lesser tightness of this pinching can allow the hydraulic fluid to creep between the membrane and the upper body 1, which will have the effect that the section D1 on which the hydraulic pressure acts will always be slightly higher to the section of the internal bore of the cylindrical body 1, but of an amount which cannot be determined in a rigorous manner. It is for this reason that in FIG. 6, the section D1 has been represented as being larger than the internal diameter of the cylindrical body 1, but this indication is an approximation.

On the other hand, since the leakage in the event of overpressure can only occur between the skirt 3a and the base of the body 1 any risk of extrusion of the fixing part of the membrane is eliminated.

We can then fix the membrane 4 by a simple elastic washer 40. This elastic washer 40 is pierced with a central hole through which pass the tie rod 17 and the upper edge of the sleeve 21. At the height of this central hole, the tie rod 17 has a groove 33; so that the end of the sleeve 21 is locked in this groove 33 by the rounded edge 40a of the washer 40. The outer edge 4b of the membrane 4 is pinched in the corner formed by the junction of the cylindrical body 1 and the plug 2 by the rounded peripheral edge 40b of the washer 40. The more the tie rod 17 will be stressed in tension, the more the elastic washer 40 will forcefully wedge the sleeve 21 in the groove 33 and the rim 4a of the membrane in the corner.

This arrangement is particularly advantageous, because it is very economical while being very reliable.

The accumulators shown in Figures 3 to 7 are prestressed. To do this (Figure 8), we screw on the threaded end 19 of the tie rod 17, the end of the rod 37a of the piston 37 a hydraulic cylinder 38 which is supported by a cylindrical wedge 39 on the plug 2. Thanks to this actuator, on the one hand a compressive force is applied to the parts 2, 1 and 3, force collected by the elasticity of the metal of which these parts are made and on the other hand a tensile force on the tie rod 17, force also cashed in by the elasticity of the metal from which the tie rod is made 17. When the predetermined maximum force is reached, the nut 20 is blocked on the plug 2 and then the pressure is released in the jack 38. The elasticity of the parts 1 , 2 and 3 on the one hand and the tie rod 17 has the result that the assembly of parts 1, 2 and 3 is carried out with prestressing.

Example

As an example, as shown in FIG. 6, an oleopneumatic accumulator having a capacity of 1 liter has been produced. In the chamber 38a of the jack 38, hydraulic fluid was introduced under a pressure of 100 bars; then, the nut 20 has been locked on the thread 19 by means of a dynamometric key so as to have a determined tightening torque; then the mounting cylinder 38 was disassembled and the accumulator was placed on a test bench and hydraulic fluid was forced at 25, 26, 24, until a hydraulic fluid leak appeared between the base 1a of the cylindrical body 1 and the skirt 3a of the plug; the pressure P1 for which this leak occurred was then noted. The operation was repeated by admitting a pressure of 120 bars in the chamber 38a; by tightening the nut 20 with the same tightening torque; by dismantling the cylinder 38 and admitting again the pressurized liquid at 25, 26, 24 until a leak appears and noting the pressure P2 for which this leak had appeared. These operations were thus repeated, increasing each time by 20 bars the pressure admitted into the chamber 38a and the pressures P3, P4 ... Pn for which the leak appeared, were noted, which made it possible to trace point by point the characteristic curve of the accumulator. These operations were stopped when the value of Pn was 400 bars.

The accumulator is then adjusted to operate with a maximum pressure of 400 bar.

When an accumulator of the usual type is intended to operate in a hydraulic circuit for a maximum pressure of N bars, the safety regulations stipulate that it must be tested at 1.5 N.

We therefore tried to subject the accumulator to a pressure of 1.5 x 400 bars, that is to say 600 bars, but it started to leak from 410 bars and it was impossible to exceed this value. : the operating safety of this accumulator is therefore absolute, which is not the case for accumulators of known type.

On the other hand, when a usual type accumulator is manufactured, it is subjected to endurance tests under which after a large number of maximum pressurization cycles, it is dismantled in order to check the deterioration that he suffered.

The accumulator tested underwent 5 million cycles and did not show the slightest trace of deterioration upon dismantling, while the usual accumulators show traces of deterioration after approximately 150,000 cycles.

The extraordinary reliability of the accumulator thus described makes it possible to use it not only as an accumulator but also as a pressure relief valve.

A pressure relief valve is a known, extremely simple member, constituted by a valve held by a calibrated spring: when the hydraulic pressure exceeds a predetermined value, the ball lifts and the hydraulic fluid flows towards the reservoir.

The well-known drawback of these devices is that they frequently cause momentary overpressures of up to 1.3 and even 1.5 times the value of the maximum allowable pressure. This is due to the fact that it often happens during a sudden increase in pressure that the valve opens too suddenly, which causes too great compression of the spring, the calibration of which increases: to this is added a rolling effect of oil whose effects are proportional to the square of the flow. In addition, the effects of oil rolling are such that the metal is hollowed out by the oil flow.

These drawbacks are well known to hydraulic engineers, but to date there are no spring-loaded pressure relief valves, which are deprived of these drawbacks.

By arranging a bypass on a hydraulic circuit a predetermined leak accumulator according to the invention, and by surrounding it with an envelope making it possible to recover the leaks to direct them towards the tank, a pressure relief valve of perfectly functioning is obtained. satisfactory. The capacity of the accumulator is determined experimentally as a function of the maximum flow rate of the circuit for which it is intended. Such a pressure relief valve will be cheaper than a large spring-loaded valve, will not be sensitive to dirt that may be in the liquid, will have a leakage section such that there will be no more fluid flow at high speed which erodes the metal and will work without causing overpressure.

Preferably, as shown in FIG. 9, the two ends of the central body 1 are bevelled as has been described for the conical surface 1a in conjunction with FIG. 1. Thus, as in the case of FIG. 1, but in an improved manner, not only self-centering of the parts and excellent sealing, but a radial component of the prestressing force.

On the other hand, it has been observed that by producing the separation membrane 4 in a form such that it is cylindrical at its upper part, as shown, and that this cylindrical part is disposed inside the upper part of the central body 1, itself cylindrical, it follows that the diameter D ₃ of the circle according to which the pressure acts on the upper plug 2 is slightly smaller (due to the thickness of the membrane) than the diameter D ₄ of the circle according to which the pressure acts on the lower plug 3; it follows that when the pressure admitted into the accumulator exerts on the central rod 17 a force greater than that of the prestress, the cylindrical body 1 and the lower plug 3 separate slightly, which causes a leakage of the liquid, the separation never occurring at the connection between the plug 2 and the body 1.

Claims (13)

1. An oleo-pneumatic accumulator comprising two enclosures separated by a flexible membrane (4), one of which is filled with gas under pressure, the other is filled with liquid, made of a cylindrical body (1) disposed between two upper (2) and lower (3) plugs with interposition of the edge (5) of the flexible membrane (4) between one of the plugs (2) and said cylindrical body (1), the two plugs (2, 3) being kept clamped onto the cylindrical body (1) by at least one tie rod (6 or 17) one end (6a, 18) of which is fast in traction with one of the two plugs (2 or 3) and the other end of which (6b, 19) is provided with a threading on which is screwed a nut (7, 20) coming to bear against the other plug (2 or 3), wherein each tie rod (6, 17) is set, prior to the screwing of its nut (7, 20), under a preliminary stress equal to the separation effort caused by the maximum hydraulic pressure which has to be admitted into the accumulator, in order to provide an assembly of the two plugs (2, 3) on the central body under a determined prestress; in such manner that when the pressure admitted into said oleo-pneumatic accumulator is exceeding said predetermined maximum pressure, the cylindrical body (1) moves slightly away from one or the other of the lower (3) and/or upper (2) plugs, thereby causing a hydraulic leak which reduces the hydraulic pressure, which therefore can never exceed said predetermined value.

2. A hydraulic accumulator according to claim 1, characterized in that it comprises a plurality of tie rods (6) disposed about the central cylindrical body (1), each tie rod (6) being screwed at one of its ends (6a) on the edge of one (2) of the plugs and formed at its other end (6b) with a threading on which is threaded a nut (7) coming to bear against the edge of the other plug (3).

3. An accumulator according to claim 2, in which the lower end of the cylindrical body (1) is formed with a conical edge (1a) coming to bear against the corresponding conical surface (3a) provided on the inner face of the lower plug (3) in order to cause the centering of said cylindrical body and to maintain said centering when, under the effect of a pressure exceeding the maximum predetermined pressure, the conical surfaces (1a, 3a) draw slightly apart from each other.

4. A method for setting under a preliminary stress the assembly means disclosed in claims 1 through 3, consisting in connecting the body (11) of a hydraulic jack to the accumulator tie rods (6) connected to one of the plugs (2) while letting the piston (13) of said jack bear against the outer face of the other plug (3); then setting the hydraulic jack under a pressure up to a desired value; then screwing the screws (7) of tie rods (6) in such manner as to bring them in contact with the outer wall of plug (3) and then releasing the pressure; in such manner that on the one hand the tie rods (6) have been set under the desired prestress and on the other hand the elements (1, 2, 3) constituent of the accumulator have been previously compressed under the effect of the same force.

5. A hydraulic accumulator according to claim 1, characterized in that it comprises a single central tie rod (17) extending through the two plugs (2, 3) and the resilient membrane (4) via a sleeve (21), said central tie rod (17) being set, prior to screwing the nut (20) against the wall of plug (2), to a preliminary stress equal to the separation effort caused by the maximum hydraulic pressure which has to be admitted into the accumulator so as to provide an assembly of the two plugs (2, 3), of the cylindrical body (1) and of membrane (4) under a determined prestress.

6. An accumulator according to claim 5, wherein the sleeve (21) extends over the whole height of the inner volume of the accumulator and is anchored at its upper end on plug (2).

7. An accumulator according to claim 5 or 6, wherein the liquid outlet openings (26) are formed in plug (3) in a crown shape about the central tie rod (17).

8. An accumulator according to claim 3, wherein the annular bottom (4a) of membrane (4) is provided, opposite the openings (26), with pellets (28, 36).

9. An accumulator according to any one of the preceding claims, wherein the inner boring of the cylindrical body (1) has a larger section at its base than at its top; so that the force exerted by the inner pressure at the base of the cylindrical body (1) is less than that applied on the top portion, the result being that the cylindrical body (1) moves slightly apart from the lower plug (3) while remaining in contact with the upper plug (2).

10. An accumulator according to claim 9, wherein the inner bore of the cylindrical body (1) is formed at its base with a chamfer (1a), the junction between said cylindrical body and plug (3) being provided by a skirt (3a) of said plug, having the same thickness as the chamfered base of the cylindrical body.

11. A hydraulic accumulator according to claim 9 or 10, wherein the membrane (4) is fixed by means of a resilient washer (40) formed with a hole in its center, the outer peripheral edge (40b) of which clamps the outer edge (4b) of the membrane in the corner defined by the junction between the upper plug (2) and the top of the cylindrical body, and the inner hole edge (40a) blocks the top of sleeve (21) of membrane (4) in a groove (33) formed in the tie rod (17).

12. A method for setting under a preliminary stress an accumulator according to claims 5 through 11, characterized in that a pressure is applied by means of a hydraulic jack (38) on the parts (1, 2, 3) constituent of the accumulator and a traction is exerted on the central tie rod (17), then the nut (20) is screwed onto the threaded stem until a blockage occurs against the upper' plug (2), whereafter the hydraulic pressure is released; in such manner that on the one hand the central tie rod (17) has been set under the desired preliminary stress and that on the other hand the constituent elements (1, 2, 3) have been previously compressed under the effect of the same force.

13. Utilization of an accumulator according to any one of claims 1 through 3 and 5 through 11, as a relief valve for a hydraulic circuit.

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