EP0060181A1 - Prestressed hydropneumatic accumulator with pressure relief - Google Patents

Abstract

The oleo-pneumatic accumulator has two chambers separated by a flexible membrane, one filled with pressurized gas, the other filled with hydraulic liquid. The different parts are held tight against each other by an assembly means previously tensioned so as to provide a preload greater than the separation force caused by the maximum hydraulic separation pressure. A central cylindrical body (1) is disposed between two plugs (2 and 3) which are connected by at least one tie rod 6 previously tensioned so as to ensure an assembly by prestressed tightening.

Description

The present invention relates to improvements in oleo-pneumatic 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 French patent application No. 80.04313 of February 27, 1981 and in its addition No. 80.09781 of April 30, 1980, an oleopneumatic accumulator has been described consisting of two substantially hemispherical shells, joined to each other with interposition 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 l 'maximum separation effort of the same parts when the accumulator thus formed is subjected to the maximum operating pressure.

On the other hand, it is known from French patent application No. 81.00740 filed on January 16, 1981 by the same applicant to manufacture a hydraulic accumulator by means of a tubular central body closed at its two ends by a plug.

The prestressed accumulators according to patent application No. 80.04319 and its addition No. 80.09781 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.

The accumulators described in patent application No. 81.00740 have the considerable advantage of being of very advantageous manufacturing price, but have the drawback of not withstanding the high operating pressures and therefore of being unusable in high and very high pressure hydraulic circuits.

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 - and therefore able to withstand very high pressures - while being much more costly to manufacture. lower 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.

According to a first embodiment, the present invention relates to an oleopneumatic accumulator comprising two enclosures separated by a flexible membrane, one of the enclosures being filled with pressurized gas, the other being connected to a hydraulic circuit also under pressure, the various parts being held tight against one another by means of assembly put beforehand under a determined tension so as to ensure a prestressing greater than the separation force caused by the maximum hydraulic pressure of use, said accumulator being constituted by a cylindrical body disposed between two plugs which are connected one to the another by a plurality of tie rods, said tie rods being tensioned beforehand so as to ensure prestressed tightening.

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

Preferably, the central cylindrical body is bevelled 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 oléo-pneumatique 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 oleo-pneumatic 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, it can be seen 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 edge of the cylindrical body 1 and the lower 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 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 to the tie rod means 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 pressure of the hydraulic fluid reaches the value of the preload pressure "p" and slightly exceeds it, the surfaces la 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 patent application no. 80.04313 decrease of the value of the prestress, by settling and elastic deformations of the parts since these deformations take place before the pressure "p" of prestressing is released.

In addition, the fact that the surface 1a of the end of the body 1 is conical and bears against an equally conical surface 3a ensures both excellent sealing, self-centering of the parts and causes a radial component of the force to appear. 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 traversing 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 is automatically set to leak as soon as this maximum use value is reached.

As seen in Figures 3 to 9, the accumulator consists of a hollow cylindrical central body 1, between two upper and lower plugs 2 and 3. The upper plug 2 is laid 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 which is supported is screwed 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 gas under pressure.

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 preload 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 the lower part, that is to say both between the plug 2 and the top of the body 1, as well as between the plug 3 and the bottom of the body 1. When the leak occurs between the plug 2 and the upper body 1, the membrane risks being entrained, being pinched between the plug 2 and the upper body 1 and being 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 is la. 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 section D1 subjected, at the top of the body 1 to the hydraulic pressure located in the space 24: this will result in 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 Dl on which the hydraulic pressure acts will always be slightly greater 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: tightness in tension, the more the elastic washer 40 will forcefully wedge the sleeve 21 in the groove 33 and the edge 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 cylinder, 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, also cashed 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 was blocked on the thread 19 by means of a torque wrench so as to have a determined tightening torque; then the assembly cylinder 38 was dismantled 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 the base 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 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 constrained 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 having a bypass on a hydraulic circuit a accumulator with predetermined leakage 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 susceptible to dirt that may be in the liquid, will have a leakage section such that there will be no more high speed fluid flow that 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 connection 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 arranged 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 less (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, causing a leakage of the liquid, the separation never occurring at the connection between the plug 2 and the body 1.

Claims (13)

1. Oleo-pneumatic accumulator comprising two chambers separated by a flexible membrane, one filled with pressurized gas, the other filled with hydraulic liquid in which the different parts are held tight against each other by means of assembly placed beforehand under a determined tension so as to ensure a prestress greater than the separation force caused by the maximum hydraulic separation pressure, characterized in that it is constituted by: a central cylindrical body (1) disposed between two plugs (2 and 3) which are connected by at least one tie rod (6, 17) previously tensioned so as to ensure an assembly by prestressed tightening. 2. Oleo-pneumatic accumulator according to claim 1, in which the upper (2) and lower (3) plugs have a diameter greater than that of the central body (1) and are connected to each other by a plurality of tie rods (6) surrounding said central body (1). 3. Oleo-pneumatic accumulator according to claim 1, in which the upper (2) and lower (3) plugs are assembled by means of a central tie (17) passing through the two plugs (2 and 3), the body cylindrical (1) and the membrane (4), the parts being held tight against each other between the head (18) of said central tie, bearing against the lower plug (3) and a nut (20) screwed on the other end of the central tie (17), bearing against the plug (2); the central tie having been put in elongation and the plugs (2 and 3) and the cylindrical body (1) having been put in compression before tightening of the nut (20). 4. Accumulator according to any one of claims 1 to 3, in which at least one of the ends of the cylindrical central body (1) has a conical surface (la) bearing on a corresponding conical surface (3a) formed on the plug (3). 5. Accumulator according to any one of claims 1 to 3, in which the two upper and lower ends of the cylindrical central body (1) have a conical surface bearing on a corresponding conical surface (2a, 3a) formed on each of the plugs ( 2, 3). 6. Accumulator according to any one of claims 1 to 5, wherein the internal bore of the cylindrical body (1) has a larger section at its base than at its upper part. 7. Accumulator according to any one of claims 1 to 5, in which the walls of the separating membrane (4) are applied to the upper part of the accumulator against the internal wall of the central body so that at the junction between the central body (1) and the upper plug (2), the internal diameter (D ₃ ) of the accumulator is less by an amount corresponding to the thickness of the membrane, than the internal diameter (D ₄ ) to the junction between the central body (1) and the lower plug (3) so that in the event of overpressure a liquid leak occurs between the central body (1) and the lower plug (3) and never between the central body ( 1) and the upper plug (2). 8. Accumulator according to claim 6, wherein the internal bore of the cylindrical body (1) comprises at its base a chamfer (la), the junction between said cylindrical body (1) and the plug (3) being by a skirt (3b) of said plug having the same thickness as the chamfered base of the cylindrical body. 9. Accumulator according to any one of claims 1 or 3 to 8, in which the membrane (4) is provided with a central sleeve (21) threaded on the central tie (17) said sleeve rising over the entire height of the interior volume of the accumulator and being anchored at its upper end to the upper plug (2). 10. Accumulator according to one of claims 1 or 3 to 9, in which the membrane (4) is fixed by means of an elastic washer (40) pierced in its center, the outer peripheral rim (40b) wedges the outer rim (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 flange (40a) of the internal bore of which blocks the top of the sleeve (21) of the membrane (4) in a groove (33) formed in the tie rod (17). 11. Accumulator according to any one of claims 1 or 3 to 10, in which the orifices (26) for the outlet of the liquid are disposed on the plug (3) in a ring around the central tie rod (17), the annular bottom (4a ) of the membrane (4) being provided, facing the orifices (26), with pellets (28, 36). 12. Accumulator according to claim 11, wherein the annular bottom (4a) of the membrane (4) rests on a circular plate (34) constrained by a spring (35). 13. A method of assembling under prestressing the elements constituting an accumulator according to any one of claims 1 to 11 consisting in compressing against each other the two upper and lower plugs (2 and 3) and the central body (1 ) by means of a hydraulic cylinder (11, 38), while exerting in the opposite direction, by means of the same hydraulic cylinder, traction on the tie rod (s) (6, 17); then blocking the screw nuts (7, 20) of the tie rod (s) as far as one of the plugs; after which the hydraulic pressure of the jack (11, 38) is released.

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