EP0035427B1 - Method of making a prestressed accumulator and accumulator according to this method - Google Patents

Abstract

A hydraulic accumulator comprises two-halfs shells, the diaphragm therebetween defining two cavities. One cavity contains presssurized gas and the cavity receives hydraulic fluid under pressure. The two-half shells are held together by a mechanical element, such as a belt which has been previously prestressed to a tension selected according to the maximum pressure to which the accumulator will be subjected. The use of the prestressed mechanical element changes the dynamic stresses ordinarily found in a hydraulic accumulator to static stresses, thus minimizing the occurrence of creeks and failure of the device.

Description

The present invention relates to a method of manufacturing an oleo-pneumatic accumulator of the type consisting of an enclosure and a gas-fluid separator and to an oleo-pneumatic accumulator produced according to this method. the enclosure is generally a metallic envelope. The separator can be produced by a flexible membrane.

It turns out that accumulators, when working at high pressures and very frequently repeated pressurization and depressurization cycles, deteriorate rapidly due to the fatigue of the metal. In the case of speakers made up of two parts screwed one inside the other, it is at the bottom of the threads that fatigue is born. It is characterized by a crack causing the rupture of the enclosure.

The object of the present invention is to transform the dynamic stresses to which the junction of the two parts of the enclosures, which cause the fatigue of the metal, is subjected, into a static stress, which eliminates the fatigue of the metal.

It is known from patent US-A-2,563,257 to produce an oleopneumatic membrane accumulator by means of two shells between which the circular edge of a membrane is pinched, these two shells being assembled by a clamping means constituted by a ring bearing on one of the shells and screwing on the other. However, this clamping means, preferably made of cast iron, does not exert any prestressed assembly of the two shells.

It is known from document DE-A-2 155 255 to arrange the two shells of an accumulator so that they each have a substantially cylindrical rim, these two rims fitting into each other and screwing one over the other. Due to the elasticity inherent in any steel part, the force-tightening of a shell one on the other inevitably leads to the appearance of a certain prestress, but it is limited by the blocking of the threads. one over the other and can only have negligible value.

It is known from document DE-A-2 755 457 to assemble the two hemispherical shells of an accumulator to one another by forcing them into a metal ring whose diameter is slightly smaller than that of the sphere produced by assembling the two shells; the ring is then placed along a meridian and is preferably held in place by the welding spots. One thus obtains a prestressing but of absolutely undetermined and uncertain value because it depends on the ratio between the dimensions of the internal diameter of said ring and those of the external diameter of the sphere. It is mentioned in this document that the preload in the metal ring is greater than the force of separation of the two shells resulting from the maximum internal test pressure.

In addition, in such an accumulator, the assembly under prestressing not being carried out along the equator, that is to say along the line of junction of the two shells, these will deviate l 'from each other, due to the elasticity of the metal, when the accumulator is subjected to high pressure and the liquid will escape, driving the membrane and therefore destroying the accumulator.

Document DE-A-2 532 807 discloses an assembly of two half-shells in a similar manner to that described in the aforementioned US-A-2,563,257 patent but by screwing the junction ring onto the two shells by means of threads having different pitches, so that the more the ring is screwed, the more it is itself prestressed. But here again, the prestressing being obtained only by screwing can only be very weak because blocking by seizing of the nets very quickly occurs and such prestressing cannot have a determined and precise value with respect to the separation forces generated. by the hydraulic pressure admitted into the accumulator.

In known devices (US-A-2,563,275, DE-A-2,155,255, DE-A-2,532,807), the two shells are screwed one on the other. However, it turns out that when such accumulators are subjected to high and highly alternating pressures, cracks in the metal appear fairly quickly at the level of the thread bottom of the threads, which can lead to breaks in the connection between the two shells.

When passing such accumulators on fatigue test benches where they are subjected to alternating pressures between atmospheric pressure and their maximum operating pressure, we see appear, and for a short cycle, of the order of a few hundred thousand cycles, cracks which very quickly lead to the rupture of the accumulator.

In order to avoid the appearance of these cracks, the two shells are, according to the present invention, assembled with a prestress.

The present invention therefore relates to a process for manufacturing an oleo-pneumatic accumulator of the type comprising two shells assembled to one another with the interposition of a flexible membrane, in which the two shells are kept in contact one on the other by an annular tightening means surrounding the two contiguous edges of the two shells, said tightening means being associated with one of said shells and screwed onto the other of said shells and being prestressed along its longitudinal axis, characterized by the fact that the clamping means is subjected, before the assembly of said shells, to a preliminary prestressing generating, after the assembly of said shells, a force for bringing the two shells together, which is greater than the force maximum separation of the two shells resulting from the maximum internal pressure that the accumulator must receive (test pressure).

The invention also relates to an accumulator produced according to this method, the clamping means of which comprises at its axial ends annular shoulders against which a tensioning device is supported.

According to a first embodiment, the two shells are assembled to each other by means of an external belt, previously tensioned; according to a second embodiment, the two shells are each provided with a skirt, one fitting into the other, one of the skirts having been previously tensioned.

According to the present invention, it is possible in one and the other embodiment to interpose between the two shells a cylindrical shim, so that when the force to which the accumulator is subjected is greater than that of the prestress, shells separate, at least slightly, from said shim, which causes a leak rate.

Thus, this arrangement plays the role of a pressure relief valve which constitutes a safety device which prevents the accumulator from being subjected to a predetermined maximum pressure.

By having one or more liquid discharge orifices, through the prestressed belt, oil is allowed to be discharged, which also makes it possible to make visible the leakage rate caused by an overpressure. The invention also relates to a device for prior tensioning of the clamping means of such an accumulator 1.

• Figure 1, une vue schématique en coupe illustrant un premier mode de mise en oeuvre du procédé;
• Figure 2, une vue schématique en coupe, illustrant une variante de la figure 1;
• Figure 3, une vue schématique en coupe illustrant un deuxième mode de mise en oeuvre du procédé;
• Figure 4, une demi-vue en coupe d'un exemple de réalisation d'un accumulateur muni d'une cale selon la présente invention;
• Figure 5, une vue à échelle agrandie d'un détail de la figure 4;
• Figure 6, un exemple de réalisation d'un mécanisme de mise en tension préalable de la ceinture représentée à la figure 4.

By way of nonlimiting example and in order to facilitate understanding of the invention, there is shown in the accompanying drawings:

• Figure 1, a schematic sectional view illustrating a first mode of implementation of the method;
• Figure 2, a schematic sectional view, illustrating a variant of Figure 1;
• Figure 3, a schematic sectional view illustrating a second embodiment of the method;
• Figure 4, a half-sectional view of an embodiment of an accumulator provided with a shim according to the present invention;
• Figure 5, an enlarged view of a detail of Figure 4;
• Figure 6, an exemplary embodiment of a prior tensioning mechanism of the belt shown in Figure 4.

Referring to Figure 1, we see that the accumulator is constituted, as is known, by two shells 1 and 2 which, when assembled together, define an interior volume which is divided into two compartments 3 and 4, separated by a flexible membrane 5. The shell 1 is closed by a plug 6 and the shell 2 is connected to a hydraulic pipe 7. By means of the plug 6, the volume 3 is filled with a pressurized gas; the volume 4 receives the hydraulic liquid coming from the pipe 7. The flexible membrane 5 is provided at its edge with a heel 5a which ensures its fixing by pinching between the two shells 1 and 2.

In the example shown in Figure 1, the two shells 1 and 2 are not assembled by direct screwing of one on the other, but by means of a belt 8, which is subjected, before the assembly, at an elongation tension of a force greater than the force generated by the pressurization of the accumulator greater than that of its maximum operating pressure.

The belt 8 comprises a thread 8a, intended to receive the thread 1a of the shell 1 and a thread 8b, intended to receive the thread 2a of the shell 2.

The belt 8 also comprises an external shoulder 9 and a groove 10 in which a ring 11 is engaged. Between the two stops constituted by the shoulder 9 and the ring 11 are arranged the two elements of a hydraulic cylinder constituted by the body of the jack proper 12, which bears against the ring 11 and an annular piston 13, which bears against the shoulder 9, the parts 12 and 13 being concentric with the belt 8.

To mount the accumulator, first place the annular piston 13 around the belt 8, then the cylinder body 12 and then the ring 11. Hydraulic fluid under pressure is then introduced between the cylinder body 12 and the piston 13, through the orifice 14, so that the belt 8 is subjected to a force tending to lengthen it longitudinally in the opposite directions F, and F ₂ .

The lower shell 2 is then put in place by screwing its thread 2a onto the internal thread 8b of the belt 8; the membrane 5 is put in place; and the shell 1 is screwed by its thread 1a onto the other internal thread 8a of the belt 8, until the two shells 1 and 2 come into tight contact one against the other. The hydraulic pressure supplied at 14 is then canceled, the ring 11 is removed, then the cylinder body 12 and the piston 13.

By its own elasticity, the belt 8 presses one against the other the two shells 1 and 2, the belt 8 constituting a fixing with preload.

In the variant shown in Figure 2 (only half of the accumulator being shown) the respective positions of the shoulder 9 and the groove 10, intended to receive the ring 11, have been reversed, but their role is identical. Only the shell 2 is screwed onto the belt 8, the shell 1 being simply held by a shoulder 15 abutting against a corresponding shoulder 16, formed inside the belt 8, in place of the thread 1a.

In this case, the belt 8 is tensioned beforehand, by means of the same jack 12-13 (not shown) as that used for the device in FIG. 1; then the shell 1 is introduced from below into the belt 8, until the shoulders 15 and 16 are in contact; the membrane 5 is put in place; then the shell 2 is screwed by its thread 2a onto the thread 8b of the belt 8 until the shell 2 is tightened against the shell 1; the pressure is then released in the cylinder 12-13 and the latter is disassembled as above.

In both cases, the two shells 1 and 2 are kept strongly pressed against each other, by the prestress created in the belt 8.

Preferably, the prior extension force to which the belt 8 is subjected is determined so as to be greater than the extension force to which it will be subjected when the accumulator is subjected to the maximum bench test pressure, pressure which is itself higher than the maximum pressure of use of the accumulator.

In the example shown in Figure 3, the belt is an integral part of one of the shells, of the shell 1 in the case shown.

In this example, the shell is provided at its base with a skirt 17, which has a length substantially equal to that of the belt 8 in Figures 1 and 2. This skirt 17 is provided at its base with an internal thread 17a. At its upper end, the skirt 17 has an internal shoulder 19, provided with a groove intended to receive the heel 5a of the membrane 5. Similarly, the shell 2 is provided with a skirt 18 having practically the same length as the skirt 17, but being of a smaller diameter, so as to fit into the interior of said skirt 17. At its lower part, the skirt 18 has a thread 18a, intended to be screwed onto the thread 17a and at its upper part a flat 20, intended to abut against the shoulder 19 and also comprising a groove for receiving the heel 5a of the membrane 5.

The skirt 17 is provided, like the belt 8, with a shoulder 9 and a groove 10, intended to receive the ring 11, so that the jack 12-13 (not shown) can be set in plaxis around the skirt 17, as it is placed around the belt 8.

The skirt 17 is tensioned beforehand by the jack 12-13 in a similar manner to that which has been described previously for FIGS. 1 and 2, then the shell 2 is screwed onto the shell 1 (with interposition of the membrane 5) until '' hard clamping of one on the other; then the pressure in the cylinder 12-13 is released and the latter is disassembled.

The two shells are then kept strongly pressed against each other, by the prestress created in the skirt 17.

By this process, an accumulator having the same capacity as a usual accumulator and subjected to the same test pressure underwent, without appearance of cracks, 5 million cycles at the test pressure, while the usual accumulator included cracks after only 150,000 cycles.

It is obvious that the length of the element tensioned before assembly (whether the belt 8 or the skirt 17) as well as its thickness are determined according to the intensity of the prestressing force that we want to get.

It is also obvious that the invention is not limited to the particular embodiment of the jack 12-13 ensuring the prior tensioning of the belt 8 or of the skirt 17.

Referring to Figures 4 and 5, we see that we can, before assembling the two shells 1 and 2, interpose between them a cylindrical shim 21, which, in the example shown is a hollow cylinder of revolution, or a portion of tube.

At its lower and upper parts, this wedge is provided with a chamfer 21a and 21b, these two chamfers fitting into chamfers of corresponding shape formed in the edges of the shells 1 and 2.

The membrane 5 is provided with a heel 5a, which engages in a correspondingly shaped groove formed in the edge of the shell 1, so that said heel is pinched between the shell 1 and the wedge 21.

The underside of the wedge 21 rests by direct contact against the edge of the shell 2.

The external prestressing belt 22 comprises, like a test piece, a central part 22c of small section and two ends 22a and 22b of larger section in which the threads 8a and 8b are formed.

To assemble the accumulator thus formed, the lower shell 2 is screwed to the base of the belt 22, by means of its thread 2a, which engages in the thread 8b of the part 22b; then the wedge 21 is placed; then the membrane 5 is placed. Next, the belt 22 is tensioned, either by means of the jack described in FIG. 1 of the main patent, or by means of the jack which will be described below, in relation to FIG. 6. When the determined preliminary tension value is reached, the upper shell 1 is screwed into the belt 22, by means of its thread 1a, which engages in the thread 8a; then release the tension created by the jack and remove it.

The two shells 1 and 2 are then strongly tightened against each other by the tension created beforehand in the structure of the belt 22 to which its thinned median shape gives better elasticity characteristics. The membrane 5 is held by pinching its heel 5a between the shell 1 and the wedge 21.

The hydraulic fluid under high pressure penetrates through the pipe 7 and lifts the membrane 5 by compressing the gas located in the enclosure 3. This hydraulic pressure and the pressure of the gas (which is equal) tend to separate each other the shells 1 and 2 and the wedge 3; but these remain applied against each other, as long as the force created by this pressure remains less than the prior tension force to which the belt 22 has been subjected.

When the pressure exceeds the predetermined maximum value, the force tending to separate the parts 1, 2 and 21 becomes greater than that tending to keep them tight and the wedge 21 separates from the shell 2, so that the liquid can leak. The greater the difference between the maximum pressure admitted and the actual pressure existing in 4, the greater the difference between the shim 21 and the shell 2 and therefore the greater the leakage rate.

This device plays the role of a safety device, preventing the deterioration of the accumulator by an overpressure.

The liquid flowing between the wedge 21 and the belt 22 is discharged through one or more orifices 23 ', formed through the latter, which, in addition makes it possible to visualize the existence of a leakage rate.

Figure 6 shows the elongation device of the belt 22. On a base 23 is screwed a crown 24, carrying two half-collars 25, by means of arms 26 articulated on axes 27, carried by the crown 24 (in FIG. 6, only a single half-collar 25, a single arm 26 and a single axis 27 have been shown).

The lower part of the base 24 constitutes a piston engaged in a cylinder body 28, comprising a pipe 29 opening into a chamber 30. This cylinder 28 carries two half-collars 31 by means of two arms 32.

To proceed with the mounting of the accumulator, the lower shell 2 is placed on the base, then the wedge 21, the membrane 5 are put in place and the belt 22 is screwed onto the lower shell. Then the two arms 26 are folded down, so that the two half-collars 25 come to bear on the shoulder separating the parts 22c and 22b of the belt 22. The two half-collars 31 are put in place, which have a shoulder coming to rest on the ends of the arms 32; the two half-collars 31 come to bear against the shoulder separating the parts 22c and 22b of the belt 22. The high pressure is introduced into the chamber 30, which has the effect that the arms 32 and the half-collars 31 are biased in direction f ₁ while the arms 26 and the half-collars 25 remain fixed: this elongates the belt 22. The shell 1 is then screwed in and the pressure in the chamber 30 is released.

Claims (8)

1. A methode for manufacturing an oleopneumatic accumulator of the type comprising two shells (1, 2) assembled to each other through the interposition of a flexible membrane (5), in which the two shells (1, 2) are kept in contact one against the other by a clamping annular means (8, 17, 22) surrounding the two contiguous edges of the two shells (1, 2), said clamping means being associated with one of said shells (1) and screwed onto the other (2) of said shells, and being prestressed along its longitudinal axis, wherein the clamping means (8, 17, 22) is subjected, before the assembly of said shells (1, 2), to a prior prestress generating, after the assembly of said shells, a force bringing the two shells (1, 2) close to each other and superior to the maximum separation force of the two shells resulting from the maximum inner pressure which the accumulator has to receive (trial pressure).

2. A method according to claim 1, wherein the prior prestressing operation is provided through a hydraulic jack (12, 13; 23, 28).

3. An oleopneumatic accumulator made according to the method described in claim 1 or 2, wherein the clamping means (8, 17, 22) is formed at its axial ends with annular shoulders (9, 11; 22a, 22b) against which comes to bear a tensioning device (12, 13; 28).

4. An oleopneumatic accumulator according to claim 3, characterized in that it is made of two shells (1, 2), each formed with an outer threading (1a, 2a) on which engage the two inner threadings (8a, 8b) of a belt-shaped clamping means (8) for assembling said shells (1, 2), said belt being formed on the one hand with an annular shoulder (9) and on the other hand with an annular groove (10) for receiving a ring (11) forming a second annular shoulder, the trensioning device being disposed between said two shoulders (9, 11).

5. An oleopneumatic accumulator according to claim 3, characterized in that it is formed of two shells (1, 2), one (1) being formed with a shoulder (15) and the other (2) with an outer threading (2a), the clamping means (8) which has to be set under a prior tension being formed with a shoulder (16) bearing against shoulder (15) of the first shell (1) and with a threading (8a) engaging the threading (2a) of the other shell (2); said belt-shaped clamping menas (8) being formed on the one hand with an annular shoulder (9) and on the other hand with an annular groove (10) for receiving a ring (11) forming a second annular shoulder, the tensioning device being disposed between said two shoulders.

6. An oleopneumatic accumulator according to claim 3, characterized in that it is made of two shells (1, 2) formed each with a substantially cylindrical skirt (17, 18), the skirt (18) of one of the shells being disposed inside the skirt (17) of the other shell, the inner skirt (18) comprising on the one hand and at its base a threading (18a) on which engages the threading (17a) formed at the end of the other skirt (17), and on the other hand at its end a flat poriton (20) coming to bear against an inner shoulder (19) provided at the base of the outer skirt (17); said outer skirt including two bearing annular elements for a tensioning device, formed of an annular shoulder (9) and of a ring (11) housed in a groove (10).

7. A hydraulic accumulator according to claim 4 or 5, wherein a hollow cylindrical wedge (21) is placed between the circular edges of the two shells (1, 2) such that, when the separation force exerted by the hydraulic liquid is superior to the force bringing the shells closer and exerted by the prestress to which is subjected the clamping means (8), the hydraulic liquid leaks outside the accumulator which plays then the part of an overpressure valve.

8. A device for the prior tensioning of the clamping means (8, 17, 22) of an accumulator according to any one of claims 3 to 7, characterized in that it is made of a hydraulic jack (12, 13; 23, 28) the piston (13, 23) and the body (12, 28) of which include annular means (13, 25; 12, 31) surrounding the clamping means (8, 12, 22) and bearing against the annular shoulders (9, 11; 22a, 22b).

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