Hydropneumatic accumulator

Abstract

The invention relates to a hydropneumatic accumulator (1) comprising: - an enclosure (2) formed by a first half-shell (3) and a second half-shell (4) which are screwed to each other by means of a first thread (6) and a second thread (8) provided respectively on the first and second half-shells, and - a membrane which is pinched between the first half-shell (3) and the second half-shell (4); the first thread (6) and the second thread (8) have the following characteristics: - a pitch P; - an angle α formed by the engagement flank (22) which is between 15 and 35°; - an angle β formed by the load flank (23) which is between 5 and 10°; - a radius of curvature R of the bottom (25) between 0.15 P and 0.25 P; - a width L which is between 0.20 P and 0.40 P; ; and - a distance D1 which is greater than or equal to the sum of the height H and the radius of curvature R. Figure for the abstract: 4

Description

Title of the invention: Hydropneumatic accumulator Technical field

[0001] The invention relates to the field of hydropneumatic accumulators.

[0002] The invention relates more particularly to hydropneumatic membrane accumulators. Technological background

[0003] Document FR2210725 discloses a hydropneumatic accumulator comprising an enclosure formed of two half-shells which are screwed to each other. An elastomer membrane is pinched between the two half-shells and separates the enclosure into two compartments, one of which is intended to receive a pressurized gas and the other is intended to be connected to a hydraulic circuit. The two half-shells are screwed to each other until abutment surfaces of the two half-shells come into contact with each other. These abutment surfaces make it possible to create a prestressing force which puts the assembly of the two half-shells under tension.

[0004] Such hydropneumatic accumulators are not fully satisfactory, in particular because of their insufficient service life. Hydropneumatic accumulators are in fact likely to be subjected, during their life, to a very large number of cycles, for example greater than 1,000,000. However, without increasing the thicknesses of the hydropneumatic accumulators of the state of the art, in particular at the level of their threads, they do not have sufficient fatigue resistance to meet such service life requirements. Increasing the thicknesses of the hydropneumatic accumulators is also not a fully satisfactory option because, on the one hand, it proves impossible for applications in which space is limited and, on the other hand, leads to an increase in the mass and manufacturing cost of the hydropneumatic accumulator. Summary of the invention

[0005] An idea at the basis of the invention is therefore to propose a hydropneumatic accumulator of the aforementioned type having excellent resistance to fatigue, without significantly increasing its mass and / or its size.

[0006] According to a first aspect, the invention provides a hydropneumatic accumulator comprising: - an enclosure formed by a first half-shell and a second half-shell which are screwed to each other, and - a membrane which is pinched between the first half-shell and the second half-shell and divides the enclosure into a first compartment and a second compartment; wherein the first half-shell has an outer surface having a first thread and the second half-shell has a skirt which is disposed around the outer surface of the first half-shell and which has a second thread cooperating with the first thread to screw the first half-shell and the second half-shell to each other along an X axis; wherein the first half-shell comprises a first abutment surface which is provided at one end of the first half-shell and which is in contact against a second abutment surface which is provided, on the second half-shell, radially inside the skirt; wherein the first thread and the second thread each comprise a thread defined by a load flank, an engagement flank positioned upstream of the load flank, an upper face connecting the engagement flank and the load flank to each other and a bottom connecting the engagement flank and the load flank of two adjacent portions of the thread; wherein the first thread and the second thread have the following characteristics: - a P step; - an angle a formed, in a transverse plane including the X axis, by the engagement flank relative to an axis perpendicular to the X axis and included in said transverse plane which is between 15 and 35°; - an angle [3 formed, in the transverse plane including the X axis, by the load-bearing flank relative to an axis perpendicular to the X axis and included in said transverse plane which is between 5 and 10°; - a radius of curvature R of the bottom, in the transverse plane, between 0.15 P and 0.25 P; - a width L corresponding to an axial dimension in the transverse plane of the upper face which is between 0.20 P and 0.40 P; - a height H corresponding to the difference between a radial distance between the bottom of one of the first and second threads and the X axis and a radial distance between the upper face of said first or second thread and the X axis; and - a distance DI corresponding to a radial distance between the bottom of the first thread and the bottom of the second thread which is greater than or equal to the sum of the height H and the radius of curvature R.

[0007] Thanks to the aforementioned characteristics of the first and second threads, the fatigue resistance of the hydropneumatic accumulator is significantly improved. Tests have notably shown that such threads make it possible to improve the fatigue resistance of the hydropneumatic accumulator by more than 60%.

[0008] In particular, the angle [3 formed by the load-bearing flank makes it possible to optimize the surface of contact between the load flanks, the radius of curvature R is large enough to limit stress concentrations in the bottom of the threads and the width L of the threads is large enough to limit the risks of deformation of the threads.

[0009] Furthermore, since the distance DI is greater than the sum of the height H and the radius of curvature R, there is a clearance between the upper face and the center of curvature of the bottom, which ensures plane contact between the load-bearing flanks of the threads and thus ensures good distribution of the stresses.

[0010] According to embodiments, such a hydropneumatic accumulator may comprise one or more of the following characteristics.

[0011] According to one embodiment, the first and second threads have a trapezoidal profile.

[0012] According to one embodiment, the pitch P is between 1.30 and 4 mm.

[0013] According to a preferred embodiment, the pitch P is between 1.50 and 2.50 mm.

[0014] According to a preferred embodiment, the angle a is between 20 and 30°.

[0015] According to a preferred embodiment, the angle [3 is between 6 and 9°.

[0016] According to a preferred embodiment, the radius of curvature R is between 0.17P and 0.23 P.

[0017] According to a preferred embodiment, the width L is between 0.25 P and 0.35 P.

[0018] According to one embodiment, a distance D2 corresponding to the difference between the radial distance between the top face of the first thread and the X axis and the radial distance between the top face of the second thread and the X axis is greater than 0.4 P.

[0019] According to one embodiment, the second thread has, towards the second surface abutment, a frustoconical end zone in which the thread is truncated so that a radial distance between the upper face of the second thread and the X axis increases in the direction of the second abutment surface. Such a frustoconical end zone thus makes it possible to limit the maximum level of stresses in the portions of the second thread closest to the second abutment surface. Tests have notably shown that this advantageous characteristic alone makes it possible to improve the fatigue resistance of the hydropneumatic accumulator by more than 60%.

[0020] According to one embodiment, in the truncated end zone, the thread has a truncated helix shape having a half-angle at the apex e which is between 0.2 and 15°.

[0021] According to one embodiment, the first half-shell has a first annular cavity which is positioned axially between the first abutment surface and the first thread and which extends radially towards the X axis beyond the first and second abutment surfaces. This makes it possible to move the area in which the maximum stresses are exerted from the first thread towards the bottom of the first annular cavity, which thus improves the fatigue resistance of the hydropneumatic accumulator.

[0022] According to one embodiment, the first annular cavity has a radial depth el, relative to the bottom of the first thread, of at least 5 mm.

[0023] According to one embodiment, the first annular cavity comprises a bottom having several curved zones, each of the curved zones joining the adjacent curved zone(s) in a portion projecting towards the first annular cavity. This makes it possible to reduce the maximum level of stresses in the bottom of the first annular cavity.

[0024] According to one embodiment, the second half-shell comprises a second annular cavity which is arranged axially between the second abutment surface and the second thread and which extends radially outwards. This makes it possible to move the zone in which the maximum stresses are exerted from the second thread towards the bottom of the second annular cavity, which thus improves the fatigue resistance of the hydropneumatic accumulator.

[0025] According to one embodiment, the second annular cavity has a radial depth e2 relative to the bottom of the second thread of at least 3 mm.

[0026] According to one embodiment, the second annular cavity comprises a bottom having several curved zones, each of the curved zones joining the adjacent curved zone(s) in a portion projecting towards the second annular cavity. This makes it possible to reduce the maximum level of stresses in the bottom of the second annular cavity.

[0027] According to one embodiment, the membrane is made of a polymer material which is chosen for example from nitrile rubber, ethylene-propylene-diene monomer rubber and fluorocarbon rubber.

[0028] According to one embodiment, the first half-shell has an orifice which opens into the interior of the first compartment and is intended for the injection of pressurized gas into the first compartment.

[0029] According to one embodiment, the second half-shell comprises an orifice intended for the passage of hydraulic fluid.

[0030] According to one embodiment, the second half-shell comprises a circular shoulder, arranged radially inside the skirt and forming the second stop surface.

[0031] According to one embodiment, the second half-shell comprises an annular groove in which an annular heel of the membrane is housed, said annular groove being positioned radially inside the circular shoulder forming the first stop surface and opposite the end of the first half-shell.

[0032] According to a second aspect capable of being implemented independently of the first aspect, the invention also provides a hydropneumatic accumulator comprising: - an enclosure formed by a first half-shell and a second half-shell which are screwed to each other, and - a membrane which is pinched between the first half-shell and the second half-shell and divides the enclosure into a first compartment and a second compartment; wherein the first half-shell has an outer surface having a first thread and the second half-shell has a skirt which is disposed around the outer surface of the first half-shell and which has a second thread cooperating with the first thread to screw the first half-shell and the second half-shell to each other along an X axis; wherein the second half-shell comprises a second abutment surface which is provided radially inside the skirt and which is in contact against a first abutment surface provided at one end of the first half-shell; wherein the first thread and the second thread each comprise a thread defined by a load flank, an engagement flank positioned upstream of the load flank, an upper face connecting the engagement flank and the load flank to each other and a bottom connecting the engagement flank and the load flank of two adjacent portions of the thread; and wherein the second thread has, towards the second abutment surface, a frustoconical end zone in which the thread is truncated so that a radial distance between the upper face of the second thread increases towards the second abutment surface.

[0033] Such a truncated end zone makes it possible to limit the maximum level of stresses in the portions of the second thread closest to the second abutment surface and in itself makes it possible to significantly improve the fatigue resistance of the hydropneumatic accumulator.

[0034] According to a third aspect capable of being implemented independently of the first aspect and / or the second aspect, the invention also provides a hydropneumatic accumulator comprising: - an enclosure formed by a first half-shell and a second half-shell which are screwed to each other, and - a membrane which is pinched between the first half-shell and the second half-shell and divides the enclosure into a first compartment and a second compartment; wherein the first half-shell has an outer surface having a first thread and the second half-shell has a skirt which is disposed around the outer surface of the first half-shell and which has a second thread cooperating with the first thread to screw the first half-shell and the second half-shells to each other along an X axis; wherein the first half-shell has a first annular cavity which is positioned axially between the first abutment surface and the first thread and which extends radially towards the X axis beyond the first and second abutment surfaces; and wherein the second half-shell comprises a second annular cavity which is arranged axially between the second abutment surface and the second thread and which extends radially outwards.

[0035] Thanks to the first and second annular cavities, the zones in which the maximum stresses are exerted are moved from the first and second threads to the bottoms of the first and second annular cavities. These annular cavities thus make it possible, by themselves, to significantly improve the fatigue resistance of the hydropneumatic accumulator. Brief description of the figures

[0036] The invention will be better understood, and other aims, details, characteristics and advantages thereof will appear more clearly during the following description of several particular embodiments of the invention, given solely for illustrative and non-limiting purposes, with reference to the appended drawings.

[0037] [Fig.l] is a sectional view of a hydropneumatic accumulator according to a first embodiment.

[0038] [Fig.2] is a detailed sectional view of the hydropneumatic accumulator of the [Fig.l] representing the threads and the abutment surfaces of the first and second half-shells.

[0039] [Fig. 3] is a detailed sectional view of the threads of the first and second half-shells according to a first variant embodiment.

[0040] [Fig.4] is a detailed sectional view of the threads of the first and second half-shells according to a second variant.

[0041] [Fig. 5] is a detailed sectional view of the threads of the first and second half-shells according to a third variant embodiment.

[0042] [Fig.6] is a sectional view illustrating in detail the net with a truncated helix shape in its lower part and the cavity of the second half-shell.

[0043] [Fig.7] is another detailed sectional view of the hydropneumatic accumulator of [Fig.l] representing the threads and the abutment surfaces of the first and second half-shells.

[0044] [Fig.8] is a detailed sectional view of the hydropneumatic accumulator of [Fig.l] showing the geometry of the cavity of the first half-shell and the cavity of the second half-shell.

[0045] [Fig.9] is a detailed sectional view of the cavities of the first half-shell and the second half-shell according to an alternative embodiment. Description of the embodiments

[0046] In the description and the claims, the "axial" and "radial" orientations will be used to describe characteristics of the hydropneumatic accumulator. By convention, the axial orientation relates to the X axis of screwing of the half-shells of the hydropneumatic accumulator and the "radial" orientation is directed orthogonally to the axial orientation.

[0047] In relation to Figures 1 and 2, a hydropneumatic accumulator 1 is described according to a first embodiment. The hydropneumatic accumulator 1 comprises an enclosure 2 which is formed by two half-shells, namely a first half-shell 3 and a second half-shell 4. The half-shells 3, 4 have, for example, a substantially hemispherical shape. They are made of steel. The geometric and dimensional characteristics which will be described subsequently relate more particularly to a hydropneumatic accumulator 1 whose enclosure has a volume of 10L. However, the invention is obviously not limited to a hydropneumatic accumulator of such a capacity and is also applicable to hydropneumatic accumulators 1 having any other capacities.

[0048] The half-shells 3, 4 are screwed to each other along an axis X. To do this, the first half-shell 3 has an external surface 5 which comprises a first thread 6 while the second half-shell 4 comprises a skirt 7 which is suitable and intended to be arranged around the cylindrical external surface 5 of the first half-shell 3 and which comprises a second thread 8. The second thread 8 is configured to be arranged outside the first thread 6 of the first half-shell 3 and to cooperate with said first thread 6 so as to screw the half-shells 3, 4 to each other.

[0049] A membrane 9 is pinched between the half-shells 3, 4 and thus divides the internal space of the enclosure 2 into a first compartment 12 and a second compartment 13. The membrane 9 is made for example from a polymer material which is for example chosen from nitrile rubber, also designated by the acronym NBR, ethylene-propylene-diene monomer rubber, designated by the acronym EPDM and fluorocarbon rubber, designated by the acronym FKM.

[0050] The first compartment 12 is intended to receive a pressurized gas, nitrogen for example, while the second compartment 13 is intended to be connected to a hydraulic circuit. The first half-shell 3 has an orifice 14 which opens into the interior of the first compartment 12 and thus allows the injection of pressurized gas into the first compartment 12. For example, the pressure of the pressurized gas in the first compartment 12 is between 10.106 and 30.106 Pa, for example of the order of 18.106 to 22.106 Pa. The orifice 14 is equipped with an inflation valve 15 and a plug 16. The second half-shell 4 has an orifice 17 intended for the passage of the hydraulic fluid.

[0051] The second half-shell 4 comprises a circular shoulder, arranged radially inside the skirt 7 and forming a second abutment surface 19 against which a first abutment surface 18 arranged on the end 20 of the first half-shell 3 is intended to come into contact. These abutment surfaces 18, 19 thus make it possible to create a prestressing force which puts the assembly of the two half-shells 3, 4 under tension.

[0052] The second half-shell 4 comprises an annular groove 10 in which an annular heel 11 of the membrane 9 is housed. The annular groove 10 is positioned radially inside the circular shoulder forming the first stop surface 18 and opposite the end 20 of the first half-shell 3. When the two half-shells 3, 4 are screwed together, the annular heel 11 of the membrane 9 is pinched between the two half-shells 3, 4 and more particularly between the annular groove 10 of the second half-shell 4 and the end 20 of the first half-shell 3.

[0053] In the embodiment shown, the membrane 9 comprises a part 21 forming a valve which is positioned in a lower part of the membrane 9 and which is configured to close the orifice 17 through which the hydraulic fluid arrives when the second compartment 13 is empty of hydraulic fluid.

[0054] In relation to [Fig. 3], the geometric characteristics of the first thread 6 and the second thread 7 are described below, making it possible to improve the fatigue resistance of the hydropneumatic accumulator 1.

[0055] The thread of each of the first and second threads 6, 7 has a trapezoidal profile. Such a profile is advantageous in that it allows high loads to be transmitted. The threads of the first thread 6 and the second thread 8 are each defined by two lateral flanks, namely an engagement flank 22 and a load flank 23, which are connected to each other by an upper face 24. The engagement flank 22 is placed upstream of the load flank 23. In other words, the engagement flank 22 of the first thread 6 faces towards the end 20 of the first half-shell 3 while the load flank 23 faces in the opposite direction to the end 20 of the first half-shell 3. Similarly, the engagement flank 22 of the second thread 8 faces towards the end 27 of the second half-shell 4 while the load flank 23 faces in the opposite direction to the end 27 of the second half-shell 4.Each of the first and second threads 6, 8 also comprises a base 25 connecting the engagement flank 22 and the load flank 23 of two adjacent portions of the thread.

[0056] The first and second threads 6, 8 advantageously have a length greater than 40 mm, and for example of the order of 46 mm.

[0057] As illustrated in [Fig. 3], the prestressing force which puts the assembly of the two half-shells 3, 4 under tension has the effect of pressing the load-bearing flanks 23 of the first thread 6 and of the second thread 8 against each other.

[0058] The pitch P of the first and second threads 6, 8, that is to say the distance along the axis X between two adjacent portions of the thread, is advantageously between 1.30 and 4 mm, and preferably between 1.50 and 2.50 mm.

[0059] The angle α, formed by the engagement flanks 22 relative to an axis perpendicular to the axis X, is between 15 and 35° and preferably between 20 and 30°.

[0060] The angle [3, formed by the load-bearing flanks 23 with respect to an axis perpendicular to the axis X, is between 5 and 10°, and preferably between 6 and 9°.

[0061] The angle Q defined between the load flank 23 and the engagement flank 22 of the thread of each of the first and second threads 6, 8 is therefore preferably between 21° and 40° and more preferably between 30 and 35°.

[0062] The radius of curvature R of the bottom 25 of the first and second threads 6, 8 is between 0.15 P and 0.25 P, preferably between 0.17 P and 0.23 P and for example of the order of 0.20 P.

[0063] The width L corresponds to the minimum thickness of the thread, that is to say to the axial dimension of the upper face 24. The width L is between 0.20 P and 0.40 P, preferably between 0.25 P and 0.35 P and for example of the order of 0.30 P.

[0064] The first and second threads 6, 8 have an identical height H. The height H corresponds to the difference between, on the one hand, the radial distance between the bottom 25 and the axis X and, on the other hand, the radial distance between the upper face 24 and the axis X.

[0065] The distance DI corresponds to the difference between, on the one hand, the radial distance between the bottom 25 of the first thread 6 and the axis X, and, on the other hand, the radial distance between the bottom 25 of the second thread 8 and the axis X. As shown in [Fig. 3], the distance DI is greater than the sum of the height H and the radius of curvature R of the bottom 25. Thus, there is a clearance J between the upper face 24 and the center of curvature of the bottom 25, which ensures plane contact between the load flanks 23 of the threads 6, 8 and ensures good distribution of the stresses.

[0066] The distance D2 corresponds to the difference between, on the one hand, the radial distance between the upper face 24 of the first thread 6 and the axis X, and, on the other hand, the radial distance between the upper face 24 of the second thread 8 and the axis X. The distance D2 is advantageously greater than 0.35 P.

[0067] In the embodiment illustrated in [Fig.3], the characteristics of the threads 6, 8 are as follows: - the step P is 2 mm; - angle a of 22°; - the angle [3 is 7.5; - the thread width L is 0.6mm; - the radius of curvature R is 0.40 mm; - the height H is 1.3 mm; - the DI distance is 1.725 mm; and - the distance D2 is 0.875 mm.

[0068] [Fig. 4] illustrates first and second threads 6, 8 according to a second alternative embodiment. In this alternative embodiment, the characteristics of the threads 6, 8 are as follows: - a P step of 4 mm; - an angle a of 30°; - an angle [3 of 10°; - the thread width L is 1.2 mm; - a radius of curvature R of 0.80 mm; - a height H of 2.2 mm; - a DI distance of 3.05 mm; and - a distance D2 of 1.4 mm.

[0069] [Fig.5] illustrates the threads 6, 8 according to a third embodiment variant. Their characteristics are as follows: - a P pitch of 1.30 mm; - an angle a of 17.5°; - an angle [3 of 6°; - the thread width L is 0.39 mm; - a radius of curvature R of 0.26 mm; - a height H of 1.150 mm; - a DI distance of 1.425 mm; and - a distance D2 of 0.88 mm.

[0070] In relation to Figures 6 and 7, other characteristics of the hydropneumatic accumulator 1 are described below, making it possible to give it increased fatigue resistance. The second thread 8 has a frustoconical end zone 26 in which the thread portions are truncated relative to those outside said frustoconical end zone 26. The frustoconical end zone 26 is the zone which is closest to the second abutment surface 19 of the second half-shell 4. In other words, it is positioned opposite the end 27 of the second half-shell 4.

[0071] In said truncated end zone 26, the radial distance between the upper face 24 and the X axis increases in a downward direction, i.e. towards the second abutment surface 19. Thus, in the frustoconical end zone 26 of the second thread 8, the upper face 24 of the second thread 8 has a frustoconical helix shape. The frustoconical helix has a half-angle at the apex e which is between 0.2 and 15° and preferably between 1 and 5°. The half-angle at the apex e is for example 2° in Figures 6 and 7. Preferably, the frustoconical end zone 26 comprises at least two complete portions of threads, that is to say it extends over more than 720° around the axis X. Such a frustoconical end zone 26 is advantageous in that it offers a better distribution of the stresses in the zone which is most subject to it, that is to say close to the second abutment surface 18, 19. It thus makes it possible to limit the maximum level of stresses in the portions of the second thread 8 closest to the second abutment surface 19.It will be observed that, to the extent that the threads are truncated in this truncated end zone 26, certain geometric characteristics and in particular the variables L, H and D2 no longer satisfy the requirements defined above.

[0072] In relation to figures 7, 8 and 9, other characteristics of the hydropneumatic accumulator 1 are described which make it possible to give it increased fatigue resistance.

[0073] The first half-shell 3 comprises a first annular cavity 28 which is arranged axially between the first stop surface 18 and the first thread 6. The annular cavity 28 extends radially inwards, i.e. in the direction of the axis X, beyond the first and second stop surfaces 18, 19. This first annular cavity 28 therefore has the effect of locally thinning the wall of the first half-shell 3 between the first stop surface 18 and the first thread 6. Advantageously, the first annular cavity 28 has a radial depth el relative to the bottom 25 of the first thread 6 of at least 6 mm and for example of the order of 8 mm. Furthermore, the bottom of the first annular cavity 28 comprises one or more curved zones.

[0074] The upper end of the first annular cavity 28 is positioned at an axial distance dl from the first abutment surface 18 which is greater than 8 mm, for example of the order of 10 mm.

[0075] Furthermore, the second half-shell 4 comprises a second annular cavity 29 which is arranged axially between the second abutment surface 19 and the second thread 8. The second annular cavity 29 aims to locally thin the wall of the second half-shell 4 between the second abutment surface 19 and the second thread 8. Advantageously, the second annular cavity 29 has a radial depth e2 of at least 4 mm and for example of the order of 6 mm relative to the bottom 25 of the second thread 8. Advantageously, the bottom of the second cavity annular cavity 29 has several curved zones 30, 31, 32, three in the embodiment shown. Each of the curved zones 30, 31, 32 joins the adjacent curved zone(s) 30, 31, 32 in a portion projecting towards the second annular cavity 29. Such a geometry makes it possible to obtain three stress concentration zones, namely in the bottom of each of the curved zones, rather than just one, which makes it possible to reduce the maximum stress level.

[0076] [Fig. 9] illustrates an alternative embodiment which differs from that described above in relation to [Fig. 8] only by geometric details of the first and second annular cavities 28, 29. Thus, in this embodiment, the first annular cavity 28 has several curved zones 34, 35, 36, three in the embodiment shown. Each of the curved zones 34, 35, 36 joins the adjacent curved zone(s) 34, 35, 36 in a portion projecting towards the first annular cavity 28.

[0077] Although the invention has been described in connection with several particular embodiments, it is quite obvious that it is in no way limited thereto and that it includes all the technical equivalents of the means described as well as their combinations if these fall within the scope of the invention, as defined by the claims.

[0078] The use of the verb “comprise”, “comprise” or “include” and its conjugated forms does not exclude the presence of other elements or other steps than those stated in a claim.

[0079] In the claims, any reference sign in parentheses cannot be interpreted as a limitation of the claim.

Claims

Claims

1. Hydropneumatic accumulator (1) comprising: - an enclosure (2) formed by a first half-shell (3) and a second half-shell (4) which are screwed to each other, and - a membrane (9) which is pinched between the first half-shell (3) and the second half-shell (4) and divides the enclosure (2) into a first compartment (12) and a second compartment (13); wherein the first half-shell (3) has an outer surface (5) having a first thread (6) and the second half-shell (4) has a skirt (7) which is arranged around the outer surface (5) of the first half-shell (3) and which has a second thread (8) cooperating with the first thread (6) to screw the first half-shell (3) and the second half-shell (4) to each other along an axis X; wherein the first half-shell (3) has a first stop surface (18) which is provided at one end of the first half-shell (3) and which is in contact against a second stop surface (19) which is provided, on the second half-shell (4), radially inside the skirt (7); wherein the first thread (6) and the second thread (8) each comprise a thread defined by a load flank (23), an engagement flank (22) positioned upstream of the load flank (23), an upper face (24) connecting the engagement flank (22) and the load flank (23) to each other and a bottom (25) connecting the engagement flank (22) and the load flank (23) of two adjacent portions of the thread; wherein the first thread (6) and the second thread (8) have the following characteristics: - a P step; - an angle a formed, in a transverse plane including the X axis, by the engagement flank (22) relative to an axis perpendicular to the X axis and included in said transverse plane which is between 15 and 35°; - an angle [3 formed, in the transverse plane including the X axis, by the load-bearing flank (23) relative to an axis perpendicular to the X axis and included in said transverse plane which is between 5 and 10°; - a radius of curvature R of the bottom (25), in the transverse plane, between 0.15 P and 0.25 P; - a width L corresponding to an axial dimension in the transverse plane of the upper face (24) which is between 0.20 P and 0.40 P; - a height H corresponding to the difference between a radial distance between the bottom (25) of one of the first and second threads (6, 8) and the axis X and a radial distance between the upper face (24) of said first or second thread (6, 8) and the axis X; and - a distance DI corresponding to a radial distance between the bottom (25) of the first thread (6) and the bottom (25) of the second thread (8) which is greater than or equal to the sum of the height H and the radius of curvature R

2. IX. Hydropneumatic accumulator (1) according to claim 1, in which the pitch P is between 1.30 and 4 mm.

3. Hydropneumatic accumulator (1) according to claim 1 or 2, wherein a distance D2 corresponding to the difference between the radial distance between the upper face (24) of the first thread (6) and the X axis and the radial distance between the upper face (24) of the second thread (8) and the X axis is greater than 0.4 P.

4. Hydropneumatic accumulator (1) according to any one of claims 1 to 3, wherein the second thread (8) has, towards the second abutment surface (19), a frustoconical end zone (26) in which the thread is truncated so that a radial distance between the upper face (24) of the second thread (8) and the X axis increases towards the second abutment surface (19).

5. Hydropneumatic accumulator (1) according to claim 4, wherein in the frustoconical end zone (26), the thread has a frustoconical helix shape having a half-angle at the apex e which is between 0.2 and 15°.

6. Hydropneumatic accumulator (1) according to any one of claims 1 to 5, wherein the first half-shell (3) has a first annular cavity (28) which is positioned axially between the first abutment surface (18) and the first thread (6) and which extends radially towards the X axis beyond the first and second abutment surfaces (18, 19).

7. Hydropneumatic accumulator (1) according to claim 6, in which the first annular cavity (28) has a radial depth el, relative to the bottom (25) of the first thread (6), of at least 5 mm.

8. A hydropneumatic accumulator (1) according to claim 6 or 7, wherein the first annular cavity (28) comprises a bottom having a plurality of curved zones (34, 35, 36), each of the curved zones (34, 35, 36) joining the adjacent curved zone(s) (34, 35, 36) in a portion projecting towards the first annular cavity (28).

9. Hydropneumatic accumulator (1) according to any one of claims 1 to 8, wherein the second half-shell (4) comprises a second annular cavity (29) which is arranged axially between the second abutment surface (19) and the second thread (8) and which extends radially outwards.

10. Hydropneumatic accumulator (1) according to claim 9, wherein the second annular cavity (29) has a radial depth e2 relative to the bottom (25) of the second thread (8) of at least 3 mm.

11. Hydropneumatic accumulator (1) according to claim 9 or 10, in which the second annular cavity (29) comprises a bottom having several curved zones (30, 31, 32), each of the curved zones (30, 31, 32) joining the adjacent curved zone(s) (30, 31, 32) in a portion projecting towards the second annular cavity (30, 31, 32).

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