Piston seal with optimized geometry

Abstract

A two-way dynamic piston seal (1) for a hydraulic machine (2) comprising at least three parts: a continuous sealing ring (11) of axial width L, made of an elastic material having a modulus of elasticity EBE; and a continuous compression ring (13). The sealing ring includes a groove (113) of axial width LG at least equal to L / 3, intended to contain a sliding ring (14) made of an elastic material having a modulus of elasticity EBG. The sliding ring has an axially discontinuous contact surface with an axial contact width Lsg of at most LG / 2 at zero pressure. The modulus of elasticity EBG is at least 3 times the modulus of elasticity EBE of the material of the sealing ring (11). (See abstract figure: Figure 1)

Description

Title of the invention: Piston seal with optimized geometry

[0001] The present invention relates to a dynamic bidirectional piston seal for a hydraulic machine whose operating pressure of the pressurized fluid is at least equal to 150 bars.

[0002] Piston seals for machines where high pressure is applied comprise, as shown in US patent 4185843, particularly in Figure 4, a plurality of elements. Each element of the seal has a function and is made of a specific material adapted to that function. The seal is fitted into a groove in the male part of the machine, which moves within a female part whose diameter is slightly larger than that of the male part and must ensure a seal during movement of the connection between the two parts. The seal must also be deformable for installation. The groove in the male part is generally only accessible to the seal by sliding and deforming it against the male part, most often to avoid complicating the machine design. A seal is "bidirectional" when the pressure exerted by the hydraulic machine fluid is likely to be applied to both axial ends of the seal.For a cylinder, the male part includes at least the piston and the rod of the cylinder, and the female part is the body of the cylinder.

[0003] For machines with low operating pressure, piston seals are often simple tori made of an elastomeric material that allows for high elastic deformation. In their unmounted state, they have an inner diameter smaller than the diameter of the groove in the male part of the hydraulic machine and an outer diameter larger than the inner diameter of the female part. The elastomeric material allows the seal to be deformed to fit into the groove of the male part. Since the inner diameter of the seal in its unmounted state is smaller than the inner diameter of the groove in the male part, the seal, once mounted, exerts pressure on the male part, ensuring a tight seal between the seal and the male part. Once mounted on the male part, the male and female parts are assembled.Since the seal has a larger outer diameter than the inner diameter of the female part in its unmounted state, once mounted it applies pressure to the inner wall of the female part, which ensures the sealing of the connection between the seal and the female part, thus making the connection between the male and female parts watertight.

[0004] When the working pressure of the hydraulic machine is increased, a point is reached where the pressure exceeds the resistance to deformation and creep of the elastomeric material of the seal that can be inserted into the gap between the male part and The female part reduces the pressure between the seal and the female part, sometimes to the point of loss of sealing. To solve this problem, support rings can be added on either side of the seal. These support rings are made of a rigid material such as polyether ether ketone or filled polyamide, which will better resist sagging and deformation. Because they are rigid, these rings must be circumferentially discontinuous to allow for their installation. The techniques for achieving this discontinuity are well known to those skilled in the art.

[0005] Another difficulty arises from the moving contact between the radially outer part of the seal and the female part of the hydraulic machine. The radially inner part of the seal is wedged in the groove of the male part; the contact is static, and using an elastomer does not pose any particular problem. However, for the moving part, elastomers are very adhesive materials that reduce the energy efficiency of the hydraulic machine. They deform and wear rapidly at this pressure level. For high pressure levels, it is therefore common to insert a polytetrafluoroethylene (PTFE) ring between the elastomer ring and the female part of the hydraulic machine. PTFE belongs to the per- or polyfluoroalkyl (PFAS) family and has excellent sliding properties.These materials, however, have two significant drawbacks: they are highly polluting, very slow to degrade, probably carcinogenic, and complex to manufacture because these sealing rings are machined, not molded. Since PTFE rings tend to relax under stress, an elastomer ring is radially inserted inside the PTFE ring. This elastomer ring provides static sealing with the male part of the hydraulic machine and also compresses the PTFE ring against the female part, thus contributing to the machine's long-term sealing. The seal between the PFAS ring and the elastomer ring is now a static seal, for which the elastomer is particularly well-suited.Because of this new compression function of a new ring - the PTFE one - on the female part of the hydraulic machine, the elastomer ring is here called a compression ring by pure convention, this compression ring ensuring static sealing between the joint and the male part.

[0006] To diversify the materials that can be used to make high-pressure sealing gaskets, the inventors sought to reduce friction by using a method other than a material with a particularly low coefficient of friction.

[0007] This objective was achieved by a dynamically sealing bidirectional piston seal for a hydraulic machine whose maximum operating pressure of the pressurized fluid is at least equal to 150 bar, having an axis of revolution XX', comprising at least three parts, said at least three parts being: - a circumferentially continuous sealing ring, comprising at least one sealing surface substantially parallel to the axis XX' intended to come into contact with a surface of the hydraulic machine and of axial width L, and a force surface radially internal to the sealing surface of the sealing ring, the sealing ring being made of an elastic material having a modulus of elasticity EBE at 20°C and 2% elongation, - a circumferentially continuous compression ring disposed internally radially to the sealing ring, the compression ring being made of an elastic material having a modulus of elasticity EBC at 20°C and 2% elongation, - the sealing ring comprising a groove in the contact surface, the groove having an axial width LG at least equal to L / 3, intended to contain a sliding ring made of an elastic material having a modulus of elasticity EBG at 20°C and 2% elongation,- the sliding ring having a contact surface, called the sliding surface, axially discontinuous in portions intended to come into contact with the surface of the hydraulic machine, the axial width Lsg of contact equal to the sum of the axial widths of the portions of the contact surface of the sliding ring being at most equal to LG / 2, , - the modulus of elasticity EBG of the sliding ring material being at least equal to 3 times the modulus of elasticity EBE of the sealing ring material.

[0008] Joints are objects with rotational symmetry about an axis XX'. They are easily described in a meridian plane containing this axis by a section which, for the simplest joints, is a disk and can take on different geometries for more complex joints. Thus, the radial, axial, and circumferential directions respectively denote the directions perpendicular to the axis of rotational symmetry of the joint, parallel to the axis of rotational symmetry of the joint, and perpendicular to the meridian plane. The circumferential direction is tangent to the circumference. Thus, "a contact surface is parallel to the axis XX'" means that the segment of the curve or straight line of the joint section which, by axial revolution, describes the contact surface, is parallel to the axis XX'.

[0009] In this document, the expressions "radially inside" and "radially outside" respectively mean "closer" and "further from the axis of revolution of the joint." By "axially inside" and "axially outside," respectively, we mean "closer" and "further from the equatorial plane of the joint," the equatorial plane of the joint being the plane passing through the middle of the contact surface sliding with the female part of the machine and perpendicular to the axis of revolution.

[0010] The invention consists of reducing frictional forces when using recyclable materials that are less polluting than PTFE, such as elastomers or thermoplastic elastomers (TPE). In accordance with the prior art, the seal comprises a compression ring and possibly support rings made of materials known in the prior art. In accordance with the prior art, the compression ring is radially internal to the sealing ring, and its dimensions, such as its inner and outer diameters, have values ​​consistent with the functions of the compression ring. The inner diameter of the compression ring is sized to exert, once the seal is mounted in the hydraulic machine, a centripetal radial force on the male part of the hydraulic machine, sufficient to ensure static sealing between the compression ring and the male part of the hydraulic machine.The outer diameter of the compression ring has a value such that, once the seal is mounted in the hydraulic machine, it exerts a radial centrifugal force on the radially inner force surface of the sealing ring so as to maintain the sealing surface of the sealing ring in contact with the surface of the female part of the hydraulic machine.

[0011] For the sealing ring, the inventors sought to use polymers that were more rigid than those used in the compression ring and had a lower coefficient of friction. Using graphite-filled elastomers or polyurethane, which has a significantly lower coefficient of friction than other elastomers used for compression rings (approximately 30%), improves the situation, but not sufficiently. Returning to the physics of contact, to reduce the frictional force, it is necessary to decrease the contact area of ​​the seal. This can be achieved by reducing the axial width of the sealing ring, but this solution results in seals with insufficient shear inertia and, consequently, unstable shapes, leading to seal failure.The inventors then considered reducing the width of the contact surface, or sealing surface, of the seal while maintaining the often rectangular shape of the seal (a rectangle whose length is its axial dimension), by using a sealing surface with circumferential recesses designed to prevent contact with the female part of the hydraulic machine. However, this solution is not functional because the materials suitable for making sealing rings that are both mountable and therefore deformable, with adequate coefficients of friction, are too deformable, causing the recesses in the sealing ring to collapse under high pressure. The sealing ring deforms in such a way that the 60% increase in contact surface at low pressure, below 10 bar, decreases linearly to less than 8% around 120 bar and decreases further beyond that. The inventors then had the idea of ​​reducing the... sealing surface of the sealing ring thanks to a sliding ring arranged in a groove of axial width LG, preferably symmetrically with respect to the equator plane of the joint for equally efficient behavior in both directions of relative movement of the male and female parts of the hydraulic machine.

[0012] The axial width LG of the sealing ring groove must be sufficiently large relative to the axial width L of the sealing surface of the sealing ring to influence the coefficient of friction; therefore, it must be at least equal to one-third of the axial width L. The sliding ring has a discontinuous contact surface to reduce the contact area, for example, with grooves, slots, etc., and the axial contact width Lsg is equal to the sum of the axial widths of the contact portions of the contact surface, at most equal to LG / 2, to achieve a certain efficiency in reducing the coefficient of friction. Measurements of Lsg will be taken on an unmounted seal geometry subjected to a pressure corresponding to the weight of the seal. Such a measurement can be made by inking the seal and placing it on a sheet of paper to measure the sum of the maximum widths of the ink marks left on the paper.A skilled professional will be able to measure this value. To prevent the contact surface of the sliding ring from increasing excessively under high pressures, the material constituting the sliding ring must be significantly stiffer than that of the sealing ring, at least three times stiffer. To avoid assembly problems, the least stiff material of both the sliding and sealing rings should be used to form the ring with the cross-sectional area exhibiting the greatest moment of inertia.

[0013] This technique of reducing the sealing surface area by using different materials with a sealing ring and a sliding ring can improve the seal's fit and its efficiency in terms of the energy dissipated as heat at the seal, which is an advantage in itself. However, it constitutes a preferred solution for eliminating PTFE rings in bidirectional dynamic piston seals for high-pressure hydraulic machines. Preferably, the per- and polyfluoroalkyl content in the materials of all parts of the seal, measured by nuclear magnetic resonance (NMR), is less than 0.1%, preferably equal to 0.

[0014] The sealing surface of the sealing ring is substantially parallel to the axis XX' intended to come into contact with a surface of the hydraulic machine. Specifically, the points of the sealing surface will be in contact with the surface of the female part of the hydraulic machine once the machine is under operating pressure, even if the points of the sealing surface are not strictly parallel to the axis XX' but substantially parallel in the unmounted state of the seal.

[0015] For a better reduction of the contact area of ​​the seal over the entire axial width L of the sealing surface, (LG-Lsg) is at least equal to L / 3, preferably at least equal to L / 2, which allows a reduction of friction of at least one third, preferably at least one half.

[0016] A preferred solution is such that the sliding surface of the sliding ring is constituted by at least three protrusions: two end protrusions disposed at each axial end of the sliding ring, at an axial distance less than 20% of the axial width LG from the groove of the sealing ring at the axial end considered, and at least one central protrusion disposed between the two end protrusions at an axial distance less than 20% of the axial width LG from the midpoint of the two end protrusions. A multitude of arrangements for the contact surface of the sliding ring are possible; they are all included in the invention.The advantage of a design with three contact points due to protrusions, two near the walls of the sealing ring's groove, is to minimize stress at this discontinuous point, which could deteriorate faster than the rest of the sealing surface. The third point prevents unwanted deformation of the sliding ring at its center.

[0017] Preferably, depending on the rigidity of the material constituting the sliding ring, the radial height of the protrusions is at least 5% and at most 50% of the radial thickness of the sliding ring. The radial thickness of the sliding ring is defined, in a meridian plane for a cross-section of the sliding ring, as the radial distance from the outermost point to the innermost point of that cross-section. The radial height of the protrusions is measured from the outermost point of the protrusion in question to the innermost point of the outermost radial surface of the sliding ring, axially between the two end protrusions.

[0018] The rectangular shape offers good resistance to deformation due to its high inertia, it is preferred that the groove of the sealing ring be of rectangular section, of greater length LG and whose radial depth is at most equal to LG / 3.

[0019] Preferably, the sliding ring has the same axial width as the groove of the sealing ring. The sliding ring thus fits perfectly into the groove of the sealing ring, the sealing ring thereby contributing to the shear stability of the sliding ring and the sliding ring contributing to the shear stability of the sealing ring.

[0020] A preferred solution is for the modulus of elasticity EBE at 20°C and 2% elongation of the sealing ring to be between 10 and 200 MPa. The modulus of elasticity is the secant modulus measured according to ISO 37. The ring The sealing material is therefore preferably made of thermoplastic elastomer (TPE) such as thermoplastic polyurethane (TPU) or thermoplastic copolyester (TPC).

[0021] A preferred solution is to have at least one circumferentially discontinuous bearing ring arranged axially against the sealing ring, each bearing ring being made of an elastic material having an elongation modulus (EBA) at 20°C and 2% elongation of between 500 and 5000 MPa. Preferably, a bearing ring is arranged axially against the sealing ring on either side of the sealing ring's median plane. The modulus of elasticity is the secant modulus measured according to ASTM D638. The bearing rings are thus preferably made of filled or unfilled polyamide (PA), polyaryl etherketone (PAEK) such as virgin or filled polyetheretherketone (PEEK), polyacetals such as filled or unfilled polyoxymethylene (POM), or filled or unfilled polyphenylene sulfide (PPS). Since the support rings are circumferentially discontinuous, their material can be very rigid without hindering their assembly.Given their functions and circumferential discontinuities, it is appropriate that the material constituting the support ring be more rigid than the materials of the sealing and compression rings (EBC). <EBA, EBE<EBA). .

[0022] A preferred solution is for the modulus of elasticity (EBC) at 20°C and 2% elongation of the compression ring to be between 5 and 50 MPa. The modulus of elasticity is the secant modulus measured according to ISO 37. The compression ring is therefore preferably made of butadiene-acrylonitrile copolymer (NBR) or hydrogenated butadiene-acrylonitrile copolymer (HNBR). Given its functions and dimensions, the compression ring is the most deformed component during assembly; therefore, it is appropriate for the material constituting the compression ring to be the least rigid of all the other materials (EBC). <EBA, EBC< EBG, EBC<EBE).

[0023] A preferred solution is for the modulus of elasticity EBG at 20°C and 2% elongation of the sliding ring (14) to be between 500 and 3500 MPa. The modulus of elasticity is the secant modulus measured according to ASTM D638. The sliding ring is therefore preferably made of filled or unfilled polyamide (PA), polyaryl etherketone (PAEK) such as virgin or filled polyetheretherketone (PEEK), polyacetals such as filled or unfilled polyoxymethylene (POM), or filled or unfilled polyphenylene sulfide (PPS).

[0024] Since the secant moduli are one of the basic data of materials, a person skilled in the art will know how to take the measurement standard corresponding to the type of material chosen.

[0025] Preferably, the sliding ring is circumferentially continuous, thus playing a role in sealing. In this case, it is preferable that the material constituting the sliding ring be less rigid than the material of the support rings (EBG). <EBA).

[0026] Preferably the sliding ring is circumferentially discontinuous, it is thus more easily mounted on the male part of the hydraulic machine.

[0027] The scope of the invention includes hydraulic machines comprising at least one joint as described above.

[0028] The features of the invention are illustrated by schematic figures 1 and 2, not shown to scale, with reference to a piston seal according to the invention inserted into a hydraulic machine.

[0029] Figure 1 represents a section of a joint according to the invention in position in a hydraulic machine, comprising a dynamic, bidirectional piston seal (1) for a hydraulic machine (2), comprising a female part (21) and a male part (22) which, during operation of the machine, move relative to each other along the axis XX' under the effect of a pressurized fluid, the maximum pressure of which is at least 150 bar in cavities not shown here. The seal is positioned in a groove (221) of the male part (22). The axis XX' is also the axis of revolution of the seal (1), which comprises five parts: - a circumferentially continuous sealing ring (11), comprising at least one sealing surface (111) parallel to the axis XX' intended to come into contact with a surface of the hydraulic machine, its female part (21) and of axial width L, and a radially internal bearing surface (112) within its sealing surface (111). The sealing ring includes a groove (113), here of rectangular cross-section, in the contact surface, groove (113) of the sealing ring (11) of the sealing ring (11) having an axial width LG. - two circumferentially discontinuous bearing rings (12), arranged axially on either side of the sealing ring (11), - a circumferentially continuous compression ring (13) arranged internally radially to the sealing ring so as to exert a radial centrifugal force on the force surface (112) of the sealing ring (11) so as to keep the sealing surface (111) of the sealing ring (11) in contact with the surface of the hydraulic machine (2), and whose inner diameter is smaller in an unmounted state than the inner diameter of the groove (221) of the male part (22) of the hydraulic machine to ensure sealing between the compression ring (13) and the male part (22) of the hydraulic machine via the sealing surface (131) of the compression ring (13), - a sliding ring (14) is positioned in the groove (113) of the sealing ring (11). The sliding ring has a contact surface, called the sliding surface, in contact with the surface of the female part (21) of the hydraulic machine (2), axially discontinuous with 3 protrusions (141), one central and two at the axial ends of the sliding ring (14). The radial height of the protrusions is measured from the outermost radial point of the protrusion central at the most radially inward point between the two end protuberances, is here on the order of 15% of the radial height of the sliding ring (14).

[0030] Figure 2 corresponds to the invention in its simplest form with 3 parts: a sealing ring, a compression ring and a sliding ring.

[0031] The invention was developed for a bidirectional dynamic seal for a hydraulic cylinder of an excavator, the operating pressure of which is on the order of 200 bar with pressure peaks of 800 bar. The seal, according to the prior art, is a PFAS seal, the manufacture and use of which are polluting and therefore should be avoided. The modulus of elasticity of PFAS at 20°C and 2% elongation of the sliding ring (14) is 600 MPa. It has a rectangular cross-section with an axial width of 6 mm and a radial height of 2.5 mm. The seal, according to the prior art, comprises a compression ring made of butadiene-acrylonitrile (NBR) copolymer with an inner diameter of 64 mm in the unmounted state, intended to be inserted into a groove in the male part of the hydraulic machine with an inner diameter of 65 mm. The modulus of elasticity EBC at 20°C and 2% elongation of the compression ring is equal to 1OMPa.The state-of-the-art seal also includes two reinforced polyamide (PA) bearing rings with an EBC modulus of elasticity at 20°C and 2% elongation equal to 3000 MPa. The bearing rings are circumferentially discontinuous.

[0032] The inventors first tried replacing the PFAS sealing ring with a ring of the same dimensions (6 mm) made of thermoplastic polyurethane with a modulus of elasticity EBE at 20°C and 2% elongation equal to 100 MPa. (test) The increase in friction force between the male and female parts of the hydraulic machine was multiplied by a factor of 10, which is not optimal, although this solution has the advantage of being less polluting than the prior art solution.

[0033] The inventors then attempted to reduce the friction forces between the male and female parts of the hydraulic machine by decreasing the contact area by 65% ​​between the female part and the polyurethane thermoplastic sealing ring by creating protrusions on the contact surface of the sealing ring (test 2). For a pressure below 5 bar, the friction forces are less than 0.35 times those of test 1, but these friction forces increase linearly with pressure, reaching 0.93 times that of test 1 at 120 bar and 95% at the operating pressure of the hydraulic machine.

[0034] The inventors then implemented the invention by replacing the sealing ring of test 1 with a sealing ring made of the same material (polyurethane thermoplastic with a modulus of elasticity EBE at 20°C and 2% elongation equal to 100 MPa) in which there is a rectangular groove centered in the contact surface of the sealing ring, with an axial width of 0.6*L and a radial depth of 1 mm. In this groove of the sealing ring, they placed a ring The sliding ring has the same dimensions but its contact surface is reduced to three protrusions: two at the axial ends of the sliding ring and one in the center, resulting in a pressureless contact area reduced to 25% of the axial width of the sliding ring. The sliding ring is circumferentially continuous due to the relative flexibility of its material and its low cross-sectional inertia, which allow it to be mounted without being circumferentially discontinuous. This solution consistently outperforms test 1 at all pressures.It is less efficient than test 2 up to a pressure of approximately 20 bars, proving that this solution is not straightforward, but the performance evolves linearly with pressure along a much shallower slope than for test 2, so that over the operating ranges of high-pressure hydraulic machines, it allows a gain in sliding performance of between 50% around 200 bars and 40% around 500 bars.

[0035] Thus the invention makes it possible, compared to the state of the art, to no longer use PFAS while optimizing the energy efficiency of the seal.

Claims

Demands

1. A bidirectional piston seal (1) for the dynamic sealing of a hydraulic machine (2) whose maximum working pressure of the pressurized fluid is at least equal to 150 bar, having an axis of revolution XX', comprising at least three parts, said at least three parts being: - a circumferentially continuous sealing ring (11), comprising at least one sealing surface (111) substantially parallel to the axis XX' intended to come into contact with a surface of the hydraulic machine and of an axial width L, and a force surface (112) radially internal to the sealing surface (111) of the sealing ring (11), the sealing ring being made of an elastic material having a modulus of elasticity EBE at 20°C and 2% elongation, - a circumferentially continuous compression ring (13) disposed internally radially to the sealing ring,the compression ring being made of an elastic material having a modulus of elasticity EBC at 20°C and 2% elongation, - Characterized in that the sealing ring comprises a groove (113) in the contact surface, the groove having an axial width LG at least equal to L / 3, intended to contain a sliding ring (14) made of an elastic material having a modulus of elasticity EBG at 20°C and 2% elongation, - in that the sliding ring has a contact surface, referred to as the sliding surface, axially discontinuous in portions intended to come into contact with the surface of the hydraulic machine (2), the axial width Lsg of contact being equal to the sum of the axial widths of the portions of the contact surface of the sliding ring being at most equal to LG / 2, - and in that the modulus of elasticity EBG of the material of the sliding ring (14) is at least equal to 3 times the modulus of elasticity EBE of the material of the sealing ring (11).,

2. Sealing joint (1) according to claim 1 wherein the rate of per- and polyfluoroalkylates in the materials of all parts (11, 12, 13, 14) of the joint (1) is less than 0.1%, preferably equal to 0.

3. Sealing joint (1) according to any one of claims 1 or 2 wherein (LG-Lsg) is at least equal to L / 3, preferably at least equal to L / 2.

4. Sealing joint (1) according to any one of the preceding claims wherein the sliding surface of the sliding ring (14) is constituted by at least 3 protrusions (141), two end protrusions disposed at each axial end of the sliding ring (14), at an axial distance less than 20% of the axial width LG of the groove (113) of the sealing ring (11) of the axial end considered, and at least one central protrusion (141) disposed between the two end protrusions (141), at an axial distance less than 20% of the axial width LG, from the middle of the two end protrusions (141).

5. Sealing joint (1) according to claim 4 in which the radial height of the protrusions is at least equal to 5% and at most equal to 50% of the radial thickness of the sliding ring (14).

6. Sealing ring (1) according to any one of the preceding claims wherein the groove (113) of the sealing ring (11) has a rectangular cross-section, length LG and a radial depth of at most LG / 3.

7. Sealing ring (1) according to any one of the preceding claims wherein the sliding ring (14) has the same axial width as the groove (113) of the sealing ring (11).

8. Sealing ring (1) according to any one of the preceding claims wherein at least one circumferentially discontinuous bearing ring (12) is arranged axially against the sealing ring, each bearing ring being made of an elastic material having an elasticity modulus EBA at 20°C and 2% elongation of between 500 and 5000 MPa.

9. Sealing ring (1) according to any one of the preceding claims wherein the modulus of elasticity EBE at 20°C and 2% elongation of the sealing ring (11) is between 10 and 200 MPa.

10. Sealing joint (1) according to any one of the preceding claims wherein the modulus of elasticity EBC at 20°C and 2% elongation of the compression ring (13) is between 5 and 50 MPa.

11. Sealing joint (1) according to any one of the preceding claims wherein the modulus of elasticity EBG at 20°C and 2% elongation of the sliding ring (14) is between 500 and 3500 MPa.

12. Sealing joint (1) according to any one of the preceding claims wherein the sliding ring is circumferentially continuous.

13. Sealing joint (1) according to any one of claims 1 to 11 wherein the sliding ring is circumferentially discontinuous.

14. Hydraulic machine (2) comprising at least one seal (1) according to the preceding claims.

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