CA2237787A1

CA2237787A1 - Piston ring

Info

Publication number: CA2237787A1
Application number: CA002237787A
Authority: CA
Inventors: Norbert Feistel
Original assignee: Individual
Current assignee: Burckhardt Compression AG
Priority date: 1995-11-17
Filing date: 1996-11-06
Publication date: 1997-05-29
Also published as: AU708508B2; EP0861391A1; ATE237083T1; EP0861391B1; NO982231D0; SK61598A3; RU2176043C2; DE59610335D1; CZ151298A3; NO982231L; BR9611460A; AU7276096A; PL181438B1; WO1997019280A1; PL326701A1

Abstract

The invention concerns a piston ring (10), in particular a piston ring for a reciprocating compressor, consisting of a first and a second annular element (10a, 10b) with a joint (10e, 10f), the rings (10a, 10b) being disposed concentrically about an axis C. The first ring (10a) has an essentially Lshaped cross-section, with a first arm (10h) extending parallel to axis C and a second arm (10g) extending radially outwards from axis C. The first arm (10h) has an inside surface (10n) and the second arm (10g) has an inside surface (10l), the inside surface (10l) of the second arm (10g) being inclined in the radial direction and the faces (10m, 10k) of the second ring (10b) which face towards the first ring (10a) physically locking with the inside surfaces (10m, 10l) of the first ring (10a).

Description

~ ~ CA 02237787 1998-05-15 r FILE, 1~1~ TH~ r~n~
T~ TRANSI A~
P.6714/Gf/Pa Machine Works Sulzer-Burckhardt AG, Basel (Switzerland) Piston Ring The invention relates to a piston ring in accordance with the preamble of claim 1.

For compressing fluids such as gases or vapours, dry running compressors are used, with sealing rings of a plastic such as, for example, PTFE being used at the piston as sealing elements. Such sealing elements have the disadvantage that they have a high wear during operation, which leads to an insufficient lifetime. Especially the free gap cross-sections in the axial and radial direction, which result from the ring gaps and the wear of the sealing elements, have a significant share in the fact that the sealing elements do not attain their full effectiveness for a desired period of time. A seal element with a good sealing effect which is constant over a long period of time is required, in particular when compressing very light gases such as hydrogen.

The object of the present invention is to propose an economically more advantageous, in particular a low-wear piston ring for a dry running piston compressor, in particular for compressing very light gases.

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This object is satisfied in accordance with the features of claim 1 or claim 2. The subclaims 3 to 6 refer to further advantageous developments of the-invention.

The piston ring in accordance with the invention for a dry running piston compressor consists of two one-piece, ring-like bodies with ring gaps or butt joints, which bodies are designated in the following as rings. These rings are arranged to extend concentric to one another with respect to an axis C. The first ring has an essentially L-shaped cross-section with a first limb extending in the direction of the axis C and a second limb extending outwards radial to the axis C. The two limbs each have an inner surface, with the second ring being executed in such a manner that its surfaces facing the first ring lie in contact with the inner surfaces of the first ring in a form-fitted manner. The second limb, extending radially outwards from the axis C, has an inner surface which is inclined in the radial direction with respect to the axis C, with the angle of inclination being less than 90~. The inner surface has a tangent extending in the radial direction, which tangent intersects the axis C at an acute angle. In an advantageous embodiment of the invention this inner surface is executed as a conical surface.

In dry running piston compressors there is no lubricant available in order to lubricate the piston rings of a piston and to additionally seal it. Accordingly metallic piston rings are unsuitable for such a dry running application. The dry running frictional pairings function as a result of solid lubricants which are contained in one of the frictional partners. The piston ring in accordance with the invention therefore consists of a plastic specially modified for dry running with solid lubricants such as PTFE, graphite or molybdenum disulphide.

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Sealing elements with a very high sealing action are required above all when compressing very-light gases, such as hydrogen for example, to very high pressures in order to keep the leakage as small as possible. A good sealing action can be achieved for example by combining two sealing rings to a twin ring in such a manner that no through-going gaps or spaces result.

The self lubricating action of the dry running frictional seals has as a consequence that the piston rings which yield the lubricant gradually abrade away. Known twin rings of plastic have the disadvantage here that the two ring parts are displaceable with respect to one another in the radial direction. Since the pressure profile in the sealing subsurfaces of the two ring surfaces near the cylinder wall is not constant, the radial displaceability enables one of the two ring parts to abrade faster than the other. This unequal wear has the effect that the two ring parts no longer fully overlap and hence that gaps arise through which large amounts of leakage gas flow, in particular for very light gases under high pressure, which considerably reduces the amount of the gas forwarded by the dry running compressor.
The piston ring in accordance with the invention is especially suitable for the dry running compression of very light gases to high compression end pressure. The piston ring has the advantage that due to the construction of two ring parts or two ring-shaped bodies, their mutual slope and the form-fitting coupling of the two ring parts a uniform, harmonised wear of the two ring parts takes place during operation. The two ring parts are pressed against one another by the pressure of the fluid to be compressed, with no relative movement or only a slight mutual relative-movement taking place due to the form-fitting coupling of the two ring parts, so that the sealing subsurfaces of the two ring parts CA 02237787 1998-0~

which, for example, face a cylinder wall, experience a uniform material abrasion. In this manner, no or only slight local leakage points can form, due to which the sealing effect of the piston ring remains approximately constant over a longer operating period in dry running piston compressors.

In the following the invention will be explained with reference to several embodiments. Shown are:

Fig. 1 a longitudinal section through a piston with a cylinder;

Fig. 2a a cross-section through a second ring;

Fig. 2b a cross-section through a first ring;

Fig. 3a a plan view of a second ring;

Fig. 3b a plan view of a first ring;

Fig. 4 a piston ring arranged in a one-piece piston;

Fig. 5 two piston rings arranged in an assembled piston;
~ig. 6a a cross-section through a further embodiment of a second ring;
~ig. 6b a cross-section through a further embodiment of a first ring.
~ig. 7a a plan view of a second ring;
~ig. 7b a plan view of a first ring.

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Fig. 1 shows a longitudinal section through a dry running piston compressor with a cylinder 1 in which a piston 2 is arranged so as to be upwardly and downwardly movable. The lower end of the piston 2 merges into a piston rod 3 which is connected in a known manner, not shown, to a crank drive.
Above the piston 2 is a compression chamber 4 into which a gas to be compressed is drawn during the downward stroke of the piston 2 in a known manner, not shown. This gas is compressed on the following upward stroke and expelled from the compression chamber. The piston 2 has a rod-like extension 5 of the piston rod 3 over which a sleeve is placed bearing seven piston rings 10 placed one above the other. For an assembled piston 2 the individual parts of the piston 2 as well as the piston rings 10 are held together by a nut 7 screwed onto the upper end of the extension 5. Likewise shown is the central axis C of the piston 2.

The section A is shown magnified in Fig. 5, where two sections A are shown. The assembled piston is fabricated from individual parts and comprises the sleeve 6 extending in axial direction, the chamber rings 8a, 8b placed about the sleeve 6 as well as the piston rings 10 placed in the grooves. Each of the piston rings 10 consists of a first ring lOa and a second ring lOb. The two rings lOa, lOb are executed in a mutually adapted manner such that the mutually contacting partial areas come to lie against one another in a form-fitted manner. Each of the rings lOa, lOb has areas lOc, lOd facing the cylinder wall 1 which are slidingly moved upwards and downwards along the cylinder wall during the operation of the piston 2.

Fig. 4 shows a section A of a one-piece piston 2 with chamber ring 8 and a piston ring 10 consisting of the two rings lOa, lOb placed in the groove. The first ring lOa is arranged in the cylinder 1 toward the pressure side, with the gas CA 02237787 1998-0~

pressure 9 exerting forces on the first ring lOa acting in the axial direction 9a as well as in the radial direction 9b so that the entire piston ring 10 is firstly pressed against the inner wall of the cylinder 1 and secondly pressed in the axial direction against the boundary surface of the groove remote from the pressure side. In this way the sealing action of the piston ring is increased during operation and the two rings lOa, lOb are held in the groove and mutually form-fitted without or with only a slight relative movement. This mutually form-fitted connection has the result that the subsurfaces lOc, lOd of the two rings lOa, lOb which slide along and rub against the cylinder wall la are uniformly worn away so that no local leakage point arises and the two rings lOa, lOb lie in contact with the cylinder wall la with the same effect over a longer operating period.

The piston ring 10 shown in Figures 4 and 5, comprising the rings lOa, lOb, is shown in detail in Figures 3a, 3b in a plan view and in the Figures 2a, 2b in cross-section. The two rings lOa, lOb extend circularly about a central axis C. Fig.
3b shows the first ring lOa of the piston ring 10, a ring-shaped body of L-shape or nearly L-shape with a ring gap or butt joint lOe. This ring has two limbs lOg, lOh, a first limb lOh extending parallel to the direction of the axis C
and a second limb lOg extending radially outwardly or approximately radially outwardly relative to the axis C. The width of the ring gap lOe is such that the first ring lOa is capable of a certain resilient movement in the peripheral direction.

A cross-section through the first ring lOa along the line A-A
is shown in Fig. 2b. The first ring lOa, executed in L-shape, has a first limb lOh extending parallel to the axis C, which has an outer surface lOo as well as an inner surface lOn. The second limb lOg extending radial to the axis C has an outer CA 02237787 1998-0~

surface lOp extending perpendicular to the axis C. The inner surface 101 is executed to extend conically and subtends in the radial direction an angle ~-to the normal to the axis C.
In the present embodiment the second limb lOg is executed in such a manner that, with respect to the direction determined by the axis C, it is made relatively narrow at the first limb lOh and broadens in the direction of the periphery, i.e.
toward the surface lOd. The cross-section of the second ring lOb, which is shown in Fig. 2a, is shaped in accordance with the shape of the two inner surfaces 101, lOn. The ring body lOi has four surfaces, the surface facing the cylinder wall lOc, the surface lOq facing the chamber ring 8, as well as the two surfaces lOm, lOk facing the first ring lOa. The two last named surfaces lOm, lOk are executed to be adapted with respect to the first ring lOa in such a manner that for rings lOa, lOb placed one within the other the surfaces lOm, lOk come to lie in a form-fitted manner on the two inner surfaces 101, lOn. Therefore the surface lOk is likewise made conical, to extend radially to the axis C at an angle of inclination ~c .

Fig. 3a shows a plan view onto the second ring-formed body lOb with ring gap or butt joint lOf, which body is designated in the following as a second ring lOb. The ring body lOi has the ring gap lOf at the one side and a protrusion lOk projecting toward the axis C at the side opposite this ring gap lOf. In the assembled piston ring 10 the second ring lOb is placed over the first ring lOa while retaining the position shown in Figs. 3a, 3b so that the protrusion lOk comes to lie in the ring gap lOe of the first ring lOa. By this means the second ring lOb is secured with respect to the first ring lOa against mutual rotation since the protrusion lOk and the limb lOh restrict their relative movement. The protrusion lOk is advantageously made narrower in the peripheral direction than the width of the ring gap lOe so CA 02237787 1998-0~

that the first ring lOa retains a certain mobility in the peripheral direction at the ring gap lOe. Figs. 7a and 7b show a further exemplary embodiment of an execution of the rings lOa, lOb for the mutual securing against a rotation. A
cylindrical fixing pin lOr which extends in the vertical direction is arranged at the limb lOg on the side of the first ring lOa lying opposite the ring gap lOe. When assembling the piston ring 10 the second ring lOb is laid over the first ring lOa, while maintaining the position illustrated in Figs. 7a, 7b, so that the fixing pin lOr comes to lie between the ring gap lOf of the second ring lOb, whereby the second ring lOb is secured with respect to the first ring lOa against a mutual rotation. In the assembled state the rings lOa, lOb can be inserted into the groove of a one-piece piston 2 by simultaneous spreading apart of the two ring parts as shown in Fig. 4 or into an assembled piston 2 without spreading apart as shown in Fig. 5.

An advantage of the piston ring 10 in accordance with the invention is to be seen in the fact that the subsurfaces 101, lOn, lOk, lOm of the two rings lOa, lOb remain lying one upon the other in a form-fitted manner over long periods of time, even during the operation of the piston ring 10. The two rings lOa, lOb are executed in such a manner that a direct action of the gas 9 under pressure on the subsurfaces 101, lOn, lOk, lOm is prevented as far as possible. The gas pressure 9 acting on the rings lOa, lOb normally acts at the ring gap lOf and exerts a force acting in the peripheral direction of the first ring lOb, which, for an angle a= O, could lead to a lifting off of the first ring lOb from the second ring lOa so that the gas pressure would act directly on the subsurfaces 101, lOn, lOk, lOm. Such a direct action of the gas pressure on the subsurfaces lOm, lOk of the second ring lOb would result in a relatively rapid wear of the second ring lOb. In order to avoid this effect the ring in CA 02237787 1998-0~

accordance with the invention is executed in such a manner that a lifting off of the first ring lOb from the first ring lOa is prevented. In accordance-with Fig. 4 the piston ring 10 is pressed in the axial direction against the chamber ring 8 by the action of the gas 9a and in the radial direction against the wall of the cylinder 1 by the action of the gas 9b. The first ring lOa subjected to this pressure exerts corresponding forces on the second ring lOb. Due to the conical execution of the subsurfaces 101, lOk the forces acting due to the gas pressure cause an increased mutual wedging together of the two rings lOa, lOb so that the subsurfaces lOk, 101, lOn, lOm are pressed against one another more strongly. A direct action of the gas pressure on the subsurfaces lOk, 101, lOn, lOm is thereby prevented, which leads to a low wear of the first ring lOb, and a uniform wear of the two rings lOa, lOb, in particular of the subsurfaces lOc, lOd, and thus to a long lifetime of the piston ring 10.

There is a great number of possibilities for the design of the subsurfaces lOk, 101, lOn, lOm in such a way that a mutual wedging together occurs between rings lOa, lOb placed together and loaded by gas pressure with the result that the subsurfaces lOk, 101, lOn, lOm in mutual contact are additionally pressed against one another. Figures 6a, 6b show a cross-section through a first ring lOa as well as a ring lOb of a further exemplary embodiment. Here the two subsurfaces lOm, lOn are executed to extend cylindrically, parallel to the axis C. The further subsurfaces 101, lOk are executed to extend in curved form in such a manner that they come to lie against one another in a form-fitted manner in the assembled piston ring 10.

The inner surface 101, lOk has an inclination in the direction extending radially to the axis C, with the angle between a tangent to the inner surface 101, lOk and the axis C amounting to about 90~ in the region of the surface lOd and becoming increasingly acute in the direction of the limb lOh, i.e. adopting an angle which becomes smaller than 90~.

The rings lOa, lOb are formed of a plastic, in particular of plastics such as polytetraflouorethylene (PTFE), a modified high-temperature polymer such as polyetheretherketone ( PEEK), polyetherketone (PEK), polyimide (PI), polyphenylene sulphide (PPS), polybenzimidazole (PBI), polyamidimide (PAI) or a modified epoxy resin. The high-temperature polymers are plastics which are not capable of dry running in pure form, so that the above named plastics are usually filled with additional solid lubricants such as e.g. carbon, graphite, molybdenum disulphide or PTFE.

Claims

1. Piston ring (10) for a dry running piston compressor, consisting of a first and a second ring-shaped body (10a, 10b) with ring gaps (10e, 10f), said rings (10a, 10b) being arranged concentric to one another with respect to an axis C, with the first ring (10a) having an essentially L-shaped cross-section, with a first limb (10h) extending in the direction of the axis C and a second limb (10g) extending radially outwardly relative to the axis C, and with the first limb (10h) having an inner surface (10n) and the second limb (10g) having an inner surface (101), with the inner surface (101) of the second limb (10g) having a slope in the radial direction, and with the surfaces (10m, 10k) of the second ring (10b) which face the first ring (10a) lying in contact with the inner surfaces (10n, 101) of the first ring (10a) in a form-fitted manner, and with the inner surface (101) of the first ring (10a) extending in the radial direction while subtending an acute angle, i.e. an angle less than 90°, with the axis (C), and with the rings (10a, 10b) consisting of a consisting of a material in the group: polytetraflouorethylene (PTFE), modified high-temperature polymer such as polyetheretherketone (PEEK), polyetherketone (PEK), polyimide (PI), polyphenylene sulphide (PPS), polybenzimidazole (PBI), polyamidimide (PAI) or modified epoxy resin, with the possibility of these materials containing additional solid lubricants such as e.g.
carbon, graphite, molybdenum disulphide or PTFE.

2. Piston ring consisting of a first and a second ring-shaped body (10a, 10b) with ring gaps (10e, 10f), said rings (10a, 10b) each having at least two mutually touching subsurfaces (101, 10k; 10m, 10n), being arranged to extend concentric to one another with respect to an axis (C), with the touching subsurfaces (101, 10k; 10m, 10n) being arranged to be mutually form-fitting, and with the subsurfaces (101, 10k; 10m, 10n) of each of the rings (10a, 10b) meeting at an acute angle in order to effect an increased mutual holding of the rings (10a, 10b) at the touching subsurfaces (101, 10k; 10m, 10n) by a fluid lying in contact with the piston ring under pressure during an operating state, and with the piston ring (10) being constructed of a plastic.

3. Piston ring in accordance with claim 1 or claim 2 characterised in that the inner surface (101) of the first ring (10a) is executed as a conically extending surface.

4. Piston ring in accordance with claim 1 or claim 2 characterised in that the inner surface (101) of the first ring (10a) is executed as a surface extending in a curve in the radial direction.

5. Piston ring in accordance with one of the claims 1 to 4 characterised in that the second ring (10b) has a part (10k) protruding in the radial direction towards the axis (C) which engages into the ring gap (10e) of the first ring (10a) in order to prevent a mutual rotation of the two rings (10a, 10b).

6. Piston ring in accordance with one of the claims 1 to 6 characterised in that the first ring (10a) of the piston ring (10) is arranged in a piston (2) towards the pressure side.

7. Dry running piston compressor with a piston ring in accordance with one of the claims 1 to 6.