Contactlessly sealing piston with piston rod

Abstract

The invention relates to a piston for use in a piston compressor for high-pressure applications, characterised in that: the piston (3) is in the form of a pot-shaped piston body (20), wherein the piston body (20) has a closed base region (21) and a hollow-cylindrical region (22) axially adjoining the base region (21); an axial end face (24) of the piston rod (4) facing the piston (3) and / or an axial base face (25) of the base region (21) facing the piston rod (4) is convexly curved, wherein the axial end face (24) of the piston rod (4) facing the piston (3) is arranged in the hollow-cylindrical region (22) of the piston body (20) and rests against the base surface (25) of the base region (21) at a contact point (K); in the radial direction, transverse to a piston axis (KA), a piston rod gap (KS) is provided between an inner circumferential surface (26) of the hollow cylindrical region (22) and the piston rod (4); and along the piston axis (KA), the contact point (K) is located closer to an axial compression face (28) of the base region (21) of the piston body (20), opposite the base face (25), than a centre of mass (M) of the piston body (20).

Description

[0001] Contactless sealing piston with piston rod

[0002] The invention relates to a contactless sealing piston with piston rod for a piston compressor, as well as the use of such a piston in a cylinder of a piston compressor.

[0003] As is well known, in a piston compressor, a piston is moved back and forth within a cylinder chamber to compress a compression medium therein. To ensure sufficient and efficient compression of the compression medium in the cylinder chamber, a sealing element is required between the moving piston and the cylinder or cylinder surface. Common sealing elements are contact seals, such as piston rings, which are inserted into grooves on the piston and rest against the cylinder surface. The purpose of a piston ring is to seal the cylinder chamber and minimize the leakage of compression medium via the piston. Typically, a plurality of piston rings are provided on the piston.

[0004] Piston rings and the cylinder bore are subject to friction and wear due to contact, piston speed, and acting transverse forces, which, in particular, limits the service life of the piston rings. Piston rings are therefore maintenance parts that must be replaced regularly.

[0005] Today's piston compressors almost exclusively use piston rings made of polymer-based materials. These are primarily high-performance polymers whose tribological properties are modified by fillers to minimize friction and wear. This can extend the service life of piston rings.

[0006] With piston compressors, a distinction must also be made between oil-lubricated and oil-free applications. In oil-lubricated applications, a thin oil film forms a boundary layer between the pressurized piston rings and the cylinder bore. This significantly reduces friction and wear compared to unlubricated applications. Furthermore, the oil film also has a sealing effect. However, in many applications, oil-lubricated piston rings are not possible because the lubricating oil can also contaminate the compression medium, which is often undesirable and impermissible.

[0007] In unlubricated, oil-free applications, tribology (friction and wear) is determined solely by the properties of the compression medium, operating conditions such as average piston speed, acting transverse forces, etc., the material and coating of the cylinder bore, and the material of the piston rings (e.g., polymer material and its fillers). This results in the formation of a transfer film between the piston ring material and the cylinder bore.

[0008] For unlubricated high-pressure applications with a final compression pressure of >250 bar, even typical high-performance polymers reach their material limits, and friction and wear on the piston rings increase disproportionately. While suitable modifications to the piston ring design can reduce the local contact pressure between the piston ring and the cylinder bore, this only allows an increase in the possible final compression pressure to approximately 350 bar, even in unlubricated applications. Furthermore, piston rings are not suitable because they would wear too quickly, thus shortening the achievable service life of the piston rings.

[0009] WO 98 / 31936 A1 describes an unlubricated piston compressor with a metallic cylinder surface and a piston with a plastic jacket. The piston rod bearing, which allows radial movement of the piston rod, ensures parallel movement of the piston in the cylinder.

[0010] To achieve industrial-grade service lives, high-pressure pistons (for final compression pressures <350 bar) of reciprocating compressors are equipped with a large number of piston rings. 10 to 20 piston rings can be provided. However, this results in very large overall lengths for the pistons and, consequently, the piston compressor, and can lead to reduced heat dissipation of the individual piston rings, which in turn limits service life.

[0011] The individual piston rings are inserted into grooves in the piston. For high-pressure applications, additional metal support rings are inserted behind the piston rings (on the side facing away from the pressure) to support them against the cylinder pressure acting on the piston ring. These support rings are designed to prevent the pressure-induced extrusion of the piston ring into the gap between the cylinder bore and the piston outer diameter.

[0012] A piston with piston rings and support rings for high-pressure applications is known, for example, from WO 2010 / 084071 A1. This document also describes the use of a support ring on the piston to guide and center the piston in the cylinder.

[0013] The typical gap width between support rings and cylinder bores is in the range of a few 0.1 mm. Narrower gap widths are not used in industry because otherwise there is a risk of metallic contact between the cylinder bore and the support rings due to tolerances and acting transverse forces transmitted to the piston via the crankshaft drive. Metallic contact would be associated with high wear and could also damage the cylinder bore. An alternative, well-known sealing concept for sealing between the piston and cylinder bore is a non-contact seal, such as a labyrinth seal or gap seal. Here, the sealing effect is achieved via a cylindrical sealing gap between the piston outer diameter and the cylinder bore. In the labyrinth seal design, this gap creates an additional throttling effect through additional grooves.To achieve sufficient sealing performance in high-pressure applications > 350 bar, this sealing gap must be <50 pm. Such a very small sealing gap causes a number of difficulties.

[0014] Unavoidable irregularities in the piston and / or cylinder bore surface due to manufacturing reasons, manufacturing tolerances on the piston and cylinder bore surface, as well as transverse forces acting on the crankshaft drive, almost inevitably lead to metallic contact between the piston and the cylinder bore surface at sealing gap heights of < 50 pm. This leads to significant wear and damage to piston compressor components after even a short operating period.

[0015] Pistons and cylinders are typically made of steel due to the high loads they are subjected to. Uneven temperature distribution between the piston and cylinder leads to changes in the effective gap height of the sealing gap due to thermal expansion. Even at small temperature differences, such as those common in piston compressors, this can lead to contact between the piston and cylinder because the effective gap height approaches zero, or to disproportionate leakage because the gap height increases.

[0016] Particularly in high-pressure applications of piston compressors with final compression pressures of >350 bar, pressure-induced deformation of the cylinder and / or piston can occur, which can lead to changes in the gap height in the 10 pm range. This can lead to similar negative effects, namely contact or an increase in the gap height.

[0017] Last but not least, particles in the micrometer range that are carried along with the compression medium can get into the sealing gap, which can lead to the piston sticking or damage the piston surface and / or the cylinder bore.

[0018] For these reasons, a gap seal for piston compressors is theoretically possible, but no industrial implementation of such a gap seal in a piston compressor is known to date.

[0019] The aforementioned problems associated with piston rings and gap seals have so far prevented the industrial use of piston compressors for unlubricated high-pressure applications, especially at final compression pressures of >350 bar. Other compressor designs, such as diaphragm compressors or ionic compressors, have previously been used for such high-pressure applications. The short service life of the diaphragm is a problem with diaphragm compressors. With ionic compressors, mixing of the ionic liquid with the compression medium can occur, which is fundamentally undesirable. Apart from this, service life is also a problem with ionic compressors.

[0020] However, the use of a piston compressor even for high-pressure applications would be advantageous because the piston compressor is a well-known, proven and, above all, simple type of compressor.

[0021] It is therefore an object of the present invention to provide a piston of a piston compressor which enables use in a piston compressor for high-pressure applications.

[0022] This is made possible with a contactless sealing piston according to claim 1. With a piston designed according to the invention, a contactless gap seal can be realized because, due to the design, in particular the floating bearing of the piston and the distance of the center of mass of the piston body to the contact point between the piston body and piston rod, the possible contact forces between piston and cylinder can be controlled and thus safe operation with a small sealing gap can be maintained. The piston with piston rod according to the invention can therefore be used in particular for high-pressure applications with final compression pressures of up to 1000 bar and even more. Such final compression pressures occur particularly in hydrogen applications. This makes it possible for the first time to use a piston compressor for such final compression pressures.

[0023] The stabilizing effect of the piston body against tilting is particularly advantageous when the ratio between the contact distance between the contact point and the center of mass and a contact surface distance between the axial compression surface and the center of mass is less than one, preferably between 0.5 and 0.8.

[0024] If at least one throttle groove is provided in an outer peripheral surface of the piston body, preferably several axially spaced throttle grooves, any particles located between the outer peripheral surface of the piston body and the cylinder bore can be trapped. This can prevent the formation of scratches on the cylinder bore and / or on the outer peripheral surface of the piston body. Furthermore, such a throttle groove can also enhance the sealing effect.

[0025] If the piston rod is elastically connected to the piston body in the hollow cylindrical area via a radial or axial elastic connecting element, lifting of the piston body from the piston rod during start-up of the piston compressor can be prevented without compromising the floating bearing of the piston body. The same can be achieved with a preload unit that actively presses the piston body against the piston rod.

[0026] The present invention will be explained in more detail below with reference to Figures 1 to 8, which show exemplary, schematic and non-limiting advantageous embodiments of the invention.

[0027] Fig.1 a piston compressor,

[0028] Fig.2 a contactless sealing piston according to the invention,

[0029] Fig.3 shows a detail of a contactless sealing piston according to the invention,

[0030] Fig.4 and Fig.5 a contactless sealing piston according to the invention with a radially elastic connecting element,

[0031] Fig.6 a contactless sealing piston according to the invention with an axially elastic connecting element,

[0032] Fig.7 a contactless sealing piston according to the invention with a pretensioning unit and

[0033] Fig.8 shows a cylinder of a piston compressor with a piston according to the invention.

[0034] Although the operating principle of a piston compressor 1 is generally well known, it will be briefly explained below with reference to Fig. 1. Fig. 1 shows a sectional view of a piston compressor 1 in an advantageous embodiment. The piston compressor 1 has a compressor housing 2 in which at least one cylinder Z1 is provided, in which a piston 3 can move reciprocally. The piston 3 is moved back and forth in the cylinder Z1 between a top dead center TDC and a bottom dead center BDC. The piston 3 partially delimits a cylinder chamber K1 in the cylinder Z1, in which a compression medium, for example hydrogen, is compressed by the movement of the piston 3. The piston 3 is connected to a reciprocating piston rod 4 and is driven by the piston rod 4 during operation.The end of the piston rod 4 opposite the piston 3 is connected to a piston drive 12, which generates the reciprocating movement of the piston rod 4 and the piston 3. In the embodiment shown in Fig. 1, the piston drive 12 is designed as a crank drive with a crosshead 5, although the piston drive 12 can also be designed differently, for example with a cam drive. In this embodiment, the piston rod 4 is connected at the end opposite the piston 3 to a crosshead 5, which is mounted for axial movement in the compressor housing 2. The crosshead 5 is driven to move the piston rod 4 and the piston 3 back and forth. In the illustrated embodiment, the crosshead 5 is driven by a crank drive. For this purpose, the crosshead 5 is articulated to one end of a push rod 6, and the other end of the push rod 6 is articulated to a crank pin 7a of a crankshaft 7. The crankshaft 7 rotates during operation of the piston compressor 1.The crankshaft 7 can be driven by a drive device not shown in Fig. 1, e.g., by an electric motor. During operation of the compressor 1, a rotary movement of the crankshaft 7 is converted into a translational movement of the crosshead 5 via the push rod 6. The resulting transverse forces are supported on the compressor housing 2 via the crosshead 5. As a result, the piston rod 4 undergoes an essentially purely translational, reciprocating movement in the direction of the piston axis KA. The piston 3 can, of course, also be driven by means other than a crank drive.

[0035] The reciprocating piston rod 4 is typically guided through a sealing unit 9, which in turn is arranged in the compressor housing 2. The sealing unit 9 seals against the piston drive housing 8, such as the crankcase in Fig. 1, in which the crosshead 5 also moves, in order to prevent compression medium from entering the piston drive housing 8. The sealing unit 9 is preferably designed as a well-known sealing packing with a plurality of sealing rings and, if necessary, additional rings, such as scraper rings, support rings, etc.

[0036] The design of the piston compressor 1 with a reciprocating piston rod 4 also makes it possible to design the piston compressor 1 as a double-acting one if required, as in the exemplary embodiment according to Fig. 1 as a double-acting piston compressor 1. In a double-acting piston compressor 1, a second cylinder Z2 with a second cylinder chamber K2 is formed on a side of the piston 3 opposite the at least one first cylinder chamber K1, wherein the piston 3 also partially delimits the second cylinder chamber K2. During the reciprocating movement of the piston 3, the cylinder volume of the first cylinder Z1 is reduced (compression stroke) and simultaneously the cylinder volume of the second cylinder Z2 is increased (suction stroke), or vice versa.

[0037] The design shown as a double-acting compressor is of course only an example. In the simplest case, the piston compressor 1 has only a single cylinder Z1, which is delimited by the end face of the piston 3 facing away from the piston drive 12. Valves SV1, DV1, SV2, DV2 are also arranged in a known manner on a cylinder Z1, Z2 of the piston compressor 1, with at least one suction valve SV1, SV2 and at least one pressure valve DV1, DV2 being provided on a cylinder Z1, Z2. The valves can be designed as automatic valves, i.e. controlled by the acting cylinder pressure. However, the valves can also be actively controlled, for example by means of known lifting grippers.

[0038] In the embodiment according to Fig.1, a double-acting piston compressor 1 is shown, in which both suction valves SV1, SV2 are connected to a suction line SL, via which compression medium KM is sucked in, and the two pressure valves DV1, DV2 are connected to a pressure line DL, via which compressed compression medium KM is discharged.

[0039] If the piston area on one side of the piston 3 is smaller than the piston area on the opposite side of the piston 3, a multi-stage piston compressor 1 can also be realized. In this case, the suction line of the first compressor stage would only be connected to the suction valve SV1 of the first compressor stage. The pressure line DL1 of the first compressor stage, connected to the pressure valve DV1 of the first compressor stage, would simultaneously be the suction line SL of the second compressor stage and thus connected to the suction valve SV2 of the second compressor stage. The pressure valve DV2 of the second compressor stage would be connected to the pressure line DL2 of the second compressor stage.

[0040] In the embodiment according to Fig.1, a piston ring 15 is arranged on the piston 3, which rests on the cylinder surface 10 of the cylinder Z1, Z2 in order to seal the respective cylinder chamber K1, k2.

[0041] Fig. 2 shows an embodiment of a contactless sealing piston 3 according to the invention with part of a piston rod 4 of a piston compressor 1 for high-pressure applications in use in a cylinder Z1 of a piston compressor 1. Other components of the piston compressor 1, such as the entire piston drive 12, are not shown for the sake of simplicity. If the piston rod 4 in the piston compressor 1 is designed in several parts (as shown in Fig. 6, for example), then the "piston rod" in combination with the piston 3 comprises at least the part of the piston rod 4 that forms the axial end facing the piston 3, but can also comprise other parts of the piston rod 4 or all parts of the piston rod 4.

[0042] A contactless gap seal with a sealing gap DS is provided for sealing between piston 3 and cylinder Z1. As is usual with piston compressors 1, piston 3 is moved reciprocatingly in cylinder chamber K1 in cylinder Z1. The piston drive 12 of piston 3 is not shown in Fig. 2 except for the reciprocating piston rod 4 because it is irrelevant to the invention. Distances and gaps are greatly exaggerated in Fig. 2 for illustrative purposes. In the case of a contactless gap seal, piston 3 is only designed to be single-acting, so that piston 3 only delimits a cylinder chamber K1 on the side facing away from piston drive 12.

[0043] The piston 3 according to the invention consists of a pot-shaped piston body 20 with a closed base region 21 and a hollow cylindrical region 22 axially adjoining the base region 21. The piston 3 is thus axially delimited at an axial end opposite the piston drive 12 by a compression surface 28. When the piston 3 is used in a cylinder Z1 of a piston compressor 1, the compression surface 28 at least partially delimits the compression chamber K1 in the cylinder Z1.

[0044] The piston body 20 is preferably constructed in one piece, meaning that the base portion 21 and the hollow cylindrical portion 22 are manufactured in one piece. However, the piston body 20 can also be constructed in multiple parts, with the individual parts of the piston body 20 being suitably connected to one another.

[0045] At least one axially extending sealing region DB is provided on the outer peripheral surface 27 of the piston body 20. The sealing region DB can also extend over the entire axial length of the piston body 20 in the direction of the piston axis KA. Several such sealing regions DB can also be provided, each axially spaced from one another in the direction of the piston axis KA.

[0046] When the piston 3 is used in a piston compressor 1, a sealing gap DS is formed between the outer circumferential surface 27 of the piston body 20 in the region of a sealing area DB and the cylinder running surface 10 on the cylinder Z1, in which the piston 3 is moved, which sealing gap DS effects the contactless gap seal. The sealing gap DS is typically between 1 pm and 50 pm wide to form a contactless gap seal. The sealing gap DS is defined by the smallest radial distance between the outer circumferential surface 27 of the piston body 20 in the region of a sealing area DB and the inner diameter Dz of the cylinder running surface 10 on the cylinder Z1, which distance results from a concentric arrangement of the piston 3 in the cylinder Z1.

[0047] The outer diameter D of the outer circumferential surface 27 of the piston body 20 in a sealing region DB is preferably constant. At least the outer diameter D of the piston body 20 along a sealing region DB is such that a sealing gap DS results along the entire sealing region. Axial regions of the piston body 20 other than a sealing region DB in any case have an outer diameter that does not result in a sealing gap DS when the piston 3 is used.

[0048] In the design according to Fig. 2, axial, conical areas are provided at both axial ends, although a conical area can also be provided at only one end or at no end. The sealing area DB extends over the remaining axial length of the piston body 20.

[0049] An advantage of the piston 3 according to the invention, in particular according to Fig.2, is that it hardly creates any additional dead space, since apart from the very small sealing gap DS no gaps are provided.

[0050] Due to the piston drive 12, transverse forces transverse to the piston axis KA could be imposed on the piston 3 via the connection of the piston rod 4 to the piston 3. Due to the small sealing gap DS, such transverse forces cause the piston 3 to come into contact with the cylinder bore 10. If the resulting contact forces become too great, this causes increased wear on the two friction pairing partners and subsequently failure of the components. Consequently, the transmitted transverse forces between the piston rod 4 and the piston 3 must be reduced as far as possible. For this reason, the piston 3 is arranged on the piston rod 4 in a floating manner in the radial direction transverse to the piston axis KA. "Floating bearing" means that the piston 3 and the piston rod 4 are freely movable relative to one another radially to the piston axis KA (apart from possible design-related frictional forces).Free movement naturally only occurs within the design-defined possible range of motion. Piston rod 4 should be able to move freely radially relative to piston 3 without radial contact with piston 3. The extent of movement of piston rod 4 in the radial direction during operation naturally depends on the design of piston drive 12, but can be assumed to be known due to the known design of piston compressor 1.

[0051] To achieve this floating bearing in the radial direction, the axial end of the piston rod 4 facing the piston 3 is arranged, according to the invention, in the hollow-cylindrical region 22 of the piston body 20 and rests at a contact point K (axially opposite the compression surface 28) on the base region 21. Thus, the axial end face 24 of the piston rod 4 facing the piston 3 and the base surface 25 of the base region 21 rest against one another at the contact point K. The floating bearing in the radial direction to the piston axis KA is achieved by providing a radial piston rod gap KS between the piston rod 4 and the inner circumferential surface 26 of the hollow-cylindrical region 22 of the piston body 20. The piston rod gap KS can be formed by the smallest inner diameter of the hollow-cylindrical region 22 being larger than the largest outer diameter of the axial end of the piston rod 4 arranged in the hollow-cylindrical region 22.The piston rod gap KS is defined by the smallest radial distance between the piston rod 4 and the hollow cylindrical area 22, which results from a concentric arrangement of the piston rod 4 and the piston body 20.

[0052] The piston rod gap KS is preferably selected in the range between 0.1 mm and 10 mm.

[0053] The piston rod 4 can therefore move radially within the piston body 20 by the piston gap KS. This also allows the piston body 20 to move radially by a maximum of twice the sealing gap DS during operation of the piston 3 in the cylinder Z1.

[0054] During operation of piston 3, the acting cylinder pressure pz also ensures that the piston body 20 is centered in the cylinder Z1. Due to hydrodynamic effects, a pressure buildup occurs on one side where the sealing gap DS between the piston body 20 and the cylinder bore becomes smaller, which in turn ensures that the piston body 20 is moved back to a central, concentric position.

[0055] During operation, the piston rod 4 can assume a slight inclination and / or a slight concentricity error relative to the piston axis KA. If this inclination and / or the concentricity error of the piston rod 4 were rigidly transmitted to the piston body 20, high contact forces could occur between the piston body 20 and the cylinder bore 10 on the cylinder Z1, which would result in high wear. To prevent this, the axial end face 24 of the piston rod 4 facing the piston and / or the axial base surface 25 of the base region 21 are convexly curved (outwards) according to the invention, as shown in Fig. 3. In the embodiment according to Fig. 3, the mutually facing axial end face 24 of the piston rod 4 facing the piston and the axial base surface 25 of the base region 21 are convexly curved, although it would be sufficient if only one of these surfaces were convex.

[0056] The convex curvature of at least one of these axial surfaces makes it possible to compensate for an angular error of the piston rod 4. A possible inclination of the piston rod 4 relative to the piston axis KA is thus not transferred to the piston body 20, and the piston body 20 remains concentric in the cylinder Z1.

[0057] During operation, the piston body 20 is pressed against the piston rod 4 by the cylinder pressure pz acting in the cylinder Z1, creating a contact force at contact point K that generates frictional forces in the transverse direction. These frictional forces depend on the unavoidable differential pressure across the piston 3, as well as the coefficient of friction between the two contact surfaces, and can become large in high-pressure applications. By selecting the appropriate geometry and materials, within the scope of the skilled craftsman's skills, to minimize this coefficient of friction, the resulting transverse forces required for centering can be controlled, thus maintaining the necessary mobility.

[0058] Due to the known structural design of the piston body 20, a specific center of mass M of the piston 3 results (see Fig. 2 and Fig. 3), the position of which can be assumed to be known. This results in an axial contact distance A of the contact point K on the piston axis KA relative to the center of mass M due to the known structural design of the piston body 20. According to the invention, the piston body 20 is designed such that the position of the contact point K on the piston axis KA is closer to the compression surface 28 on the piston 3 in the direction of the piston axis KA than the center of mass M.

[0059] With this position of the contact point K relative to the center of mass M, it can be achieved that during the compression stroke (i.e., when the piston 3 moves toward top dead center TDC), during which the greatest forces occur, the contact force (pressure force) acting on the piston 3 between the piston rod 4 and the piston body 20, in the event of an inclined position of the piston body 20, exerts a restoring effect on the piston body 20, which attempts to rotate the piston body 20 back into the concentric position. This also reduces possible torques acting on the piston body 20 due to the cylinder pressure pz and the compression surface 28. The inventive position of the contact point K on the piston axis KA thus improves the stability of the angular conformity (relative to the piston axis KA) of the piston body 20.This is particularly important for a high-pressure piston with ultimate pressures of up to 1000 bar and with a gap seal, because even slight inclinations could lead to high contact forces between the piston body 20 and the cylinder bore 10 on cylinder Z1, resulting in significant wear. A contact point K on the opposite side of the center of mass M, on the other hand, would have a destabilizing effect and would even exacerbate inclinations.

[0060] Typically, the piston 3 is designed to be rotationally symmetrical, so that the center of mass M will lie on the piston axis KA. In the case of a center of mass M that is not located on the piston axis KA, the position of the radial projection of the center of mass M onto the piston axis KA relative to the compression surface 28 is decisive. The position of the radial projection of the contact point K onto the piston axis KA is considered in the same way if the contact point K is not located on the piston axis KA (e.g., in a design such as that shown in Fig. 6).

[0061] For the stabilizing effect, it is advantageous if the ratio A / AF between the contact distance A and the contact surface distance AF between the center of mass M and the compression surface 28 on the piston axis KA is as large as possible, although this ratio A / AF is always less than one. The preferred ratio A / AF is 0.5 to 0.8.

[0062] Relative to the outer diameter D of the outer circumferential surface 27 of the piston body 20, the geometric ratios on the piston 3 are selected such that AF / D = 0.7 to 1.5 and A / D = 0.35 to 1.2 (which leads to the above ratio A / AF = 0.5 to 0.8). The axial length L of the piston body 20 relative to the outer diameter D also follows a preferred size ratio, namely L / D = 1.5 to 3.

[0063] Fig.4 and 5 show an alternative floating bearing of the piston 3 on the piston rod 4. Fig.5 shows an enlarged view of the floating bearing of Fig.4 for illustration purposes. In Fig.5, a convex curvature of the axial end face 24 of the piston rod 4 can also be seen.

[0064] 4 and 5, the piston rod 4 and the piston 3 are radially elastically connected to one another via a radial elastic connecting element 46. The piston rod gap KS remains unaffected. In the embodiment shown, an elastic ring, such as an O-ring, is arranged radially between the piston rod 4 and the hollow cylindrical region 22 of the piston body 20 as a radial elastic connecting element 46. For this purpose, a ring receiving groove 47a, 47b can be provided on the outer peripheral surface of the piston rod 4 and on the inner peripheral surface of the hollow cylindrical region 22, as in Fig. 4 and Fig. 5. When the piston rod 4 is arranged in the hollow cylindrical region 22, the elastic ring sits in these two ring receiving grooves 47a, 47b and is held therein.However, the elasticity of the radial elastic connecting element 46 and, if applicable, the dimensions of the ring receiving grooves 47a, 47b are selected such that the forces generated by the connecting element 46 in the radial direction are so small that they can be neglected and, as a result, the floating bearing of the piston 3 in the radial direction is not impaired.

[0065] During operation of the piston compressor 1, the cylinder pressure pz acting in the cylinder acts on the piston 3, which also ensures that the piston 3 moves with the piston rod 4 during the return movement from top dead center (TDC). During operation of the piston 3, the radial elastic connecting element 46 therefore has no function and does not impair the function of the piston 3.

[0066] However, when starting up the piston compressor 1, for example after maintenance, it cannot be expected that the cylinder Z1 is already pressurized. If the piston drive 12 is driven in this state, the reciprocating movement at top dead center TDC can lead to a loss of contact between the piston rod 4 and the piston 3. This would cause the piston 3 to periodically strike the cylinder cover until the pressure built up by the compression movement is sufficient to compensate for this loss of contact. The radial elastic connecting element 46 also serves to prevent such a loss of contact when starting up the piston compressor 1. The radial elastic connecting element 46 represents an elastic connection between the piston rod 4 and the piston 3, by means of which the piston 3 can be easily retracted with the piston rod 4.

[0067] Instead of a radial elastic connecting element 46 between piston rod 4 and piston 3, an axial elastic connecting element 46 can also be provided between piston rod 4 and piston 3, as described with reference to Fig. 6. Such an axial elastic connection also allows the retraction of piston 3 via piston rod 4, but does not impair the floating arrangement of piston 3 on piston rod 4.

[0068] At the axial end of the piston rod 4 facing the piston 3, an axial elastic connecting element 46 in the form of an elastic body is provided. The arrangement of the piston rod 4 in the hollow cylindrical region 22 with piston rod gap KS is not changed as a result. Likewise, the base surface 25 of the bottom region 21 and the end surface 24 of the piston rod 4 abut one another at the contact point K. In this embodiment, a contact ring is created around the piston axis KA due to the axial elastic connecting element 46 and a convexly curved end surface 24 / or base surface 25. The radial projection onto the piston axis KA is considered the contact point K. The axial elastic connecting element 46 is merely provided additionally and does not impair the stabilizing effect of the position of the contact point K described above.At the axial end of the piston rod 4, a connecting element recess 48 can be provided for the axial elastic connecting element 46, in which the axial elastic connecting element 46 is arranged. A threaded pin 49 protrudes from the axial elastic connecting element 46 at each of its axial ends, with one of the threaded pins 49 being screwed into a threaded bore 50 on the piston body 20, and the opposite threaded pin 49 being screwed into a threaded bore 50 in the piston rod 4. Fig. 6 also shows a design of a multi-part piston rod 4, with the parts of the piston rod 4 being screwed into one another.

[0069] Fig. 7 describes a further embodiment of a floating arrangement of the piston 3 on the piston rod 4. In this embodiment, too, the axial end of the piston rod 4 is arranged with a piston rod gap KS in the transverse direction in a hollow cylindrical region 22 on the piston body 20. Likewise, the base surface 25 of the bottom region 21 and the end face 24 of the piston rod 4 abut each other axially at the contact point K.

[0070] In the embodiment according to Fig.7, the piston rod 4 is additionally actively pressed axially against the base surface 25 of the base region 21 by a preloading unit 51, without impairing the floating bearing, in particular without impairing the piston rod gap KS.

[0071] In the embodiment shown, the preload unit 51 is designed with a clamping sleeve 52. The clamping sleeve 52 is fastened to the axial end of the piston body 20, specifically the hollow cylindrical region 22, for example by means of a screw connection as in Fig. 7, or glued or pressed in or secured in some other way. A radially inwardly projecting shoulder 53 is provided on the axial end of the clamping sleeve 52 facing away from the piston 3. A radial web 54 is provided on the piston rod 4 between the shoulder 53 and the piston body 20. The radial web 54 can be formed by a ring fastened to the piston rod 4, but can also be formed integrally with the piston rod 4. The radial web 54 is firmly connected to the piston rod 4 and does not move relative to the piston rod 4. A spring element 55 is provided between the radial web 54 and the shoulder 53, which preloads the radial web 54 and thus the piston rod 4 in the direction of the piston 3.This ensures a certain axial contact force on the contact surface 23 between piston rod 4 and piston 3, which is particularly advantageous in the case of force reversal, since it prevents a loss of contact between piston rod 4 and piston 3 during the pressureless start-up phase of the compressor.

[0072] In the versions of the piston 3 according to Figs. 4, 6, and 7, throttle grooves 30 in the form of circumferential grooves are provided on the outer circumferential surface 27 of the piston 3. Such throttle grooves 30 are optional, and only one throttle groove 30 may be provided. At least one throttle groove 30 may also be provided in the version according to Fig. 2 or Fig. 3.

[0073] If present, the at least one throttle groove 30 serves to intercept any particles located between the outer circumferential surface 27 of the piston body 20 and the cylinder bore 10 and to collect them in the throttle groove 30. This can prevent the formation of scratches on the cylinder bore 10 and / or on the outer circumferential surface 27 of the piston body 20 or the lifting of a chip by such particles. In addition, the throttle groove 30 can be designed in such a way that a throttling effect occurs, similar to a contactless labyrinth seal, which increases the contactless sealing performance on the piston body 20 during operation of the piston 3. In the circumferential groove 30, turbulence occurs in the compression medium flowing along the outer circumferential surface 27, leading to losses that in turn reduce the acting pressure. This throttling effect is particularly effective when the ratio between the sealing gap DS and the radial groove depth of the throttle groove 30 is at least 1.5, preferably at least 2.

[0074] The materials of the cylinder bore 10 and the piston body 21 are selected so that the thermal deformations caused by different thermal expansion coefficients, together with the respective deformation due to the applied pressure, enable the maintenance of a functional sealing gap DS. This means that it is ensured that the sealing gap DS cannot become too large or too small due to thermal expansion and load-induced deformation. It is advantageous if the thermal expansion coefficients of the materials used differ by a maximum of 1 x 10' 61 / K. Preferably, but not necessarily, the thermal expansion coefficient of the material of the cylinder bore 10 is greater than the thermal expansion coefficient of the material of the piston body 20. This can prevent the piston body 20 from expanding more than the cylinder bore 10, thus reliably preventing the thermally induced closing of the sealing gap DS between piston 3 and cylinder bore 10. Possible material combinations to ensure this are ceramic for the cylinder bore 10 and Invar (iron-nickel alloy with a very low thermal expansion coefficient) for the piston body 20 or steel for the cylinder bore 10 and ceramic for the piston body 20. Likewise, with a suitable design, a piston body 20 made of steel can be combined with a ceramic cylinder bore 10. A design of both elements from ceramic materials is also possible.

[0075] Fig. 8 shows a cylinder Z1 of a piston compressor 1, in which a piston 3 according to the invention with a piston rod 4 is moved back and forth. In this embodiment, the cylinder chamber K1 in the cylinder Z1 is delimited by a cylinder head 60, in which the suction valve SV1 and the pressure valve DV1 are also arranged. The piston 3 with piston rod 4 can be designed as described above with reference to Figures 2 to 6. In this embodiment, the cylinder running surface 10 is formed on a cylinder liner 61, which is arranged in the cylinder Z1, which corresponds to the usual design.

[0076] In the embodiment shown in Fig. 8, cylinder Z1 is cooled. Temperature control lines 62, through which a temperature-controlled temperature control medium is passed, are provided in the cylinder housing of cylinder Z1 and around cylinder running surface 10 or cylinder liner 41. Using temperature control lines 62 and the temperature control medium, cylinder running surface 10 can be heated to a specific temperature, allowing the thermal expansion of cylinder running surface 10 and thus also the sealing gap DS between cylinder running surface 10 and piston body 20 to be controlled. For this purpose, a temperature sensor 63 can also be provided in cylinder Z1 to measure the temperature in the area of ​​cylinder running surface 10. Temperature sensor 63 can be integrated into a closed control loop to regulate the temperature in the area of ​​cylinder running surface 10 by adjusting the temperature of the temperature control medium.The temperature control medium can be both cooled and heated in a controlled manner. Cooling will be necessary in particular during operation of the piston compressor 1, when the cylinder Z1 heats up due to the compression of the compression medium. When starting the piston compressor 1 or when operating in low ambient temperatures, it may also be necessary to heat the temperature control medium. The temperature of the temperature control medium can also or additionally be controlled depending on the current sealing gap DS in order to keep the sealing gap DS within a desired range via the thermal expansion of the cylinder bore 10 and the piston body 20. For this purpose, the sealing gap DS can also be measured, for example using an ultrasonic measurement.

Claims

Patent claims 1. Contactless sealing piston (3) with piston rod (4) for a piston compressor (1), characterized in that the piston (3) is designed as a pot-shaped piston body (20), wherein the piston body (20) has a closed bottom region (21) and a hollow-cylindrical region (22) axially adjoining the bottom region (21), that an axial end face (24) of the piston rod (4) facing the piston (3) and / or an axial base surface (25) of the bottom region (21) facing the piston rod (4) is convexly curved, wherein the axial end face (24) of the piston rod (4) facing the piston (3) is arranged in the hollow-cylindrical region (22) of the piston body (20) and bears against the base surface (25) of the bottom region (21) at a contact point (K), that in the radial direction, transversely to a piston axis (KA),a piston rod gap (KS) is provided between an inner circumferential surface (26) of the hollow cylindrical region (22) and the piston rod (4), and that along the piston axis (KA) the contact point (K) is closer to an axial compression surface (28) of the base region (21) of the piston body (20) opposite the base surface (25) than a center of mass (M) of the piston body (20).

2. Contactless sealing piston (3) with piston rod (4) according to claim 1, characterized in that a ratio A / AF between a contact distance (A) between the contact point (K) and the center of mass (M) and a contact surface distance (AF) between the axial compression surface (28) and the center of mass (M) is less than one, preferably between 0.5 and 0.

8.

3. Contactless sealing piston (3) with piston rod (4) according to claim 1 or 2, characterized in that in an outer peripheral surface (27) of the piston body (20) at least one throttle groove (30) is provided, preferably a plurality of axially spaced throttle grooves (30) are provided.

4. Contactless sealing piston (3) with piston rod (4) according to one of claims 1 to 3, characterized in that the piston rod (4) is elastically connected to the piston body (20) in the hollow cylindrical region (22) via a radial or axial elastic connecting element (46).

5. Contactless sealing piston (3) with piston rod (4) according to claim 4, characterized in that an elastic ring is provided as a radial elastic connecting element (46) between the piston rod (4) and the piston body (20).

6. Contactless sealing piston (3) with piston rod (4) according to claim 5, characterized in that a ring receiving groove (47a, 47b) is provided on the piston body (20) and on the piston rod (4) and the elastic ring is arranged in the ring receiving grooves (47a, 47b).

7. Contactless sealing piston (3) with piston rod (4) according to claim 4, characterized in that the axial elastic connecting element (46) is designed as an elastic body, wherein a connecting element recess (48) is provided on the axial end of the piston rod (4) facing the piston (3), in which connecting element recess the axial elastic connecting element (46) is arranged, wherein an axially projecting threaded pin (49) is provided on each of the two axial ends of the axial elastic connecting element (46), and wherein one threaded pin (49) is screwed into a threaded bore (50) on the piston body (20) and the opposite threaded pin (49) is screwed into a threaded bore (50) on the piston rod (4).

8. Contactless sealing piston (3) with piston rod (4) according to one of claims 1 to 3, characterized in that at the piston rod side axial end of the piston body (20) a clamping sleeve (52) is fastened to the piston body (20), wherein at the axial end of the clamping sleeve (52) facing away from the piston (3) a shoulder (53) projects radially inwards, wherein on the piston rod (4) axially between the shoulder (53) and the piston body (20) a radial web (54) is provided and wherein between the radial web (54) and the shoulder (52) a spring element (55) is arranged, which presses the piston rod (4) in the direction of the piston (3).

9. Cylinder for a piston compressor (1), wherein a contactless sealing piston (3) with piston rod (4) according to one of claims 1 to 8 is reciprocally arranged in the cylinder (Z1), and the piston (3) with the compression surface (28) at least partially delimits a compression chamber (K1) formed in the cylinder (Z1), wherein a sealing gap (DS) is provided between the outer circumferential surface (27) of the piston body (20) and a cylinder running surface (10) of the cylinder (Z1).

10. Cylinder according to claim 9, characterized in that in the cylinder (Z1) around the cylinder running surface (10) a tempering line (62) is arranged, through which a tempered tempering medium is passed.

11. Cylinder according to claim 9 or 10, characterized in that the material provided is ceramic for the cylinder running surface (10) and Invar for the piston body (20) or steel for the cylinder running surface (10) and ceramic for the piston body (20) or ceramic for the cylinder running surface (10) and steel for the piston body (20) or ceramic is provided for the cylinder surface (10) and ceramic for the piston body (20).

12. Piston compressor with at least one cylinder (Z1) according to one of claims 9 to 11, characterized in that the piston rod (4) is connected to a piston drive (12) of the piston compressor (1) at the axial end facing away from the piston (3).

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