Piston/cylinder unit

Abstract

A piston-cylinder unit including a piston that is fluid pressure supported and movable in a linear manner in a cylinder, wherein the cylinder, a face wall of the piston and a face wall of the cylinder define a compression cavity which is at a minimum size in a portion of a top dead center of the piston, wherein the compression cavity is connected in a fluid transferring manner with a bearing gap which is formed between a cylinder inner circumferential wall and a piston outer circumferential wall, wherein a plurality of fluid outlet nozzles are arranged in at least one cross-sectional plane of the cylinder in the cylinder inner circumferential wall along a circumference, which fluid outlet nozzles open into the bearing gap and are connected with a supply conduit for a pressurized fluid.

Claims

1. A piston-cylinder unit, comprising: a piston that is fluid pressure supported and movable in a linear manner in a cylinder, wherein the cylinder, a face wall of the piston and a face wall of the cylinder define a compression cavity which is at a minimum size in a portion of a top dead center of the piston, wherein the compression cavity is connected in a fluid transferring manner with a bearing gap which is formed between a cylinder inner circumferential wall and a piston outer circumferential wall, wherein a plurality of fluid outlet nozzles are arranged in at least one cross-sectional plane of the cylinder in the cylinder inner circumferential wall along the circumference, which fluid outlet nozzles open into the bearing gap, wherein the piston is provided with a ventilation groove configured as a circumferential groove into which a ventilation conduit opens, wherein the ventilation groove is configured in a circumferential section of the piston that is adjacent to the piston face wall, wherein the ventilation conduit reduces pressurized fluid entering the ventilation groove to a pressure level which is lower than a pressure in the compression cavity when the piston is in its top dead center or when it moves towards the top dead center in proximity to the top dead center, and wherein a pressure compensation circumferential groove is provided between the piston face wall and the ventilation groove.

2. The piston-cylinder unit according to claim 1, wherein the ventilation groove is in fluid transferring connection with a cavity where the lower pressure level is provided.

3. The piston-cylinder unit according to claim 1, wherein the piston includes a piston section with a reduced diameter in the portion of the piston face wall, and wherein the ventilation groove is provided with a non-reduced diameter in a remaining piston portion.

4. The piston-cylinder unit according to claim 1, wherein the diameter of the piston section with the reduced diameter increases in axial direction of the piston starting from the piston face wall.

5. The piston-cylinder unit according to claim 4, wherein an increase of the diameter in the piston section with the reduced diameter is linear.

6. The piston-cylinder unit according to claim 4, wherein an increase of the diameter in the piston section with reduced diameter is non-linear.

7. A piston-cylinder unit, comprising: a piston that is fluid pressure supported and movable in a linear manner in a cylinder, wherein the cylinder, a face wall of the piston and a face wall of the cylinder define a compression cavity which is at a minimum size in a portion of a top dead center of the piston, wherein the compression cavity is connected in a fluid transferring manner with a bearing gap which is formed between a cylinder inner circumferential wall and a piston outer circumferential wall, wherein a plurality of fluid outlet nozzles are arranged in at least one cross-sectional plane of the cylinder in the cylinder inner circumferential wall along the circumference, which fluid outlet nozzles open into the bearing gap, wherein the piston is provided with a ventilation groove configured as a circumferential groove into which a ventilation conduit open, wherein the ventilation groove is configured in a circumferential section of the piston that is adjacent to the piston face wall, wherein the ventilation conduit reduces pressurized fluid entering the ventilation groove to a pressure level which is lower than a pressure in the compression cavity when the piston is in its top dead center or when it moves towards the top dead center in proximity to the top dead center, wherein a plurality of fluid outlet nozzles is arranged in the piston outer circumferential wall along the circumference at least in one cross sectional plane of the piston on a side of the ventilation groove that is oriented away from the piston face wall, and wherein the plurality of fluid outlet nozzles opens into the bearing gap.

8. The piston-cylinder unit according to claim 7, wherein the at least one cross sectional plane of the piston with the fluid outlet nozzles is arranged in any position of the reciprocating piston between the at least one cross sectional plane of the cylinder with the fluid outlet nozzles and the cylinder face wall.

9. A piston-cylinder unit, comprising: a piston that is fluid pressure supported and movable in a linear manner in a cylinder, wherein the cylinder, a face wall of the piston and a face wall of the cylinder define a compression cavity which is at a minimum size in a portion of a top dead center of the piston, wherein the compression cavity is connected in a fluid transferring manner with a bearing pap which is formed between a cylinder inner circumferential wall and a piston outer circumferential wall, wherein a plurality of fluid outlet nozzles are arranged in the cylinder inner circumferential wall along a circumference at least in a cross sectional plane of the cylinder where the fluid outlet nozzles open into the bearing gap, wherein a section of the bearing gap that is adjacent to the compression cavity has a greater radial extension than a section of the bearing gap that is oriented away from the compression cavity, at least when the piston approaches to dead center, and wherein the section of the bearing gap with the greater radial extension is formed by a cylinder section with an increased diameter.

10. The piston-cylinder unit according to claim 9, wherein the section of the bearing gap with the greater radial extension is formed by a piston section with a reduced diameter.

11. The piston-cylinder unit according to claim 10, wherein a diameter of the piston section with the reduced diameter increases starting from the piston face wall in axial direction of the piston.

12. The piston-cylinder unit according to claim 11, wherein a diameter increase in the piston section with the reduced diameter is linear.

13. The piston-cylinder unit according to claim 11, wherein the diameter increase in the piston section with reduced diameter is non-linear.

14. The piston-cylinder unit according to claim 9, wherein the diameter of the cylinder section with the increased diameter decreases from the cylinder face wall in an axial direction of the cylinder.

15. The piston-cylinder unit according to claim 14, wherein a decrease of the diameter in the cylinder section with the increased diameter is linear.

16. The piston-cylinder unit according to claim 14, wherein a decrease of the diameter in the cylinder section with increased diameter is non-linear.

17. The piston-cylinder unit according to claim 9, wherein a plurality of fluid outlet nozzles are arranged in the piston outer circumferential wall along a circumference at least in a cross sectional plane of the piston, the piston face wall or adjacent to the face side piston section with reduced diameter, and wherein the fluid outlet nozzles open into the bearing gap.

18. The piston-cylinder unit according to claim 17, wherein the at least one cross sectional plane of the piston with the fluid outlet nozzles is arranged in any position of the reciprocating piston between the at least one cross sectional plane of the cylinder with the fluid outlet nozzles and the cylinder face wall.

19. A piston-cylinder unit, comprising: a piston that is fluid pressure supported and movable in a linear manner in a cylinder, wherein the cylinder, a face wall of the piston and a face wall of the cylinder define a compression cavity which is at a minimum size in a portion of a top dead center of the piston, wherein the compression cavity is connected in a fluid transferring manner with a bearing gap which is formed between a cylinder inner circumferential wall and a piston outer circumferential wall, wherein a plurality of fluid outlet nozzles are arranged in the cylinder inner circumferential wall along a circumference at least in a cross sectional plane of the cylinder where the fluid outlet nozzles open into the bearing gap, wherein a section of the bearing gap that is adjacent to the compression cavity has a greater radial extension than a section of the bearing gap that is oriented away from the compression cavity, at least when the piston approaches top dead center, wherein the piston is provided with at least one circumferential groove in a circumferential section that is adjacent to the piston face wall or the piston section with reduced diameter, and wherein at least one circumferential groove of the piston is configured as a ventilation groove into which a ventilation conduit opens.

20. The piston-cylinder unit according to claim 19, wherein the ventilation conduit is in fluid transferring connection with a space in which a fluid pressure is provided that is lower than the pressure in the compression cavity when the piston is in its top dead center or moves towards its top dead center.

21. The piston-cylinder unit according to claim 19, wherein the ventilation groove is configured in a circumferential section of the piston that is adjacent to the piston face wall or the piston section with the reduced diameter.

22. The piston-cylinder unit according to claim 7, wherein the ventilation groove is in fluid transferring connection with a cavity where the lower pressure level is provided.

23. The piston-cylinder unit according to claim 19, wherein the section of the bearing gap with the greater radial extension is formed by a piston section with a reduced diameter.

24. The piston-cylinder unit according to claim 23, wherein a diameter of the piston section with the reduced diameter increases starting from the piston face wall in axial direction of the piston.

25. The piston-cylinder unit according to claim 24, wherein a diameter increase in the piston section with the reduced diameter is linear.

26. The piston-cylinder unit according to claim 24, wherein the diameter increase in the piston section with reduced diameter is non-linear.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is subsequently described in more detail based on embodiments with reference to the drawing figure, wherein:

(2) FIG. 1 illustrates a prior art piston cylinder unit;

(3) FIG. 2 illustrates a piston cylinder unit according to a first embodiment of the invention;

(4) FIG. 3 illustrates a first variant of a second embodiment of a piston cylinder unit according to the invention;

(5) FIG. 4 illustrates a second variant of the second embodiment of the piston cylinder unit according to the invention;

(6) FIG. 5 illustrates a third variant of the second embodiment of the piston cylinder unit according to the invention;

(7) FIG. 6 illustrates a piston of the third variant of the second embodiment with a conical front end section;

(8) FIG. 7 illustrates a piston of the third variant of the second embodiment with concave front end section;

(9) FIG. 8 illustrates a piston of the third variant of the second embodiment with convex forward end section;

(10) FIG. 9 illustrates a fourth variant of the second embodiment of the piston cylinder unit according to the invention;

(11) FIG. 10 illustrates a first variant of a third embodiment of the piston cylinder unit according to the invention;

(12) FIG. 11 illustrates a piston of the first variant of the third embodiment with conical forward end section;

(13) FIG. 12 illustrates a piston of the first variant of the third embodiment with concave forward end section;

(14) FIG. 13 illustrates a piston of the first variant of the third embodiment with convex forward end section;

(15) FIG. 14 illustrates a piston cylinder unit according to the invention with a piston of the first variant of the third embodiment which includes a ventilation groove;

(16) FIG. 15 illustrates the variant of FIG. 14 wherein the piston includes an additional pressure compensation circumferential groove;

(17) FIG. 16 illustrates a piston cylinder unit of the first variant of the third embodiment with a piston which is provided with a piston side fluid pressure bearing in the forward piston section;

(18) FIG. 17 illustrates the piston cylinder unit of FIG. 16, wherein the piston is provided with an additional ventilation groove; and

(19) FIG. 18 illustrates a second variant of the third embodiment of the piston cylinder according to the invention with a cylinder section with expanded diameter.

DETAILED DESCRIPTION OF THE INVENTION

(20) FIG. 1 illustrates the prior art piston cylinder unit according to DE 10 2004 061 904 A1 that is already described in the introduction of the description.

(21) FIG. 2 illustrates a first embodiment of the piston cylinder unit according to the invention, wherein the same reference numerals are used for the elements in FIG. 2 that are identical to the elements in FIG. 1.

(22) The piston 103 is arranged in a center position between its bottom dead center UT and its top dead center OT. The second ring channel 32 and the third ring channel 34 are arranged in the cylinder similar to the piston cylinder unit illustrated in FIG. 1. The position of the micro holes 32′ associated with the second ring channel 32 and forming fluid outlet nozzles in the cross sectional plane Q2 and the position of the micro holes 34′ associated with the third ring channel 34 and forming fluid outlet nozzles in the cross sectional plane Q3 and the distance between the second annular nozzle arrangement 32″ and the third annular nozzle arrangement 34″ in axial direction are selected so that the micro holes 32′ and 34′ are covered by the circumferential wall 136 of the piston 103 during the entire axial movement of the piston 103. The two cylinder side air bearings, namely the second air bearing formed by the second annular nozzle arrangement 32″ and the third air bearing formed by the third annular nozzle arrangement 34″ are thus active during the entire piston movement and support the piston 103 in a rear piston section 103′ and in a forward piston section 103″ in radial direction.

(23) The first air bearing contrary to the embodiment of FIG. 1 is not configured in the cylinder but in the piston 103. Thus, the piston 103 is provided with micro holes 130′ distributed over the circumference, evenly offset and forming fluid outlet nozzles in the piston outer circumferential wall 136 in a cross sectional plane Q1 directly adjacent to the piston face wall 116, wherein the micro holes lead into a ring channel 130 configured in an interior of the piston 103 and form a first forward annular nozzle arrangement 130″. The ring channel 130 in the interior of the piston 103 is connected through a channel 131 extending in an interior of the piston rod 104 and through a non-illustrated supply conduit with the connection channel 28. The pressurized fluid flowing into the connection channel 28 is thus also conducted into the ring channel 130 in the interior of the piston and flows from the first micro holes 130′ into the bearing gap 19.

(24) This way a fluid bearing, for example an air bearing is formed in the most forward portion of the forward piston section 103″ of the piston 103 by the annular nozzle arrangement 130″ provided at this location, wherein the air bearing supports the piston 103 radially directly adjacent to the piston face wall 16 relative to the cylinder inner circumferential wall 14 forming the bearing surface 15. Since this most forward fluid bearing moves with the piston the forces applied in this area for radially supporting the piston 103 are almost constant over the entire piston movement. Laterally deflecting the piston transversal to the longitudinal axis X is therefore almost impossible even when fluid compressed in the compression cavity 18 penetrates under pressure into the bearing gap 19.

(25) FIG. 3 illustrates a second embodiment of the piston cylinder unit according to the invention, wherein identical reference numerals are used for the elements in FIG. 3 that are identical with FIG. 1.

(26) The piston 203 is illustrated in a center position between its bottom dead center UT and its top dead center TDC. The second ring channel 32 and the third ring channel 34 are arranged in the cylinder similar to the piston-cylinder unit illustrated in FIG. 1. A position of the micro holes 32′ associated with the second ring channel 32 forming fluid outlet nozzles in the cross sectional plane Q2 and a position of the micro holes 34′ forming fluid outlet nozzles in the cross sectional plane Q3 and associated with the third ring channel and a distance between the second annular nozzle arrangement 32″ and the third annular nozzle arrangement 34″ in axial direction are selected so that the micro holes 32′ and 34′ are covered during the entire axial movement of the piston 203 by the outer circumferential wall 236 of the piston 230. The two cylinder side air bearings, namely the second air bearing formed by the second annular nozzle arrangement 32′ and the third air bearing formed by the third annular nozzle arrangement 34″ are thus active during the entire piston movement and support the piston 203 in a rear piston section 203′ and in a forward piston section 203″ in radial direction.

(27) The piston 203 is provided with a circumferentially extending ventilation groove 233 in a forward piston section 203″ in the piston outer circumferential wall 236 directly adjacent to the piston face wall 216, wherein a ventilation opening 233′ leads into the ventilation groove 233 wherein the ventilation opening is provided with a fluid connection through a channel 233″ which extends in an interior of the piston rod 204 with a space in which a fluid pressure prevails which is lower than the pressure in the compression cavity 18 when the piston 203 is in its top dead center TDC or moves towards its top dead center TDC; at least the pressure provided in the ventilation air groove 233 must be lower than the pressure in the bearing gap 19 in front and behind the ventilation groove 233.

(28) FIG. 4 illustrates a variation of the embodiment according to FIG. 3 in which another circumferential groove 235 is configured in the piston outer circumferential wall 236 between the piston face wall 216 and the ventilation groove 233 directly adjacent to the piston face wall 216. This additional circumferential groove 235 forms a pressure balancing circumferential groove which provides that a pressure compensation along the circumference of the piston 203 is provided in the pressurized fluid entering the bearing gap 19 on one side from the compression cavity 18 so that the piston remains in its centered position with reference to the cylinder axis X and is not displaced laterally.

(29) FIG. 5 illustrates another variant of the piston 203 provided with the ventilation groove 233 in which the piston 203 in its forward piston section 203″ in the portion of the piston face wall 216 includes a piston section 237 with reduced diameter. This piston section 237 with reduced diameter is offset from the ventilation groove 233 in axial direction so that the ventilation groove 233 is configured in the remaining portion of the forward piston portion 203″ with a non-reduced diameter.

(30) By providing the piston section 237 with reduced diameter an annular gap 19′ is provided between the cylinder inner circumferential wall 14 and the outer circumferential wall 237′ of the piston section 237 with reduced diameter, wherein a radial extension of the annular gap, thus its radial thickness is greater than a thickness of the bearing gap 19. When compressed fluid exits during the compression movement of the piston 203 from the compression cavity 18 into the forward ring cavity 19′ the pressurized fluid entering the annular gap 19 centers the piston 203.

(31) In the variant according to FIG. 5 the piston section 237 with the reduced diameter is configured cylindrical. The piston section 237 however can also be configured with increasing diameter starting from the piston face wall 216 in axial direction of the piston. This can be for example implemented as piston section with a conical circumferential contour 239 as illustrated in FIG. 1, wherein the increase of the diameter in the piston section 237 with reduced diameter is linear. The increase of the diameter in the piston section 237 with reduced diameter however can also be nonlinear as illustrated in FIGS. 7 and 8. Thus, the piston section can also have a concave circumferential contour 239′ (FIG. 7) or a convex circumferential contour 239″ (FIG. 8).

(32) The configuration of the piston 203 with the forward piston section 237 with reduced diameter can also be provided in the variant illustrated in FIG. 4 of the piston with an additional pressure compensation circumferential groove 235.

(33) By the same token as illustrated in FIG. 9 the piston 203 that is provided according to the invention with the ventilation groove 233 can be additionally provided with a forward piston side fluid bearing in its embodiments according to FIGS. 3-8 described herein.

(34) Thus, the piston 203 is provided with micro holes 230′ distributed over the circumference and forming fluid outlet nozzles evenly offset from one another in a cross sectional plane Q1′ in the piston exterior circumferential wall 236 directly adjacent to the ventilation groove 233 but axially offset therefrom on a side of the ventilation groove 233 that is oriented away from the piston face wall 216. These micro holes 230′ lead into a ring channel 230 configured in an interior of the piston 203 and form a first forward annular nozzle arrangement 203″. The ring channel 240 in the interior of the piston 203 is connected with the connection channel 28 through a channel 231 that also extends in an interior of the piston rod 204 and through a non-illustrated supply conduit. The pressurized fluid flowing into the connection channel 28 is thus also run into the ring channel 230 in the interior of the piston 203 and flows from the first micro holes 230′ into the bearing gap 19.

(35) This way a fluid bearing, for example an air bearing is also formed in the forward piston section 203 by the annular nozzle arrangement 230″ provided at this location wherein the fluid bearing supports the piston 203 in the forward piston section 203″ in radial direction against the cylinder inner circumferential wall 14 forming the bearing surface 15. Since the forward fluid bearing moves with the piston the forces applied in this portion for the radial support of the piston 203 are almost constant over the entire piston movement. A lateral displacement of the piston transversal to the longitudinal axis X is therefore almost impossible even when an asymmetrical entry of compressed fluid from the compression cavity 18 into the bearing gap should occur in spite of the additional measures described supra (pressure compensation circumferential groove 235, piston section 237 with reduced diameter).

(36) FIG. 10 illustrates a third embodiment of the piston cylinder unit according to the invention, wherein identical reference numerals are used for elements of FIG. 10 that are identical with FIG. 1.

(37) The piston 303 is illustrated in a center position between its bottom dead center UT and its top dead center TDC. The second ring channel 32 and the third ring channel 34 are arranged in the cylinder similar to the piston-cylinder unit illustrated in FIG. 1. The position of the micro holes 32′ associated with the second ring channel 32 and forming fluid outlet nozzles in the cross sectional plane Q2 and the position of the micro holes 34′ associated with the third ring channel 34 and forming fluid outlet nozzles in the cross sectional plane Q3 and the distance between the second annular nozzle arrangement 32″ and the third annular nozzle arrangement 34″ in axial direction are selected so that the micro holes 32′ and 34′ during the entire axial movement of the piston 303 are covered by the exterior circumferential wall 336 of the piston 303. The two cylinder side air bearings, namely the second air bearing formed by the second annular nozzle arrangement 32″ and the third air bearing formed by the third annular nozzle arrangement 34″ are thus active during the entire piston movement and support the piston 303 in a rear piston section 303′ and in a forward piston section 303″ in radial direction.

(38) The piston 303 is provided with a piston section 337 with reduced diameter in its forward piston section 303″ in the portion of the piston face wall 316, wherein the bearing gap 19 in this section forms an annular gap 19′ with a greater radial extension than the section of the bearing gap 19 oriented away from the compression cavity 18.

(39) Providing the piston section 337 with reduced diameter provides an annular gap 19′ between the cylinder inner circumferential wall 14 and the outer circumferential wall 337′ of the piston section 337 with reduced diameter, wherein the radial extension of the annular gap, thus its radial thickness is greater than the radial thickness of the bearing gap 19. When pressurized fluid enters into the forward annular gap 19′ from the compression cavity 18 during the compression movement of the piston 303 the pressurized fluid entering the annular gap 19 centers the piston 303.

(40) In the embodiment according to FIG. 10 the piston section 337 is configured cylindrical with a reduced diameter. However, the piston section 337 can also be provided with an increased diameter in axial direction of the piston starting from the piston face wall 316. This can be implemented for example as a piston section with a conical circumferential contour 339 as illustrated in FIG. 11 wherein the increase of the diameter in the piston section 337 with reduced diameter is linear. This increase of the diameter in the piston section 337 with reduced diameter, however, can also be nonlinear as illustrated in FIGS. 12 and 13. The piston section 337 can also include a concave circumferential contour 339′ (FIG. 12) or a convex circumferential contour 339″ (FIG. 13).

(41) FIG. 14 illustrates another variant of the piston 303 provided with the piston section 337 with reduced diameter. The piston 303 in its forward piston section 303″ adjacent to the piston section 337 with reduced diameter is provided with a ventilation groove 333 extending along the circumference wherein the ventilation groove leads into a ventilation opening 333′ which is in fluid connection through a channel 333″ in the interior of the piston rod 304 with a cavity in which a fluid pressure is provided which is lower than the pressure in the compression cavity 18 when the piston 303 is in its top dead center TDC or when it moves towards the top dead center TDC; at least the pressure prevailing in the ventilation groove 333 has to be lower than the pressure in the bearing gap 19 in front and behind the ventilation groove 333. The ventilation groove 333 is offset in axial direction from the piston section 337 with reduced diameter so that the ventilation groove 333 is not configured with the reduced diameter in the remaining portion of the forward piston portion 303″.

(42) FIG. 15 illustrates a variation of the embodiment according to FIG. 14 in which an additional circumferential groove 335 is configured in the piston outer circumferential wall 336 adjacent to the piston section 337 with reduced diameter between the piston section 337 with reduced diameter and the ventilation groove 333. This additional circumferential groove 335 forms a pressure balancing circumferential groove which provides a pressure balancing in the pressurized fluid entering from the compression cavity 18 on one side into the bearing gap 19, wherein the pressure balancing is provided along the circumference of the piston 303 so that the piston remains in its centered position with reference to the cylinder axis X and is not laterally displaced.

(43) FIG. 16 illustrates another alternative embodiment of the piston cylinder unit according to the invention in which the piston 303 includes a piston side fluid bearing in its forward piston section 303″ adjacent to the piston section 337 with reduced diameter.

(44) For this purpose the piston 303 is provided with micro holes 330′ that are distributed over the circumference and evenly offset from one another and which form fluid outlet nozzles in a transversal plane Q1″ in the piston outer wall 336 directly adjacent to the piston section 337 with reduced diameter but offset there from. These micro holes 330′ lead into a ring channel 330 configured in an interior of the piston 303 and form a first forward annular nozzle arrangement 330″. The ring channel 330 in the interior of the piston 303 is connected with the connection channel 28 through a channel 331 extending in an interior of the piston rod 304 and through a non illustrated supply conduit. The pressurized fluid flowing into the connection channel 28 is also conducted into the ring channel 330 in an interior of the piston 303 and flows from the first micro holes 330′ into the bearing gap 19.

(45) As illustrated in FIG. 17 the piston 303 that is illustrated in FIG. 16 and which is provided with the piston side air bearing can be additionally provided with a ventilation groove 333 as described in the context of FIGS. 14 and 15. In addition to or as an alternative to the ventilation groove 333 also the pressure compensation circumferential groove 335 can be provided which is described in the context with FIG. 15. The ventilation groove 333 and also the pressure compensation circumferential groove 335 are configured between the piston section 337 with reduced diameter and the forward annular nozzle arrangement 330″ in the portion of the piston 303 which does not have a reduced diameter.

(46) This way a fluid bearing, for example a gas or air bearing is also formed in the forward piston section 303″ by the annular nozzle arrangement 330″ provided at this location, wherein the gas or air bearing supports the piston 303 in the forward piston section 303″ in radial direction relative to the cylinder inner circumferential wall 14. Since this forward fluid bearing moves with the piston, forces applied for a radial support of the piston 303 in this area are almost constant over the entire piston movement. A lateral displacement of the piston transversal to the longitudinal axis X is therefore almost impossible even in case compressed fluid enters in an asymmetric manner from the compression cavity 18 into the bearing gap in spite of the additional measures recited supra, thus the pressure compensation circumferential groove 335 and piston section 337 with reduced diameter.

(47) Eventually FIG. 18 illustrates a second variation of the third embodiment of the piston cylinder unit according to the invention in which the section 19″ of the bearing gap 19 with greater radial extension is formed by a forward portion 10′ of the cylinder bore hole 10 that is arranged proximal to the cylinder face wall 12 in which portion the cylinder bore hole 10′ is increased in diameter towards the cylinder face wall 12 (cylinder section 2′). This portion 10′ of the cylinder bore hole with increasing or increased diameter envelops at least a portion of the forward piston section 303′ of the piston 303 when the piston as represented in FIG. 18 in dashed lines is in its top dead center TDC.

(48) In the variant of the third embodiment of the piston cylinder unit illustrated in FIG. 18 it is not required that the piston is provided with the piston section 337 with reduced diameter in the portion of its piston face wall, though this is not impossible either.

(49) Also in the variant according to FIG. 18, the piston 303 can be provided with a ventilation groove 333, a pressure compensation circumferential groove 335, a piston side fluid bearing (forward annular nozzle arrangement 330″) or with combinations thereof as has already been described in conjunction with the first variant of the third embodiment.

(50) The piston cylinder unit according to the invention, and this also applies for all embodiments, forms an element of a linear compressor in an advantageous embodiment, wherein the compressed fluid is a gas, for example air. The fluid bearings are thus configured as gas pressure bearings, for example air bearings. An advantageous embodiment is a refrigeration system linear compressor wherein the fluid is a gaseous refrigerant.

(51) The invention is not limited to the embodiments recited supra which only provide a general description of the core idea of the invention. Within the scope of the invention the device according to the invention can also be provided in embodiments that differ from the embodiments recited supra. The device can thus in particular include features which represent a combination from the respective individual features of the patent claims.

(52) Reference numerals in the patent claims, the description and the drawings are intended for better comprehension of the invention and do not limit the scope thereof.