Patent Publication Number: US-7913613-B2

Title: Piston/cylinder unit

Description:
The invention relates to a piston/cylinder unit, in particular for a compressor for producing a pressure fluid, comprising a cylinder, a piston which can reciprocate in the axial direction of the cylinder between a first piston position in which the cylinder volume enclosed by the piston and the cylinder is a maximum and a second piston position in which this cylinder volume is a minimum, and a fluid bearing provided between the piston and the cylinder which supports the piston such that it can be displaced axially in the cylinder and which defines a piston-side bearing surface, enclosing the circumference of the piston at least over a part of the axial extension of the piston, the fluid bearing comprising a plurality of outlet nozzles for the fluid provided in the inner circumferential wall of the cylinder. 
     BACKGROUND OF THE INVENTION 
     Such a piston/cylinder unit is known from U.S. Pat. No. 5,525,845 A. In this known piston/cylinder unit outlet nozzles are provided in the cylinder wall which support the piston in its first piston position and in its second piston position. In order to make this possible, the outlet nozzles are located relatively far from the cylinder base, that is from the front inner wall of the cylinder bore. This has the consequence that the fluid cushion formed between the piston circumference and the inner circumference of the cylinder for bearing the piston in the cylinder in the area of the front circumferential region adjacent to the piston base becomes weaker, the further the piston migrates into its second piston position, that is the compression position. As a result of the high pressure produced simultaneously during the compression in the cylinder volume, compressed fluid penetrates from the cylinder volume into the bearing gap between the outer circumference of the piston and the inner circumference of the cylinder which, when this penetrates asymmetrically along the circumference, results in a lateral deflection of the piston and therefore in undesired tipping of the piston. 
     Known from JP 2002-349 435 A is a piston/cylinder unit which is driven by a linear motor and is guided freely on a gas cushion in the piston-ring-free piston. For stabilising this gas cushion, the piston is provided with a circumferential groove on its circumference. This circumferential groove is designed to reduce the risk of the piston tilting in the cylinder. The circumferential groove not only weakens the transverse force disadvantageously for the bearing of the piston but also the air bearing as a whole so that the effect of the circumferential groove relative to the air bearing is rather disadvantageous. 
     SUMMARY OF THE INVENTION 
     It is thus the object of the present invention to provide a generic piston/cylinder unit in such a manner that even when the piston moves into the compression position or is located in the compression position, sufficiently reliable mounting of the piston in the cylinder and therefore security against lateral deflection of the piston is ensured. 
     This object is achieved by a piston/cylinder unit having the features illustrated in the exemplary embodiments. 
     The arrangement of the outlet nozzles in such a manner that that when the piston is in the second position thereof, first outlet nozzles provide the front or middle region of the piston-side bearing surface relative to the longitudinal extension of the piston and second outlet nozzles provide the middle region of the piston-side bearing surface relative to the longitudinal extension of the piston with pressure fluid, ensures reliable mounting and radial positioning of the piston in the cylinder without the piston being able to come in contact with the cylinder. As a result of the arrangement of the outlet nozzles in the central region or in the front and central region, it is achieved that during penetration of pressure from the compression chamber into the bearing gap surrounding the piston, the centre of gravity or centre of force of the bearing remains in the central or front region of the piston and in any case only migrates slightly towards the back, thus ensuring reliable radial support of the piston via the bearing fluid in the middle and also in the front region of the piston so that the influence of the pressure in the compression chamber on the pressure prevailing in the bearing gap is reduced significantly compared with conventional solutions. 
     It is advantageous in this case if the nozzle arrangements are arranged such that outlet nozzles are also provided in the region of the inner circumferential wall of the cylinder to which the piston lies opposite in the second piston position but not in the first piston position. As a result, in the compression state a fluid cushion is reliably formed between the inner circumferential wall of the cylinder and the outer circumferential wall of the piston without this being expelled from the cylinder volume by penetration of compressed fluid. In this embodiment, the piston is more reliably supported against the inner circumferential wall of the cylinder on the fluid cushion in the second piston position, that is, in the compression position of the piston. 
     In a preferred embodiment, the outlet nozzles are arranged such that when the piston is located in its second piston position, first outlet nozzles provide the front region of the piston-side bearing surface relative to the longitudinal extension of the piston and second outlet nozzles provide the middle or rear region of the piston-side bearing surface relative to the longitudinal extension of the piston with pressure fluid. If the outlet nozzles are provided in the front and rear region of the piston-side bearing surface in this case, in the compression position of the piston a particularly uniform support of the piston via its longitudinal extension is achieved. However, it is also advantageous if the first outlet nozzles are provided in the front region and the second outlet nozzles in the middle of the piston-side bearing surface, so that the centre of gravity or centre of force of the bearing extends forwards, that is towards the piston base. As a result, in the area of the front end of the ring gap between the piston and cylinder, that is towards the cylinder volume, a higher pressure is built up in the fluid bearing between the piston and cylinder which offers a higher resistance to the compressive pressure in the cylinder volume and thus more efficiently prevents the compressed pressure fluid from penetrating into the bearing gap from the cylinder volume. 
     In another optional embodiment, the outlet nozzles are arranged in such a manner that when the piston is located in its first piston position, the second outlet nozzles provide the front region of the piston-side bearing surface relative to the longitudinal extension of the piston and third outlet nozzles provide the rear region of the piston-side bearing surface relative to the longitudinal extension of the piston with pressure fluid. These optionally provided third outlet nozzles in the rear region can effect improved support of the piston in its withdrawn position. 
     It is particularly preferred if the fluid bearing is formed by a gas pressure bearing, the outlet nozzles being formed by gas outlet nozzles; an advantageous and particularly preferred embodiment is the air bearing. 
     Preferably, a plurality of outlet nozzles form nozzle arrangements in each case. 
     The nozzle arrangements are preferably spaced apart from one another in the axial direction of the piston/cylinder unit and are preferably formed in a ring shape around the cylinder axis. A particularly uniform fluid or gas cushion is hereby formed between the piston and the cylinder. 
     It is also advantageous for the formation of a particularly uniform fluid or gas cushion between the piston and the cylinder if each nozzle ring comprises a plurality of outlet nozzles uniformly spaced apart from one another in the circumferential direction. 
     The outlet nozzles are formed preferably formed by micro-holes drilled by an energetic beam, which are preferably configured as conical, wherein the narrowest cross-section is located at the mouth into the cylinder-side bearing surface. The micro-holes produced in this way produce a fluid or gas cushion having high uniformity and high bearing capacity. 
     These micro-holes are preferably drilled by means of a laser beam. 
     If the pressure fluid for supplying the outlet nozzles is removed from a fluid flow compressed by compression of the cylinder volume, for example, from the outlet channel, a simple structure of the piston-cylinder unit can be achieved and at the same, an additional pressure generator for the pressure fluid for supplying the outlet nozzles can be dispensed with, helping to make such a piston/cylinder unit cost-effective to produce. 
     This piston/cylinder unit is particularly preferred if the piston is acted upon by a movable part of a linear drive for the reciprocating drive. 
     An advantageous application of the piston-cylinder unit according to the invention which is particularly to be stressed, is in a compressor for generating a pressure fluid, preferably in a linear compressor driven by a linear motor. 
     Further advantageous embodiments of the invention are specified in the remaining dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in detail hereinafter using an example with reference to the drawings; in the figures: 
         FIG. 1  is a schematic longitudinal section through a piston-cylinder arrangement according to the invention with the piston in a first piston position and 
         FIG. 2  is the same piston-cylinder unit with the piston in the compression position. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS OF THE PRESENT INVENTION 
       FIG. 1  is a longitudinal section through a piston-cylinder unit  1  comprising a cylinder  2  and a piston  3 . The cylinder  2  is provided with a cylinder bore  10  which accommodates the piston  3  so that it can move to and fro and be freely guided in the direction of the longitudinal axis X of the cylinder bore  10 . The front wall  12  of the cylinder bore  10  formed on the head side at a cylinder head  23 , the inner circumferential wall  14  of the cylinder bore  10  and piston base  16  define the cylinder volume  18 . 
     An inlet channel  22  provided with a valve  20  shown schematically opens into the head-side front wall  12  of the cylinder bore  10 . Also provided in the head-side front wall  12  is an outlet channel  24  which has a corresponding valve  26 ; this outlet channel also opens into the cylinder bore  10 . 
       FIG. 1  also shows that a cylinder-side bearing surface  15  extends from a front boundary plane Z 1  which coincides with a front piston-side boundary plane K 1  of a piston-side bearing surface  38  when the piston  3  is in its second piston position shown in  FIG. 3 , and a rear boundary plane Z 2  which coincides with a rear boundary line K 2  of the piston-side bearing surface  38  facing away from the piston base  16  when the piston  3  is located in is first piston position shown in  FIG. 1 . The length of the cylinder-side bearing surface  15  is divided into two halves each of length L/2 by a bearing surface central plane E which is at right angles to the cylinder-side bearing surface  15 . 
       FIG. 1  also shows that more outlet nozzles  30 ′,  32 ′ are provided in the front region of the cylinder-side bearing surface  15  than in its rear region where merely the optionally provided outlet nozzles  34 ′ are shown. This asymmetric arrangement of the outlet nozzles relative to the bearing surface central plane E has the effect that the distribution of the nozzle cross-sectional areas of the outlet nozzles over the length L of the cylinder-side bearing surface  15  is also asymmetrical relative to the bearing surface central plane E. Such asymmetry can be achieved not only by providing a different number of outlet nozzles in the front or rear region of the cylinder-side bearing surface  15  but, for example, also by the outlet nozzles in the front area of the cylinder-side bearing surface  15  having a larger diameter and therefore a larger cross-sectional area than those outlet nozzles located in the rear region of the cylinder-side bearing surface  15 . 
     During a movement of the piston  2  to the left in  FIG. 2 , fluid is sucked into the cylinder space  16  through the inlet channel  22  and the inlet valve  20  and during a movement of the piston to the right, this fluid is expelled in the compressed state through the outlet valve  26  and the outlet channel  24 . The piston/cylinder unit  1  shown is part of a piston machine in which the expelled fluid is gaseous, as is the case for example in a compressor. The invention can fundamentally be applied, however, to other piston machines such as, for example, internal combustion engines or pumps. 
     Some of the expelled gaseous fluid is guided from the outlet channel  24  through a connecting channel  28  provided in the cylinder head  23  and in the housing  21  of the cylinder  2 , into ring channels  30 ,  32 ,  34  which are likewise provided in the housing  21  of the cylinder  2  and which surround the cylinder bore  10  in an annular configuration. The ring channels  30 ,  32 ,  34  are spaced apart from one another in the direction of the longitudinal axis X of the cylinder bore  10 . Each of the ring channels  30 ,  32 ,  34  is provided with a plurality of micro-holes  30 ′,  32 ′,  34 ′ which are distributed uniformly over the circumference of the cylinder bore  10  and connect the respective ring channel  30 ,  32 ,  34  to the interior of the cylinder bore  10  and thereby penetrate through the inner wall  14  of the cylinder. The micro-holes  30 ′,  32 ′,  34 ′ of each ring channel  30 ,  32 ,  34  thus form a respective annular nozzle arrangement  30 ″,  32 ″,  34 ″. Pressurised gas is passed through the connecting channel  28  into the ring channels  30 ,  32 ,  34  and can thus escape through the micro-holes  30 ′,  32 ′,  34 ′ and form a gas cushion which laterally supports the piston between the cylinder-side bearing surface  15  on the inner circumferential wall  4  of the cylinder  2  and a piston-side bearing surface  38  on the outer circumferential wall  36  of the piston  3 . 
     The first ring channel  30  with the micro-holes  30 ′ assigned thereto is located in a region in which the piston only covers the micro-holes  30 ′ when it is close to the compression position, that is when the cylinder volume  18  is minimised, as shown in  FIG. 2 . In this case, the piston  3  covers the front, first micro-holes with the bearing surface  38  in the front region  3 ″. 
     In the position shown in  FIG. 1  in which the cylinder volume  18  is greatest, the front-most micro-holes  30 ′ do not contribute to the formation of a gas cushion between the inner circumferential wall  14  of the cylinder  2  and the outer circumferential wall  36  of the piston. However, as a result of the extremely small cross-section of the micro-holes  30 ′, the pressure loss thus produced is not serious. However, there can also be provided a valve arrangement (not shown) which only acts upon the first ring channel  30  with pressure gas when the piston  3  covers the micro-holes  10 . 
     The second ring channel  32  is arranged so that the micro-holes  32 ′ allocated to it are always covered by the piston  3  so that over the entire axial movement path of the piston  3  the micro-holes  32 ′ contribute to the formation of the gas cushion between the inner circumferential wall  14  of the cylinder  2  and the outer circumferential wall  36  of the piston  3 . 
     The third ring channel  34  is furthest removed from the head-side front wall  12  of the cylinder bore. The micro-holes  34 ′ allocated to the third ring channel  34  are thus only covered by the piston  3  and specifically by the bearing surface  38  in the rear region  3 ′ of the piston when the piston  3  is located in the area of its withdrawn position in which the cylinder volume  18  is greatest. The provision of the third ring channel  34  with the micro-holes  34 ′ allocated to it is optional and is merely used to further improve the running properties of the piston  3  in the cylinder bore  10 . 
     In this case, the rear region  3 ′ of the piston is defined as a region facing away from the piston base  16  relative to a central plane M ( FIG. 2 ) orthogonal to the piston-side bearing surface  38 . The front piston region  3 ″ is accordingly a region facing the front end of the piston  3  on the piston base side relative to the central plane M. Between the rear piston region  3 ′ and the front piston region  3 ″ is a central piston region  3 ′″ defined as a region in front of and behind the piston central plane M. The piston central plane M is orthogonal to the piston-side bearing surface  38  and lies at the centre at half the bearing surface length a/2 relative to the bearing surface length a of the piston-side bearing surface  38 . The central piston region  3 ′″ is delimited from the front piston region  3 ″ by a front circumferential line U 1  which is an imaginary circumferential line running in a plane parallel to the piston central line M. Similarly, the central piston region  3 ′″ is delimited from the rear piston region  3 ′ by a rear circumferential line U 2  which is an imaginary line running in a plane parallel to the piston central plane M. The front circumferential line U 1  and the rear circumferential line U 2  each have an axial distance of up to 20%, preferably up to 15%, more preferably up to 10% of the bearing surface length a from the piston central plane M. In this case, the distance of the front circumferential line U 1  to the piston central plane M must not be the same as the distance from the rear circumferential line U 2  to the piston central plane M although a symmetrical arrangement of the circumferential lines U 1 , U 2  to the piston central plane M is preferred. 
     Further annular nozzle arrangements having a similar structure can be provided in the inner wall  14  of the cylinder bore  10  between the ring channels  30 ,  32 ,  34  with their allocated micro-holes  30 ′,  32 ′,  34 ′, each forming the annular nozzle arrangements  30 ″,  32 ″,  34 ″. 
     In one embodiment of the piston/cylinder unit according to the invention which has proved useful in practice, the first outlet nozzles  30 ′ and the second outlet nozzles  32 ′ are arranged such that in the second front piston position shown in  FIG. 2 , they act upon the middle region  3 ′″ of the piston  3  with pressure fluid whilst in this piston position, no outlet nozzles act upon the rear piston region  3 ′. In this case, as shown in  FIG. 2 , the outlet nozzles  30 ′,  32 ′ can be slightly offset relative to the piston central plane M in the direction of the front piston region  3 ′. 
     The invention is not restricted to the above exemplary embodiment which merely serves to give a general explanation of the basic idea of the invention. Rather, the device according to the invention can have embodiments other than those described above within the scope of protection. In particular, the device can have features which represent a combination of the respective individual features of the claims. 
     Reference numerals in the claims, the description and the drawings merely serve to give a better understanding of the invention and should not restrict the scope of protection.