Patent Publication Number: US-6910406-B1

Title: Axial piston compressor

Description:
DESCRIPTION 
   The invention relates to an axial piston compressor for a refrigerant, in particular for a vehicle air conditioner, with a housing in the interior of which at least one bearing is disposed, with an outlet conduit for the compressed refrigerant, and with a lubricant present within the interior of the housing. 
   Such an axial piston compressor is known, for example, from the German patent DE 196 21 174 A1. It is used to compress the refrigerant in a vehicle air conditioner and serves to suck in the refrigerant from a heat-transfer compartment in which it evaporates while taking up heat at low pressure, and to compress it to a higher pressure at which it is passed into another heat-transfer compartment where, while releasing heat, the refrigerant is returned to the liquid state and/or cooled. 
   Such compressors have been produced in a great variety of constructions; for various reasons, the most generally accepted axial piston compressors are those that operate with a swash plate. In this construction the axial movement of the piston is generated by a swash plate, tilted relative to the drive shaft at an angle that can be controlled. The pistons are connected to the swash plate so that they cannot be shifted by forces of compression or traction; because the cylinders within which the pistons move are fixed in place while the swash plate is in motion, as a coupling mechanism between the swash plate and the pistons there are provided either sliding blocks supported in sliding bearings situated on the pistons, or a wobble plate with piston rods seated on sliding bearings on the pistons. When the pistons are directly connected to the swash plate, on each piston there are formed two hemispherical bearings in which the two sliding blocks are disposed so that each contacts a slideway, one on one side of the swash plate and the other on the other side. In contrast, when a wobble plate is used, it is mounted on the swash plate so that it can be rotated with respect thereto, so that what is transmitted to the wobble plate is only the angle of tilt of the swash plate but not its rotational movement. The piston rods are seated on both the wobble plate and the piston by way of ball-and-socket joints. 
   In the case of axial piston compressors employed in motor vehicles, it is impossible to lubricate their components by circulating oil with a pump. For one thing, the axial piston compressor can in some circumstances become much more expensive when a lubricant pump is included. Furthermore, such a pump impairs performance, which in the case of an axial piston compressor for cooling vehicles, which tends to have a low output at best, is more significant than in the case of a higher-output axial piston compressor. Finally, the volume of the overall structure would be considerably increased by providing a pump that has to suck the lubricant in from an oil sump, and by the sump itself. For all these reasons the lubrication in the interior of the housing is accomplished not by a pump-driven circulation of oil but rather by a mist of oil generated within the housing. Furthermore, it is known from the European patent application 0 738 832 that an oil sump can be employed to collect oil droplets that are produced in the interior of the housing. This oil sump is connected to a reservoir in the interior of the housing by way of a lubricant-oil channel. Because the oil sump is positioned above the corresponding reservoir, the oil flows into the reservoir under the influence of gravity. 
   The situation is quite similar in the case of the axial piston compressor according to the patent DE 198 21 265 A1. There, too, gravity causes oil to drop onto moving parts within a wobble-plate chamber. Accordingly, this construction also allows the bearings to be lubricated without applying pressure. 
   The same applies to the construction according to the patent U.S. Pat. No. 4,283,997. Although in this case oil is separated out on the high-pressure side and sent from there to the bearings of the moving parts of the axial piston compressor, there is a throttle behind the oil separator on the high-pressure side, and furthermore the oil channel opens into an oil-collecting chamber connected to the drive-mechanism chamber by way of a radial bearing for the drive shaft. Therefore the pressure of the lubricant oil falls nearly to the same level as that in the drive-mechanism chamber, with the consequence that the supply of lubricant oil to the bearings is considerably reduced. This construction, too, is thus distinguished by a nearly unpressurized application of lubricant. 
   However, the previously provided means of lubrication, by a mist of oil or by the application of lubricant without pressure, is not satisfactory under all operating conditions. In particular in the case of sliding bearings, which involve only a slight amount of oscillatory relative movement, the lubrication can be deficient because it is impossible to provide enough lubricant in the form of a mist. 
   The objective of the invention is thus to lubricate the bearings of an axial piston compressor of the kind described above with pressurized oil, reliably and by the simplest means, with no need for a separate oil pump. Nevertheless, the lubrication thus achieved is designed to be of higher quality than the lubrication of bearing sites by the lubricant mist present in the interior of the housing. 
   ADVANTAGES OF THE INVENTION 
   An axial piston compressor of the kind cited above with the features given in the characterizing part of claim  1  offers the advantage that a pressurized lubricant circulation is created that can conduct the required lubricant to the bearing sites in the interior of the housing. Expressed simply, this lubricant circulation is based on recycling the amount of lubricant that unavoidably leaves the axial piston compressor along with the compressed refrigerant, as well as on utilizing the pressure difference between the compressor side and the interior of the housing of the axial piston compressor. Because the pressure on the compressor side is much greater than that in the interior of the housing, the resulting throughput of lubricant is very high, with no need to provide a separate energy source or even a pump for the purpose. The lubricant separator provided in accordance with the invention is a comparatively simple component, which involves no great expenditure. 
   Within the lubricant circulation so created a suitable throttling ensures the resistance necessary to prevent the lubricant from flowing out of the separator too rapidly. Such throttling results automatically, for example, when a sliding bearing is being lubricated; the narrow gap in the bearing limits the rate at which lubricant can flow through. In contrast, when a roller bearing is being lubricated, in some circumstances a cover plate must be used to restrict the flow cross section by a suitable amount. 
   The construction in accordance with the invention thus is characterized by the fact that even the last bearing to be supplied with lubricant is supplied under high pressure, i.e. compression pressure. This is achieved because the bearings themselves represent throttling sites towards the low-pressure side; that is, they are either sealed off towards the low-pressure side or on the basis of their construction—this applies in particular to sliding bearings—constitute an extremely efficient throttle towards the low-pressure side. The oil pressure is applied to the bearing or gap in the bearing. With respect to their supply of lubricant oil, the bearings are arranged in series such that the pressure of the oil supplied to even the last bearing is nearly unchanged, i.e. is still high. Only on the basis of this construction is it possible to replace an oil pump by an oil separator on the high-pressure side, without impairing the supply to the bearings of oil under high pressure. 
   When both sliding and roller (in particular needle) bearings are employed, preferably the oil is first sent to the sliding bearings with “narrow gaps”, because sliding bearings on account of their construction serve as efficient throttling sites towards the low-pressure side. In case the construction is such that roller or needle bearings must come first in the series, these should be sealed off towards the low-pressure side. In the sense of the above explanation, at the core of the present invention is the fact that the lubricant is propelled from the lubricant separator to the lubricant channel entirely on the basis of the pressure difference between the compression pressure of the refrigerant and the internal pressure of the housing, and is conducted by the channel under corresponding pressure to the bearings. This means that all the bearings are supplied with lubricant under high pressure, with no need for a separate oil pump. 
   In this arrangement, the lubricant separator is disposed on the pressure side of the circulation, either between the compressor and a pressure-side heat exchanger or between the pressure-side heat exchanger and an expansion valve. 
   According to one preferred embodiment of the invention, the feed line is provided with a controllable valve. With this valve the feed line can be closed while the axial piston compressor is not operating, so that the high pressure on the compressor side cannot press the lubricant that is within the collection chamber out into the housing of the axial piston compressor and thereby eventually empty the collection chamber. If that were to happen, no lubricant would be available when the axial piston compressor was put into operation. On the other hand, if the valve is opened as soon as the axial piston compressor becomes operative, the compressor can be adequately lubricated immediately, by the lubricant that has accumulated in the collection chamber. 
   The controllable valve could be eliminated if outflow of the lubricant from the collection chamber were counteracted by a sufficient resistance, namely the internal resistances and throttling sites of the system. That is, if the compressor is only briefly inoperative, there is not enough time for equilibration of the pressures on the high- and low-pressure sides of the compressor. Hence when the compressor is turned on again, it must work against a high pressure; in compensation, however, lubricant is immediately available. On the other hand, after the compressor has been inactive for a long time, so that the pressures have equilibrated and pressurized lubricant is not delivered immediately, it takes some time for the pressure to be built up; that is, the compressor is initially not working under a heavy load, so that complete lubrication is not immediately required. As the load on the compressor increases, the lubrication improves accordingly. 
   Alternatively, the controllable valve could be replaced by a throttling site, if it is ensured that the pressure equilibration during compressor standstill is brought about primarily by other parts of the circulation system, for example by a separate valve. 
   Instead of a separate throttling site, a throttle conduit could be used, in which the required throttling is produced by the various pressure losses in the lubricant supply system, in particular by channels in the compressor. 
   Preferably an overflow conduit is provided, which connects the lubricant separator to the interior of the housing. By this means excess lubricant that has accumulated in the collection chamber can be removed when necessary. For this purpose a controllable valve can be provided in the overflow conduit, which is opened in dependence, for example, on the signal from a sensor that detects the level of the contents in the collection chamber. 
   According to one embodiment of the invention it can be provided that the lubricant separator is integrated into the housing. This enables a particularly compact construction. 
   Alternatively, it can be provided that the lubricant separator is separate from the housing and the feed line acts as a lubricant cooler. This ensures that the lubricant fed back to the bearing sites, which has been warmed by the refrigerant during the compressor stroke, is returned to its initial temperature. 
   According to one preferred embodiment, a drive shaft with an axial distributor bore is provided. An axial distributor bore in the drive shaft enables nearly all the important bearing sites in the interior of the housing of the axial piston compressor to be reached with a particularly simple arrangement. The complexity in this case is distinctly less than in the case of an arrangement such that individual lubricant channels are provided for all the bearing sites in the compressor housing. 
   Preferably the distributor bore opens at an end face of the drive shaft situated in the interior of the housing, i.e. the face at the end opposite to the drive end of the drive shaft. Given an axial delivery route of the lubricant, because of the low circumferential velocities a small axial sealing element can be used, so that the whole structure can be made compact. According to a preferred embodiment a swash plate is provided that is disposed on the drive shaft so that it can move along a sliding bearing, and in the region of the sliding bearing a branch bore is provided in the drive shaft to connect the sliding bearing with the distributor bore. The sliding bearing is very difficult to lubricate by means of the lubricant mist present in the interior of the housing; the branch bore makes it possible to conduct the necessary amount of lubricant to the sliding bearing. The amount thus conducted can be determined by adjusting the cross-sectional area of the branch bore. 
   Preferably a supply bore is formed within the swash plate, into which lubricant passing through the sliding bearing flows, so that sliding blocks in contact with the swash plate can be supplied with lubricant by way of the supply bore. By this means the sliding blocks, which move with only a slight oscillatory component and hence are difficult to lubricate by means of a lubricant mist, are supplied in a targeted manner with pressurized lubricant. 
   According to a preferred embodiment the drive shaft is seated in at least one subassembly comprising radial and axial bearings, this subassembly being supplied with lubricant by a branch bore from the drive shaft such that the lubricant flows first through the radial bearing and then through the axial bearing. This sequential arrangement of the bearings with respect to the lubricant flow makes it possible for both bearings to be lubricated in a relatively uncomplicated manner. Because the space available limits the size of the radial bearing, this bearing is likely to have the shortest working life and hence is the first to be supplied with lubricant; the lubricant flowing out of the radial bearing is then sent to the axial bearing. As is preferably the case, this order of lubrication can be produced by sealing disks that form leakage gaps of specified dimensions. By suitably dimensioning the sites through which the lubricant passes, the function of a non-controllable valve can be simulated, so that when the compressor is not operating, it is impossible for too much lubricant to be transferred from the separator into the compressor. 
   According to one preferred embodiment of the invention, CO 2  is used as the refrigerant. Apart from various technical advantages of CO 2  in comparison to conventionally employed refrigerants such as R134a, an air conditioner with CO 2  as refrigerant operates at a much higher pressure than does an air conditioner with a conventional refrigerant. When CO 2  is used, the suction pressure is about 50 bar and the compression pressure, about 120 bar. In contrast, the suction pressure for the refrigerant R234a is about 5 bar and the compression pressure, about 20 bar. As a result, when CO 2  is used as the coolant the pressure difference between the lubricant separator and the interior of the housing of the axial piston compressor is much greater than in conventional axial piston compressors, namely about 70 bar as compared with 15 bar. This increase in pressure differential in accordance with the invention produces an improved supply of lubricant to the bearings. 
   Advantageous embodiments of the invention will be apparent from the subordinate claims. 

   
     DRAWINGS 
     In the following the invention will be described with reference to various embodiments illustrated in the attached drawings, wherein 
       FIG. 1  is a schematic sectional view of an axial piston compressor according to a first embodiment of the invention; 
       FIG. 2  shows a schematic section of an axial piston compressor according to a second embodiment of the invention; and 
       FIG. 3  shows a schematic section of an axial piston compressor according to a third embodiment of the invention. 
   

   DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
   In  FIG. 1  an axial piston compressor according to a first embodiment is shown schematically. It comprises a drive shaft  10  seated in a housing  12 . To the drive shaft  10  there is nonrotatably connected a swash plate  14 , which can be pivoted between a position in which it is approximately perpendicular to the long axis of the drive shaft  10  and a maximally tilted position shown in  FIG. 1 . The position occupied by the swash plate  14  while in operation is adjusted in dependence on the difference between the intake pressure of the compressor and the pressure in the interior of the housing  12  as well as on the pretensioning of a spring  16  that can shift the swash plate along the drive shaft  10 ; the swash plate is braced against a holder  18  so that as it is shifted along the drive shaft, it is also pivoted. 
   A wobble plate  20  is rotatably seated on the swash plate by means of radial and axial roller bearings  22 ,  24 . Engaged with the wobble plate  20  are several ball-and-socket joints  26 , each of which provides a tension- and pressure-resistant connection between a piston  28  and the wobble plate  20 . Each piston  28  can move within a cylinder  30 , the central axis of which is parallel to the long axis of the drive shaft  10 . In the drawing only two pistons are shown; in fact, the compressor can contain as many as seven pistons. 
   When the drive shaft  10  is put into rotation and the swash plate is tilted with respect to the drive shaft, each piston  28  moves back and forth within its cylinder  30 . This movement can be utilized to compress a refrigerant, for example CO 2 . Under evaporative or suction pressure, the refrigerant is extracted from an input conduit  32 , and under condensation or evaporation pressure it is sent into an output conduit  34 . During the compression process the refrigerant takes up small amounts of a lubricant that is present in the interior of the housing and is also deposited on the inner wall of the cylinder  30 . 
   The output conduit  34  opens into a lubricant separator  36 . This comprises, firstly, a separator compartment  38  in which the flow velocity of the refrigerant, which is present as a pressurized gas, is lowered and as a result the lubricant is separated out by the force of gravity; in addition it comprises a chamber  40  in which the separated lubricant is collected. The lubricant contained in the collection chamber  40  is under the pressure of the refrigerant. From the separator compartment  38 , the compressed refrigerant is conducted to a heat exchanger through a compressor conduit  39 . 
   As an alternative to a gravity separator, in principle any generally used means of separation can be used to implement the lubricant circulation. 
   Attached to the lowest part of the collection chamber  40  is a feed line  42 , which is provided with a controllable valve  44 . The feed line  42  leads to a supply channel  46  in the housing  12 , which opens into a radial bearing  48  for the drive shaft  10 . To the separator compartment  38  is attached an overflow conduit  43 , provided with a valve  45 . Opening of the valve  45  enables an excess volume of lubricant contained in the collection chamber  40  to be returned to the housing. 
   The drive shaft  10  is provided with a distributor bore  50  that extends axially and is connected by way of a radially extending supply bore  52  to the radial bearing  48 . The drive shaft  10  is further provided with two radially extending branch bores  54 , one of which is associated with a sliding bearing  56  by means of which the swash plate is seated on the drive shaft  10 , while the other is associated with a radial bearing  58  which, together with an axial bearing  60 , supports the end of the drive shaft  10  disposed in the interior of the housing  12 , namely the end opposite the drive end of the shaft. 
   When during operation of the axial piston compressor described here the valve  44  of the feed line  42  is opened, the lubricant contained in the collection chamber  40  flows through the feed line  42  to the supply channel  46 , because of the difference between the pressure in the separator compartment  38  and the interior of the housing  12 . From the supply channel it flows into the distributor bore  50  of the drive shaft  10 , by way of the radial bearing  48  and the supply bore  52 . From the distributor bore it can reach the various bearing sites in the interior of the housing by way of the branch bores  54 . In this way the sliding bearing  56  as well as the subassembly consisting of radial bearing  58  and axial bearing  60  are lubricated. The radial bearing  58  is so constructed that the lubricant that has flowed through it continues to the axial bearing. For this purpose, the radial bearing can be integrated into the housing in such a way that a projection of the housing, together with the rotating drive shaft, forms a narrow gap that allows only as much lubricant to escape as can ensure an acceptable lubricant pressure throughout the entire “serial arrangement” of bearing sites. 
   That the lubricant introduced to the interior of the housing will be returned to the lubricant separator is ensured by the fact that, because of the rotation of the components of the axial piston compressor, a mist of lubricant is always present in the interior of the housing. This is deposited on the inner wall of the cylinder  30 , and from there it is carried by the compressed refrigerant into the lubricant separator. 
   In  FIG. 2  a second embodiment of an axial piston compressor is diagrammed. For the components already described in the first embodiment the same reference numerals are used here, so that the explanations given above also apply here. 
   In contrast to the first embodiment, in the second embodiment the lubricant is conducted axially in the distributor bore  50  within the drive shaft  10 , having been introduced at the end of the drive shaft that is on the right in  FIG. 2 . For this purpose, on the end face of the drive shaft  10  a sealing element  62  is provided, which can have small dimensions because the circumferential velocity there is so low. 
   In this embodiment, a branch bore  54  is now provided in the region of the radial bearing  48  associated with the drive end of the drive shaft, so that this bearing is reliably provided with lubricant. From this bearing the lubricant flows to an axial bearing  64 , which supports the holder  18 . 
     FIG. 3  is a diagram of an axial piston compressor according to a third embodiment. Here, again, for known components the same reference numerals are used as in  FIG. 1 , so that for these reference is made to the explanations given above. 
   Here the lubricant is conducted radially, as in the first embodiment, but it is introduced in the region of the radial bearing  58 . From there it can flow through the distributor bore  50  to the sliding bearing  56  and the radial bearing  48 . 
   A difference from the first embodiment is that in the third embodiment a supply bore  66  is provided both in the swash plate  14  and in the wobble plate  20 . Hence the lubricant emerging from the branch bore  54  can pass through the sliding bearing  56 , the radial bearing  22  and the wobble plate  20  to reach the ball-and-socket joints  26  and lubricate the latter, in particular the sliding blocks disposed in the ball-and-socket joints. 
   It is also possible to supply the pistons  28  in the cylinders  30  with pressurized oil, so as to produce a better film of lubricant in the region of the friction pairing there, which can be regarded as a sliding bearing. For this purpose, a lubricant pocket is formed in the cylinder face, which is supplied with lubricant through a suitable channel. The narrow gap between cylinder and piston ensures the required throttling of the lubricant throughput. 
   LIST OF REFERENCE NUMERALS 
   
       
         10  Drive shaft 
         12  Housing 
         14  Swash plate 
         16  Spring 
         18  Holder 
         20  Wobble plate 
         22  Roller bearing 
         24  Roller bearing 
         26  Ball-and-socket joint 
         28  Piston 
         30  Cylinder 
         32  Input conduit 
         34  Output conduit 
         36  Lubricant separator 
         38  Separator compartment 
         39  Compressor conduit 
         40  Collection chamber 
         42  Feed line 
         43  Overflow conduit 
         44  Valve 
         45  Valve 
         46  Supply channel 
         48  Radial bearing 
         50  Distributor bore 
         52  Supply bore 
         54  Branch bore 
         56  Sliding bearing 
         58  Radial bearing 
         60  Axial bearing 
         62  Sealing element 
         64  Axial bearing 
         66  Supply bore