JP2014163299A - Hermetic type compressor and refrigerator using the same - Google Patents

Abstract

Provided are a hermetic compressor that improves the retention of lubricating oil between a piston and a cylinder, and has improved compression efficiency and reliability due to the dynamic pressure effect of oil due to the reciprocating motion of the piston, and a refrigerator including the same. With the goal.
A compression element and an electric element housed in an airtight container, the compression element having a piston and a cylinder, and a lower end portion of a crankshaft driven by the electric element being stored in the airtight container The lubricating oil that is immersed in oil, raised by the pumping action of an oil pump or the like provided at the lower end portion of the crankshaft, and ejected from the lubricating oil radiation hole of the eccentric shaft formed eccentrically on the upper portion of the crankshaft In the hermetic compressor for supplying oil between the cylinder and the piston, the piston has a lubricating oil holding portion, and the lubricating oil is provided on the sliding surface by the reciprocating motion of the piston in communication with the lubricating oil holding portion. It has a fine groove to guide.
[Selection] Figure 6

Description

  The present invention relates to a hermetic compressor and a refrigerator using the same.

  Conventionally, in a small-capacity hermetic compressor used in a home refrigerator, the demand for energy saving of the refrigerator has increased, and the compressor has been operated at a low speed by inverter control. For this reason, the further efficiency improvement at the time of the low-speed driving | operation of the closed compressor for refrigerators and the reliability improvement of a sliding part are calculated | required. One of the measures to improve the energy efficiency of hermetic compressors is to stabilize the piston behavior in the reciprocating sliding part of the cylinder and piston, which are the main components of the compression element, to improve lubrication and reduce mechanical friction loss. In addition, configurations for improving the efficiency of the compressor by improving the sealing performance between the two are disclosed in Japanese Patent Laid-Open Nos. 7-63264 (Patent Document 1) and 2005-264740 (Patent Document 2). ing.

  In Patent Document 1, a triangular pocket-shaped recess opening at each end surface is formed at both upper and lower end edges of the outer peripheral surface of the piston that reciprocates along the inner peripheral surface of the cylinder, and the recess is formed in the circumferential direction of the outer peripheral surface. It is described that they are arranged at a plurality of positions at equal intervals.

  In Patent Document 2, a plurality of grooves are formed on the outer periphery of the piston that have a length exceeding at least half of the total length of the piston, one end communicates with the skirt side of the piston, and the other end does not reach the top surface of the piston. Therefore, it is described that the oil film is stably formed between the piston and the cylinder by supplying the lubricating oil through the groove.

JP 7-63264 A JP 2005-264740 A

  In Patent Document 1, even if the piston is inclined with respect to the cylinder, the lubricating oil flows into the pocket-shaped recess to generate dynamic pressure, and the inclination of the piston is corrected and stable reciprocating motion is possible. However, in Patent Document 1, since the lubricating oil is supplied to the sliding surface of the piston only through a narrow gap between the outer peripheral surface of the piston and the inner peripheral surface of the cylinder, the lubricating oil is not sufficiently supplied. The cylinder may be worn out and reliability may be reduced.

  In Patent Document 2, the stroke in which the dynamic pressure is generated in the lubricating oil taken into the groove is limited to the suction stroke in which the piston moves from the top dead center to the bottom dead center, and the piston moves from the bottom dead center to the top dead center. Since the dynamic pressure generation effect in the compression stroke in which a large side pressure acts on the piston with compression cannot be expected, the reduction of the sliding loss of the piston is also limited.

  In addition, oil is supplied between the piston and cylinder only while the lubricating oil reaches the side surface of the notch of the cylinder, and is not always supplied during one rotation of the rotating shaft. Since it is open to the surface, it has a structure in which it is difficult for the lubricating oil injected from the upper end of the crankshaft to be taken into the groove, and the oiling time is also short.

  Accordingly, the formation of an oil film in the compression stroke between the piston and the cylinder becomes insufficient, the sealing performance decreases, the amount of compressed gas leaked from the compression chamber increases, the efficiency of the compressor decreases, and The lubricity of the sliding part is lowered and the reliability of the compressor is likely to be lowered.

  Therefore, the present invention provides a hermetic compressor that improves the retention of lubricating oil between the piston and the cylinder and improves the compression efficiency and reliability due to the dynamic pressure effect of the oil due to the reciprocating motion of the piston, and a refrigerator including the same. The purpose is to provide.

  In order to solve the above problems, for example, the configuration described in the claims is adopted. The present application includes a plurality of means for solving the above-mentioned problems. For example, the present application includes a compression element and an electric element housed in a closed container, and the compression element includes a piston and a cylinder, and the electric element The lower end portion of the crankshaft driven by the cylinder is immersed in the lubricating oil stored in the sealed container, and the lubricating oil is raised by a pumping action of an oil pump or the like provided at the lower end portion of the crankshaft. In the hermetic compressor for supplying the lubricating oil ejected from the lubricating oil radiating hole of the eccentric shaft formed eccentric to the upper portion between the cylinder and the piston, the piston has a lubricating oil holding portion, and the lubricating oil There is a fine groove that communicates with the holding portion and guides the lubricating oil to the sliding surface by the reciprocating motion of the piston.

  According to the present invention, there is provided a hermetic compressor that improves the retention of lubricating oil between the piston and the cylinder, and that has improved compression efficiency and reliability due to the dynamic pressure effect of the oil due to the reciprocating motion of the piston, and a refrigerator including the same. Can be provided.

1 is a longitudinal sectional view of a hermetic compressor according to a first embodiment of the present invention. It is a cross-sectional view of the hermetic compressor corresponding to the AA cross section of FIG. It is a principal part expanded sectional view near the piston bottom dead center position of the hermetic compressor concerning this embodiment. It is a piston external view of the hermetic compressor concerning this embodiment. It is a principal part expanded sectional view which carries out the BB cross section of FIG. It is a piston external view which shows other embodiment of the fine groove formed in the piston which concerns on this embodiment. It is a principal part expanded sectional view of the hermetic compressor concerning a 2nd embodiment of the present invention. It is a piston external view of the hermetic compressor concerning a 2nd embodiment of the present invention. It is a piston external view which shows other embodiment of the fine groove formed in the piston which concerns on the 2nd Embodiment of this invention. It is a principal part expanded sectional view which shows an example of the process of the fine groove | channel shown in FIG. It is a piston external view of the hermetic compressor concerning a 3rd embodiment of the present invention. It is a principal part enlarged view of FIG. It is a longitudinal cross-sectional view of the refrigerator in which the hermetic type compressor concerning this embodiment was carried.

(Embodiment 1)
Hereinafter, a first embodiment of the present invention will be described in detail with reference to FIGS. First, the overall configuration of the hermetic compressor will be described with reference to FIGS. 1 and 2. FIG. 1 is a longitudinal sectional view of a hermetic compressor according to the first embodiment of the present invention, and FIG. 2 is a transverse sectional view of the hermetic compressor corresponding to the AA section of FIG.

  The hermetic compressor 50 of the present embodiment houses an electric element 2 and a compression element 3 including a stator 2 a and a rotor 2 b in a hermetic container 1. In the compression element 3, the piston 7 connected to the eccentric shaft 8b of the crankshaft 8 by the connecting rod 6 reciprocates in the cylinder 5a formed in the cylinder block 5 in which the bearing portion 5c and the frame portion 5b are integrally formed. , So-called reciprocating type. The crankshaft 8 is fixed to the rotor 2 b of the electric element 2 at the lower part of the main shaft 8 a that is rotatably fitted to the bearing portion 5 c of the cylinder block 5. The compression element 3 is elastically supported on the bottom of the hermetic container 1 by a coil spring via a stator 2a of the electric element 2 fixed to the lower part of the frame part 5b.

  As the rotor 2b of the electric element 2 rotates, the crankshaft 8 rotates, and the piston 7 reciprocates in the bore of the cylinder 5a via the connecting rod 6 that is eccentrically rotated and connected to the eccentric shaft 8b. It has become.

  The upper end portion of the cylinder 5 a is closed by a cylinder head 10 in which a suction valve and a discharge valve (not shown) are incorporated, and a compression chamber 12 is formed between the cylinder 5 a and the piston 7. In the compression chamber 12, the refrigerant is sucked and compressed by the reciprocating motion of the piston 7 and discharged from the discharge valve.

  Note that the lubricating oil 4 stored at the bottom of the sealed container 1 is pulled up by the centrifugal pump action of the oil pump 8 c attached to the lower end of the crankshaft 8 by the rotation of the crankshaft 8. A ball joint portion connected to the piston 7 and the large end bearing portion of the connecting rod 6 guided upward through a spiral groove 8d formed on the outer periphery of the main shaft 8a and rotatably fitted to the eccentric shaft 8b. And is injected from a lubricating oil radiation hole 8bb provided in a form that partially penetrates the balance weight 9 attached to the upper end of the eccentric shaft 8b, and finally injected from the upper end portion of the eccentric shaft 8b to the periphery. . Lubrication and sealing between the piston 7 and the cylinder 5a are mainly performed by the injected lubricating oil 4.

  The refrigerant that has flowed in through the suction pipe 14 connected in the sealed container 1 passes through the plastic suction silencer 15 and enters the compression chamber 12 of the cylinder 5a. Here, the compressed refrigerant enters the discharge chamber 11 a formed in the head cover 11 from the cylinder head 10, passes through the discharge silencer 16 integrally formed with the cylinder block 5 from here, and is discharged through the discharge pipe 17. The pipe 18 flows out to the external refrigeration cycle.

  Next, the lubrication structure between the piston and the cylinder bore will be described in detail with reference to FIGS. 3 is an enlarged cross-sectional view of the main part near the piston bottom dead center position of the hermetic compressor according to this embodiment, FIG. 4 is an external view of the piston of the hermetic compressor according to this embodiment, and FIG. FIG. 6 is an external view of a piston showing another embodiment of the fine groove formed in the piston according to the present embodiment.

  Two annular grooves 7b and 7c are formed on the outer periphery of the piston 7 of the present embodiment as a lubricating oil holding portion. The annular groove 7b mainly functions as a holding portion for lubricating oil leaking from the compression chamber 12 that the piston top portion 7a faces, and the annular groove 7c functions as a holding portion for lubricating oil injected from the eccentric shaft 8b of the crankshaft 8. The space between the inner peripheral surface of the cylinder 5a and the outer peripheral surface of the piston 7 is a sliding surface (sliding portion length Ls), and the bore inner diameter of the cylinder 5a is usually smaller than the outer diameter of the piston 7 in the diameter gap. It is formed large by about 10 μm. The annular grooves 7b and 7c are recessed from the outer peripheral surface of the piston 7 by about 50 μm, and the lubricating oil is held in the groove inner space. The holding amount of the lubricating oil 4 is determined by the width and depth dimensions of the annular grooves 7b and 7c. When the groove depth is about 50 μm, the width dimension is about 1 mm, but these dimensional relationships can be arbitrarily set. The part excluding the sliding part length Ls from the total length Lp of the piston is the skirt part 7d. This part is about 200 μm in diameter smaller than the outer diameter of the piston 7, and is injected from the eccentric shaft 8b of the crankshaft 8 in the same manner as the annular groove 7c. Functions as a lubricant holding part. A plurality of substantially V-shaped fine grooves 7e1 and 7e2 are continuously formed in the circumferential direction of the outer circumferential surface of the piston 7 on the piston sliding surfaces on both sides of the annular groove 7c so as to communicate with the annular groove 7c. Here, as shown in the cross-sectional shape of FIG. 5, the dimensions of the fine grooves 7e1 and 7e2 are a groove width W of about 0.5 mm, and a groove depth δ is an oleophilic surface treatment applied to the surface of the piston 7. This corresponds to about 5 μm, which corresponds to the thickness of a certain manganese phosphate coating 7f.

  An oil reservoir 13 for receiving the lubricating oil 4 scattered from the eccentric shaft 8b of the crankshaft 8 by the partition wall 13a is provided on the upper surface of the outer periphery of the cylinder 5a. By inclining toward the side, the lubricating oil 4 is supplied to the sliding surfaces of the cylinder and the piston 7.

  When the piston 7 is positioned near the bottom dead center, as shown in FIG. 4, the annular groove 7c on the lower side (crankshaft side) of the piston 7 is changed in two steps by changing the chamfer angle to the end surface on the main shaft side of the cylinder 5a. The entire surface is opened into the space inside the sealed container 1 through a chamfer 5aa provided on the inside. The lubricating oil 4 pumped up by the rotation of the crankshaft 8 is injected from the lubricating oil radiation hole 8bb of the eccentric shaft 8b, passes through the chamfer 5aa located at substantially the same height as the lubricating oil radiation hole 8bb, and below the piston 7 To the annular groove 7c.

  In the compression stroke in which the piston 7 moves from the bottom dead center to the top dead center, the lubricating oil 4 held in the annular groove 7c of the piston 7 is taken into the fine groove 7e1 by the (upward) movement of the piston 7 (see FIG. 4). As shown in the broken line arrow), the end of the fine groove 7e1 is closed, so that dynamic pressure is generated in the lubricating oil, and the piston 7 and the cylinder 5a are separated by an oil film, and good lubrication is maintained.

  Further, in the suction stroke in which the piston 7 moves from the top dead center to the bottom dead center, the lubricating oil 4 held in the annular groove 7c of the piston 7 is now moved to the opposite fine groove 7e2 by the (downward) movement of the piston 7. The piston 7 and the cylinder 5a are separated by an oil film due to the same dynamic pressure effect, and the lubrication between them is kept good.

  In the lubricating structure of the present embodiment, the oil reservoir 13 of the cylinder 5a that is an oil supply part is released in the hermetic container 1, and the annular groove 7c of the piston 7 that is an oil holding part is also rotated once in the crankshaft 8. Since it opens once in the airtight container 1, it is possible to prevent a vapor lock phenomenon in which the gas stays in the annular groove 7 c and inhibits the inflow of the lubricating oil, and the lubricating oil 4 is distributed throughout the groove. Can do. Further, by reciprocating the piston 7, the lubricating oil is guided to the entire sliding surface of the piston 7 by the fine grooves 7e1 and 7e2, thereby improving the lubricating oil retention of the sliding surface of the piston 7 and preventing the oil film from being cut. it can.

  Next, FIG. 6 shows another embodiment of the fine groove formed in the piston according to the present embodiment. FIG. 6A shows the fine groove 7e1 when the fine grooves 7e1 and 7e2 are machined. The fine grooves 7e2 are arranged so that they can be continuously processed with a single stroke, and the processing path of the processing tool is shortened to reduce the processing cost. FIG. 6B shows the arrangement of the fine grooves 7e1 and 7e2 limited to the side pressure support portion of the piston 7 that accompanies gas compression. Thereby, further simplification of processing can be achieved, maintaining the dynamic pressure effect by a fine groove.

(Embodiment 2)
Next, a second embodiment of the present invention will be described with reference to FIGS. 7 is an enlarged cross-sectional view of a main part of a hermetic compressor according to the second embodiment of the present invention, FIG. 8 is an external view of the piston of the hermetic compressor according to the second embodiment of the present invention, and FIG. FIG. 10 is an external view of a piston showing another embodiment of the fine groove formed in the piston according to the second embodiment of the present invention, and FIG. 10 is an enlarged cross-sectional view of an essential part showing an example of processing of the fine groove shown in FIG. FIG.

  In the present embodiment, the position where the fine groove is disposed is the sliding portion on the skirt portion 7d side (crankshaft side) from the annular groove 7c of the piston 7.

  Also in this embodiment, the basic lubrication function between the piston and the cylinder bore is realized in the same manner as in the first embodiment. That is, in the compression stroke in which the piston 7 moves from the bottom dead center to the top dead center, the lubricating oil 4 held in the annular groove 7c of the piston 7 is taken into the fine groove 7e1 by the (upward) motion of the piston 7 and lubricated. A dynamic pressure is generated in the oil, and the piston 7 and the cylinder 5a are separated by an oil film, and good lubrication is maintained.

  Further, in the suction stroke in which the piston 7 moves from the top dead center to the bottom dead center, the lubricating oil 4 held by the skirt portion 7d of the piston 7 is moved by the downward movement of the piston 7 and this time, the fine groove 7e2 on the opposite side is formed. The piston 7 and the cylinder 5a are separated by an oil film by the same dynamic pressure effect that is taken in by the oil film, and the lubrication between the two is kept good.

  In the second embodiment, since the fine groove is not provided on the sliding surface on the piston top portion 7a side from the annular groove 7c, the sealing performance of the piston 7 is maintained better than that in the first embodiment.

  Next, FIG. 9 shows another embodiment of the fine groove formed in the piston according to the second embodiment of the present invention, and the groove exhibiting the dynamic pressure effect is not limited to the above-mentioned substantially V shape. The groove may have any shape as long as the cross-sectional area of the flow path is reduced in the flow direction of the lubricating oil. In the figure, in consideration of workability in machining, the outer peripheral surface of the piston 7 is deleted and removed by using a relatively large processing tool (grinding stone) 20 whose outer peripheral surface is tapered (angle α) as shown in FIG. The substantially elliptical cut surfaces are defined as fine grooves 7e1 ′ and 7e2 ′. Thereby, the lifetime of a processing tool can be lengthened and it contributes to processing cost reduction.

(Embodiment 3)
Next, a third embodiment of the present invention will be described with reference to FIGS. FIG. 11 is an external view of a piston of a hermetic compressor according to the third embodiment of the present invention, and FIG. 12 is an enlarged view of a main part (C1 part) of FIG.

  In the present embodiment, an inclined surface (tapered surface) structure capable of exhibiting a dynamic pressure effect by simpler processing is employed instead of the fine groove shown in the previous embodiments. FIG. 11A shows an example in which inclined surfaces 7e1 ″ and 7e2 ″ are provided instead of the fine grooves 7e1 and 7e2 shown in the first embodiment of the present invention, and FIG. Instead of the fine grooves 7e1 ″ and 7e2 ″ shown in the second embodiment, inclined surfaces 7e1 ″ and 7e2 ″ are provided. As can be seen from the enlarged view of C1 portion of FIG. 11 shown in FIG. 12, it is a relatively simple structure that forms an inclined surface at the corner of the sliding surface of the piston 7, and is formed on the inclined surface by the reciprocating motion of the piston 7. Due to the dynamic pressure effect of the wedge-shaped oil film, the piston 7 and the cylinder 5a are separated from each other by the oil film, and the lubrication between the two is kept good. Thereby, further simplification of processing can be achieved, maintaining the dynamic pressure effect by an inclined surface.

  In the embodiment described above, the processing of the fine groove and the inclined surface on the outer peripheral surface of the piston 7 has been described as after the manganese phosphate coating treatment, which is a lipophilic surface treatment applied to the surface of the piston 7. This processing can also be performed before the surface treatment. Further, the processing of the fine groove and the inclined surface is not necessarily limited to the mechanical processing, and can be performed, for example, by removing the manganese phosphate film by a chemical method such as etching.

  Next, FIG. 13 is a longitudinal sectional view of a refrigerator in which the hermetic compressor according to this embodiment is mounted. In FIG. 13, the hermetic compressor 50 according to the present embodiment includes a cooler 66 and is mounted on a refrigerator 60 using a natural refrigerant R600a having a small warming coefficient, and includes a refrigerator compartment 62, an upper freezer compartment 63, and a lower freezer compartment. 64 and the vegetable compartment 65 are cooled by operating a refrigeration cycle (not shown) by driving the hermetic compressor 50.

  In the embodiment described in detail above, the formation of an oil film between the piston and the cylinder is ensured, the sealing performance is improved, and the compressor efficiency at the time of low speed operation in the rotation speed control compressor by the inverter having a large influence of leakage is improved. In addition, it is possible to provide a hermetic compressor in which the piston and the cylinder are separated by an oil film due to the dynamic pressure effect of oil and the reliability of the compressor is improved. Further, the system to which the hermetic compressor of the present invention is applied is not limited to a refrigerator, and can be applied to a system such as a room air conditioner or a refrigerator for refrigeration and air conditioning applications. Can be improved.

DESCRIPTION OF SYMBOLS 1 ... Sealed container, 2 ... Electric element, 2a ... Stator, 2b ... Rotor, 3 ... Compression element, 4 ... Lubricating oil, 5 ... Cylinder block, 5a ... Cylinder, 5b ... Frame part, 5c ... Bearing part, 6 ... Connecting rod 7 ... Piston, 7a ... Piston top, 7b ... Annular groove 1, 7c ... Annular groove 2, 7d ... Skirt, 7e1, 7e1 ', 7e2, 7e2' ... Fine groove, 7e1 ", 7e2" ... Inclined surface , 7f ... manganese phosphate coating, 8 ... crankshaft, 8a ... main shaft, 8b ... eccentric shaft, 8ba ... oil supply groove, 8bb ... lubricating oil radiation hole, 8c ... oil pump, 8d ... spiral groove, 9 ... balance weight, 10 ... Cylinder head, 11 ... Head cover, 11a ... Discharge chamber, 12 ... Compression chamber, 13 ... Oil sump, 13a ... Partition wall, 14 ... Suction pipe, 15 ... Suction silencer, 16 ... Discharge silencer, 17 ... Outlet pipe, 18 ... discharge pipe, 20 ... processing tool (grinding stone), 50 ... hermetic compressor, 60 ... refrigerator, 61 ... refrigerator body, 62 ... refrigerator compartment, 63 ... upper freezer, 64 ... lower freezer, 65 ... Vegetable room, 66 ... Cooler

Claims (7)

A compression element and an electric element housed in an airtight container are provided, the compression element has a piston and a cylinder, and a lower end portion of a crankshaft driven by the electric element is immersed in lubricating oil stored in the airtight container. The lubricating oil is raised by a pumping action of an oil pump or the like provided at the lower end of the crankshaft, and the lubricant is ejected from a lubricating oil radiation hole of an eccentric shaft formed eccentrically at the upper portion of the crankshaft. And a hermetic compressor supplied between the piston and
A hermetic compressor having a lubricating oil holding portion in the piston, and a fine groove that communicates with the lubricating oil holding portion and guides the lubricating oil to a sliding surface by a reciprocating motion of the piston. A compression element and an electric element housed in an airtight container are provided, the compression element has a piston and a cylinder, and a lower end portion of a crankshaft driven by the electric element is immersed in lubricating oil stored in the airtight container. The lubricating oil is raised by a pumping action of an oil pump or the like provided at the lower end of the crankshaft, and the lubricant is ejected from a lubricating oil radiation hole of an eccentric shaft formed eccentrically at the upper portion of the crankshaft. And a hermetic compressor supplied between the piston and
The piston has an annular groove exposed from the inner circumferential surface of the cylinder when the piston is located near the bottom dead center on the outer circumferential surface of the piston, and is connected to the annular groove to lubricate the sliding surface by the reciprocating motion of the piston. A hermetic compressor having a fine groove that guides the pressure and generates dynamic pressure. A compression element and an electric element housed in an airtight container are provided, the compression element has a piston and a cylinder, and a lower end portion of a crankshaft driven by the electric element is immersed in lubricating oil stored in the airtight container. The lubricating oil is raised by a pumping action of an oil pump or the like provided at the lower end of the crankshaft, and the lubricant is ejected from a lubricating oil radiation hole of an eccentric shaft formed eccentrically at the upper portion of the crankshaft. And a hermetic compressor supplied between the piston and
The piston has an annular groove exposed from the inner circumferential surface of the cylinder when the piston is located near the bottom dead center on the outer circumferential surface of the piston, and is connected to the annular groove to lubricate the sliding surface by the reciprocating motion of the piston. A hermetic compressor having an inclined surface that guides the pressure and generates dynamic pressure.   The partition wall which comprises an oil reservoir part is provided in the said cylinder upper surface, The bottom face of the said oil reservoir part inclines below toward the said crankshaft side, The any one of Claim 1 thru | or 3 characterized by the above-mentioned. Hermetic compressor.   3. The hermetic compressor according to claim 1, wherein the fine groove is provided in a side pressure support portion of the piston accompanying gas compression.   The hermetic compressor according to any one of claims 1 to 5, wherein the hermetic compressor is driven by an inverter at a plurality of operation frequencies including at least an operation frequency equal to or lower than a power supply frequency. In a refrigerator comprising a hermetic compressor, and a cooler connected to the hermetic compressor and a refrigerant pipe,
The hermetic compressor includes a compression element and an electric element housed in a hermetic container, the compression element includes a piston and a cylinder, and a lower end portion of a crankshaft driven by the electric element is stored in the hermetic container. The lubricating oil is immersed in the lubricating oil, lifted by the pumping force of an oil pump or the like provided at the lower end portion of the crankshaft, and ejected from the lubricating oil radiation hole of the eccentric shaft formed eccentrically on the upper portion of the crankshaft. Supplying the lubricating oil between the cylinder and the piston,
The piston has an annular groove exposed from the inner circumferential surface of the cylinder when the piston is located near the bottom dead center on the outer circumferential surface of the piston, and is connected to the annular groove to lubricate the sliding surface by the reciprocating motion of the piston. A refrigerator characterized in that it has fine grooves for generating dynamic pressure.