Hermetic compressor

Abstract

A hermetic compressor stores oil (106), and houses compression mechanism (105) that compresses a refrigerant gas, in housing (101). Piston (140) is formed with at least two grooves (144) that do not include the edges of top surface (142) and skirt surface (143), on outer circumferential surface (145) of the piston, where groove (144) communicates with the space in housing (101), at least near the bottom dead center. Such a makeup allows the sliding part to be supplied with oil through groove (144), increasing efficiency owing to high sealability and a low sliding loss.

Description

TECHNICAL FIELD

The present invention relates to a hermetic compressor used in refrigeration cycles for such as a refrigerator with freezer.

BACKGROUND ART

In recent years, price-reduction of hermetic compressors is remarkable. In addition to implementing cost reduction, reduction of power consumption is further demanded. Reducing the cost involves improving the productivity, and reducing power consumption needs improving the sliding characteristic. To solve these problems, Japanese translation of PCT publication No. 2004-501320 (hereinafter, document 1) discloses a hermetic compressor in which the outer shape of the piston has been improved to reduce a sliding loss between the piston and the cylinder for higher efficiency. Here, improvement of the sliding characteristic mainly means reduction of friction coefficient.

Hereinafter, a description is made for a conventional hermetic compressor mentioned above, referring to drawings.

FIG. 8 is a perspective view of a piston used for the conventional hermetic compressor described in document 1. In FIG. 8, piston 1 includes: sealing surface 3 formed on outer circumferential surface 2 so as to contact the inner circumferential surface of the cylinder; at least two guide surfaces 4 formed on outer circumferential surface 2 so as to contact a part of the inner circumferential surface of the cylinder, and also extending roughly in parallel with the direction of movement of piston 1; and cut portion 5 that does not contact the inner circumferential surface of the cylinder, characterized that the angle 4ab formed with the two lines connecting two boundary edges 4a and 4b of guide surface 4, to cylinder central axis 6 of piston 1, in the radial direction of the piston, is 40° or less, desirably 30° or less.

Next, a description is made for the action of the hermetic compressor mentioned above.

While in operation, piston 1 is moving reciprocally. Near the bottom dead center, a part of the skirt side of piston 1 deviates from the cylinder. Guided by guide surface 4, piston 1 can enter the cylinder smoothly. However, because the above-mentioned conventional piston has cut portion 5, the area on the outer circumferential surface of piston 1, that is closer to the skirt side than sealing surface 3, in other words, the lower side in FIG. 8 is not cylindrical but multilevel. In such a case, a rotative honing stone while processing does not stay stably, and thus a through-feed centerless grinder cannot be used, reducing productivity.

In addition, with the above-mentioned conventional piston, the vertical posture of piston 1 to the cylinder is regulated by the clearance between outer circumferential surface 2 of piston 1 and the inner circumferential surface of the cylinder, only in the short section between the edge of the top surface and the lower edge of sealing surface 3, causing the piston to easily tilt relative to the direction of the cylinder. Consequently, leakage of a large amount of refrigerant decreases the freezing capacity, so does the efficiency.

DISCLOSURE OF INVENTION

In a hermetic compressor according to the present invention, at least two grooves are formed on the outer circumferential surface of the piston, where the grooves do not communicate with the top and skirt surfaces of the piston, and at the same time do communicate with the space in the housing at least near the bottom dead center. This makeup stabilizes a rotative honing stone while processing, owing to the outer circumferential surface, substantially cylindrical, on the top and skirt sides of the piston; causes the piston to be resistant to vertically tilting relative to the direction of the cylinder; and supplies the sliding part with oil through the grooves.

The above-mentioned makeup provides a hermetic compressor with high productivity, freezing capacity, and efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a hermetic compressor according to the first exemplary embodiment of the present invention.

FIG. 2 is a perspective view of a piston used for the hermetic compressor according to the first exemplary embodiment of the present invention.

FIG. 3 is a characteristic diagram showing the freezing capacity and coefficient of performance (C.O.P.) of the hermetic compressor according to the first exemplary embodiment of the present invention.

FIG. 4 is a longitudinal sectional view of a hermetic compressor according to the second exemplary embodiment of the present invention.

FIG. 5 is a perspective view of a piston used for the hermetic compressor according to the second exemplary embodiment of the present invention.

FIG. 6 is a longitudinal sectional view of a hermetic compressor according to the third exemplary embodiment of the present invention.

FIG. 7 is a perspective view of a piston used for the hermetic compressor according to the third exemplary embodiment of the present invention.

FIG. 8 is a perspective view of a piston used for a conventional hermetic compressor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a hermetic compressor with high productivity, and high freezing capacity and efficiency by suppressing leakage of a refrigerant from between the piston and cylinder.

A hermetic compressor according to the present invention can store oil in the housing, and a compression mechanism for compressing a refrigerant gas is housed in the housing. The compression mechanism is provided with: a crankshaft allocated substantially vertically, having a main axis part and a eccentric axis part; a block forming a cylinder; a piston moving reciprocally in the cylinder; a connector for connecting the eccentric axis part with the piston; and an oil supplying structure for supplying the outer circumferential surface of the piston with oil. The piston has an outer circumferential surface formed with grooves that do not communicate with the top and skirt surfaces of the piston, where the grooves communicate with the space in the housing when the piston is positioned near the bottom dead center. Here, the grooves that do not communicate with the top and skirt surfaces of the piston refer to two grooves formed independently each other, on the outer circumferential surface of the piston, but excluding the edges of the top and skirt surfaces. In other words, the grooves are not formed on an area with a predetermined width from the top and skirt surfaces, meaning the outer circumferential shape of the piston is retained. This makeup stabilizes a rotative honing stone while processing, owing to the outer circumferential surface, substantially cylindrical, on the top and skirt sides of the piston. In addition, tilting of the piston in an off-axis direction relative to that of the central axis of the cylinder is regulated on the edges of top surface and on the skirt side, suppressing the tilting. Further, supplying the sliding part with oil through the grooves brings high productivity, freezing capacity, and efficiency.

Additionally, in the hermetic compressor according to the present invention, the area of the grooves is to be half or more of the area of the outer circumferential surface of the piston. This suppresses leakage of a refrigerant gas from between the piston and the cylinder. In addition, a large size of an area that does not contact the cylinder extensively decreases a sliding loss occurring between the piston and the cylinder. In addition, supplying a wide range of the outer surface of the piston with oil through the grooves increases efficiency.

Additionally, in the hermetic compressor according to the present invention, the crankshaft has a secondary axis part which is coaxial with the main axis part, and the eccentric axis is sandwiched by the secondary axis part and the main axis part. The hermetic compressor further has a secondary bearing pivotally supporting the secondary axis part. This makeup prevents the crankshaft from inclining away from the vertical direction, and the piston connected to the crankshaft by means of a connector becomes substantially parallel with the direction of the cylinder axis. The structure prevents the edge of the groove on the skirt side from touching the cylinder edge, when the piston moves from the bottom dead center to the top dead center, stabilizing the behavior of the piston. The structure further suppresses leakage of a refrigerant, increasing freezing capacity and efficiency as well as reducing noise.

Additionally, in the hermetic compressor according to the present invention, for the shape of the grooves in a case where the outer circumferential surface of the piston is developed on a plane, the shape of the groove on the skirt side includes a shape that does not form a parallel line with the skirt surface of the piston. This makeup allows the edge of the groove on the skirt side to act as a vertical guide between the piston and the cylinder, when the piston moves from the bottom dead center to the top dead center, stabilizing the behavior of the piston to reduce noise.

Additionally, the hermetic compressor according to the present invention is inverter-driven with a plurality of operation frequencies including at least a frequency or lower. Applying the frequency suppresses leakage of a refrigerant by storing oil in the grooves, even though less oil is supplied during a slow operation, further increasing the freezing capacity and efficiency.

Additionally, the hermetic compressor according to the present invention uses R600a for its refrigerant. Using the refrigerant causes the piston to be resistant to tilting in the off-axis direction relative to that of the cylinder. As a larger cylinder volume is required for a compressor using R60a, external diameter of the piston becomes larger and leakage of the refrigerant usually tends to becomes larger as compared with the compressor using conventional R134a. As the present invention prevents the piston from tilting against the cylinder, leakage of the refrigerant is suppressed, and the freezing capacity and efficiency increase.

Additionally, in the hermetic compressor according to the present invention, oil with its viscosity grade of VG10 to VG5 can be selected. A conventional compressor using low-viscosity oil leaks a large amount of refrigerant gas, while the makeup according to the present invention suppresses leakage of the refrigerant, further increasing the freezing capacity and efficiency.

Hereinafter, a description is made for some exemplary embodiments according to the present invention, referring to drawings.

FIRST EXEMPLARY EMBODYMENT

FIG. 1 is a longitudinal sectional view of a hermetic compressor according to the first exemplary embodiment. FIG. 2 is a perspective view of a piston used for the hermetic compressor according to the first exemplary embodiment. FIG. 3 is a characteristic diagram showing the freezing capacity and coefficient of performance (C.O.P.). C.O.P. represents efficiency of a compressor, namely the ratio of freezing capacity (W) to input power (W) of the electromotive element of a compressor, where a higher freezing capacity tends to raise C.O.P.

In FIGS. 1 and 2, housing 101 houses electromotive element 104 composed of stator 102 and rotor 103, able to inverter-drive with a plurality of operation frequencies including a power-line frequency or lower, by using such as a control circuit; and compression mechanism 105 driven by electromotive element 104. In addition, housing 101 stores oil 106 therein.

The refrigerant used for this compressor is R600a, which is a natural hydrocarbon refrigerant with a low global warming potential. Oil 106 is a combination of oils from ISO VG10 to ISO VG5, low-viscosity grades, having compatibility one another.

Crankshaft 110, substantially vertically allocated, includes: main axis part 111 with rotor 103 press-fitted; eccentric axis part 112 formed eccentrically from main axis part 111; and secondary axis part 113 provided coaxially with main axis part 111, and eccentric axis part 112 is sandwiched between main axis part 111 and secondary axis part 113.

Oil supplying structure 120 is composed of oil cone 122 fixed to the bottom end of main axis part 111, where one end of the cone is open in oil 106, and the other end communicates with oil supplying duct 121; and oil supplying duct 121 formed inside crankshaft 110, where the duct communicates with oil cone 122, and is open at the top end of eccentric axis part 112.

Block 130 has substantially cylindrical cylinder 131, as well as main bearing 132 pivotally supporting main axis part 111, and secondary bearing 133 pivotally supporting secondary axis part 113. Cylinder 131 has a top provided with dash-board 134 thereon, and is further equipped with notch 135 provided on the top edge on the crankshaft 110 side, and chamfered part 136 provided on the circumferential edge on the crankshaft 110 side.

Piston 140 is inserted to cylinder 131 of block 130 reciprocally slidably, connected to eccentric axis part 112 by means of connector 141.

Piston 140 is further provided with at least two grooves 144 that do not communicate with top surface 142 and skirt surface 143 of piston 140, on outer circumferential surface 145 of the piston. Here top surface 142 and skirt surface 143 refer to surfaces vertical to the direction of the reciprocal movement of the piston, as shown in FIG. 1, and outer circumferential surface 145 refers to a side parallel with the reciprocal movement. Top surface 142 and skirt surface 143 correspond to the surfaces on the top dead center and the bottom dead center of piston 140, respectively. Groove 144 is substantially quadrangle in a state where the outer circumferential surface of the piston is developed on a plane. For example, edge 441 on the skirt side of groove 144 is to be positioned at piston 140, approximately 2 mm away from skirt surface 143 in arrow T direction. Edge 442 on the top side of groove 144 is to be positioned 7 mm to 8 mm away from top surface 142 in arrow S direction. The depth of groove 144 is to be 0.1 mm to 0.5 mm from outer circumferential surface 145 of piston 140.

As a part of groove 144 on the skirt side of piston 140 protrudes from the cylinder 131 to the inside of housing 101, near the bottom dead center, the part of groove 144 is exposed in the space inside housing 101 near at least the bottom dead center. At that position groove 144 communicates to the space inside housing 101. Grooves 144 are made so that their total area exceeds half the area of outer circumferential surface 145 of piston 140.

Hereinafter, a description is made for the operation or action of the hermetic compressor structured as mentioned above.

Rotor 103 of electromotive element 104 rotates crankshaft 110, and transmitting the rotational movement of eccentric axis part 112 to piston 140 through connector 141, causes piston 140 to move reciprocally in cylinder 131. This movement causes the refrigerant gas to be suctioned and compressed from the cooling system (not illustrated) to the inside of cylinder 131, and again discharged to the cooling system.

Meanwhile, oil supplying structure 120 raises oil 106 in oil supplying duct 121 with a centrifugal force generated by the rotation of oil cone 122 involved in the rotation of crankshaft 110, and sprays oil 106 into housing 101 from near the top end of secondary axis part 113. Oil 106 having been sprayed strikes dash-board 134, travels through notch 135, and attaches to outer circumferential surface 145 and chamfered part 136 of the piston. Attached Oil 106 penetrates into outer circumferential surface 145 of the piston and groove 144 according to the reciprocal movement of piston 140, acting as a lubricant between outer circumferential surface 145 of the piston and cylinder 131.

In this case, oil 106 attached to chamfered part 136, and oil 106 penetrated into groove 144 are gathered by means of edge 441 on the skirt side of groove 144, when piston 140 moves from the bottom dead center (S direction end) to the top dead center (T direction end), and stored near the edge of groove 144 of piston 140. When piston 140 moves from the top dead center to the bottom dead center, the movement of piston 140 causes oil 106 stored between cylinder 131 and outer circumferential surface 145 of the piston, to be attracted to a clearance between piston 140 and cylinder 131, and the entire clearance near top surface 142 is filled with oil 106, improving sealability. In addition, a wide range of outer circumferential surface 145 of the piston can be supplied with oil 106 through groove 144, allowing oil 106 to be supplied more effectively.

When piston 140 is positioned near the top dead center, the inside of cylinder 131 is at high pressure due to the compressed refrigerant, which is then discharged. At this moment, the difference between the pressure in housing 101 and that in cylinder 131 becomes the maximum value, and thus a large amount of refrigerant gas leaks from a clearance between cylinder 131 and piston 140. In addition, pressure distribution occurs due to the flow of the refrigerant generated in cylinder 131, and consequently a force arises to force piston 140 tilt against cylinder 131.

However, according to the first exemplary embodiment, piston 140 faces to the inner surface of cylinder 131, owing to the two edges: one near top surface 142 and the other near skirt surface 143, and thus piston 140 can be regulated so as to suppress a tilt. Consequently, the clearance between cylinder 131 and the outer circumferential surface 145 of the piston can be limited, suppressing leakage of the refrigerant from cylinder 131 to the inside of housing 101, to increase the freezing capacity. Meanwhile, groove 144 does not contact cylinder 131, reducing a sliding loss. The synergistic effect allows increasing the efficiency. Better still, the area of grooves 144 is more than half the area of outer circumferential surface 145 of the piston, and thus the area that does not contact cylinder 131 can be enlarged, while suppressing leakage of the refrigerant from the clearance between piston 140 and cylinder 131. This makeup significantly reduces a sliding loss occurring between piston 140 and cylinder 131.

When grinding piston 140 in the manufacturing process, along the area from skirt surface 143 to edge 441 on the skirt side of groove 144, and from edge 442 on the top side of groove 144 to outer circumferential surface 145 of the substantially cylindrical piston on top surface 142, a honing stone is less influenced by groove 144, resulting in a stable process. Consequently, accurate processing and high productivity are achieved even if using through-feed centerless grinder, suitable for mass production.

Without secondary axis part 113, crankshaft 110 tilts when eccentric axis part 112 of crankshaft 110 is applied with a compressive load from piston 140. Consequently, piston 140 vertically tilts, and particularly for piston 140 according to this exemplary embodiment, the edge on the skirt side strikes the edge of cylinder 131, possibly making a noise.

However, in the first exemplary embodiment, crankshaft 110 includes secondary axis part 113 provided coaxially with main axis part 111, where eccentric axis part 112 is sandwiched between main axis part 111 and secondary axis part 113; and secondary bearing 133 pivotally supports secondary axis part 113, and a tilt of crankshaft 110 is suppressed. Therefore, piston 140 connected to crankshaft 110 with connector 141 is retained substantially in parallel with the axial direction of cylinder 131. Consequently, this prevents the edge on the skirt side of groove 144 provided on the skirt surface of piston 140, from striking the edge of cylinder 131 on the crankshaft 110 side, when piston 140 moves from the bottom dead center to the top dead center. This suppresses a striking noise, and also stabilizes the behavior of piston 140, suppressing leakage of the refrigerant, and increasing the freezing capacity and efficiency.

Further, when inverter-driving at a plurality of operation frequencies including at least power-line frequency or lower, the reciprocal movement of piston 140 becomes slow in a slow operation, and the amount of oil 106 is reduced sprayed from near the top end of secondary axis part 113 to the inside of housing 101. This causes a large amount of refrigerant to leak from the clearance between outer circumferential surface 145 of the piston and cylinder 131. Meanwhile, in the first exemplary embodiment, groove 144 can store oil 106 therein, sustaining high freezing capacity and efficiency.

The density of R600a refrigerant is low as compared to R134a refrigerant conventionally used for a refrigerator. A hermetic compressor using R600a refrigerant needs to have a larger cylinder volume and a larger external diameter of piston 140, if the same level of freezing capacity as the compressor using R134a refrigerant is required. Therefore, more refrigerant may leak from cylinder 131 to the inside of housing 101 because of the larger area of the duct in case of using R600a refrigerant. However, a hermetic compressor according to the first exemplary embodiment suppresses a tilt of piston 140 relative to the direction of cylinder 131 and prevents R600a refrigerant from leaking and the efficiency is improved.

Low-viscosity oil with an ISO viscosity grade from VG10 to VG5 can be used for oil 106. Generally, low viscosity oil reduces a sliding loss, while oil 106 becomes discontinuous in the clearance between cylinder 131 and outer circumferential surface 145 of the piston, causing the seal to break and the refrigerant to leak into housing 101. The first exemplary embodiment, however, can store oil 106 in groove 144 and can supply the clearance between cylinder 131 and piston 140 with oil 106, preventing the seal from breaking and the refrigerant from leaking and the efficiency is improved.

Next, a description is made for the result in improving the efficiency of a hermetic compressor according to the first exemplary embodiment.

In FIG. 3, the vertical right axis indicates the freezing capacity of the compressor, and the vertical left axis indicates the coefficient of performance (W/W) characteristic. Both the conventional compressor and the compressor in the first exemplary embodiment use R600a refrigerant. The operation frequency of the reciprocal movement of the piston is 50 Hz. The operating temperature is −25° C. for vaporization temperature, and 55° C. for condensation temperature, which are usual conditions in a refrigerator operation.

FIG. 3 clearly shows that using the compressor in the first exemplary embodiment significantly improves the freezing capacity, C.O.P., and efficiency.

SECOND EXEMPLARY EMBODYMENT

FIG. 4 is a longitudinal sectional view of a hermetic compressor according to the second exemplary embodiment of the present invention. FIG. 5 is a perspective view of a piston used for the hermetic compressor according to the second exemplary embodiment. Here, for an identical component as in the first exemplary embodiment, the identical mark is given to omit its detailed description.

In FIGS. 4 and 5, housing 101 houses compression mechanism 205 driven by electromotive element 104 Piston 240 included in compression mechanism 205 is provided with at least two grooves 244 that do not communicate with top surface 242 and skirt surface 243 of piston 240 on outer circumferential surface 245 of the piston. The grooves 244 do not communicate each other.

Groove 244 communicates with the space inside housing 101 at least near the bottom dead center, and the right side is wider than the left one, viewing piston 240 from its top. As shown in FIG. 5, a line formed by edge 246 on the skirt side of groove 244 is curved or tilted so as not to form a line parallel with a line formed by skirt surface 243 of piston 240, if outer circumferential surface 245 of the piston is developed on a plane. FIG. 5 shows an example where edge 246 on the skirt side is processed into a curved shape having a predetermined curvature at the lower-left corner of the figure. Here, the shape is not limited to a curve with a curvature, but it may be a gradient straight line.

Next, a description is made for the operation and action of the hermetic compressor with the above-mentioned makeup.

Rotor 103 of electromotive element 104 rotates crankshaft 110 clockwise, and the rotational movement of eccentric axis part 112 is transmitted to piston 240 through connector 141, causing piston 240 to move reciprocally in cylinder 131.

Edge 246 on the skirt side of piston 240 is curved so as not to form a line parallel with skirt surface 243 of piston 240. With this makeup, edge 246 on the skirt side of groove 244 provided on the skirt surface of piston 240, acts as a guide part continuously for the edge of cylinder 131 on a side of crankshaft 110, when piston 240 moves from the bottom dead center to the top dead center. The effect prevents edge 246 on the skirt side from hitting the edge of cylinder 131 periodically, the behavior of piston 240 is stabilized, and occurrence of noise is suppressed. In addition to this advantage, the right side of groove 244 is wider, viewing from the top in FIG. 5, and thus a wide range of the area most vulnerable to wear due to a side pressure can be supplied with oil 106 through groove 244, to increase reliability.

Still, in the compressor according to the second exemplary embodiment, crankshaft 110 includes secondary axis part 113 provided coaxially with main axis part 111, eccentric axis part 112 is sandwiched between main axis part 111 and secondary axis part 113, and secondary axis part 113 is pivotally supported by secondary bearing 133. Meanwhile, even if secondary axis part 113 and secondary bearing 133 are not provided, in other words, crankshaft 110 is pivotally supported only by main bearing 132, and even if piston 240 tends to vertically tilt relative to the direction of cylinder 131 due to a tilt from the vertical direction, the makeup as described in the second exemplary embodiment may bring the same actions and effects.

THIRD EXEMPLARY EMBODYMENT

FIG. 6 is a longitudinal sectional view of a hermetic compressor according to the third exemplary embodiment of the present invention. FIG. 7 is a perspective view of a piston used for the hermetic compressor according to the third exemplary embodiment.

Here, for an identical component as in the first exemplary embodiment, the identical mark is given to omit its detailed description.

In FIGS. 6 and 7, housing 101 houses compression mechanism 305 driven by electromotive element 104.

Piston 340 included in compression mechanism 305 is provided with at least two grooves 344 that do not communicate with top surface 342 and skirt surface 343 of piston 240 on outer circumferential surface 345 of the piston.

Groove 344 communicates with the space in housing 101 at least near the bottom dead center. The shape of groove 344 is arcuate and the top side is widened as shown in FIGS. 6 and 7. FIG. 7 shows the shape of edge 346 on the skirt side of groove 344. The shape does not form a line parallel with skirt surface 343, and is arcuate having a predetermined curvature. Here, the shape is not limited to an arc with a curvature, but may be a gradient straight line.

Next, a description is made for the operation and action of the hermetic compressor according to the third exemplary embodiment.

Rotor 103 of electromotive element 104 rotates crankshaft 110, and the rotational movement of eccentric axis part 112 is transmitted to piston 340 through connector 141, causing piston 340 to move reciprocally in cylinder 131.

The shape of edge 346 on the skirt side of piston 340 does not form a line parallel with skirt surface 343 of piston 340. With this makeup, edge 346 on the skirt side of groove 344 provided on the skirt surface of piston 340 continuously acts as a guide part to the edge of cylinder 131 on the crankshaft 110 side, when piston 340 moves from the bottom dead center to the top dead center. The effect prevents edge 346 on the skirt side from periodically hitting the edge of cylinder 131, the behavior of piston 340 is stabilized, and noise caused by the hitting is suppressed. In addition to this advantage, oil 106 gathers to the top of piston 340 along edge 346 on the skirt side of groove 344, further increasing sealability.

Still, in the compressor according to the third exemplary embodiment, crankshaft 110 has secondary axis part 113 provided coaxially with main axis part 111, eccentric axis part 112 is sandwiched between main axis part 111 and secondary axis part 113, and secondary axis part 113 is pivotally supported by secondary bearing 133. Meanwhile, even if secondary axis part 113 and secondary bearing 133 are not provided, in other words, crankshaft 110 is pivotally supported only by main bearing 132, and even if piston 240 tends to vertically tilt relative to the direction of cylinder 131 due to a tilt from the vertical direction, the makeup as described in the third exemplary embodiment may bring the same actions and effects.

It will be obvious to those skilled in the art that various changes may be made in the above-described embodiments of the prsent invention. However, the scope on the present invention should be determined by the following claims.

INDUSTRIAL APPLICABILITY

As described above, a hermetic compressor according to the present invention allows increasing productivity, efficiency, and reliability, and thus can be also used for an air conditioner and combined refrigerator with freezer.

Claims

1. A hermetic compressor comprising: a compression mechanism including: a crankshaft allocated vertically, having a main axis part and an eccentric axis part; a cylinder; a piston that moves reciprocally in the cylinder, and includes a top surface and a skirt surface, both vertical to a direction of the reciprocal movement, and an outer circumferential surface parallel with a direction of the reciprocal movement; a connector that connects the eccentric axis part and the piston; and an oil supplying structure for supplying an outer surface of the piston with oil, wherein the piston has at least two grooves on an outer circumferential surface thereof, the grooves are independent each other and formed on a part of the circumferential surface, excluding an edge of the top surface and an edge of the skirt surface, and the grooves communicate with a space in the housing, when the piston is positioned at a bottom dead center; an electromotive element, and a housing containing the compression mechanism and the electromotive element.

2. A hermetic compressor as claimed in claim 1, wherein the groove can store the oil therein.

3. A hermetic compressor as claimed in claim 1, wherein an area of the grooves is not less than half an area of the outer circumferential surface of the piston.

4. A hermetic compressor as claimed in claim 1, wherein the crankshaft further includes a secondary axis part coaxially with a main axis part, and the eccentric part is sandwiched by the secondary axis part and the main axis part, and the compressor further includes a secondary bearing pivotally supporting the secondary axis part.

5. A hermetic compressor as claimed in claim 1, wherein a shape of an edge on the skirt side of the groove includes a shape that does not form a line parallel with the skirt surface, in a case where the outer circumferential surface of the piston is developed on a plane.

6. A hermetic compressor as claimed in claim 5, wherein the shape of the edge on the skirt side of the groove includes a curve having a predetermined curvature.

7. A hermetic compressor as claimed in claim 1, wherein the electromotive element is inverter-driven at a plurality of operation frequencies including at least power-line frequency or lower.

8. A hermetic compressor as claimed in claim 1, wherein the compression mechanism compresses a refrigerant gas, and the refrigerant gas is R600a.

9. A hermetic compressor as claimed in claim 1, wherein the oil is selected from ISO viscosity grades from VG10 to VG5, both included.