WO2007046593A1

WO2007046593A1 - Linear compressor

Info

Publication number: WO2007046593A1
Application number: PCT/KR2006/004056
Authority: WO
Inventors: Jong Koo Lee
Original assignee: Lg Electronics Inc.
Priority date: 2005-10-17
Filing date: 2006-10-09
Publication date: 2007-04-26

Abstract

Disclosed related to a linear compressor compressing working fluid such as refrigerant and etc in a cylinder as a piston is reciprocated in the cylinder with the reciprocating force of a linear motor, and the linear compressor has some advantage in that the entire size is minimized with the reduction of the volume of the back cover and the planning limitation in size and etc of the elastic member is reduced as comprising a piston coupled with the cylinder to be reciprocated, an elastic member applying the elastic force to the piston, and a back cover arranged on the outer side of the elastic member and having a deleting unit on the part corresponded to the elastic member.

Description

LINEAR COMPRESSOR

Technical Field

[1] The present invention relates to a linear compressor and, more particularly, to a linear compressor used for refrigerators as an apparatus compressing a working fluid in a cylinder such as refrigerant and etc with a piston reciprocated in the cylinder due to the reciprocating driving force of a linear motor.

Background Art [2] FIG. 1 is a cross-sectional view illustrating a linear compressor according to the conventional art. [3] The linear compressor illustrated in FIG. 1 according to the conventional art has a shell 2 which is the external appearance having a cylinder block 4 and a back cover 6 in the inside, and has a compression unit between the cylinder block 4 and the back cover 6 to compress the working fluid with a predetermined pressure ratio. [4] The shell 2 has a fluid suction pipe 8 inhaling the working fluid to be compressed into the compression unit and a fluid discharge pipe 9 discharging the working fluid compressed in the compression unit to out side of the shell 2. [5] The compression unit includes a cylinder 10 inhaling the working fluid through the fluid suction pipe 8, a piston 20 compressing the working fluid inhaled into the cylinder 10 as reciprocated in the cylinder 10, and a linear motor 30 reciprocating the piston 20. [6] The cylinder 10 has a discharge valve assembly 12 discharging the working fluid compressed in the cylinder 10 into the fluid discharge pipe 9. [7] The piston 20 has a suction passage 21 inhaling the working fluid passed through the fluid suction pipe 8 into the cylinder 10, and has a suction valve 22 opening and shutting the suction passage 21. [8] The linear motor 30 is broadly composed of a stator 32 and a rotor 34 connected with the piston 20 and reciprocated with the electromagnetic interaction with the stator

32. [9] On the other hand, the compression unit has an elastic member 40 giving elastic force to the piston 20 in the reciprocating direction of the piston 20 to advance and reverse the piston 20 during the reciprocating movement. [10] The elastic member 40 may be composed of a first piston spring 42 positioned between the back cover 6 and the piston 20, and of a second piston spring 44 supported by the cylinder block 4 and the piston 20 as positioned between the cylinder 10 and a linear motor 30. [11] Reference will now be made in detail as for the operation of the linear compressor according to the conventional art configured as above.

[12] When the linear motor 30 is operated, the piston 20 is reciprocated in the cylinder

10 with the driving force of the linear motor 30. Further, the piston 20 is advanced and reversed as with the repetitive compression and release of the first and second piston springs 42 and 44 interlocked with the reciprocating movement of the piston 20, and the discharge valve assembly 12 and the suction valve 22 achieve the opening and shutting operation repetitively.

[13] Then, the working fluid is inhaled into the cylinder 10 through the fluid suction pipe 8, the working fluid inhaled into the cylinder 10 is inhaled by the piston 20 with a high pressure, and the working fluid compressed in the cylinder 10 is discharged to outside of the shell 2 through the discharge valve assembly 12 and the fluid discharge pipe 9.

[14] The processes of inhaling, compressing, and discharging the working fluid as above is repeated in order while the linear motor 30 is operated.

[15] However, the above-mentioned linear compressor according to the conventional art has a disadvantage in that the volume and size of a space not used is relatively enlarged as a prescribed gap between the first piston spring 42 and the back cover 6 is needed to prevent the interference of the first piston spring 42 with the back cover 6 as well as needing a prescribed gap among the second piston spring 44, the cylinder 10 and the linear motor 30 to prevent the interference of the second piston spring 42 with the cylinder 10 and the linear motor 30.

[16] Furthermore, the linear compressor has a disadvantage in that the capacity of the linear compressor is hardly maximized as the first and second piston springs 42 and 44 are not planned in the most suitable way due to the relative limitation in the planning of the size and etc of the first and second piston springs 42 and 44 when the size of the linear compressor is limited. Disclosure of Invention Technical Problem

[17] The present invention is contrived to overcome the above-mentioned problems of the conventional art, and an object of the invention is to provide a linear compressor that the capacity is maximized as possible to minimize the planning limitation of the elastic member, the volume and the size as composing an elastic member in the back cover, and a deleting unit in the back cover to prevent the interference of the elastic member with the back cover.

[18] Further, another object of the present invention is to provide a linear compressor minimizing the entire size and the planning limitation of the elastic member as improving the elastic member in an organic way, such as positioning the elastic member only between the back cover and the piston. Technical Solution

[19] The present invention to overcome the above-mentioned technical problems provides a linear compressor comprising a piston coupled with the cylinder to be reciprocated; an elastic member giving the piston an elastic force; and a back cover arranged on the outer side of the elastic member, and having a deleting unit on the part corresponded to the elastic member.

[20] The elastic member may be a piston spring capable of changed elastically toward the reciprocating direction of the piston.

[21] The back cover includes a base supporting the elastic member as adjacent to the piston, and at least one of the side walls extended toward the piston from the base. Further, and a deleting unit is arranged on the at least one of the side walls.

[22] The side wall may be arranged as a pair corresponded to each other with the base as the central.

[23] The side wall is arranged as a pair corresponded to each other with the base as the central, and the deleting unit may be arranged on each of the side walls.

[24] The linear compressor comprises a supporter supporting the elastic member as coupled with the piston, the side wall is arranged as a pair corresponded to each other with the base as the central, and the distance between the pair of side walls may be longer or shorter than the maximum length of the supporter in the same direction to the adjacent direction of the pair of side walls.

[25] The deleting unit of the back cover is approximately same to the length of the side wall in the extended direction of the side wall from the base.

[26] The deleting unit may be a slit into which a part of the elastic member is inserted.

[27] The back cover may have a direction decision protrusion to support the elastic member.

[28] Furthermore, the present invention to overcome the above-mentioned technical problems comprises a shell; a cylinder block arranged on a side in the side; a cylinder supported by the cylinder block; a piston coupled with the cylinder to be capable of reciprocated; a supporter coupled with the piston; an elastic member giving the piston an elastic force as supported by the supporter; and a back cover arranged on the outer side of the elastic member as corresponded to the cylinder block with the piston as the central and having a deleting unit on a part corresponded to the elastic member.

Advantageous Effects

[29] The linear compressor has some advantages in that the back cover supports the elastic member giving the piston the elastic force during reciprocating of the piston, the entire size is minimized with the reduction of the volume of the back cover, and the planning limitation such as for the size of the elastic member is reduced as the deleting unit is arranged on the back cover to prevent the mutual interference of the back cover and the elastic member.

Brief Description of the Drawings [30] FIG. 1 is a cross-sectional view illustrating a linear compressor according to the conventional art, [31] FIG. 2 is a cross-sectional view illustrating the state that the piston of the linear compressor according to the present invention is advanced, [32] FIG. 3 is a cross-sectional view illustrating the state that the piston of the linear compressor according to the present invention is reversed, [33] FIG. 4 is a view illustrating a part of a section in another direction of the linear compressor according to the present invention, [34] FIG. 5 is a perspective view illustrating the elastic member and the back cover in the linear compressor according to the present invention, [35] FIG. 6 is a plane view illustrating the elastic member and the back cover in the linear compressor according to the present invention, [36] FIG. 7 is a view comparing the elastic member and the back cover in the linear compressor according to the present invention with the case that the back cover in the linear compressor according to the present invention not having the deleting unit is applied. [37] [38] <REFERENCE NUMERALS FOR THE PRINCIPLE PARTS OF THE

DRAWINGS>

[39] 50: SHELL 52: FLUID SUCTION PIPE

[40] 54: FLUID DISCHARGE PIPE 60: CYLINDER BLOCK

[41] 62: BACK COVER 66: DELETING UNIT

[42] 70: CYLINDER 75: DISCHARGE VALVE

[43] 80: PISTON 84: SUCTION VALVE

[44] 90: LINEAR MOTOR 92: MAGNET

[45] 94: MAGNET FRAME 95: OUTER STATOR

[46] 96: COIL 98: INNER STATOR

[47] 100: OIL SUPPLY DEVICE

[48] 102:LUBRICATING OIL SUCTION PATH

[49] 104: LUBRICATING OIL DISCHARGE PATH

[50] 106: LUBRICATING OIL PUMP

[51] 110: MUFFLER 120: ELASTIC MEMBER [52] 122: SUPPORTER 124: FIRST PISTON SPRING

[53] 126: SECOND PISTON SPRING

Best Mode for Carrying Out the Invention

[54] FIG. 2 is a cross-sectional view illustrating the state that the piston of the linear compressor according to the present invention is advanced, and FIG. 3 is a cross- sectional view illustrating the state that the piston of the linear compressor according to the present invention is reversed.

[55] The linear compressor according to the present invention comprises the shell 50 inhaling and discharging the working fluid and filled with the lubricating oil, the cylinder block 60 and the back cover 62 arranged in the shell 50, the compression unit discharging the working fluid inhaled into the shell 50 after compressing with a prescribed pressure ration as arranged between the cylinder block 60 and the back cover 62, and the damper spring 68 supporting the compression unit elastically.

[56] A fluid suction pipe 52 is arranged at the shell 50 to inhale the working fluid from the outside of the shell 50 into the shell 50. Further, A fluid discharge pipe 54 is arranged at the shell to lead the working fluid discharged after compressed in the compression unit to outside of the shell 50 after.

[57] The back cover 62 is relatively closer to the fluid suction pipe 52 than to the cylinder block 60.

[58] The compression unit includes a cylinder 70 fixed on the cylinder block 60 and having a predetermined space inhaling the working fluid, a piston 80 compressing the working fluid inhaled into the cylinder 70 with a predetermined pressure ratio as reciprocated in the cylinder 70, and a linear motor 90 reciprocating the piston 80 as connected with the piston 80 to be interlocked.

[59] The cylinder 70 is formed as a cylindrical shape that the both ends are opened to insert the piston 80 through the inlet and to discharge the working fluid compressed in the cylinder 70 through the outlet.

[60] A discharge assembly 75 is arranged on the outlet of the cylinder to discharge the working fluid compressed in the cylinder 70 through the fluid discharge pipe 54.

[61] The discharge valve assembly 75 includes a discharge plenum 76 having a discharge plenum outlet 76a communicated with the outlet of the cylinder 70 and discharging the working fluid as arranged on the outlet of the cylinder 70, a discharge cover 77 having a discharge cover outlet 77a connected with the of the fluid discharge pipe 54 and discharging the working fluid as arranged on the outer side of the discharge plenum 76a, and a discharge pipe 78 opening and shutting the outlet of the cylinder 70 as arranged in the discharge plenum 76.

[62] The discharge valve 78 is composed of a discharge valve body 78a arranged on the outlet of the cylinder 70 to be advanced and reversed in the in and out direction of the cylinder 70, and a discharge valve spring 78b supporting the discharge valve body 78a elastically as positioned on the opposite side of the cylinder 70 with the discharge valve body 78a as the central.

[63] The piston 80 has a fluid passage 81 in the inside to inhale the working fluid inhaled through the fluid suction pipe 52.

[64] The piston 80 has a fluid intake 82 on the discharged side of the working fluid to inhale the working fluid in the fluid passage 81 of the piston 80 into the cylinder 70.

[65] The fluid intake 82 of the piston 80 has a suction valve 84 opening and shutting the fluid intake 82 of the piston 80 as interlocked with the reciprocating movement of the piston 80.

[66] The linear motor 90 is broadly composed of a rotor connected with the piston 80 to be interlocked, and a stator interacted with the rotor electromagnetically to reciprocate the rotor.

[67] The rotor is composed of a magnet 94 arranged on the outer side in the radius direction of the cylinder 70 and reciprocated in the stator, and a magnet frame 92 on which the magnet 94 is fixed and transferring the reciprocating force of the linear motor 90 to the piston 80 as connected with the piston 80 to be interlocked.

[68] The stator is composed of a ring-shaped outer stator 95 arranged on the outer side in the radius direction of the cylinder 70 and fixed between the cylinder block 60 and the back cover 62, a coil 96 forming the magnetic field as arranged in the outer stator 95, and a ring-shaped inner stator 98 arranged in the outer stator 95 to have a predetermined gap and pressurized on the outer side of the cylinder 70.

[69] The rotor is arranged on the gap between the outer stator 95 and the inner stator 98.

[70] A side of the linear motor 90 is supported with the cylinder block 60, and a stator cover 99 is arranged on the other side. The stator cover 99 has the main spring location deciding protrusions 99a and 99b to decide the location of the first and second piston springs 124 and 126 described on the following.

[71] An oil supply device 100 is arranged between the cylinder block 60 and the stator cover 99 to supply the lubricating oil in the shell 50 to the cylinder 70 and the piston 80 for lubricating and refrigerating of the cylinder 70 and the piston 80.

[72] The oil supply device 100 is composed of a lubricating oil suction path 102 leading the working fluid in the shell 50 to be inhaled between the cylinder 70 and the piston 80, a lubricating oil discharge path 104 leading the lubricating oil inhaled between the cylinder 70 and the piston 80 to be discharged into the shell 50, and a lubricating oil pump 106 pumping the lubricating oil in the shell 50 to the lubricating oil suction path 102 as arranged on the lubricating oil suction path 102.

[73] Further, a muffler 110 is arranged on the rear of the piston 80 to reduce the noise of the working fluid inhaled into the shell 50 as penetrated on the back cover 62 and as installed to connect the fluid suction pipe 52 and the fluid passage 81 of the piston 80.

[74] A part of the muffler 110 is fixed on the piston 80 to move with the piston 80 in a single organization, and the other side is fixed on the shell 50.

[75] Furthermore, a piston spring assembly 120 is arranged on the rear of the piston 80 to give the piston 80 the elastic force in the reciprocating direction of the piston 80 to accelerate the reciprocating movement while the piston 80 is reciprocated.

[76] Reference will now be further made in detail as for the piston spring assembly 120 and the back cover 62 with reference to FIGS. 2 to 6. Reference will be made in detail as for the back cover 62 first, and then, the piston spring assembly 120 will be described in detail for the convenience of describing.

[77] FIG. 4 is a view illustrating a part of a section in another direction of the linear compressor according to the present invention, FIG. 5 is a perspective view illustrating the elastic member and the back cover in the linear compressor according to the present invention, and FIG. 6 is a plane view illustrating the elastic member and the back cover in the linear compressor according to the present invention.

[78] The back cover 62 is composed of a base 63 supporting the piston spring assembly

120 as arranged on the rear of the piston 80, and a pair of side walls 64 and 65 extended toward the piston 80 from the base 63 and covering at least a part of the piston spring assembly 120 as corresponded to each other with the base 63 as the centeral.

[79] The base 63 is formed as a rectangular form that the adjacent direction of a pair of side walls 64 and 65 of the back cover 62 is longer.

[80] The base 63 has the location deciding protrusions 63b and 63c protruded on the edges of a pair of side walls toward the piston spring assembly 120 to decide the location of the first piston spring 124 described on the following.

[81] The free ends of the pair of side walls 64 and 65 which are the opposite side of the base 63 have flange units 64a and 65a bended perpendicularly toward the outer side of the back cover 62.

[82] Each of the flange units 64a and 65a of the pair of side walls 64 and 65 is coupled with the stator cover 99 with bolts and etc.

[83] The distance Ll between the pair of side walls 64 and 65 of the back cover 62 is planned to be the same to or shorter than the maximum length L2 of the supporter of the same direction to the adjacent direction of the pair of side walls 64 and 65 to minimize the size of the linear compressor and the planning limitation of the piston spring assembly 120.

[84] The back cover 62 has a deleting unit 66 arranged at least one of the pair of side walls 64 and 65 to prevent the interference with the piston spring assembly 120. [85] The deleting unit 66 is embodied as a slit in which a part of the piston spring assembly 120 faced with the pair of side walls 64 and 65 is inserted to be capable of moving.

[86] The height of the deleting unit 66 in the direction of the pair of side walls 64 and 65 of the back cover 62 may be the same to or taller than the maximum extended length of the piston spring assembly 120 during the reciprocating movement of the piston 80 to prevent the interference with the spring assembly 120.

[87] Therefore, the height of the deleting unit 66 may be the same to the height of the pair of the side walls 64 and 65 in the height direction of the pair of the side walls 64 and 65 of the back cover 62.

[88] The deleting unit 66 may be arranged on each of the pair of the side walls 64 and 65 for balancing the back cover 62.

[89] On the next place, the piston spring assembly 120 may includes a supporter 122 coupled with the piston 80, and an elastic member composed of a first piston spring 124 positioned between the supporter 122 and the back cover 62 and the second piston spring 126 positioned between the supporter 122 and the stator cover 99 of the linear motor 90.

[90] The supporter 122 is coupled with the piston 80 and is composed of a supporter base 122a having a center hole 122a through which the muffler 110 is penetrated, a first spring arm 122b supporting the first piston spring 124 as arranged on the edge of the supporter base 122a, and a second spring arm 122c supporting the second piston spring 126 as arranged on the edge of the base 122a. Each of the first and second piston springs 124 and 126 may be composed as plural such two or four of them, and is described as limited to be composed as a pair on the following for the convenience of describing.

[91] The first spring arm 122b of the supporter is arranged on the supporter base 122a with same intervals as composed as the number of the piston springs 124.

[92] That is, the first location deciding protrusions 122d and 122d' to decide the location of the first piston spring 124 are arranged on the part corresponded with the location deciding protrusion of the back cover 62 are arranged on each of the first spring arm 122b of the support 122 for arranging the first piston spring side by side between the first spring arm 122b and the back cover 62 of the support 122 in the reciprocating direction of the piston 80.

[93] A part of the first spring arm 122b of the supporter 122 is inserted into the deleting unit 66 of the back cover with a part of the first piston spring 124.

[94] The second spring arm 122c of the supporter 122 is also composed as two as the number of the piston springs 124, and they are arranged on the supporter base unit 122a with intervals of 180 degrees to each other. [95] At this time, it is desirable for the first spring arm 122b of the supporter 122 and the second spring arm 122c of the supporter 122 to be arranged on the supporter base 122a with the same intervals from each other, so they are arranged as maintaining 90 degrees from each other.

[96] Each of the second spring arm 122c of the supporter 122 has the second location deciding protrusion 122e and 122e' on the part corresponded to the location deciding protrusion 99a and 99b of the supporter 99 to decide the location of the second piston spring 126 for arranging the second piston spring 126 side by side between the second spring arm 122c of the supporter 122 of the stator cover 99 in the reciprocating direction of the piston 80.

[97] The supporter is arranged as the center is accorded with the center of the center hole

63a of the base 63 of the back cover 62 in the reciprocating direction of the piston 80.

[98] Among the back cover 62, piston spring assembly 120, and the supporter 122 according to the present invention configured as above, the size of the back cover 62 may be reduced relatively or the size of the piston spring assembly 120 may be enlarged relatively as illustrated in FIG. 7.

[99] That is, the A of FIG. 7 is a plane view illustrating the state that the back cover 62 having the deleting unit 66 according to the present invention is coupled with the piston spring assembly 120, and the B of FIG. 7 is a plane view illustrating the state that the back cover 62' not having the deleting unit is coupled with the piston spring assembly 120' same to the present invention, and they show that the back cover 62 having the deleting unit 66 according to the present invention may be formed as smaller than the back cover 62' not having the deleting unit when the piston spring assembly 120 and 120' are the same.

[100] Of course, when the size of the back cover 62 having the deleting unit 66 according to the present invention is formed as same to the size of the back cover 62' not having the deleting unit, the piston spring assembly 120 coupled with the back cover 62 having the deleting unit 66 according to the present invention may be formed relatively bigger than the main spring assembly 120' coupled with the back cover not having the deleting unit 66.

[101] Reference will now be made in detail as for the operation of the linear compressor according to the present invention configured as above.

[102] When the linear compressor 90 is operated, the magnet 94 is reciprocated with the magnet frame 92 due to the electromagnetic interaction between the stator and the rotor, and the reciprocating force of the linear motor 90 is transferred to the piston 80 connected with the magnet frame 92. Then, the piston 80 is reciprocated in the cylinder 70, and the processes of inhaling, compressing, and discharging of the working fluid is repeated as the first and second piston springs 124 and 126 are compressed and released in turns.

[103] That is, when the piston 80 is advanced toward the inside of the cylinder 70 as illustrated in FIG. 2, the suction valve 84 closes the fluid intake 82 of the piston 80 due to the pressure difference between the fluid passage 81 of the piston 80 and the cylinder 70.

[104] Further, the working fluid in the cylinder 70 is compressed by the piston 80 moving toward the inside of the cylinder 70 with a predetermined pressure ratio, and the discharge valve 78 opens the outlet of the cylinder 70 in accordance with the force balancing relation between the pressure in the cylinder 70 and the discharge valve spring 78b. Then, the working fluid compressed in the cylinder 70 is discharged to outside of the shell 50 after passing through the discharge plenum 76, discharge cover 78, and the fluid discharge pipe 54 in order.

[105] At this time, the piston 80 is advanced and reversed with the elastic variation force of the first and second piston spring 124 and 126 as the first piston spring 124 is released and the second piston spring 126 is compressed.

[106] On the other hand, when the piston 80 is reversed toward the outside of the cylinder

70 as illustrated in FIG. 3, the suction valve 84 opens the fluid intake 82 with the pressure difference between the fluid passage 81 of the piston 80 and the cylinder 70. Therefore, the working fluid is inhaled into the cylinder 70 after passing through the fluid suction pipe 52, the fluid suction path 62, the muffler 110, the fluid passage 81, and the fluid intake 82 of the piston 80 in order.

[107] At this time, the discharge valve 78 closes the outlet of the cylinder 70 in accordance with the force balancing relation between the inner pressure of the cylinder 70 and the discharge valve spring 78b.

[108] Further, the piston 80 is advanced and reversed, with the elastic variation force of the first and second piston springs 124 and 126 as the first piston spring 124 is compressed and the second piston spring 126 is released. Industrial Applicability

[109] The linear compressor according to the present invention configured as above has some advantage in that the entire size is minimized as the volume of the back cover is reduced or the planning limitation of the size and etc of the elastic member is reduced as the back cover supports the elastic member applying the elastic force to advance and reverse the piston during the reciprocating movement of the piston, but as preventing the interference between the back cover and the elastic member to each other.

[110] Further, the present invention has some advantages in that the entire size is minimized as the volume between the cylinder and the linear motor is reduced and the planning limitation of the size of the elastic member and etc as composing all of the first and second piston springs of the elastic member between the piston and the back cover. Furthermore, the present invention has an advantage in that the costs and weight are reduced as the size of the back cover and the linear motor is reduced with the reduction of the volume of the back cover, and between the cylinder and the linear motor.

Claims

[ 1 ] A linear compressor comprising: a piston coupled with a cylinder to be reciprocated; an elastic member applying elastic force to the piston; and a back cover arranged on the outer side of the elastic member, and has a deleting unit on the part corresponded to the elastic member.

[2] The linear compressor according to claim 1, wherein the elastic member is a piston spring which is may be elastically deformed in the reciprocating direction of the piston.

[3] The linear compressor according to claim 1, wherein the back cover includes a base supporting the elastic member as spaced apart from the piston and at least one of side walls extended toward the piston from the base, and the deleting unit is arranged at least one of the side walls.

[4] The linear compressor according to claim 3, wherein the side wall is arranged as a pair opposite to each other about the base.

[5] The linear compressor according to claim 3, wherein the side wall is arranged as a pair opposite to each other about the base, and the deleting units are arranged on each of the pair of side walls.

[6] The linear compressor according to claim 3, wherein the linear compressor comprises a supporter supporting the elastic member as coupled with the piston, the side wall is arranged as a pair opposite to each other about the base, and the distance between a pair of side walls is the same to or shorter than the maximum length of the supporter of the same direction to the spaced apart direction of a pair of side walls.

[7] The linear compressor according to claim 3, wherein the length of the deleting unit of the back cover is approximately the same to the length of the side walls in the extended direction of the side walls from the base.

[8] The linear compressor according to claim 3, wherein the deleting unit is a slit in which a part of the elastic member may be inserted.

[9] The linear compressor according to claim 3, wherein the back cover has location deciding protrusion to support the elastic member.

[10] A linear compressor comprising: a shell; a cylinder block arranged on a side in the shell; a cylinder supported by the cylinder block; a piston coupled with the cylinder to be reciprocated; a supporter coupled with the piston; an elastic member applying elastic force to the piston as supported by the supporter; and a back cover arranged on the outer side of the elastic member as opposite to the cylinder block about the piston and has a deleting unit on a part corresponded to the elastic member.