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A piston assembly for a shock absorber includes: a bypass passage formed along a vertical length direction of a piston rod in a small-diameter portion that is stepped in a lower portion of the piston rod reciprocating within a cylinder; a main piston connected to the small-diameter portion; an orifi

A piston assembly for a shock absorber includes: a bypass passage formed along a vertical length direction of a piston rod in a small-diameter portion that is stepped in a lower portion of the piston rod reciprocating within a cylinder; a main piston connected to the small-diameter portion; an orifice assembly coupled to the small-diameter portion and disposed under the main piston to form an orifice passage, which communicates with the bypass passage; a housing coupled to the small-diameter portion and disposed under the orifice assembly, the housing having an opened upper side and forming a pressure chamber; and a sliding valve coupled to the orifice assembly and accommodated in the pressure chamber to generate a damping force by selectively opening and closing the orifice passage while reciprocating through a high-frequency section and a low-frequency section. Accordingly, the piston assembly is capable of improving a ride comfort by exhibiting a damping performance over a low-frequency section and a high-frequency section.

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1. A piston assembly for a shock absorber, comprising: a bypass passage formed along a vertical length direction of a piston rod in a small-diameter portion that is stepped in a lower portion of the piston rod reciprocating within a cylinder;a main piston connected to the small-diameter portion;an o

1. A piston assembly for a shock absorber, comprising: a bypass passage formed along a vertical length direction of a piston rod in a small-diameter portion that is stepped in a lower portion of the piston rod reciprocating within a cylinder;a main piston connected to the small-diameter portion;an orifice assembly coupled to the small-diameter portion and disposed under the main piston to form an orifice passage, which communicates with the bypass passage;a housing coupled to the small-diameter portion and disposed under the orifice assembly, a top of the housing having an opened portion and the housing forming a pressure chamber; anda sliding valve coupled to the orifice assembly and accommodated in the pressure chamber to generate a damping force by selectively opening and closing the orifice passage while reciprocating through a high-frequency section and a low-frequency section,wherein the orifice assembly includes: a cylindrical body which is coupled to the small-diameter portion and through which the sliding valve slidingly reciprocates over the low-frequency section and the high-frequency section;at least one first orifice penetrating through an upper portion of the body such that the bypass passage is coupled to an upper space of the pressure chamber, the pressure chamber being partitioned into the upper space and a lower space by the sliding valve; andat least one second orifice passing through a lower portion of the body and connecting the bypass passage to the lower space of the housing. 2. The piston assembly according to claim 1, wherein the orifice assembly is integrally formed with the housing. 3. The piston assembly according to claim 1, wherein a first plane passes through centers of the at least one first orifice, and a second plane is formed separately from the first plane and passes through centers of the at least one second orifice. 4. The piston assembly according to claim 1, wherein a volume of the lower space is larger than a volume of the upper space. 5. The piston assembly according to claim 1, wherein a diameter of the second orifice is larger than a diameter of the first orifice. 6. The piston assembly according to claim 1, wherein the sliding valve is configured to maintain the first orifice and the second orifice in an opened state in the high-frequency section. 7. The piston assembly according to claim 1, wherein the sliding valve is configured to move upward to close the first orifice in the low-frequency section. 8. The piston assembly according to claim 1, wherein an amount of a working fluid introduced from the bypass passage through the second orifice in the low-frequency section is larger than an amount of a working fluid introduced from the bypass passage through the second orifice in the high-frequency section. 9. The piston assembly according to claim 1, wherein the sliding valve includes: an elevating piece reciprocating along an outer periphery of the orifice assembly;a valve sheet protruding from a top surface of the elevating piece in a ring shape, a valve disk being mounted on the valve sheet; anda guide piece extending along an edge of the elevating piece and facing an inner side of the housing. 10. The piston assembly according to claim 9, wherein the sliding valve further includes at least one sheet slit cut along a forming direction of the valve sheet and allowing the flow of the working fluid that is introduced from the bypass passage through at least one first orifice passing through the upper portion of the orifice assembly, wherein the sliding valve is configured to maintain the first orifice in an opened state in the high-frequency section such that the working fluid is discharged through the sheet slit, andthe sliding valve is configured to maintain the first orifice in a closed state in the low-frequency section such that the working fluid is not discharged through the sheet slit. 11. The piston assembly according to claim 9, further comprising an O-ring disposed between the guide piece and the inner side of the housing and preventing leakage of the working fluid. 12. The piston assembly according to claim 1, wherein the housing includes: a bottom plate coupled to the small-diameter portion to form a bottom surface;a partition wall extending upward from an edge of the bottom plate to form the pressure chamber; andat least one second communication hole passing through the bottom plate and allowing the flow of the working fluid to the outside of the bottom plate in the low-frequency section. 13. The piston assembly of claim 1, further comprising: a elevating piece reciprocating along an outer periphery of the orifice assembly; anda stopper formed at an upper edge of the second orifice passing through a lower portion of the orifice assembly and preventing the elevating piece from moving downward. 14. The piston assembly of claim 1, further comprising a valve disk provided above the orifice assembly and the housing. 15. A piston assembly for a shock absorber, comprising: a bypass passage formed along a vertical length direction of a piston rod in a small-diameter portion that is stepped in a lower portion of the piston rod reciprocating within a cylinder;a main piston connected to the small-diameter portion;an orifice assembly coupled to the small-diameter portion and disposed under the main piston to form an orifice passage, which communicates with the bypass passage;a housing coupled to the small-diameter portion and disposed under the orifice assembly, the housing having an opened upper side and forming a pressure chamber; anda sliding valve coupled to the orifice assembly and accommodated in the pressure chamber to generate a damping force by selectively opening and closing the orifice passage while reciprocating through a high-frequency section and a low-frequency section,wherein the sliding valve includes: an elevating piece reciprocating along an outer periphery of the orifice assembly;a valve sheet protruding from a top surface of the elevating piece in a ring shape, a valve disk being mounted on the valve sheet; anda guide piece extending along an edge of the elevating piece and facing an inner side of the housing, andwherein the piston assembly further comprises a stopper formed at an upper edge of at least one second orifice passing through the lower portion of the orifice assembly and preventing the elevating piece from moving downward. 16. A piston assembly for a shock absorber, comprising: a bypass passage formed along a vertical length direction of a piston rod in a small-diameter portion that is stepped in a lower portion of the piston rod reciprocating within a cylinder;a main piston connected to the small-diameter portion;an orifice assembly coupled to the small-diameter portion and disposed under the main piston to form an orifice passage, which communicates with the bypass passage;a housing coupled to the small-diameter portion and disposed under the orifice assembly, a top of the housing having an opened portion and the housing forming a pressure chamber; anda sliding valve coupled to the orifice assembly and accommodated in the pressure chamber to generate a damping force by selectively opening and closing the orifice passage while reciprocating through a high-frequency section and a low-frequency section,wherein the orifice assembly is detachably connected to the housing.