VALVE STRUCTURE OF SHOCK ABSORBER

1 . A valve structure of a shock absorber which has a cylinder filled with a working fluid and a piston rod provided with one end located within the cylinder and the other end extending to the outside of the cylinder, the valve structure comprising:

a main piston valve assembly installed at the end of the piston rod and operated to generate damping force varied according to moving velocity under the condition that the inside of the cylinder is divided into an upper chamber and a lower chamber; and

a sub piston valve assembly moving together with the main piston valve assembly to generate damping force varied according to frequency,

wherein the sub piston valve assembly includes:

a free piston, the moving distance of which is varied according to frequency, a hollow housing fixing a sub piston body to the lower portion of the main piston valve assembly, and a connection passage formed within the piston rod so as to communicate an inner space of the housing with the upper chamber;

the free piston is supported by an upper elastic unit and a lower elastic unit so as to be movable vertically according to frequency within the inner space of the housing; and

at least one slit is formed on the inner surface of the housing.

2 . The valve structure according to claim 1 , wherein:

the free piston is maintained at a height where the slit is formed, in a state in which external force is not applied to the free piston; and

the length of the slit is greater than the thickness of a portion of the free piston contacting the inner surface of the housing so as to facilitate free flow of the working fluid through the slit.

3 . The valve structure according to claim 1 , wherein:

the upper elastic unit and the lower elastic unit are one selected from the group consisting of a spring, a disc and a clip, which support the free piston by elasticity; and

if the upper and lower elastic units are upper and lower springs, the moduli of elasticity of the upper and lower springs are different.

4 . The valve structure according to claim 1 , wherein the main piston valve assembly includes a main piston body provided with at least one main compression passage through which the working fluid passes when the shock absorber is compressed and at least one main rebound passage through which the working fluid passes when the shock absorber is expanded, a main compression valve unit arranged on the main piston body and generating damping force against pressure of the working fluid having passed through the at least one main compression passage, and a main rebound valve unit arranged under the main piston body and generating damping force against pressure of the working fluid having passed through the at least one main rebound passage.

5 . The valve structure according to claim 1 , wherein the sub piston valve assembly further includes a sub piston body provided with at least one sub compression passage through which the working fluid passes when the shock absorber is compressed and at least one sub rebound passage through which the working fluid passes when the shock absorber is expanded, a sub compression valve unit arranged on the sub piston body and generating damping force against pressure of the working fluid having passed through the at least one sub compression passage, and a sub rebound valve unit arranged under the sub piston body and generating damping force against pressure of the working fluid having passed through the sub rebound passage.