Frequency/pressure sensitive shock absorber

1. A frequency/pressure sensitive shock absorber for generating a damping force varying according to a frequency and a pressure, comprising:

a cylinder filled with a working fluid;

a piston rod having one end located inside the cylinder and the other end extending outward from the cylinder;

a main piston valve assembly installed at one end of the piston rod and configured to operate in a state that the inside of the cylinder is divided into an upper chamber and a lower chamber, and generate a damping force varying according to a moving speed; and

a sensitive unit installed at one end of the piston rod under the main piston valve assembly and configured to generate a damping force varying according to the frequency and the pressure,

wherein the sensitive unit comprises:

a hollow housing fixed to one end of the piston rod;

an inner tube installed within the hollow housing;

a free piston disposed to be vertically movable within the inner tube; and

an auxiliary valve assembly mounted at a lower end of the housing and configured to generate a damping force by a flow of the working fluid between an inner space of the housing and the lower chamber,

wherein the inner space of the housing is divided into an upper space and a lower space by the free piston,

wherein the working fluid of the lower chamber may pressurize the free piston after passing through the auxiliary valve assembly when the shock absorber is compressed,

wherein an upper concave portion and a lower concave portion are formed on an inner surface of the inner tube,

wherein when an amplitude of the piston rod is small and a frequency thereof is high, the free piston is disposed between the upper concave portion and the lower concave portion to prevent the working fluid from flowing between the upper space and the lower space, and

wherein when the amplitude of the piston rod is large and the frequency thereof is low, the free piston vertically moves toward the upper concave portion or the lower concave portion to allow the working fluid to flow between the upper space and the lower space.

2. The frequency/pressure sensitive shock absorber according to claim 1 , wherein the free piston moves vertically within the inner space of the housing according to the frequency and the pressure, and is supported by an upper elastic member and a lower elastic member, such that the free piston is returned to an initial position.

3. The frequency/pressure sensitive shock absorber according to claim 1 , wherein the inner space of the housing communicates with the upper chamber through a connection passage formed inside the piston rod.

4. The frequency/pressure sensitive shock absorber according to claim 1 , wherein the lower concave portion is aligned in a straight line and spaced-apart from the upper concave portion.

5. The frequency/pressure sensitive shock absorber according to claim 1 , wherein a plurality of upper concave portions and a plurality of lower concave portions are formed on the inner surface of the inner tube.