Power steering apparatus

1. A power steering apparatus comprising:

a housing having a hydraulic chamber;

an input shaft adapted to receive a driver's steering input;

an output shaft connected with the input shaft by a torsion bar;

a piston slidably mounted on the output shaft, and arranged to partition the hydraulic chamber into a first hydraulic chamber and a second hydraulic chamber;

a ball screw mechanism disposed between the output shaft and the piston, and configured to convert a rotary motion of the output shaft into an axial motion of the piston;

an output member to receive the axial motion of the piston and to deliver an output motion for steering;

a control valve configured to supply hydraulic fluid selectively to the first hydraulic chamber and to the second hydraulic chamber, in accordance with an amount of torsion of the torsion bar caused by relative rotation between the input shaft and the output shaft;

a first connecting passage connecting the control valve and the first hydraulic chamber;

a second connecting passage connecting the control valve and the second hydraulic chamber;

a portion defining a first valve receiving hole and provided in the first connecting passage;

a portion defining a second valve receiving hole and provided in the second connecting passage;

a first damper valve provided in the first valve receiving hole, and configured to open the first connecting passage in response to a hydraulic pressure from the control valve when the hydraulic pressure from the control valve exceeds a first valve opening pressure of the first damper valve, and to open the first connecting passage in response to a hydraulic pressure from the first hydraulic chamber when the hydraulic pressure from the first hydraulic chamber exceeds a second valve opening pressure of the first damper valve which is higher than the first valve opening pressure of the first damper valve; and

a second damper valve provided in the second valve receiving hole, and configured to open the second connecting passage in response to a hydraulic pressure from the control valve when the hydraulic pressure from the control valve exceeds a first valve opening pressure of the second damper valve, and to open the second connecting passage in response to a hydraulic pressure from the second hydraulic chamber when the hydraulic pressure from the second hydraulic chamber exceeds a second valve opening pressure of the second damper valve which is higher than the first valve opening pressure of the second damper valve.

2. The power steering apparatus as claimed in claim 1 , wherein the housing is formed with the first connecting passage and the second connecting passage.

3. The power steering apparatus as claimed in claim 1 , wherein the first and second valve receiving holes of the housing extend in an axial direction of the input shaft and open in an end face of the housing, and each of the first and second receiving holes of the housing is closed with a plug member.

4. The power steering apparatus as claimed in claim 1 , wherein the housing includes an inlet port opening radially outwards; and the first and second valve receiving holes of the housing are located between the inlet port and a side face diametrically opposite to the inlet port, and arranged circumferentially at an acute angle.

5. The power steering apparatus as claimed in claim 1 , wherein the first and second receiving holes of the housing are located radially outside the control valve; the first connecting passage includes a first radial passage section connecting the control valve and a side surface of the first valve receiving hole, and a first axial passage section connecting the first hydraulic chamber and a bottom of the first valve receiving hole; and the second connecting passage includes a second radial passage section connecting the control valve and a side surface of the second valve receiving hole, and a second axial passage section connecting the second hydraulic chamber and a bottom of the second valve receiving hole.

6. The power steering apparatus as claimed in claim 5 , wherein the first radial passage section and the first axial passage section of the first connecting passage both extend straight from the first valve receiving hole; and the second radial passage section and the second axial passage section of the second connecting passage both extend straight from the second valve receiving hole.

7. The power steering apparatus as claimed in claim 5 , wherein at least one of the first and second radial passage sections is formed by drilling from an open end of a respective one of the first and second valve receiving holes which is open in an end surface of the housing.

8. The power steering apparatus as claimed in claim 5 , wherein the housing includes an inside cavity which is divided by the piston into the first hydraulic chamber on one side of the piston closer to the input shaft than the second hydraulic chamber, and the second hydraulic chamber on the other side of the piston; and the first radial passage section is formed by drilling from an open end of the first valve receiving hole which is open in an end surface of the housing.

9. The power steering apparatus as claimed in claim 8 , wherein the housing is formed with the bearing hydraulic passage; and the first radial passage section is connected through the bearing hydraulic passage to the control valve.

10. The power steering apparatus as claimed in claim 8 , wherein the input shaft is supported in the housing by a bearing held by an adjusting plug member; and the power steering apparatus further includes an O-ring and a seal ring which are disposed in a sliding portion between the adjusting plug member and the output shaft.

11. The power steering apparatus as claimed in claim 10 , wherein the housing includes a first housing member supporting the input shaft rotatably, and a second housing member joined with the first housing member; and the second housing member includes an inside cavity which is divided by the piston into the first hydraulic chamber between the first housing member and the piston, and the second hydraulic chamber.

12. The power steering apparatus as claimed in claim 11 , wherein the first housing member is formed with the first and second valve receiving holes extending axially, and opening in an outside surface of the first housing member.