The present invention relates to a damping force control type hydraulic shock absorber attached to a suspension system of a vehicle, for example, an automobile.
Hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles include damping force control type hydraulic shock absorbers that are designed so that the level of damping force can be properly controlled in accordance with road surface conditions, vehicle running conditions, etc. with a view to improving ride quality and steering stability.
In general, a damping force control type hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein. A piston is slidably fitted in the cylinder to divide the inside of the cylinder into two chambers. The piston is connected with a piston rod to form a piston assembly. The piston assembly is provided with a main hydraulic fluid passage and a bypass passage, which provide communication between the two chambers in the cylinder. The main hydraulic fluid passage is provided with a damping force generating mechanism including an orifice and a disk valve. The bypass passage is provided with a damping force control valve for controlling the passage area of the bypass passage.
When the bypass passage is opened through the damping force control valve, the flow resistance to the hydraulic fluid flowing between the two chambers in the cylinder is reduced, thereby reducing damping force. When the bypass passage is closed, the flow resistance between the two chambers is increased, thereby increasing damping force. Thus, damping force characteristics can be appropriately controlled by opening and closing the damping force control valve.
In the above-described damping force control type hydraulic shock absorber, in which damping force is controlled by varying the passage area of the bypass passage, the damping force characteristics can be changed to a considerable extent in a low piston speed region because damping force depends on the restricting action of the orifice in the hydraulic fluid passage. However, the damping force characteristics cannot greatly be changed in intermediate and high piston speed regions because in these regions damping force depends on-the degree of opening of the damping force generating mechanism (disk valve, etc.) in the main hydraulic fluid passage.
To solve the above-described problem, U.S. Pat. No. 5,655,633, for example, discloses a damping force control type hydraulic shock absorber in which a pressure chamber (pilot chamber) is formed at the back of a disk valve serving as a damping force generating mechanism in a main hydraulic fluid passage common to the extension and compression sides. The pressure chamber is communicated with a cylinder chamber on the upstream side of the disk valve through a fixed orifice and also communicated with a cylinder chamber on the downstream side of the disk valve through a variable orifice (flow control valve).
According to the above damping force control type hydraulic shock absorber, the area of the communicating passage between the two chambers in the cylinder is controlled by opening and closing the variable orifice.
Moreover, the valve opening initial pressure of the disk valve can be varied by changing the pressure in the pressure chamber by the pressure loss in the variable orifice. Thus, it is possible to control orifice characteristics (in which damping force is approximately proportional to the square of the piston speed) and valve characteristics (in which damping force is approximately proportional to the piston speed) and hence possible to widen the control range for damping force characteristics.
However, the damping force control type hydraulic shock absorber having a pilot chamber, disclosed in above-mentioned U.S. Patent, suffers from the following problem.
When the direction of stroke of the piston rod changes from the extension side to the compression side or vice versa, the pressure of hydraulic fluid in the cylinder changes sharply, and the change in the pressure is transmitted to the pilot chamber-through the hydraulic fluid passage, causing the valve opening pressure of the disk valve to change. Therefore, it becomes difficult to obtain stable damping force.
The present invention was made in view of the above-described circumstances.
An object of the present invention is to provide a damping force control type hydraulic shock absorber capable of suppressing the change in the pressure in the pilot chamber when the piston rod stroke direction changes and thus providing stable damping force.
The present invention provides a damping force control type hydraulic shock absorber including a cylinder having a hydraulic fluid sealed therein. A piston is slidably fitted in the cylinder. A piston rod is connected at one end thereof to the piston. The other end of the piston rod extends to the outside of the cylinder. A main passage and a sub-passage are communicably connected to the cylinder to allow the hydraulic fluid to flow therethrough in response to the sliding movement of the piston. A pilot-operated damping valve generates damping force by controlling the flow of the hydraulic fluid in the main passage. A fixed orifice is provided at the upstream side of the sub-passage. A regulating valve is provided at the downstream side of the sub-passage. A pilot passage allows a portion of the sub-passage between the fixed orifice and the regulating valve to communicate with a pilot chamber of the pilot-operated damping valve. A restrictor is provided in the pilot passage.
With this arrangement, damping force is controlled by controlling the flow of the hydraulic fluid in the sub-passage with the regulating valve, and the valve opening pressure of the pilot-operated damping valve is controlled by varying the pilot pressure transmitted to the pilot chamber through the pilot passage. At this time, a change in pressure of the hydraulic fluid transmitted to the pilot chamber through the pilot passage is attenuated by the restrictor.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.