1. Field of the Invention
The present invention relates to a pressure regulating valve, and, more particularly, to a pressure regulating valve in a hydraulic control system for an automatic transmission which raises output pressure responsive to output pressure leakage.
2. Related Art
In a prior art pressure regulating valve for an automatic transmission hydraulic control system, as shown in FIG. 20, a spool S is controlled to control an output pressure by taking a balance among a spring force of spring means B, a feedback force resulting from an output pressure, and an electromagnetic attraction established in response to an electric signal input to an electromagnetic unit R. The controlled output pressure is fed to a change-over valve and a regulator valve to control the application/release of a clutch or a brake.
The leakage of oil from the clearances between the spools and valve bodies of a change-over valve and a regulator valve, for example, is increased as a result of thermal expansion and other changes in the pressure medium (or oil). As a result, the output pressure of the linear solenoid is lowered, and shift shock is created at the time of applying/releasing the clutch or brake.
As a method of raising the output pressure of the regulator valve, therefore, it is conceivable to increase the valve diameter and accordingly the displacement. This concept raises a problem in that the space for mounting the regulator valve has to be enlarged. Another approach would be to reduce the size of, or omit, each valve to which the output pressure is applied from the regulator valve, or to lower the control pressure for the clutch or brake. However, either concept is technically or practically difficult to execute.
There is, therefore, a need in the art for a compact, simple and cost-effective design for a pressure regulating valve for automatic transmission hydraulic control systems which automatically increases output pressure in response to leakage.