Normally closed solenoid valve includes a fixed core, a valve portion including a valve seat and a valve body, a movable core disposed between the fixed core and the valve seat, a resilient biasing member for biasing the movable core in a valve-closing direction and a coil.

When energized, the coil generates an attracting force between the movable core and the fixed core against a biasing force by the resilient biasing member to cause the movable core to retreat from an initial position where the valve seat is in a closed state. When a characteristic graph line of spacing distance/attracting force that defines the attracting force in relation with a spacing distance between the fixed core and the movable core provides an inclination A (N/mm) at the initial position, the compression resiliency coefficient of the resilient biasing member is set to A (N/mm).

Conventionally, there are two types of solenoid valves for opening and closing a flow path of a hydraulic fluid, one of which is a normally closed solenoid valve that is normally closed and is opened by excitation of a fixed core and a movablecore when a current is rendered to flow into a coil, and the other of which is a normally open valve that is normally opened and is closed by excitation of a fixed core and a movable core when a current is rendered to flow into a coil. Such a normallyclosed solenoid valve is provided with a valve seat for opening and forming a flow path of a hydraulic fluid and a valve body formed at the tip end of a movable core capable of being brought into contact with the valve seat and moving away therefrom. Ina demagnetized state, the movable core is seated on the valve body by a spring force of a spring member secured between the movable core and fixed core and is pressed into the direction of closing the valve. In a magnetized state where a current isrendered to flow into a coil, an attracting force works between the fixed core and movable core, and when the attracting force exceeds the spring force of the spring member, the valve body is separated from the valve seat, wherein the flow path of ahydraulic fluid can be opened. Herein, a force in which the attracting force and the spring force working in the direction opposite the attracting force are synthesized or composed is called a "thrust force" working on the movable core.

Such a normally closed solenoid valve is used for a fluid pressure control unit such as, for example, an anti-lock brake unit, etc., in a vehicle. In such a fluid pressure control unit, in order to use a normally closed solenoid valve in a highfluid pressure range, it was necessary to select a spring member having an intensive spring force in order to maintain a valve closed state against a high fluid pressure in a demagnetized state. Accordingly, if a spring member having an intensive springforce is selected, it was necessary to secure a great attracting force in order to secure a thrust force of the movable core against the spring force.

Therefore, the inventor, et al. commenced development of a normally closed solenoid valve that is able to set the attracting force to a greater level by increasing the area of the plane of the movable core opposed to the fixed core. Where acharacteristic line S of spacing distance/attracting force is obtained in association with the attracting force thus obtained and the spacing distance between the fixed core and movable core, such a characteristic line as shown in FIG. 3 was obtained.

The characteristic lines S1 and S2 of spacing distance/attracting force are based on the normally closed solenoid valve in which the attracting force is set to a greater level, and the characteristic line S3 of spacing distance/attracting force is basedon a prior art normally closed solenoid valve. As has been made clear from the characteristic lines shown in the same drawing, in the assembling allowance range from L1 to L2, the characteristic line S3 of spacing distance/attracting force producesalmost the same attracting force, and the normally closed solenoid valve in the assembling allowance range produces almost the same initial performance. However, the characteristic lines S1 and S2 of spacing distance/attracting force produce steepslopes, and even in the assembling allowance range, there is a comparatively large difference in the initial performance. That is, a difference will be brought about in the initial thrust force of the normally closed solenoid valve in an assembledstate.

With the structure as described above, it is possible to obtain a rough inclination even if the characteristic line of the spacing distance and attracting force in the assembling allowance range is not straight, and it is possible to makeconstant the thrust force of the movable core, which works against the biasing force of a resilient biasing member, in the assembling allowance range. Therefore, in the normally closed solenoid valve assembled in the assembling allowance range, it ispossible to make roughly constant the initial thrust force of at least the movable core, wherein respective products will be almost freed from any unevenness. In addition, even in a case where the current value supplied to the coil is made variable bycurrent controlling means, it is possible to secure a planned initial thrust force of the movable core, wherein the respective products will be free from any unevenness.

It is therefore an object of the present invention to provide a normally closed solenoid valve in which the thrust force of a movable core working against a resilient biasing member in the initial position or assembling allowance range is fixed.