The present invention relates to a vacuum power servo booster which is used as a power servo booster for a brake or a clutch. More particularly, the invention relates to such a servo booster having an improved interior structure of a valve housing which extends from the axial part of a piston in the vacuum power servo booster.
FIG. 1 shows a conventional brake power servo booster. In FIG. 1, reference numeral 1 designates a shell, 2, a piston slidably arranged in the shell 1, and 3 a diaphragm which, together with the piston 2, divides the interior of the shell 1 into a constant pressure (or negative pressure) chamber 4 and a variable pressure chamber 5. A valve housing 2A incorporating a control valve 6 extends from the axial part of the piston 2. Under normal conditions, the control valve 6 communicates the constant pressure chamber 4 with the variable chamber 5. However, when a control valve body 7 made of elastic material is displaced to the left-hand side in FIG. 1 by an input shaft 8, the control valve 6 interrupts the communication between the two chambers 4 and 5 and then introduces air into the variable pressure chamber 5. As a result, a thrust attributed to the pressure difference between the two chambers is applied to the piston 2 and a push rod 9.
A skirt 10 and an annular seat 11 are formed at the rear of the control valve body 7. The annular seat 11 is held in abutment with a step 15 formed on the inner wall of the valve housing 2A by coiled tensioned compression spring 14 provided between the rear end spring seat 12A of a retainer 12 covering the rear end portion and the inner wall portion of the annular seat and a spring seat 13 locked to the input shaft 8. Thus, the constant pressure chamber 4 and the variable pressure chamber 5 are maintained airtight by the annular seat 11. A typical example of such a conventional structure is shown in U.S. Pat. No. 3,452,646.
Recently, there has been a strong demand for reducing the weight and size of a brake power servo booster of this type and related components. In order to increase the space in the operating chamber, a method of decreasing the mounting length (L) of the power servo booster which protrudes into the operating chamber has been proposed. However, with the above-described conventional structure, the reduction of the length of the valve housing 2A is limited to a certain value. Accordingly, the reduction must be carried out at the sacrifice of a space (S) for incorporating a filter or a silencer 16.
In view of the foregoing, an object of the invention is to provide a vacuum power servo booster in which the length of the portion thereof which protrudes into the operating chamber is substantially reduced, or otherwise if the length of the protruding portion is equal to that of a conventional booster, the thickness of a filter or silencer used therewith can be increased. It is a still further object of the invention to provide such a booster in which the control valve can be readily inserted into the valve housing.