| 摘要 |
Pneumatic brake servo comprising a housing (10) with an axis of symmetry (X-X') sealingly separated by a movable wall structure (12) into a front chamber (14) permanently linked to a low pressure source and a rear chamber (16) selectively linked to the front chamber (14) or to a high pressure source by a three-way valve (36) operated by a control rod (30) capable of being in contact, through the front face of a plunger (28), with the rear face of a thrust rod (56) integral with a reaction disc (58), the plunger sliding in a bore (26) within the movable wall (12). The three-way valve (36) includes a valve (36) disposed in a rear tubular part (22) of the movable wall (12), cooperating at an annular front face (40) with a first annular valve seat (28a) formed on the plunger (28) and a second annular valve seat (20a) formed on the movable wall (12), the first valve seat (28a) being concentric with the second valve seat (20a) and having a smaller diameter, the front annular face (40) of the valve (36) being movable in the tubular rear part (22) of the movable wall (12) and being sealed therein by its outer edge and by its inner edge, the front annular face (40) of the valve (36) including at least one opening (46) by which a chamber (44) located behind the annular front face (40) of the valve (36) communicates with a space (35) located between the first (28a) and second (20a) valve seats. According to the invention, the resultant of the forces produced by the low pressure and high presure acting on the plunger (28) is permanently nil or negligible.
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