Brake-pressure control device for a road vehicle

A brake-pressure control device for a road vehicle with a hydraulic multi-circuit brake system, comprising a master cylinder for generating the brake pressures to be fed into the wheel brakes and a pedal-controlled pneumatic brake booster arranged to actuate the master cylinder and provided with a change-over device operative to respond to actuating behavior of a vehicle driver and to cause an automatic change-over of the brake booster from a boost factor value required for limited braking to a higher boost factor value required for full braking, at least in the event that the speed with which the vehicle driver actuates a brake pedal associated with the brake booster in an introductory phase of a braking operation exceeds a threshold value,

• (a) the brake booster having a vacuum chamber connected to a suction pipe of an engine of the vehicle and a driving chamber limited movably relative to said vacuum chamber via a driving piston and, via a control part functioning overall as a proportional valve, comprising an inlet valve and a compensating valve and actuatable by the brake pedal, is configured to be loaded with a pressure higher than the pressure prevailing in the vacuum chamber and being proportional to a regulating force which is exerted by the vehicle driver and which is introduced into the master cylinder via a pedal tappet, a reaction piston, a flexible reaction element and a push rod, which are supported axially with respect to one another in a mechanical series connection and which form a force-transmission train via which an actuating force is introduced into the master cylinder, and the reaction force resulting from the generation of brake pressure reacts on the brake pedal,
• (b) the inlet valve comprising a flanged portion of the reaction piston arranged axially displaceably in a depression of a control-part housing, said depression being communicatingly connected to the driving chamber, and a valve element configured as an annular disc, sealingly supportable at an inner edge region on the flanged portion and connected in a pressure-tight manner, via a sealing cuff, to a radially outer boundary, formed by a tubular portion of the control-part housing, of an ambient pressure space through which the pedal tappet engaging on the reaction piston passes centrally, and
• (c) the compensating valve comprising a seat coaxially surrounding the depression, in which a sealing flange of the reaction piston is axially displaceably arranged, and which delimits the depression from an outer coaxial annular space communicatingly connected to the vacuum chamber via a housing channel, and a radially outer region of the annular disc valve element arranged to be urged into bearing contact with the seat by a valve spring, wherein a housing-side valve element of the compensating valve, on which the valve element is arranged to be at the radially outer region, comprises a sleeve-shaped part guided displaceably in a pressure-tight manner in an axial direction on the control-part housing and, via a thread extending in the axial direction over a portion of the length of a casing of the sleeve-shaped part, is in engagement with a complementary thread of a rotatably mounted hollow shaft which, in a tubular portion of the control-part housing coaxially surrounds and is radially spaced from the pedal tappet, and is sealed off from the tubular portion of the control-part housing, said hollow shaft being configured and arranged to be rotationally driven by means a reversible electric motor constituting an actuating device of the change-over device, said hollow shaft being secured against axial shifts within the control-part housing such that, in the event of actuation of the brake pedal taking place with excessive speed, the electric motor arranged to be actuated in a direction of rotation in which an axial shift, resulting from rotation of the hollow shaft, of the sleeve-shaped part, on which the valve element is supported and further constitutes a valve element of the inlet valve, results in a lifting off of the valve element from the inner edge region of the reaction piston, and, at the latest at an end of the brake actuation, the electric motor is actuated in a direction of rotation in which the resulting axial displacement of the sleeve-shaped part again returns to a basic position of the valve element which corresponds to a blocking state of the inlet valve.