GB2133848A

GB2133848A - Adjustable transmission ratio between brake servo and master cylinder

Info

Publication number: GB2133848A
Application number: GB08334031A
Authority: GB
Inventors: Hans-Dieter Reinartz; Hajo Pickel
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co oHG
Priority date: 1983-01-15
Filing date: 1983-12-21
Publication date: 1984-08-01
Also published as: FR2539368A1; GB8334031D0; IT8419104A0; DE3301185A1; IT1172998B

Abstract

In an auxiliary power-assisted actuating arrangement for the master cylinder (3) of a hydraulic vehicular brake system with a vacuum servo motor (5) governed by the brake pedal (4), there is provided a force- transmitting lever assembly (6,7,8,12) which transmits the movement of the output drive member (9) of the servo vacuum motor (5) onto the piston rod (7) of the master cylinder (3) in a specific transmission ratio. The transmission ratio is automatically adjustable at the force-transmitting lever assembly by a special servo cylinder (11) connected to the vacuum source. The servo cylinder (11) has a piston (10) with actuating rod (13), an adjustable bearing (8) being pivotally fastened to the actuating rod (13) which latter is held and guided by the frame (19), on which bearing a two-armed lever (6,12) is slidably supported whose first arm (6) is articulated at the output member (9) and whose second arm (12) is articulated at the piston rod (7). As soon as the vacuum prevailing at the vacuum servo motor (5) varies, the effective lever ratio of the force- transmitting lever assembly (6,7,8,12) will likewise vary, i.e. a lower level of vacuum increases the pedal travel required at the brake pedal (4). <IMAGE>

Description

SPECIFICATION Master cylinders for hydraulic vehicular braking systems This invention relates to master cylinders for hydraulic vehicular braking systems and in particular to an auxiliary power-assisted actuating device for the master cylinders of such a brake system of the kind comprising a servo vacuum motor governed by the brake pedal and a forcetransmitting lever assembly acting upon the piston of the master cylinder.

The pedal ratio which is suitable for a normal power-assisted actuation of the master cylinder may be inadequate upon failure of the servo effect, since the driver might not be able to generate the force required for the actuation of the master cylinder without the servo effect. It is known for this reason to ensure that there is a different pedal ratio in the condition of failure of the servo effect so that the driver is enabled to apply the desired actuating force onto the master cylinder; this, however, necessitates a larger pedal travel.

Thus, a power-assisted pedal assembly is known (German printed and published patent application 25 49 194) with a pedal which is pivotally connected with a lever, tiltable about a pivot, and which is coupled at a point intermediate its ends with an actuating member of a master cylinder, in order to exert an actuating force on the actuating member, while the pedal assembly is furnished with a power-actuated servo unit which is able to apply an auxiliary force onto the brake pedal when the latter moves, and which has a force-transmitting device isolated from the actuating member wherein, during operation of the brake boosting device, the brake pedal and the lever swivel about the pivot, wherein, upon failure of the brake boosting device, the brake pedal swivels about a point disposed between the pivot and the junction between the lever and the actuating member, and wherein the forcetransmitting device is in force-transmitting engagement with the lever at or close to the line of effect of the force which is applied by the brake pedal onto the actuating member.

Furthermore, there is known a power-assisted actuating device for master cylinders of vehicular brake systems (U.S. patent 2,229,247), wherein a two-armed lever pivotable about a stationary pivot is articulated with its first lever arm via a tie rod at a one-armed lever moved by the brake pedal, while it acts with its second lever arm directly upon the piston rod of the master cylinder, wherein the lever connected to the brake pedal acts with its free end on the valve device of a servo motor whose piston is supported on the piston rod of the master cylinder.

These known devices have the disadvantage that, through varying servo forces (resulting from pressure variations in the lines connected to the servo motor) which act on the piston rod of the master cylinder assist the pedal forces to be generated at the brake pedal, this is achieved without adaptation of the transmission ratio of the pedal lever in relation to the lever acting on the piston rod of the master cylinder.

It is an object of the present invention to arrange for an actuating device of the kind referred to, wherein, with the vacuum reduced, the transmission ratio of the force-transmitting lever assembly is increased and thus counteracts the reduction of the boosting force of the servo vacuum motor. Upon complete failure of the vacuum source, the transmission ratio is, in this arrangement, desired to adjust automatically in such a way that the reduction of the boosting force will be approximately compensated by variation in the transmission and the pedal force required for braking will not increase considerably.

Finally, the device is to be designed in such a way that the housing of the servo vacuum motor will not receive any supporting forces generated by the force-transmitting lever assembly.

According to the invention in its broadest aspect, an auxiliary power-assisted actuating device for the master cylinder of a hydraulic vehicular brake system of the kind referred to is characterised in that the off-drive member of the servo vacuum motor is articulated at the first arm of a two-armed lever which latter is pivotable about a sliding fulcrum or adjustable bearing, wherein the second arm is connected to the piston rod of the master cylinder and the adjustable bearing is held and guided by the actuating rod of the piston of a servo cylinder acted upon by differential pressure.

Preferably, the vacuum chamber of the servo vacuum motor and the chamber in front of the piston of the servo cylinder are permanently connected to the same vacuum source via pipe or hose lines.

To accomplish a design as simple as possible and to avoid an elaborate and heavy-weight housing for the servo vacuum booster, the piston rod of the master cylinder and the off-drive member of the servo vacuum motor are arranged in parallel, and the movement of the actuating rod of the servo cylinder takes place in a plane which is located perpendicularly relative to the plane of the piston rod and to the plane of the off-drive member.

Advantageously, in the event of decrease of the vacuum in the lines connected to the vacuum chamber of the servo vacuum motor and to the chamber in front of the piston of the servo cylinder, a spring accommodated in the servo cylinder displaces the actuating rod together with the sliding fulcrum or adjustable bearing arranged thereon in the direction of the adjusting cylinder, for the purpose of enlarging the first lever arm articulated at the off-drive member.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing which shows a servo vacuum motor (for example a vacuum brake booster), which is mounted together with the master cylinder onto a wall, the servo cylinder being flanged to the end of the master cylinder close to the piston.

The auxiliary power-assisted actuating device illustrated is substantially composed of the pedal force boosting device or servo vacuum motor 5 which latter utilises as a power source the pressure gradient between the vacuum generated in the intake manifold of the four-stroke cycle engine and the atmospheric pressure, of the generator or master cylinder 3 which is arranged on the bottom plate 1 8 together with the servo vacuum motor 5, of the servo cylinder 11 carried by the frame 19, of the force-transmitting lever assembly 6, 8, 10 inserted between the off-drive member 9 of the servo vacuum motor 5 and the piston rod 7 of the master cylinder 3, and of the lines 15, 1 6 which are connected to the intake manifold of the motor (not illustrated in detail).

The actuating device operates as follows: upon depression of the brake pedal 4, the off-drive member 9 moves in the direction of the arrow and swivels the two-armed lever 6, 12 about the adjustable bearing 8 which latter is fitted on the actuating rod 13 of the servo cylinder 1 When the levers 6, 12 are swivelled, the piston rod 7 is moved to the right, and braking pressure develops in the working chamber of the master cylinder 3.

Upon reduction of the vacuum in the lines 15, 1 6 and thus also in the chamber 14 of the servo member 11, the compression spring 1 7 will urge the piston 10 downwardly, i.e. into the position illustrated in the drawing. The downward movement of the actuating rod 1 3 likewise causes displacement of the sliding fulcrum or adjustable bearing 8, as a result whereof the ratio of the lengths of the lever arms 6 and 12 in respect of each other will change. In the position illustrated in the drawings, the transmission ratio of the lever arms 6 and 12 amounts to about 2:5, that means that a relatively large travel of the off-drive member 9 of the servo vacuum motor corresponds to a comparatively small travel at the piston rod 7.

Consequently, if the vacuum source fails and hence also boosting of the pedal force by the servo vacuum motor, the high pedal force required for braking will be reduced to an admissible amount by variation of the lever transmission by way of bringing about the transmission ratio 'pedal travel travel of the piston rod 7' which is the optimum for this case. If there is boosting of the pedal force to some extent only, the adjustable bearing 8 will move to assume an intermediate position, and if there is maximum boosting of the pedal force, the adjustable bearing will be placed in its upper position 8 that is indicated in broken lines in the drawing.

Claims

1. An auxiliary power-assisted actuating device for the master cylinder (3) of a hydraulic vehicular brake system of the kind comprising a servo motor (5) governed by the brake pedal (4) and a forcetransmitting lever assembly (6, 7, 8, 12) acting upon the piston of the master cylinder (3), characterised in that the off-drive member (9) of the servo vacuum motor (5) is articulated at the first arm (6) of a two-armed lever (6, 12) which latter is pivotable about a sliding fulcrum or adjustable bearing (8), wherein the second arm (12) is connected to the piston rod (7) of the master cylinder (3) and the adjustable bearing (8) is held and guided by the actuating rod (13) of the piston (10) of a servo cylinder (11) acted upon by differential pressure.

2. An auxiliary power-assisted actuating drvice as claimed in claim 1, characterised in that the vacuum chamber of the servo vacuum motor (5) and the chamber (14) in front of the piston (10) of the servo cylinder 1) are permanently connected to the same vacuum source via pipe or hose lines (15,16).

3. An auxiliary power-assisted actuating device as claimed in claim 1, characterised in that the piston rod (7) of the master cylinder (3) and the off-drive member (9) of the servo vacuum motor (5) are arranged in parallel and in that the movement of the actuating rod (1 3) of the servo cylinder (11) takes place in a plane which is located perpendicularly relative to the plane of the piston rod (7) and to the plane of the off-drive member (9).

4. An auxiliary power-assisted actuating device as claimed in any one of the preceding claims, characterised in that, in the event of decrease of the vacuum in the lines (15, 16) connected to the vacuum chamber of the servo vacuum motor and to the chamber (14) in front of the piston (10) of the servo cylinder 1), a spring (17) accommodated in the servo cylinder (11) displaces the actuating rod (13) with the sliding fulcrum or adjustable bearing (8) arranged thereon in the direction of the adjusting cylinder (11), for the purpose of enlarging the first lever arm (6) that is articulated at the off-drive member (9).

5. An auxiliary power-assisted actuating device substantially as described with reference to the accompanying drawing.