VACUUM BRAKE BOOSTER ASSEMBLY

Abstract: A vacuum brake booster assembly comprising a brake booster (10) and a hydraulic master-­ cylinder (12), the booster comprising an input rod (14), a valve assembly (28) and piston means (26) defining first and second booster chambers, the piston means being connected to an output rod (33) having adjustment means to vary the length thereof ; the piston means having a fist spring (58) to urge it towards a rest position, the master cylinder comprising at least one piston assembly (42) associated with a respective hydraulic pressure chamber for a vehicle braking circuit, the piston assembly having a second spring (50) to urge it into a rest position, the piston assembly and the output rod (33) being adapted to cooperate when the brake booster assembly is in operation; characterised in that the force exerted by the first spring (58) is less than the force exerted by the atmosphere on the piston means when a vacuum is applied to the first and second booster chambers, the combined force of the first (58) and second (50) springs being greater than the said force exerted by the atmosphere.

Description: The present invention relates to a vacuum brake booster assembly for automotive vehicles. The majority of vacuum brake boosters have the inherent problem that that the dead stoke, i.e. the movement of the brake pedal experienced by the vehicle driver before any braking force is produced, is unacceptably long. Various modifications and methods of adjustment have been proposed to reduce the dead stroke. However in many cases a limited dead stroke remains which is attributable to the distance, in the rest position of the assembly, between the end of the booster output rod and the adjacent end of the master cylinder piston. This dead stroke arises from the need to take into account the maximum possible combination of manufacturing tolerences for both the master cylinder and the booster in order to be sure that the push rod does not touch the master cylinder piston when assembled and in the rest position. It is therefore an object of the present invention to provide a vacuum brake booster assembly in which the above described dead stroke is minimised. According to the invention there is provided a vacuum brake booster assembly as claimed in present claim 1. The present invention will now be described by way of example with reference to the accompanying drawings in which : Figure 1 is a longitudinal section through a vacuum brake booster assembly, in accordance with the invention, when in its rest position ; and Figure 2 is a longitudinal section of the assembly of figure 1 but showing its position where the dead stroke has been taken up. As shown in Figure 1 a vacuum brake booster assembly comprises a vacuum booster, shown generally 10 and a master cylinder 12 which, in the example shown is a tandem master cylinder. The booster includes an input rod 14 intended to be connected to a vehicle brake pedal (not shown) by an eye 16. A booster housing is formed by a first shell 18 and a second shell 20 on which are mounted retaining bolts 22 for securing the booster to a vehicle fire-wall (not shown). The first and second shells together support a flexible diaphragm 24 which is connected to a plastics piston 26 which, in turn, is linked to a valve assembly 28. The diaphragm 24 and the plastics piston 26 separate the interior of the housing into first and second chambers 30 and 32. An output push rod 33 is mounted on the plastics piston 26. The master cylinder 12 is mounted on the first shell 18 by means of bolts 34 (only one of which is shown). It should be noted that the master cylinder does not project more than a nominal distance into the housing but rather is of the type where the end plane of the master cylinder is closely adjacent the end wall 36 of the first shell 18. The master cylinder 12 comprises a body 38 enclosing a bore 40 in which are slideably mounted primary and secondary piston assemblies 42, 44. Each piston assembly defines with the bore 40 a respective chamber 46, 48 each of which is associated with a respective vehicle braking circuit (not shown). Each piston assembly 42, 44 has a respective return spring 50, 52 to urge the pistons back into their rest position in which each abuts a respective stop 54, 56 fixedly mounted in the master cylinder body 38. The booster further comprises a return spring 58 for the plastics piston 26 which it assi$ts to return to the rest position illustrated in Figure 1 when the vehicle engine is switched off resulting in the presence of air in the first and second booster chambers 30, 32. When the vehicle engine is started a vacuum is applied to the first and second booster chambers 30, 32. The atmospheric pressure acting on the annular surface 60 of a tubular extension 62 of the plastics piston 26 tends to push the plastics piston to the left (when viewing the drawings). It is this force on the plastics piston 26 which is used in the present invention to minimise the dead stroke. In accordance with the present invention the force of spring 58 is selected to be less than the atmospheric force exerted on the annular surface 60 of the plastics piston 26 when the vehicle engine is started. Furthermore, the force of spring 50 which acts on the primary piston 42 is selected so that the combined forces of springs 50 and 58 are greater than the atmospherie force exerted on plastics piston 26. A typical example of the spring force is 10 Kg for spring 50 and 5 Kg for spring 58. Before the master-cylinder 12 is mounted on the booster 10, a screw 64 on the end of the push rod is adjusted, taking into account the combination of the possible manufacturing tolerences, to ensure that when assembled and in the rest position the end of the push-rod 33 does not touch the adjacent end of the primary piston 42. This position is shown in Figure 1. Thus, when the vehicle engine is started and a vacuum applied to chambers 30 and 32 of the booster, the choice of the springs 50 and 58 allows the plastics piston 26 to move to the left under the influence of atmospheric pressure until the end of the push rod 33 comes into contact with the primary piston 42. This position is illustrated in Figure 2. Thus, in accordance with the present invention the dead stroke is automatically minimised as soon as the vehicle engine is started. In order that the booster piston may be free to move forward and take up the dead stroke, the vehicle brake pedal (not shown) must not have a return spring. It is envisaged that the present invention be also applied to a brake assembly where the master-cylinder 12 projects into the interior of the booster 10.

Claims: 1/ A vacuum brake booster assembly comprising a brake booster and a hydraulic master-cylinder, the booster comprising an input rod, a valve assembly and piston means defining first and second booster chambers, the piston means being connected to an output rod having adjustment means to vary the length thereof ; the piston means having a fist spring to urge it towards a rest position, the master cylinder comprising at least one piston assembly associated with a respective hydraulic pressure chamber for a vehicle braking circuit, the piston assembly having a second spring to urge it into a rest position, the piston assembly and the output rod being adapted to cooperate when the brake booster assembly is in operation ; characterised in that the force exerted by the first spring is less than the force exerted by the atmosphere on the piston means when a vacuum is applied to the first and second booster chambers, the combined force of the first and second springs being greater than the said force exerted by the atmosphere. 2/ Vacuum brake booster assembly as claimed in Claim 1 characterised in that the force of the first spring is approximately 5 Kg, and the force of the second spring is approximately 10 Kg.

Inventors: CADEDDU, LEONARDO
Applicants: BENDIX ITALIA S.P.A.
Application year: 1987
Main classification: B60T13
IPC codes: B60T13