GB2261712A

GB2261712A - Hydraulic master cylinder

Info

Publication number: GB2261712A
Application number: GB9124713A
Authority: GB
Inventors: Philip Woolvin
Original assignee: Automotive Products PLC
Current assignee: Automotive Products PLC
Priority date: 1991-11-21
Filing date: 1991-11-21
Publication date: 1993-05-26
Anticipated expiration: 2011-11-21
Also published as: GB2261712B; GB9124713D0

Abstract

A hydraulic master cylinder for a vehicle braking system, has a body with a bore (15) therein in which two pistons (21) (22) are reciprocable, the pistons being biased apart by a spring (37) and each having a coaxial passageway (28) therein which houses a centre valve (30). One valve (30) is located in a hollow housing (35) which is fixed to one piston (21) and the housing (35) includes an integral coaxial connector (41) which is connected to an end cap (40) for the spring (37) which fits on the second piston (22) so that the connector (41) sets the distance between the two pistons. <IMAGE>

Description

11HYDRAULIC MASTER CYLINDER" This invention relates to hydraulic tandem master cylinder assemblies used for, but not exclusively for, the braking systems of automobiles.

The hydraulic fluid housed in the reservoir is fed into the braking system to replenish lost fluid due to leaks, or to top up the system as it expands due to for example, wear on brake linings in the case of a braking system. The feeding of fluid into the high pressure side of the master cylinder actuating piston is called recuperation, and typically involves a one way valve mechanism so that fluid can feed from the reservoir to the braking system when the brakes are released, but fluid is not displaced into the reservoir when the brakes are applied.

One known recuperation means is shown in British Patent No.

513, 639, is a one way valve arranged along the axis of the master cylinder piston and which abuts a hollow pin extending from the reservoir into the centre of the master cylinder bore.

The present generation of master cylinders are generally manufactured with pistons having centre valves located in a passageway extending through the piston and which allow fluid to feed an unpressurised supply chamber to a pressure chamber.

With tandem brake systems, in order to synchronise the operation of the pistons, it is necessary to accurately locate the two pistons with respect to the master cylinder bore and to each other. In order that the lost travel is reduced to a minimum it is known to have a tolerance compensating mechanism acting between the pistons to accurately position the two pistons relative to each other.

The present invention provides an accurate means of locating the two pistons in relation to each other.

According to the invention there is provided a piston assembly for a brake master cylinder comprising a piston having a coaxial passageway therein in which a valve is located for opening and closing the passageway, the valve comprising a stem which is located in the passageway and a head which is biased by a spring to close the passageway the valve head being accommodated in a hollow housing which is fixedly secured to the piston, and against which the valve spring reacts.

A hydraulic master cylinder preferably includes a piston assembly as described above, and the piston assembly is biased to an at-rest position in the master cylinder bore by a return spring, and in the at-rest position the centre valve is in abutment with a pin extending transversely of the bore to open the centre-valve.

The master cylinder is preferably a tandem master cylinder comprising two such piston assemblies, and the primary piston assembly return spring is located between these two pistons, one end of the return spring acting against the primary piston and its other end acting against an end cap, the axial spacing between the primary piston and the end cap being limited by a coaxial connection which is formed integrally with the primary piston valve housing.

The invention will be described by way of example and with reference to the accompanying drawings in which: Fiq. 1 is a longitudinal cross-section through a hydraulic master cylinder assembly according to this invention; anci Fig 2 is an enlargement of detail in Fig 1.

A hydraulic master cylinder and reservoir assembly 11 is shown in the drawings and comprises a polypropylene hydraulic reservoir 11 secured to a master cylinder body 12. The master cylinder body 12 has a bore 15 which is closed at one end 13 and is open at its other end 14. Two fluid connections are provided between the reservoir 11 and master cylinder bore 15 by hollow spigots 16 in the base of the reservoir which are each connected through a seal 20 with a cylindrical recess 17 in the external surface of the master cylinder body 12.

The pair of pistons 21 and 22 are sealingly slideable in the bore 15, each piston 21 and 22 has an axially extending diametral slot 19 thereon to accommodate a solid pin 18 transverse of the bore 15. Each pin 18 has one end located in the recess 17 and extends diametrally across the bore 15, so as to engage in the axially extending slots 19 in the master cylinder pistons 21 and 22. A fluid passageway (not shown) connects each recess 17 to the bore 15 and is substantially parallel with the respective pin 18.

The master cylinder bore 15 is a blind bore having an end wall 13 and a mouth 14. Each piston 21 and 22 comprises a spool shaped piston having two cylindrical portions 25 and 26, which seal against the wall of the bore, and which are joined hy a axially extending reduced diameter middle portion 27.

The axially inner end portions 26 of the two pistons 21 and 22 have a coaxial stepped bore 28 (see Fig 2) therein which has its smaller diameter portion connecting with the respective axial slot 19 in the middle portion 27 of the piston.

Each axial stepped bore 28 has a recuperation means constituted by a one way centre valve 30 located therein.

The valve 30 is loaded by a spring 31 towards the shoulder 29 of the stepped bore 28 and each valve 31 has a stem 32 and a head 33 with a seal 34 thereon for sealing against said shoulder 29.

Teach valve head 33 is accommodated in a hollow cup like housing 35 which is fixedly secured to the end face of the respective pistons 21 or 22. The valve springs 31 react against the base of the housing 35 and its open end is swaged around an undercut surface 36 on the piston.

Teach piston 21 and 22 is biased towards an 'at-rest' position by a respective return spring 37 and 38. The axial stem 34 of each valve 30 projects through the stepped bore 28 to contact the pin 18, when the respective piston 21 or 22 is moved towards the mouth of the bore 14 by the respective return springs 37,38 to an at-rest condition, so that each valve 30 is held open.

The return spring 37 on the primary piston 21 is a higher rated spring than the return spring 38 of the secondary piston 22 so that when a load is applied to the piston 21 to move it axially inwardly, the two pistons 21,22 and spring 37 move axially inwardly compressing the spring 38 and each valve stem 34 is moved out of abutment with its respective pin 18 and each valve 30 is biased by its respective spring 31, seating the seal 34 against the shoulder 29 of the stepped bore 28, and closing it to fluid flow. Thereafter both pistons operate to pressurise separate hydraulic circuits.

When the hydraulic pressure is released, the pistons 21 and 22 are moved by the spring 37,38 towards the mouth 14 until the piston 21 is held in an at-rest condition (as shown) at which position the valves 30 are opened by abutment with the pins 18 and allow fluid to flow through the axial bore and recuperate the hydraulic systems.

In the at-rest condition the distance between the two pistons 21 and 22 is accurately determined by controlling the length of the primary return spring 37. The return spring 37 has one end seated on the inner end of the piston 21, and its other end seated on a spring end cap 40 connected to the valve housing 35 of the primary piston 21 by a coaxial connector 41 formed integrally with the housing 35.

The end cap 40 fits snugly onto the adjacent end portion of the secondary piston 22, with the head 43 of the connector accommodated in a blind bore 44 in the piston 22. The distance between the pistons 21 and 22 as set by the spring 37, determines the relative movements of closure of the two valves 30 in the two pistons.

Claims

1. A piston assembly for a brake master cylinder comprising a piston having a coaxial passageway thereon in which a valve is located for opening and closing the passageway, the valve comprises a stem which is located in passageway and a head which is biased by a spring to close the passageway, the valve head being accommodated in a hollow housing which is fixedly secured to the piston and against which the valve spring reacts.

2. A piston assembly as claimed in Claim 1 wherein the valve housing is deformed around an indulent surface on the piston to secure the housing to the piston.

3. A hydraulic master cylinder comprising a master cylinder body having bore therein in which the piston assembly, according to claim 1 or claim 2, is slideable in the bore, the piston assembly being biased to an at-rest position by a return spring and in the at-rest position the centre valve is in abutment with a pin extending transversely of the bore to open the centre valve.

4. A hydraulic master cylinder as claimed in Claim 3 and further comprising a second piston assembly as claimed in Claim 1 or Claim 2 also slideable in the bore, the second piston assembly also being biased to an at-rest position by a respective return spring, and in the at-rest position the respective centre valve is in abutment with a second pin extending transversely of the bore to open the second piston centre valve.

5. A hydraulic master cylinder as claimed in Claim 4 wherein the primary piston assembly return spring is located between the two pistons, and has one end acting against the primary piston and and its other end acting against a spring end cap, the axial spacin between said primary piston and end cap being limited by a coaxial connection which is formed integrally with said primary piston valve housing.