GB2052011A

GB2052011A - Piston assembly for hydraulic master cylinder

Info

Publication number: GB2052011A
Application number: GB7924269A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1979-07-12
Filing date: 1979-07-12
Publication date: 1981-01-21
Also published as: JPS5621947A; AU6022880A; FR2461172B1; DE3026428C2; ES493349A0; AU540652B2; IN150673B; FR2461172A1; BR8004327A; GB2052011B; ES8103316A1; DE3026428A1

Abstract

A piston assembly for the master cylinder of a hydraulic braking system comprises a rigid support member (1) which has a boss (4) and flange (5) to support a conventional flexible master cylinder seal. The maximum diameter of the support member is defined by circumferentially spaced lands (8A, B and C) which slidably engage the master cylinder bore wall. During recuperation strokes fluid can flow between the lands and past the seal. The support member (1) is molded from plastics material or die-cast from metal with sufficient accuracy to obviate the need for any subsequent machining of the lands (8A, B, C). The central body portion (2) of the support member is cruciform. <IMAGE>

Description

SPECIFICATION Piston assembly for hydraulic master cylinder This invention relates to a piston assembly for the hydraulic master cylinder of a braking system.

In a hydraulic master cylinder a piston assembly is slidably mounted in a bore for axial movement along the bore. During the pressure generating stroke of the piston assembly, i.e. the stroke which causes pressurised hydraulic fluid to be delivered from the master cylinder, the piston assembly is in sealing contact with the wall defining the cylinder bore. During the return or recouperation stroke it is desirable to permit some flow of hydraulic fluid past the piston.To this end a piston assembly typically comprises a rigid support member which slidingly engages the bore wall in order to guide the piston assembly in the bore, and a flexible seal mounted on the support member for providing a fluid seal between the support member and the bore wall during the pressure generating stroke, and for flexing away from the bore wall to allow fluid flow past the seal during the recouperation stroke.

Because of the extremely high pressure differential to which the flexible seal is subjected during the pressure generating stroke it is essential that the seal is adequately supported over substantially the entire area of the cylinder bore. Accordingly, it has been known to manufacture the rigid support member with a flange for supporting the flexible seal, the flange having a diameter substantially equal to that of the cylinder bore. In order to provide for flow across the piston assembly during the recouperation stroke axially extended drillings are provided in the flange. An arrangement of this sort is shown, for example, in Figure 3 of British Patent Specification 1,354,987.

Whilst a piston assembly of this type functions satisfactorily, it is expensive to manufacture since the seal supporting flanges must be accurately machined to provide satisfactory sliding contact with the bore wall, and the axially extending passages through the flange must be formed by a separate drilling operation.

According to one feature of the present invention there is provided a piston assembly for the hydraulic master cylinder of a braking system, the piston assembly comprising: a rigid support member the maximum diameter of which is defined by a plurality of circumferentially spaced apart lands which, in use, slidingly engage the bore wall of the master cylinder to guide the piston assembly in the bore; and a flexible seal mounted on the support member for providing a fluid seal between the support member and the bore wall during the pressure generating stroke of the piston assembly and for flexing away from the bore wall to allow fluid to flow between the lands and past the seal during the recuperation stroke of the piston assembly.

An embodiment of the present invention can be moulded from a synthetic plastics material, or precision diecast from a metal, for example Mazak 3. The external diameter of the support member is defined by the lands which can be accurately moulded. Circumferential spaces between the lands enable fluid to flow past the support member during the recouperation stroke, and are formed automatically during the moulding process by the formation of the lands. By making the lands of relatively small circumferential extent, the circumferential spaces or recesses between the lands have a relatively large circumferential extent, and accordingly can provide an adequate flow cross section even if their radial thickness is very small. Thus, the seal can be adequately supported during the pressure generating stroke.

The above and further features and advantages of the invention will become clear from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawings, wherein: FIGURE 1 is a side view of the support member of a piston assembly; FIGURE 2 is an end view in the direction of the arrow A of Figure 1; FIGURE 3 is an end view in the direction of the arrow B of Figure 1; FIGURE 4 is a section along the line IV-lV of Figure 2; FIGURE 5 is an enlarged view of the circled portion C of Figure 4; FIGURE 6 is a section on the line VI--VI of Figure 1; FIGURE 7 is a partial cross sectional view on the line VIl-VIl of Figure 6; and FIGURE 8 is a partial sectional view on the line VIlI-VIlI of Figure 3.

Referring now to the drawings the support member 1 shown is an integral moulding formed of a suitable synthetic plastics material, for example glass fibre reinforced nylon. The support member includes a central generally cruciform body part 2 formed at the leading end with an outwardly extending flange 3. A circular boss 4 extends forward of the flange 3, and the forward end of the support member terminates in a reduced diameter portion 5. In use a flexible seal (not shown) is mounted on the boss 4 to ensure a fluid seal between the support member 1 and the bore wall of a hydraulic master cylinder during each forward or pressure generating stroke of the piston assembly.

A flange 6 is formed at the rear end of the body part 2, and to the rear of the flange 6 a coupling part 7 is provided for coupling the piston to an operating rod (not shown).

A plurality of lands 8A-8F are formed integrally with the body part immediately to the rear of the flange 3. As can best be seen in Figure 2 the lands 8A-8F define the maximum diameter of the support member, this diameter being substantially equal to the diameter of the cylinder bore so that in use the lands slidingly engage the bore wall to guide the piston assembly. The radial projection of the lands beyond the flange 3 is small, and may typically be approximately 0.08 mm, i.e. the diameter defined by the lands may typically exceed the diameter of the flange 3 by approximately 0.16 mm.

When viewed in the axial direction a plurality of circumferentially extending spaces or recesses 9A-9F are defined between the lands 8A-8F.

These recesses ensure that whilst the lands are in sliding engagement with the bore wall a plurality of fluid flow passages are defined to permit fluid to flow past the support member.

During the forward or pressure generating stroke of the piston assembly the flexible seal is deformed outwardly into sliding fluid tight engagement with the bore wall as is well understood in the art. However, during the recouperation or return stroke the seal can flex away from the bore wall, and thereby permit fluid to flow through the recesses 9A-9F and past the seal to the forward side of the piston assembly.

The fact that the recesses are shallow ensures that the flange 3 is large enough to provide adequate support for the seal during the pressure generating stroke. The significant circumferential extent of the recesses ensures that they provide sufficient flow area despite their small radial extent.

The above described support member can be integrally moulded from plastics material or precision die-cast from a suitable metal in a simple four-part mould. The mould comprises a forward end part terminating in the plane P 1 shown in Figure 5; a rear end part terminating in the plane P2 marked on Figure 1; and two central parts extending between the planes P1 and P2 and split along the plane P3 marked on Figure 2.

Retracting cores are provided in the rear end part to form slots 10. The provision of a forward end part to the mould enables an axial passage 11 to be formed in the support member and ensures that the seal supporting flange 3 is free of flash. By splitting the central parts of the mould along the plane P3 the lands 8A-8F are formed free of flash. The provision of the rear end part of the mould ensures that the flange 6 is substantially free of flash. Material is preferably injected into the die in a region 12 on the cruciform body part where the existence of sprue is immaterial.

After the completed support member has been removed from the die the only finishing operation necessary is to tap the passage 11 to receive a screw for holding the seal in place on the boss 4.

Claims

CLAIMS 1. A piston assembly for the hydraulic master cylinder of a braking system, the piston assembly comprising: a rigid support member for the maximum diameter of which is defined by a plurality of circumferentially spaced apart lands which, in use, slidingly engage the bore wall of the master cylinder to guide the piston assembly in the bore; and a flexible seal mounted on the support member for providing a fluid seal between the support member and the bore wall during the pressure generating stroke of the piston assembly and for flexing away from the bore wall to allow fluid to flow between the lands and past the seal during the recouperation stroke of the piston assembly. 2. A piston assembly, substantially as hereinbefore described with reference to and as shown in the accompanying drawings. New claims or amendments to claims filed on 3/7/80. Superseded claims 1 and 2. New or amended claims: CLAIMS

1. A piston assembly for the hydraulic master cylinder of a braking system, the piston assembly comprising: a rigid support member the maximum diameter of which is defined by a plurality of circumferentially spaced apart lands which, in use, slidingly engage the bore wall of the master cylinder to guide the piston assembly in the bore; and a flexible seal mounted on the support member for providing a fluid seal between the support member and the bore wall during the forward, pressure generating, stroke of the piston assembly and for flexing away from the bore wall to allow fluid to flow between the lands and past the seal during recuperation stroke of the piston assembly.

2. A piston assembly according to claim 1 wherein the support member includes an axially central body portion and with a radially outwardly extending support flange formed at the leading end of the central body portion for supporting the flexible seal, the lands being located immediately axially rearwardly of the support flange.

3. A piston assembly according to claim 2 wherein a further flange is formed at the trailing end of the central body portion, the further flange being of substantially the same diameter as the support flange.

4. A piston assembly according to claim 2 or claim 3 wherein the central body portion is of cruciform in transverse cross-section.

5. A piston assembly according to any of claims 2 to 4 wherein the diameter of the support flange is approximately 0.16 mm smaller than the diameter defined by the lands.

6. A piston assembly according to any preceding claim wherein there are six equiangularly spaced lands.

7. A piston assembly according to any preceding claim wherein the support member is moulded from a synthetic plastics material.

8. A piston assembly according to any preceding claim wherein the support member is die-cast from a metal.

9. A piston assembly, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.