GB2082274A - Hydraulic master cylinders - Google Patents

Abstract

A hydraulic master cylinder (1) comprises a body (2) formed from tubular stock. A closure plug (4) is secured to the body (2) and includes an outlet port (9) in communication with a working chamber (8), and a reservoir port (9A) connected to a fluid reservoir. In the rest position of the piston (6) the working chamber (8) communicates with the reservoir via radial passages (19) and axial passage (16) of a tubular member (15) which is integral with the closure plug. At the commencement of a pressure generating stroke the passages (19) are closed by main seal (7). The design obviates the need for ports which pierce the body (2) and by providing a blind bore (17) in which the tubular member (15) is received obviates the need for the usual secondary atmospheric seal between the piston and the body. <IMAGE>

Description

SPECIFICATION Hydraulic master cylinders This invention relates to hydraulic master cylinders suitable for use, for example, in a hydraulic brake or hydraulic clutch system.

Conventional hydraulic master cylinders used in the automotive industry comprise a body defining a cylindrical bore and having a piston slidably mounted in the bore to define a working chamber.

An outlet port is provided through which hydraulic fluid is delivered from the working chamber during pressure generating strokes of the piston, and a seal is provided between the piston and the cylindrical bore in order to prevent the flow of hydraulic fluid past the piston during the pressure generating stroke. At least one port pierces the wall of the bore in order to provide communication between the bore and a hydraulic fluid reservoir. In general, during pressure generating strokes all such ports are isolated from the working chamber by the pressure generating seal, and communicate with the region of the bore behind the pressure generating seal.

Such hydraulic master cylinders suffer from two significant disadvantages. Firstly, it is relatively expensive and difficult to provide ports which pierce the wall of the body since the provision of such ports necessitates either casting of the body in order to provide a suitable connection to the port, or the fabrication of the body from a simple cylindrical tube and some member secured to the side of the tube to provide a connection to a port drilled in the side of the tube. Additionally, a port so drilled has a sharply burred or frazed edge at the point where it opens into the bore of the tube.

This sharp-edged opening must be smoothed, and the drill fraze removed before assembling the master cylinder in order to minimise the possibility of seal damage. This necessitates a great deal of extra machining and polishing of the cylinder bore.

Secondly, the fact that the port in the wall of the body communicates with the region behind the pressure generating seal during pressure generating strokes necessitates the use of a secondary or atmospheric seal between the piston and the body in order to prevent the loss of hydraulic fluid from the body during pressure generating strokes.

A typical example of a prior art master cylinder is shown in British Patent Specification 938185.

In this specification the body of the master cylinder is formed by a tube the wall of which is pierced by two ports. In order to provide communication between the ports and a reservoir a reservoiris secured to the tube by a clip and a seal is provided between the reservoir and the exterior wall of the tube. During pressure generating strokes both ports communicate with the region of the bore behind the pressure generating seal, and accordingly a secondary atmospheric seal is provided at the end of the piston remote from the pressure generating seal in order to prevent the loss of hydraulic fluid.

According to one feature of the present invention there Is provided a hydraulic master cylinder comprising: a body defining a cylindrical bore; a closure plug secured to the body and closing one end of the bore; a piston slidably mounted in the bore to define a working chamber and movable from a rest position towards the closure plug to displace hydraulic fluid from the working chamber through an outlet port formed in the closure plug; a tubular member secured to the closure plug and extending through the working chamber towards the piston; a bore in the piston into which the tubular member is received as the piston advances towards the closure plug; a reservoir port formed in the closure plug and in communication with the interior of the tubular member; a seal mounted on the piston for sealingly engaging the body bore and the exterior surface of the tubular member; and at least one passage in the tubular member for connecting the interior of the tubular member to the working chamber when the piston is in the rest position whereby the working chamber is in communication with the reservoir port, such communication being interrupted by the seal at the commencement of the working stroke of the piston.

The preferred embodiment of the present invention overcomes the difficulties referred to above in that no port need pierce the wall of the body, and accordingly the body can be formed from tubular stock without the need for complex exterior seals or the securing of connecting means to the body by other means, e.g. welding. Further, in the preferred embodiment the passages in the tubular member communicate with a blind bore in the piston during pressure generating strokes, and accordingly the area of the bore behind the pressure generating seal is dry, i.e., it is not flooded with hydraulic fluid. Accordingly the need for a secondary or atmospheric seal at the end of the piston remote from the pressure generating seal is obviated.In addition, since the only ports or openings, over which the pressure generating seal must slide, are formed on an external cylindrical surface drill fraze or burring is minimised and any such sharp-edged opening may be avoided by using a small counter-sink. This then obviates the likelihood of seal damage encountered in the known master cylinder.

The preferred embodiment of the invention accordingly provides a cheap and simple master cylinder the body of which can readily be fabricated from tube stock and which employs only a single piston seal.

The above and further features and advantages of the invention will become clear from the following description of preferred embodiments thereof, given by way of example only, reference being had to the accompanying drawings, wherein: Figure 1 is a longitudinal cross section through a preferred embodiment of hydraulic master cylinder according to the present invention; and Figure 2 is a cross section corresponding to Figure 1 of an alternative embodiment.

Referring firstly to Figure 1 the hydraulic master cylinder 1 shown comprises a body 2 having a cylindrical bore 3. The body 2 may conveniently be formed from tubular stock cut to an appropriate length.

A closure plug 4 is secured to the body by an inwardly rolled end portion 5 of the body and closes one end of the bore 3. A piston 6 is slidably mounted in the bore and is provided with a pressure generating cup seal 7. A working chamber 8 is defined between the cup seal 7 and the closure plug 4 so that upon displacement of the piston 6 towards the closure plug 4 during a pressure generating stroke of the piston, hydraulic fluid is displaced from the working chamber 8 through an outlet port 9.

An input member 10 is provided for advancing the piston 6 towards the closure plug during a pressure generating stroke, and a spring 11 acts through a locating collar 12 on the seal 7 in order to maintain the seal in position and to bias the piston 6 towards the input member 10. When no input force is applied to the input member 10 the piston 6, seal 7, and input member 10 adopt the rest position shown in Figure 1 under the influence of the bias applied by spring 11. In this rest position the bulbous head of the input member 10 rests against an end cap 13 which is secured to an outwardly flared end portion 14 of the body 2.

A tubular member 1 5 is secured to the closure plug 4 and includes a central passage 1 6 in communication with a reservoir port 9A formed in the closure plug 4. The tubular member 1 5 is co axial with the piston 6 and a blind bore 1 7 is provided in the piston aligned with the tubular member 1 5 to receive the tubular member as the piston advances during a pressure generating stroke. During a pressure generating stroke the inner lip 1 8 of the seal 7 slidingly and sealingly engages theexterior surface of the tubular member 1 5.

A plurality of radially extending passages 1 9 pass through the wall of the tubular member 1 5 at a location such that, when the piston is in the rest position, the working chamber 8 is connected to the reservoir port 9A via the passages 1 9 and the central passage 1 6. The passages 19 are, however, located adjacent the lip 18 of the seal 7 so that at the commencement of a pressure generating stroke of the piston 6 the lip 1 8 passes over the passages 19 to isolate the working chamber 8 from the reservoir port 9A.

In use, a hydraulic fluid reservoir is in free communication with the reservoir port 9A, so that, each time the piston 6 returns to the rest position illustrated the working chamber 8 is connected to the reservoir to provide for make-up of hydraulic fluid and to accommodate thermal expansion of hydraulic fluid.

It will be appreciated that the above described master cylinder is particularly simple to construct since no apertures need be formed in the wall of the body 2, and no exterior connections associated with such apertures need be provided.

Further, the region 20 of the bore 2 on the side of the pressure generating seal 7 remote from the working chamber 8 is dry, i.e. it does not contain hydraulic fluid. Accordingly, the need for the conventional secondary or atmospheric seal at the end of the piston remote from the pressure * generating seal 7 is obviated. Since the master cylinder only requires a single seal between the piston and the cylinder bore the cost of construction of the cylinder is correspondingly reduced.

In the above described embodiment of the invention the tubular member 1 5 is formed integrally with the closure plug 4. The alternative embodiment illustrated in Figure 2 shows an arrangement in which a tubular member 1 5A is provided which is a separate member from the closure plug 4A. The tubular member 1 5A is formed of light gauge metal and includes a flange 21 which is received within a bore in the closure plug 4A. The flange is retained within the bore by an O-ring 22 and a washer 23 which is acted upon by the piston spring 11 A. The tubular member 1 5A is a clearance fit within the washer 23A, and this fact combined with the resiiience of the O-ring 22 enables the tubular member 1 5A to align itself with the piston bore 1 7A in use and accordingly accommodate any eccentricity of the bore 1 7A.

Operation of the embodiment of Figure 2 is as described above with reference to the embodiment of Figure 1.

Advantageously, the closure plug 4 and tubular member 1 5 are integrally formed as a one-piece plastics moulding, or as a one-piece die casting of a suitable ailoy. Similarly, the closure plug 4A of the embodiment of Figure 2 can be moulded from plastics material or die cast from a suitable alloy.

In the case of both embodiments an O-ring seal 24 is provided between the closure plug and the master cylinder body.

Claims (10)

1. A hydraulic master cylinder comprising: a body defining a cylindrical bore; a closure plug secured to the body and closing one end of the bore; a piston slidably mounted in the bore to define a working chamber and movable from a rest position towards the closure plug to displace hydraulic fluid from the working chamber through an outlet port formed in the closure plug; a tubular member mounted on the closure plug and extending through the working chamber towards the piston; a bore in the piston into which the tubular member is received as the piston advances towards the closure plug; a reservoir port formed in the closure plug and in communication with the interior of the tubular member; a seal mounted on the piston for sealingly engaging the body bore and the exterior surface of the tubular member; and at least one passage in the tubular member for connecting the interior of the tubular member to the working chamber when the piston is in the rest position whereby the working chamber is in communication with the reservoir port, such communication being interrupted by the seal at the commencement of the working stroke of the piston.

2. A hydraulic master cylinder according to claim 1 wherein the body is formed from tubular stock.

3. A hydraulic master cylinder according to claim 1 or claim 2 wherein the closure plug is secured to the body by an inwardly deformed portion of the body and an elastomeric seal is provided between the closure plug and the cylindrical bore.

4. A hydraulic master cylinder according to any preceding claim wherein the bore in the piston is closed except for an entrance through which the tubular member passes and wherein the portion of the body bore which is located on the side of the piston seal remote from the working chamber is dry.

5. A hydraulic master cylinder according to any preceding claim wherein the said at least one passage comprises a plurality of radial bores extending through the wall of the tubular member.

6. A hydraulic master cylinder according to any preceding claim wherein the tubular member is integral with the closure plug.

7. A hydraulic master cyclinder according to claim 6 wherein the tubular member and closure plug are a one-piece plastics moulding.

8. A hydraulic master cylinder according to any of claims 1 to 5 wherein the tubular member is movably mounted on the closure plug to facilitate self-alignment of the tubular member with the piston bore.

9. A hydraulic master cylinder according to claim 8 wherein the tubular member has an outwardly directed flange located in an over-size bore in the closure plug and wherein an elastomeric seal is held located in said over-size bore to provide a fluid seal between the tubular member and the closure plug and to accommodate relative movement between the tubular member and the closure plug.

10. A hydraulic master cylinder substantially as hereinbefore described with reference to and as shown in the accompanying drawings.