GB2176552A

GB2176552A - A hydraulic master cylinder

Info

Publication number: GB2176552A
Application number: GB08515056A
Authority: GB
Inventors: Maurice John Wimbush
Original assignee: Automotive Products PLC
Current assignee: Automotive Products PLC
Priority date: 1985-06-14
Filing date: 1985-06-14
Publication date: 1986-12-31
Also published as: GB8515056D0

Abstract

The master cylinder has a body 13 and a piston 11 sealed with the body by a primary seal 13 and movable in the body to pressurise a cylindrical high pressure chamber 42 for connecting to a hydraulically operated device. The piston 11 has a tail portion 15 sealed with the body by a secondary seal 18. An annular low pressure chamber 47 is defined by the body and the piston between the primary seal 13 and the secondary seal 18, the body having a main feedport 34 opening into the low pressure chamber for connection to a hydraulic fluid reservoir 27. The high pressure chamber 42 and low pressure chamber 47 are of equal diameter. <IMAGE>

Description

SPECIFICATION Hydraulic master cylinder This invention relates to hydraulic master cylinders, particularly but not exclusively for vehicle hydraulic braking systems.

A typical known hydraulic master cylinder comprises a body, a piston sealed with the body by a primary seal and movable in the body to pressurise a high pressure chamber for connection to a hydraulically operated device, such as a vehicle brake, a piston tail portion sealed with the body by a secondary seal and and annular low pressure chamber for connection to a hydraulic fluid reservoir.

Such a master cylinder is described in our British Patent Application No. 8505773 and the present invention is concerned with improvements in such a master cylinder.

According to one aspect of the invention, there is provided a hydraulic master cylinder comprising a body, a piston sealed with the body by a primary seal and movable in the body to pressurise a cylindrical high pressure chamber for connection to a hydraulically operated device, a piston tail portion sealed with the body by a secondary seal, an annular low pressure chamber defined by the body having a main feedport opening into the low pressure chamber for connection to a hydraulic fluid reservoir, the high pressure chamber and low pressure chamber being of equal diameter.

Such an arrangement assists in the manufacture of the body and in assembly of the master cylinder. Preferably a removable end closure is provided on the body to facilitate assesmbly of the hydraulic master cylinder.

The tail portion may comprise an elongate stem having one end in engagement with the piston and its other end rigid with a cylindrical member carrying the secondary seal.

According to another aspect of the invention there is provided a hydraulic master cylinder comprising a body, a piston sealed with the body by a primary seal and movable in the body to pressurise a high pressure chamber for connection to a hydraulically operated device, a piston tail portion sealed with the body by a secondary seal, an annular low pressure chamber defined by the body and the piston between the primary seal and the secondary seal, the body having a main feed port opening into the low pressure chamber for connection to a hydraulic fluid reservoir, the piston tail portion comprising an elongate stem having one end in engagement with the piston head and its other end rigid with a cylindrical member carrying the secondary seal.

Preferably the stem and cylindrical member are formed as a one piece element.

The piston may include an axial projection having means therein which is located remote from the piston by support means in the high pressure chamber.

Preferably the axial projection and piston are formed as a one piece element. The support means is preferably located on a secondary piston in the high pressure chamber.

The low pressure chamber may be divided into two portions by an auxiliary seal in which there is encapsulated a reinforcing element.

A hydraulic master cylinder in accordance with the invention will now be described by way of example with reference to the accompanying drawing which is a cross-section through a tandem master cylinder and hydraulic fluid reservoir in accordance with the invention.

The tandem master cylinder includes a primary piston 11 having a primary seal 9 and a secondary piston 12 both slidable in a body 13 in the manner of a conventional tandem master cylinder.

The primary piston 11 is formed with a socket 14 extending from its lower end as viewed in the drawing which locates the upper end of a piston tail portion stem 15. The stem 15 extends sealingly through an annular auxiliary seal 16 which is described in detail below. The lower end of the stem 15 is integral with a cylindrical member 17 which carries a secondary seal 18. A pushrod 36 is retained by a washer 19 and circlip 20 and is arranged for connection to a driver's brake pedal.

The primary piston 11 is also formed with an upwardly extending projection 22 which locates at its upper end in the lower end of a sleeve 23 locates at its upper end on the secondary piston 12. The sleeve 23 is biassed normally away from the primary piston by a spring 24 and is held captive on the projection 22 by a screw 25.

A reservoir 27 is secured to the body 13 at flanger 28 and 29 and has a filler cap 30.

Rubber gaskets 31,31a seal the reservoir 27 at the flanges 28, 29.

The auxiliary seal is secured in a bore 32 in the body by a groove 33 between a main feed port 34 and an auxiliary feed port 35 in the body. The reservoir 27 incorporates an internal wall 36 for separating a volume of fluid which is directly connected to the main feed port 34 from a volume of fluid which is directly connected to the auxiliary feed port 35. The gasket 31a seals against the wall 36.

The primary piston 11, secondary piston 12 and cylindrical member 17 are slidable in the bore 32 and it will be noted that the bore is of constant diameter throughout its entire length. This feature is particularly useful because it makes the body easier to manufacture compared to a cylinder where the body has a bore comprising sections of differing diameters. For example, the use of a constant diameter bore avoids concentricity problems between bore sections. Furthermore, the use of a constant diameter bore enables the com plete assembly of pistons to be assembled into the body from the upper end of the bore 32 which can be closed by a detachable closure cap 40.

The bore 32 includes a high pressure chamber 42 above the primary piston 11 and a breather port 43 connects the reservoir 27 to the high pressure chamber 42. The high pressure chamber 42 is arranged for connection to a hydraucally operated device (not shown), in this case vehicle brakes, through a first outlet port 45 which opens into the high pressure chamber 42 tangentially. A second outlet port 46 is formed towards the upper end of the cylinder in known manner.

The body 13 and the primary piston 11 between the primary seal 9 and the secondary seal 18 define an annular low pressure chamber 47 divided by the auxiliary seal 16 into an upper portion 48 between the primary seal 9 and the auxiliary seal 16 and a lower portion 49 between the auxiliary seal 16 and the secondary seal 18. Hence the main feed port 34 is between the primary seal 9 and the auxiliary seal 16 and the auxiliary feeds port 35 opens into the lower portion 49 between the auxiliary seal 16 and the secondary seal 18.

The secondary seal 18 provides a seal between the lower portion 49 and the ambient air at atmospheric pressure on the other side of the seal, thereby inhibiting leakage of air into the low pressure chamber.

When the primary piston 11 is moved upwards by the pushrod 36, the primary seal 9 wipes over the breather port 43 to increase pressure in the high pressure chamber 25 and direct fluid to the appropriate brake through the port 45.

If air enters the lower chamber portion 49 past the secondary seal 18, it is directed into the fluid reservoir 27 through auxiliary feed port 35 by the auxiliary seal 16. To assist this, the seal 16 has a frusto-conical surface 50 so as to present a convex surface towards the lower chamber portion 49. Furthermore, the volume of the lower chamber portion 49 varies during the working stroke of the piston 11, in this case decreasing during movement of the piston 11 in the direction to increase pressure in the high pressure chamber 42.

The flow of hydraulic fluid out of the lower chamber portion 49 through the auxiliary feed port 35 during this part of the working stroke of the piston helps to sweep air bubles into the reservoir 27. The wall 36 helps to ensure that only air-free hydraulic fluid can enter the upper chamber portion 48 through the main feed port 34.

The auxiliary seal 16 is of annular form and is moulded from suitable resilient material. In order to reduce flexing of the seal during movement of the stem 15, an annular metal plate 60 is encapsulated in the washer during moulding so that it lies generally midway between the upper and lower ends of the washer.

The arrangement of integral stem 15 and cylindrical member 17 is useful in that the location of the upper end of the stem 15 in the piston socket 14 prevents tipping of the member 17 in the cylinder bore due to thrusting of the pushrod 36. The primary piston 11 is itself prevented from tipping by the projection 22 located in the sleeve 23.

The use of a constant diameter bore 32 is also useful in that it keeps transfer of fluid between chamber 47 and the reservoir 27 to a minimum during movement of the piston.

Claims

1. A hydraulic master cylinder comprising a body, a piston sealed with the body by a primary seal and movable in the body to pressurise a cylindrical high pressure chamber for connection to a hydraulically operated device, a piston tail portion sealed with the body by a secondary seal, an annular low pressure chamber defined by the body and the piston between the primary seal and the secondary seal, the body having a main feedport opening into the low pressure chamber for connection to a hydraulic fluid reservoir, the high pressure chamber and low pressure chamber being of equal diameter.

2. A hydraulic master according to Claim in which a removable end closure is provided on the body to facilitate assembly of the hydraulic master cylinder.

3. A hydrualic master cylinder according to Claim 1 or 2 in which the piston tail portion comprises an elongate stem having one end in engagement with piston and its other end rigid with a cylindrical member carrying the secondary seal.

4. A hydraulic master cylinder comprising a body, a piston sealed with the body by a primary seal and movable in the body to pressurise a high pressure chamber for connection to a hydraulically operated device, a piston tail portion sealed with the body by a secondary seal an annular low pressure chamber defined by the body and the piston between the primary seal and the secondary seal, the body having a main feedport opening into the low pressure chamber for connection to a hydraulic fluid reservoir, the piston tail portion comprising an elongate stem having one end in engagement with the piston head and its other end rigid with a cylindrical member carrying the secondary seal.

5. A hydraulic master cylinder according to claim 3 or 4 in which the stem and cylindrical member are formed as a one-piece element.

6. A hydraulic master cylinder according to claims 3,4 or 5 in which the piston includes an axial projection having meanstherein which is located remote from the piston by support means in the high pressure chamber.

7. A hydraulic master cylinder according to Claim 6 in which the axial projection and pis ton are formed as a one-piece element.

8. A hydraulic master cylinder according to Claim 6 or 7 in which the support means is located on a secondary piston in the high pressure chamber.

9. A hydraulic master cylinder according to any preceding claim in which the low pressure chamber is divided into two portions by an auxiliary seal in which there is encapsulated a reinforcing element.

10. A hydraulic master cylinder comprising a body, a piston sealed with the body by a primary seal and movable in the body to presurise a high pressure chamber for connection to a hydraulically operated device, a piston tail portion sealed with the body by a secondary seal,an annular low pressure chamber defined by the body and the piston between the primary seal and the secondary seal, the body having a main feedport opening into the low pressure chamber for connection to a hydraulic fluid reservoir, the low pressure chamber being divided into two portions by an auxiliary seal in which there is encapsulated a reinforcing element.

11. A hydraulic master cylinder according to Claim 9 or 10 in which the auxiliary seal is annular and the reinforcing element is an annular stiffening plate arranged substantially midway between the ends of the auxiliary seal.

12. A hydraulic master cylinder constructed and arranged substantially as described herein with reference to the accompanying drawings.