GB2246178A - Hydraulic master cylinder - Google Patents

Abstract

A master cylinder/serve combination includes a primary piston formed by an inner portion (33) slidingly and sealingly mounted in an annular outer portion (34), reaction forces being fed back by the inner portion (33) through an elastomeric reaction disc (37) to the input member (40) and the outer piston (34) being connected to the serve piston by a tubular rod (6). An expander chamber (10) houses piston (11) which is driven to the right when pressure in the primary chamber (7) reaches a predetermined value to cause fluid to flow into a secondary chamber (9) located behind the primary piston seal (30) to reduce the effective area of the primary piston to that (A2) of the tubular rod (6). Because feedback forces are always generated only by the inner piston (33) there is no discontinuity in pedal feel when the expander piston (11) commences movement. <IMAGE>

Description

HYDRAULIC MASTER CYLINDER This invention relates to a hydraulic master cylinder, and in the preferred embodiment provides a hydraulic master cylinder suitable for the braking system of a motor vehicle.

It is known to provide hydraulic master cylinders in which, upon initial application of the brakes, hydraulic fluid is displaced by a relatively large effective piston area in order to take up, clearances within the braking system rapidly and with minimum piston travel. When the fluid pressure within the braking system reaches a predetermined value the effective area of the master cylinder piston used for pressure generation is reduced so that a high fluid pressure can be produced at an acceptably low level of input force.

A master cylinder of the above type is illustrated in our British patent publication GB-A-2072281.

In this publication there is described a hydraulic master cylinder comprising: a reservoir for hydraulic fluid; a body defining a primary bore and a secondary bore co-axial with and having a smaller cross-sectional area than the primary bore; a piston rod slidingly and sealingly mounted in the secondary bore; a piston connected to the piston rod and slidingly and sealingly mounted in the primary bore, the piston dividing the primary bore into a primary chamber located on the side of the piston remote from the piston rod and a secondary annular chamber located between the piston rod and the wall of the primary bore; an outlet connected to the primary chamber for supplying fluid from the primary chamber to hydraulic apparatus; an expander chamber in fluid communication with both the primary chamber and the secondary chamber; a movable expander member located within the expander chamber and dividing the expander chamber into a primary portion which is in fluid communication with the primary chamber and a secondary portion which is in fluid communication with the secondary chamber, the expander member being movable in response to a predetermined pressure differential across the primary and secondary chambers to expand the volume of the primary portion of the expander chamber to receive fluid from the primary chamber and to contract the volume of the secondary portion of the expander chamber and thereby supply fluid to the secondary chamber; means permitting fluid to flow from the reservoir into the secondary chamber until said predetermined pressure differential is reached; and means thereafter preventing fluid flow between the reservoir and the secondary chamber whereby, during brake application, after said predetermined pressure differential is attained said secondary chamber is pressurized and the effective cross-section area of said master cylinder is the crosssectional area of the secondary bore.

Whilst such a hydraulic master cylinder fulfils the objective of coupling rapid take up of clearance within the braking system with the ability to generate a high pressure from an acceptably small input force, the master cylinder suffers from the disadvantage that the ratio of output pressure to input force changes when the predetermined pressure is reached. The effect of this is that when the brakes are applied the reaction force felt on the brake pedal is initially characteristic of a brake system requiring a large pedal force coupled with relatively little pedal travel, but changes to be characteristic of a braking system requiring a relatively smaller pedal force but larger travel when the change over from pressure generation over a large area to a small area occurs.This can be disconcerting and can lead to an inadvertent over application of the brakes as the brake pedal "gives" when the predetermined changeover pressure is reached.

The present invention is characterized in that the piston comprises two axially arranged piston parts, the outer piston part being connected to the diaphragm piston of a brake servo device, and the inner piston part being connected to the elastomeric reaction disc of the servo device to provide the pedal reaction force to the brake pedal. In this way, the area of the piston which is used to provide the reaction force to the brake pedal is independent of the overall area of the piston and may be selected to provide the desired feedback force to the brake pedal. Since the effective area of the inner piston part is not affected by the change in effective piston area which occurs at the predetermined pressure, there is no discontinuity in the feel of the brakes as sensed by the reaction force of the brake pedal.

The invention will be better understood 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 schematic cross-sectional view of a preferred embodiment of the invention; Figure 2 is a graph illustrating the output pressure verses input force characteristics of the embodiment of Figure 1 and of a conventional master cylinder of the type shown in GB-A-2072281; Figure 3 is a second embodiment of the invention; and Figure 4 illustrates the third embodiment of the invention.

Referring firstly to Figure 1 the illustrated hydraulic master cylinder 1 comprises a body 2 defining a primary bore 3 having an area A1 and a secondary bore 4 having an area A2 which is smaller than the area A1 of the primary bore. A piston assembly 5 is slidingly and sealingly mounted in the primary bore 3 and is connected to a piston rod 6 which is slidingly and sealingly mounted in the secondary bore 4. The piston assembly 5 divides the primary bore into a primary chamber 7 which is permanently connected to a braking circuit by way of an outlet 8, and a secondary chamber 9 which is annular and is located between the piston rod 6 and the wall of the primary bore 3.

An expander chamber 10 is located parallel to the bores 3,4 and has slidably mounted therein an expander piston 11 which divides the expander chamber into a primary portion 12 which is in fluid communication with the primary chamber 7 by way of a passage 13, and a secondary portion 14 which is connected to the secondary chamber 9 by way of a passage 15.

The expander piston 11 is biased to the left as viewed in Figure 1 by a spring 16. The zone 17 between seals 18 and 19 of the expander piston is connected to a hydraulic fluid reservoir (not shown) by means of a suitable passage. A poppet valve 20 is mounted within the expander piston 11 and, in the illustrated rest position of the components, is held open by a bar 21 positioned within a slot 22 formed in the expander piston. The bar 21 is permanently fixed to the body 2 to provide a fixed abutment for the poppet valve 20 when the expander piston 11 is in the illustrated position. The poppet valve 20 is provided with a light spring 23 which biases the poppet valve into engagement with a seat 24 provided on the expander piston as soon as the expander piston moves to the right of the illustrated position.

A secondary piston 25 is slidably mounted in a bore 26 to define a chamber 27 having an outlet 28 for connection to a further brake circuit.

In the illustrated rest position of the components the secondary portion 10 of the expander chamber is connected to the hydraulic fluid reservoir via the poppet valve 20, and the primary and secondary chambers 7,9 are connected to the secondary expander chamber 10 via the passage 15 and a branch passage 29 which enters the primary chamber 7 via a port located just forward of the leading edge of the piston seal 30. The chamber 27 is connected to reservoir via a poppet valve 31 held open by a bar 32 and via a passage (not shown) which connects the zone between the seals of the secondary piston 25 to the reservoir.

The piston assembly 5 comprises co-axially arranged radially inner and outer piston portions 33 and 34 respectively. The inner and outer piston portions are arranged for limited relative axial movement, and are in sliding sealing contact via a seal 35. The area of the inner piston portion is, in the illustrated embodiment, A2 and is thus the same as the area of the piston rod 6.

However, this is not a requirement of the invention and the area of the inner piston portion may be selected as desired. The outer piston portion 34 is rigidly connected to the piston rod 6. The zone to the left (as illustrated) of the inner piston portion is exposed to the pressure in the primary chamber 7, and the area to the right of the inner piston portion is exposed to atmosphere.

A push rod 36 is slidably mounted within the piston rod 6 and extends from abutting engagement with the inner piston portion 33 to abutting engagement with an elastomeric reaction disc 37 of a vacuum servo 38. The piston rod 6 is rigidly connected to the diaphragm piston 39 of the servo. An input rod 40 connected to a conventional brake pedal provides the input to the servo as will be understood by those skilled in the art.

In use, when the brakes are not applied the various components are in the illustrated positions and equal vacuum is present on both sides of the diaphragm piston 39. When the brakes are to be applied a force is applied to the input member 40 in the direction of the arrow A thereby isolating the chamber 41 to the right of the diaphragm piston 39 from the vehicle manifold, and admitting atmospheric air to this chamber. The piston 39 is accordingly moved to the left carrying with it the piston assembly 5 to displace hydraulic fluid from the master cylinder over the effective area A1 of the primary piston. The pressure in the primary chamber 7 will be applied to the left face of the inner piston portion 33 to produce a force which is transmitted via push rod 36 to the elastomeric reaction disc 37.The force will be apportioned by the disc 37 in conventional manner to provide a reaction force on the input member 40.

During initial leftward movement of the piston 5 hydraulic fluid will be supplied to the secondary chamber 9 from the reservoir via the poppet valve 20, secondary expander chamber 10, and passage 15. Any clearance within the brake system will rapidly be taken up because of the large volume of fluid displaced from the master cylinder owing to its large effective area A1. When all brake clearances have been taken up the pressure within the primary chamber 7 will rise, and this will be communicated via the passage 13 and primary expander chamber 12 to the piston 11. When the pressure in the primary chamber 7 has reached a predetermined level, typically 10 bar, the expander piston 11 will move against the force of spring 16 thereby permitting the poppet valve 20 to seat and isolate the secondary expander chamber 10 from the reservoir.Any further movement of the piston to the left will cause fluid to flow from the primary chamber 7 into the primary expander chamber 12, and from the secondary expander chamber 10 into the secondary chamber 9 as described in more detail in GB-A-2072281. The effect of this will be to reduce the net effective area of the master cylinder to the area of the piston rod 6, namely A2. Although the effective area of the master cylinder changes, the reaction force on the brake pedal is still produced by the action of fluid pressure over the area of the left face of the inner piston portion 33, and adcordingly there will be no change in the characteristics of the master cylinder, as detected by reaction on the brake pressure, at the point when the pressure generating area changes from A1 to A2.

Referring now to Figure 2, the characteristics of the master cylinder of Figure 1 are plotted in solid line, whilst the characteristics of the master cylinder of Figure 1 of GB-A-2072281 are plotted in broken line. In both cases, output pressure is plotted against pedal force. It will be noted that in the case of the prior art design there is a change in characteristics at the threshold pressure of 10 bar. No such change in characteristics occurs in the illustrated embodiment of the present invention.

Referring now to Figure 3, the illustrated embodiment is substantially the same as that described above with reference to Figure 1 except that the elastomeric reaction disc 37' of the brake servo is located within the master cylinder piston, and the servo input rod 40' has an extension which passes through the piston rod 6 to engage the elastomeric disc 37'. This arrangement offers a number of advantages, and in particular simplifies the construction of the vacuum servo whilst at the same time positioning the reaction disc within the metal components of the master cylinder piston which are well able to cope with the stresses imposed by the reaction disc in use. Further, the fact that the reaction disc is housed within the main master cylinder piston will reduce the valve mechanism hysteresis whilst air is being metered into the servo chamber because the piston will move very little.

Referring now to Figure 4, the arrangement is similar to that of Figure 1 except that the inner piston portion 33' is provided with a second seal 41 which, together with the first seal 35' defines a chamber 42 which is connected to the secondary chamber 9' by a suitable bore 43. Thus, the reaction force applied to the push rod 36' is the sum of the pressure in the primary chamber 7' acting over the area of the seal 41 and the pressure in the secondary chamber 9' acting over the annular area between the seals 41 and 35'. With this design, the reaction force felt by the driver is less during the initial phase of taking up clearances because during this phase the secondary chamber 9' is connected to reservoir. At high pressures, the reaction force produced is, however, substantially the same as that of the Figure 1 embodiment because the pressure within the secondary chamber 9' is only slightly less than that in the primary chamber 7'.

Claims (5)

1. A hydraulic master cylinder comprising: a reservoir for hydraulic fluid; a body defining a primary bore and a secondary bore co-axial with and having a smaller cross-sectional area than the primary bore; a piston rod slidingly and sealingly mounted in the secondary bore; a piston connected to the piston rod and slidingly and sealingly mounted in the primary bore, the piston dividing the primary bore into a primary chamber located on the side of the piston remote from the piston rod and a secondary annular chamber located between the piston rod and the wall of the primary bore; an outlet connected to the primary chamber for supplying fluid from the primary chamber to hydraulic apparatus; an expander chamber in fluid communication with both the primary chamber and the secondary chamber; a movable expander member located within the expander chamber and dividing the expander chamber into a primary portion which is in fluid communication with the primary chamber and a secondary portion which is in fluid communication with the secondary chamber, the expander member being movable in response to a predetermined pressure differential across the primary and secondary chambers to expand the volume of the primary portion of the expander chamber to receive fluid from the primary chamber and to contract the volume of the secondary portion of the expander chamber and thereby supply fluid to the secondary chamber; means permitting fluid to flow from the reservoir into the secondary chamber until said predetermined pressure differential is reached; and means thereafter preventing fluid flow between the reservoir and the secondary chamber whereby, during brake application, after said predetermined pressure differential is attained said secondary chamber is pressurized and the effective crosssection area of said master cylinder is the cross-sectional area of the secondary bore; characterized in that the piston comprises two coaxially arranged piston parts, the outer piston part being connected to the diaphragm piston of a brake servo device, and the inner piston part being connected to the elastomeric reaction disc of the servo device to provide the pedal reaction force to the brake pedal.

2. A master cylinder according to claim 1, wherein said inner piston part abuts a push rod which is slidably mounted in the piston rod and which extends to abutting engagement with the reaction disc.

3. A master cylinder according to claim 1, wherein said reaction disc is located within said piston.

4. A master cylinder according to any preceding claim, wherein said inner piston part is slidable within an inner chamber, said inner chamber being in fluid communication with said secondary chamber.

5. A master cylinder substantially as hereinbefore described with reference to the accompanying drawings.