SERVO ASSISTED HYDRAULIC MASTER CYLINDER ASSEMBLY

Abstract: A servo assisted hydraulic master cylinder assembly comprising a tandem master cylinder and a vacuum servo has the elastomeric reaction disc (42) for providing pedal feedback located within the master cylinder body (38), rather than in the more usual position adjacent the servo valving arrangement (43). Repositioning of the elastomeric member in this manner enables a stop pin (34) for determining the rest position of the servo valve member to be located within the master cylinder body to abut an end wall (37) provided for this purpose within the master cylinder body. The master cylinder primary piston consists of an inner part (41) to provide a reaction force to the elastomeric disc (42) and an outer annular part connected directly to the servo piston by a tubular strut (36) to provide for quick fill of the braking system.

Description: SERVO ASSISTED HYDRAULIC MASTER CYLINDER ASSEMBLY This invention relates to a servo assisted hydraulic master cylinder assembly suitable for use in a motor vehicle braking system and comprising: a master cylinder having a bore and a primary piston mounted in the bore for displacing hydraulic fluid from the master cylinder; an input rod axially movable by a brake pedal to apply an operating force to the primary piston via an elastomeric member positioned between the input rod and the primary piston; and a servo device operable in response to movement of the input rod to apply an operating force to the primary piston via the elastomeric member to supplement the operating force provided by the input rod, the elastomeric member being operable to apportion the reaction force produced by the primary piston in response to the input forces between the input rod and the servo device. Servo assisted hydraulic master cylinder assemblies of the above type are well known and are shown, for example, in GB-A-2009871. In assemblies of this type of the prior art the elastomeric member is located close to the valving arrangement which controls the admission of working fluid to the servo device, and within the body of the servo device. According to one aspect, the present invention is characterised in that the elastomeric member is located within the bore of the master cylinder. By locating the elastomeric member within the bore of the master cylinder and thus displaced from the valving arrangement associated with the servo device a number of significant advantages are derived. In particular, in the absence of the elastomeric member it is considerably easier to manufacture the piston of the servo device since firstly the passages associated with the valving arrangement may be made larger and less restrictive, and secondly the servo piston is no longer required to withstand the burst stresses generated by the elastomeric disc during braking. As a result of this simplification in servo piston design the servo piston can readily be produced as a plastics moulding devoid of metal reinforcing inserts. Further, because the elastomeric member is shifted to the zone of the master cylinder the servo piston can be positively supported from both sides. In one embodiment of the invention the primary piston forms part of a primary piston assembly comprising the primary piston itself and an annular piston within which the primary piston is slidingly and sealingly mounted. The annular piston is connected to the servo piston of the servo device and the elastomeric member is housed within the annular piston in engagement with a shoulder which transmits to the elastomeric member the operating force provided by the servo device. With such an arrangement the effective area of the primary piston assembly is the sum of the effective area of the primary piston and the effective area of the annular piston. However, the feedback force to the input rod is derived solely from the primary piston. If the master cylinder is provided with a secondary piston mounted in the bore to divide the bore into primary and secondary chambers serving different brake circuits, this arrangement enables the secondary piston to be manufactured with an effective area equal to that of the primary piston and as a result in the event of failure of the brake circuit served by the chamber located between the primary and secondary pistons the feedback to the brake pedal produced by the fluid pressure in the secondary chamber will be the same as that produced during normal operation by the fluid pressure in the primary chamber. In one embodiment of the invention the stop provided for determining the rest position of the valve seat of the servo is also located within the bore of the master cylinder. In prior art arrangements in which the elastomeric member is located close to the valving arrangement of the servo it was necessary also to locate the stop which determined the rest position of the valve seat of the servo close to the valving arrangement. Positioning of the elastomeric member within the master cylinder bore enables the stop also to be located in the master cylinder bore and to react against a portion of the master cylinder body to determine the rest position of the seat of the valving arrangement of the servo. This obviates the need to position the stop within the servo body. By positioning the stop within the master cylinder bore the accumulated manufacturing tolerances are substantially less and the primary piston seal can accordingly be accurately positioned immediately adjacent the recuperation port. A servo assisted hydraulic master cylinder assembly is disclosed in GB-A-2220239 in which the primary piston assembly comprises inner and outer portions connected respectively to the input rod and the piston of the servo assembly, and in which a resilient reaction dice is located in the bore of the master cylinder. However, the resilient reaction dice does not function in the manner of the present invention to transfer the operating forces provided by the input rod and the servo device to the primary piston, and to apportion the reaction force - 6. produced by the primary piston between the input rod and the servo device. Accordingly he assembly of GB-A-2220239 does not fall within the pre-characterising portion of the main claim of this specification. The invention will be better understood and further advantages thereof will become apparent from the following descriptions of preferred embodiments 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 first embodiment of the invention; Figure 2 is a schematic cross-sectional view of a second embodiment of the invention; and Figure 3 is a schematic cross-sectional view of a third embodiment of the invention. Referring firstly to Figure 1 the illustrated servo assisted master cylinder assembly 1 comprises a master cylinder body 2 having a bore 3 and a primary piston 4 slidably mounted within the bore for displacing hydraulic fluid from the master cylinder. A secondary piston 5 is also slidably mounted in the bore and divides the bore into a primary chamber 6 and a secondary chamber 7 which are connected to respective use circuits as will be understood by those skilled in the art. Connections 8 are provided for connecting a tandem reservoir to the master cylinder body 2. The master cylinder body 2 is mounted on the casing 9 of a servo device 10 of the vacuum type. The servo device 10 comprises a servo piston 11 connected by way of a diaphragm 12 to the casing 9 to divide the casing into a first chamber 13 and a second chamber 14. As will be understood by those skilled in the art when the brake system is not in use both chambers 13,14 are connected to a suitable vacuum source, for example the vehicle manifold. A valving arrangement 15 is provided for selectively connecting the chamber 14 to the vacuum source or to atmosphere. If the chamber 14 is connected to atmosphere the pressure differential across the servo piston 11 produces a force which is transmitted via a tubular strut 16 to an elastomeric member 17 mounted within the master cylinder body 2 between the rear face of the primary piston 4 and a shoulder 18 provided by the strut. An input rod 20 which, in use, is connected to a O 92/01594 PCT/GB91/0116 - 6 - brake pedal serves to control the valving arrangement 15 and, when moved to the left as viewed in Figure 1 applies a force to the elastomeric member 17. Hence, when the brakes are in use the leftward acting force applied to the elastomeric member 17 is the sum of the force applied by the input rod 20 and the force applied by the servo piston 11. The elastomeric member 17 which under load substantially retains its original volume completely fills the space within the tubular strut 16 between the shoulder 18 and input rod 20, and the rear face of the primary piston 4. Hence, the force applied to the elastomeric member by the tubular strut 18 and the input rod 20 is transferred to the primary piston 4. In use, when the brakes are to be applied the brake pedal is depressed which applies a force to the input rod 20 and displaces the input rod to the left as viewed in Figure 1. As will be understood by those skilled in the art movement of the rod 20 controls the valving arrangement 15 to isolate the chamber 14 from the vacuum source and to admit atmosphere to the chamber 14. The pressure differential thus created across the servo piston 11 drives the servo piston to the left and the resultant forces applied to the elastomeric member 17 by the shoulder 18 and the input rod 20' displace the primary piston 4 to the left to pressurize the chambers 6,7. Whilst the brakes are being applied in this manner there is very little relative movement between the input rod 20 and the servo piston 11 , that is to say the input rod 20 and the servo piston 11 move forward substantially together. Pressure within the chamber 6 acting on the primary piston 4 produces a reaction force which is applied by the piston 4 to the elastomeric member 17. As will be understood by those skilled in the art the elastomeric member 17 apportions the reaction force between the shoulder 18 and the end face 20 of the input rod substantially in the ratio of the respective areas of the shoulder 18 and the end face of the input rod. Thus the driver feels a reaction force proportional to but substantially less than the total reaction force generated by the piston 4. When the brakes have been applied to a sufficient extent the driver ceases further downward movement of the brake pedal and maintains the brake pedal and thus the input rod 20 at a constant position. Slight forward movement of the servo piston 11 relative to the input rod 20 then occurs with the result that the valving arrangement 15 is closed to maintain all the components in the resultant position until the brakes are either applied further, or are released. Upon release of the brakes rearward movement of the input rod 20 controls the valving arrangement 15 to isolate the chamber 14 from atmosphere and connect the chamber to the vacuum source with the result that the components are restored to the illustrated position in which the primary piston 4 is fully retracted and a key 21 secured to the input rod 20 is in engagement with the casing 9 of the servo device. Referring now to Figure 2 there is illustrated a servo assisted hydraulic master cylinder assembly similar to that illustrated in Figure 1 except that the primary piston 22 forms part of a primary piston assembly 23 comprising the primary piston 22 itself and an annular piston 24 within which the primary piston 22 is slidingly and sealingly mounted. The annular piston 24 is connected directly to the servo piston 25 by a tubular strut 26. An elastomeric member 27 is mounted within the annular piston 24 between the rear face of the primary piston 22 and the end face of the input rod 28 and a shoulder 29 provided by the tubular strut 26. Operation of the device is similar to that described above except that during brake application hydraulic fluid is displaced over the entire _ 8 - area of the primary piston assembly 23. The force necessary to displace the annular piston 24 is received direct from the servo piston 25 whilst the force necessary to displace the primary piston 22 comes in part from the servo piston 25 and in part from the input rod 28. The reaction force on the annular piston 24 is transmitted direct to the servo piston 25, whilst the reaction force produced by the primary piston 22 is apportioned between the servo piston 25 and the input rod 28 by the elastomeric member. This reaction force is determined by the area of the primary piston 22 rather than the area of the primary piston assembly 23. So far as the driver is concerned, therefore, the "feel" of the brakes is consistent with a master cylinder cross-sectional area equal to that of the primary piston 22, although the actual master cylinder cross-sectional area is somewhat larger. The master cylinder bore 30 is stepped and includes a portion 30A in which a secondary piston 31 is slidably mounted, the bore section 30A having a cross- sectional area corresponding to that of the primary piston 22. In the event of failure of the hydraulic circuit supplied by the primary chamber 32 the primary piston 22 will move forward to engage the secondary piston 31 and cause pressure generation within the secondary chamber 33. The reaction force on the brake pedal will, under these circumstances, be generated by the secondary chamber pressure acting on the secondary piston 31 , and since the effective area of the secondary piston 31 is the same as that of the primary piston 22 there will be no change of brake feel as compared with normal operation of the braking system. In the Figure 2 embodiment a key 21 is provided for determining the rest position of the components, in a manner similar to that of Figure 1. Turning now to the Figure 3 embodiment, a - Q - modified arrangement for controlling the rest position of the components is illustrated. In this embodiment the pin 21 of the previous embodiments is omitted and is replaced by a pin 34 which is frictionally retained within a cross bore provided in the composite input rod 35. The pin 34 passes through clearance holes provided in the tubular strut 36 to engage an inwardly directed face 37 of the master cylinder body 38 when the components are in the rest position. This arrangement has the advantage that the tolerance of the various components can be relatively easily controlled to ensure that in the rest position of the various components the leading edge of the primary piston assembly seal 39 is immediately to the rear of the primary chamber recuperation passage 40. Hence, very little pedal travel is necessary to displace the primary piston assembly 41 before pressure generation commences. A stop arrangement similar to that illustrated in Figure 3 may, of course, be utilized in the embodiments of Figures 1 and 2 in order to provide the advantage of precise conditioning of the primary piston seal assembly in those embodiments. It will be noted that in each of the above embodiments the elastomeric member is located within the master cylinder body. In this location, the elastomeric member can readily be housed within a metal component which is well able to withstand the bursting forces generated by the elastomeric member as a result of deformation thereof in use. By relieving the servo device servo piston of these forces the servo piston can be manufactured from plastics material without need for elaborate reinforcement. Further, by positioning the elastomeric member away from the valving arrangement which controls the admission of air to the servo device the various passages necessary for effective operation of the valving arrangement may be more readily produced. It will be noted that in the case of the Figure 2 and Figure 3 embodiments the apparent boost ratio, as determined by pedal feel, is determined by the area of the primary piston and is independent of the actual boost ratio which is determined by the primary piston assembly effective working area. Thus, a servo assisted master cylinder assembly can be produced having an actual boost ratio of, say, 4:1 but having an apparent boost ratio as determined by pedal feel of 2:1. The elastomeric member may thus be made more readily deformable reducing hysterisis losses and improving general feel of the braking system. Also, movement of the annular piston relative to the primary piston which occurs as the brakes reach an equilibrium state after the brakes have been applied will be amplified by the elastomeric member resulting in greater travel for the valve mechanism. Thus the primary piston assembly need only move within the flexure range of its own seal and a reversal of seal friction is avoided. This again reduces the hysterisis of the system and improves brake pedal feel.

Claims: CLAIMS 1. A servo assisted hydraulic master cylinder assembly suitable for use in a motor vehicle braking system and comprising: a master cylinder having a bore and a primary piston mounted in the bore for displacing hydraulic fluid from the master cylinder; an input rod axially movable by a brake pedal to apply an operating force to the primary piston via an elastomeric member positioned between the input rod and the primary piston; and a servo device operable in response to movement of the input rod to apply an operating force to the primary piston via the elastomeric member to supplement the operating force provided by the input rod, the elastomeric member being operable to apportion the reaction force produced by the primary piston in response to the input forces between the input rod and the servo device, characterised in that the elastomeric member is located within the bore of the master cylinder. 2. An assembly according to claim 1 characterised in that the primary piston forms part of a primary piston assembly comprising the primary piston itself and an annular piston within which the primary piston is slidingly and sealingly mounted. 3. An assembly according to claim 2 characterised in that the annular piston is connected to the servo piston of the servo device and the elastomeric member is housed within the annular piston in engagement with a shoulder which transmits to the elastomeric member the operating force provided by the servo device. 4. An assembly according to claim 3 characterised in that the annular piston is connected to the servo by a tubular strut which surrounds the input rod. 5. An assembly according to claim 3 or claim 4 characterised in that the master cylinder is provided with a secondary piston mounted in the bore to divide the bore into primary and secondary chambers serving different brake circuits, and the secondary piston has an effective area equal to that of the primary piston. 6. An assembly according to any preceding claim characterised in that a stop provided for determining the rest position of the valve seat of the servo is also located within the bore of the master cylinder. 7. An assembly according to claim 6 characterised in that the stop comprises a pin fast with the input rod.

Inventors: FARR GLYN PHILLIP REGINALD, FARR, GLYN, PHILLIP, REGINALD
Applicants: LUCAS IND PLC, LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1991
Main classification: B60T13
IPC codes: B60T13