GB2091363A

GB2091363A - Improvements in power- operated boosters

Info

Publication number: GB2091363A
Application number: GB8200549A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1981-01-21
Filing date: 1982-01-08
Publication date: 1982-07-28
Also published as: GB2091363B

Abstract

An hydraulic booster has a housing (1) with a bore (2) in which a boost piston (4) works. Pressurisation of a boost chamber (8) by fluid from a pressure source under the control of valve means (9) advances the boost piston (4) to augment a force applied to an input member (5). The augmented force is transmitted to an output member (7) which acts on a master cylinder (74) of a braking system. The output member (7) works in a bore (13, 100) in the boost piston (4) and is movable relative to the boost piston (4) in response to pressurisation of an auxiliary chamber (14). Initial movement of the input member (5) causes the valve means (9) to pressurise the auxiliary chamber (14), moving the output member (7) relative to the boost piston (4) through a distance determined by limiting means (15), to take up the lost-motion in the system. On further movement of the input member (5) the output member (7) and the boost piston (4) move together in response to pressurisation of the boost chamber (8). The limiting means (15) may be an auxiliary valve (16) or a stop member (103) on the boost piston (4). <IMAGE>

Description

SPECIFICATION Improvements in power-operated boosters This invention relates to a power-operated booster of the kind comprising a housing provided with a bore, an inlet for connection to a source of hydraulic pressure fluid and an outlet for connection to a reservoir for fluid, a pedal-operated input member, an output member, and a boost piston working in the bore, and in operation a force applied to the input member is transmitted to the output member and is augmented by the boost piston, which is advanced in the bore in response to pressurisation of a boost chamber by fluid pressure from the source, pressurisation of the boost chamber being controlled by control valve means operative to control communication between the inlet and the boost chamber, and between the boost chamber and the outlet, the control valve means being operable in response to relative movement of parts of the booster on movement of the pedal-operated input member.

Known boosters of the kind set forth are used in vehicle braking systems, where the output member applies an output force from the booster to a master cylinder in order to pressurise the braking system. It is a problem that considerable initial movement of the pedal is necessary to take up lost-motion in the braking system before the output force can be used to actuate brakeapplying means. Lost-motion in a braking system includes not only clearances between mechanical parts of the system, but also elasticity in low stiffness parts, such as seal and hoses and any springs in a brake actuator.

One attempt at solving this problem is shown in British Patent Application No. 2 049 850, in which the output member also works in the bore in the housing, and on movement of the input member pressurised fluid is applied to the output member.

This causes movement of the output member, but not of the boost piston, and so the output member applies to the master cylinder an output force which is sufficient to take up the lost-motion in the braking system. This construction, however, has the disadvantage that the length of the booster is increased by the extra stroke length required by the output member.

According to our invention, in a booster of the kind set forth, the output member works in a bore in the boost piston, and is movable in response to pressurisation of an auxiliary chamber, and on an initial movement of the input member in a brakeapplying direction the control valve means is operative to admit pressurised fluid from the source to the auxiliary chamber such that the output member is advanced relative to the boost piston through a distance determined by a limiting means, without the boost piston being advanced in the bore, and on further movement of the input member the boost piston and the output member move together in response to pressurisation of the boost chamber.

Only a small initial pedal movement is required to cause the initial movement of the input member, and the further lost-motion normally taken up by movement of the pedal is instead compensated for by movement of the output member relative to the boost piston. Arranging for the output member to work in a bore in the boost piston means that the length of the booster is not increased.

The distance through which the output member moves relative to the boost piston may be designed to take up all the lost-motion in the braking system and to pressurise the system to a relatively low level.

The auxiliary chamber is defined in the bore in the boost piston between the boost piston and the output member.

The limiting means may comprise an auxiliary valve means which is operative, when the pressure supplied to the auxiliary chamber reached a predetermined value, to isolate the auxiliary chamber to prevent further movement of the output member relative to the boost piston.

The pressure in the auxiliary chamber depends on the reaction applied to the booster from the pressure in the braking system. Thus the pressure in the auxiliary chamber at which the auxiliary valve means operates is chosen to correspond to a desired level of pressurisation of the braking system.

Alternatively, the limiting means may comprise a stop member mounted on the boost piston with which the output member engages. The output member thus moves a fixed distance relative to the boost piston.

In one construction on initial movement of the input member the control valve means is operative to open communication between the auxiliary chamber and the inlet, to supply fluid through an auxiliary valve means comprising an auxiliary valve which closes at a predetermined level of pressure supplied to the auxiliary chamber to isolate the auxiliary chamber, the control valve means then being operative to open communication between the inlet and the boost chamber.

The auxiliary valve has a valve member adapted to close at the predetermined pressure. The valve member may comprise a member biased to closure by a spring, but held open by a springloaded auxiliary piston, the piston being responsive to the pressure supplied to the chamber to allow the valve to close when the predetermined pressure is reached.

The control valve means may comprise relatively movable parts including a valve mechanism working in a bore. The valve mechanism may work in a bore in the boost piston, with one part of the valve mechanism being movable relative to the input member and having a pressure-responsive face. Closure of the auxiliary valve may result in the increase in pressure of the fluid applied to the pressureresponsive face, to move the part relative to the input member to place the inlet in communication with the boost chamber. Alternatively, closure of the auxiliary valve acts directly on the part of the valve mechanism to move it in order to place the inlet in communication with the boost chamber.In this case the auxiliary piston may be coupled to the part, and movement of the piston in response to the predetermined level of pressure supplied to the auxiliary chamber to allow the auxiliary valve to close also moves the part relatively towards the input member to place the inlet in communication with the boost chamber.

In another construction permanent communication between the boost chamber and the auxiliary chamber is provided. The limiting means may then comprise a stop member. On initial movement of the input member the control valve means is operative to supply pressurised fluid both to the auxiliary chamber and the boost chamber. In this case the relative areas of the output member and the boost piston are arranged so that the force acting on the boost piston in the forward direction is less than or equal to the force acting on the boost piston in the rearward direction until the output member has moved through the fixed distance relative to the boost piston.

The control valve means again may comprise relatively movable parts including a valve mechanism working in the bore. Preferably the valve mechanism works in a bore in the housing separate from the bore in which the boost piston works, but the valve mechanism may work in a bore in the boost piston.

Operation of the control valve means may be controlled by pressure in a control chamber, which is pressurised in response to relative movement of the input member and the boost piston.

Some embodiments of our invention are illustrated in the accompanying drawings, in which: Figure I is a longitudinal section through a pedal-operated booster; Figure 2 is a cross-section along the line 2-2 of Figure 1; Figure 3 is a fragmentary section of the booster of Figure 1 showing it combined with a master cylinder; Figure 4 is a longitudinal section through a modified booster; Figure 5 is a fragmentary section of the booster of Figure 4 showing it combined with a master cylinder; and Figure 6 is a longitudinal section through a further booster.

The booster shown in Figures 1 and 2 comprises a housing 1 provided with a longitudinally extending stepped through-bore 2, in a larger diameter portion 3 of which a boost piston 4 works. A pedal-operated input member in the form of an input piston 5 works in a smaller diameter portion 6 of the bore 2. In operation a force applied to the input piston 5 is transmitted to an output member, in the form of an output piston 7, and is augmented by the boost piston 4 which is advanced in the bore 2 in response to pressure in a boost chamber 8. Pressurisation of the boost chamber 8 is controlled by control valve means 9, which is operable in response to relative movement of the input piston 5 and the boost piston 4.The control valve means 9 controls communication between the boost chamber 8 and an inlet 10 in the housing 1 for connection to a source of hydraulic fluid, suitably an accumulator (not shown), and between the boost chamber 8 and an outlet 11 in the housing 1 for connection to a fluid reservoir (not shown).

The output piston 7, which is adapted to actuate a master cylinder (not shown) works in a larger diameter portion 1 2 of a stepped bore 1 3 in the boost piston 4. The output piston 7 is movable relative to the boost piston 4 in response to pressurisation of an auxiliary chamber 14 defined in the bore 1 3 between the output piston 7 and the boost piston 4. Limiting means 1 5, in the form of an auxiliary valve means 16, determine the distance through which the output piston 7 moves relative to the boost piston 4, the output piston 7 and the boost piston 4 thereafter moving together in response to pressurisation of the boost chamber 8.

The inlet 10 in the housing 1 is connected to an inlet port 17 in the boost piston 4 through an annular recess 1 8 and a radial passage 1 9 in the boost piston 4. The outlet 11 is connected to an exhaust port 20 in the boost piston 4 through an annular recess 21, an inclined passage 22, axial passage 23 and inclined passage 24 in the boost piston 4. The inclined passage 22 also provides communication between a portion 25 of the bore 13 and the outlet 11 via the annular recess 21.

The boost piston 4 is provided with a port 26 which is in communication with the boost chamber 8 through an axial passage 27. The port 26 is opposite the exhaust port 20 and as shown in Figure 2, there are three ports 26 and three ports 20 equiangularly spaced round a portion 28 of the bore 13. A further port 29 in the boost piston 4 leads to a passage 30 which provides communication with the auxiliary chamber 14.

The boost piston 4 carries seals 31, 32, 33, while the input piston 5 is sealed in the bore portion 6 by a seal 34.

The control valve means 9 comprises a valve mechanism 35, which works in the portion 28 of the bore 1 3 of intermediate diameter. The mechanism 35 is also located in a blind bore 37 in the input piston 5 by a circlip 38. The control valve mechanism 35 is a two-part assembly comprising an outer sleeve member 39 which is provided with a blind bore 40 in which an inner spool member 41 works. The sleeve 39 is located at its rearward end in the bore 37 of the input piston 5, and is also provided with three spaced ports. The first port 42 at the forward end of the sleeve 39 is in communication with the recess 29 in the boost piston 4. The second intermediate port 43 controls communication with the inlet port 1 7 or the exhaust port 20, and also controls communication between the port 26 and the exhaust port 20, to control communication between the boost chamber 8 and the outlet 11.

The third port 44 is in communication with the boost chamber 8. A spring 45, acting between the boost piston 4 and an abutment member 46 held by a circlip 47 on the sleeve member 39, biases the sleeve member 39 rearwardly.

The inner spool member 41 slides in the bore 40, and is urged rearwardly by a spring 48 acting against an abutment member 49 located adjacent the open end of the bore 40. The spool 41 is provided with an axially extending blind bore 50 which opens rearwardly. A first diametral port 51 at its forward end controls communication between the bore 50 and the first port 42 on the sleeve 39, a second diametral port 52 controls communication between the bore 50 and the intermediate port 43 on the sleeve 39, and a third diametral port 53 at the rearward end controls communication between the bore 50 and the third sleeve port 44.The sleeve 39 also carries at its forward end a seal 54 to separate the bore portion 25, which is in communication with the outlet 11, from the ports in the control valve means.' A member 55 is also disposed in bore 13 of the boost piston 4 forward of the control valve means.

The member 55 is located between a shoulder 56 at a step in diameter of the bore 13, and a circlip 57, and houses the auxiliary valve 1 6. The valve 1 6 is located in an axially extending bore 58 in the member 55, and comprises a valve member 59 provided with a seal 60 adapted to seat on a conical portion 61 of the bore 58. The valve member 59 is biased to closure by a spring 62 acting against an abutment member 63 at the forward end of the bore 58, and is also provided with a rearward extension 64 which engages a piston 65 located at the rearward end of the bore 58. The piston 65 is biased forwardly by a spring 66 into engagement with the extension 64 on the valve member 59 to keep the valve 16 open.The auxiliary chamber 14 is defined between the seal 60 on the valve member 59 and a seal 68 between the output piston 7 and the portion 12 of the bore 1 3 of the boost piston 4. Fluid communication to the auxiliary chamber 14 is provided through a recess 69 in the member 55, which is in communication with the passage 30 in the boost piston 4, radial passages 70 and through the auxiliary valve 1 6. The member 55 also carries seals 71 and 72 to seal the recess 69 from the auxiliary chamber 14 and the bore portion 25 respectively. A seal 67 on the valve piston 65 also seals the recess 69 from the bore portion 25.

In the inoperative position shown in Figure 1 all the parts are retracted, and the boost piston 4 abuts the rear end 73 of the housing. The boost chamber 8 and the auxiliary chamber 14 are connected to the outlet 11, as is the bore portion 25.

When the brakes are to be applied movement of a pedal (not shown) is transmitted to the input piston 5. The input piston 5 acts on the sleeve 39, which acts on the spool 41 so that all three initially move together against the force in the spring 45. Initial movement causes the intermediate port 43 of the sleeve 39 to move out of registry with the ports 20 and 26, cutting off communication between the boost chamber 8 and the outlet 11 so that the boost chamber 8 is isolated at reservoir pressure. Further movement then opens communication between the port 43 and the inlet port 1 7 which allows pressure fluid to flow to the auxiliary chamber 14 through the port 52, bore 50, ports 51,42 and 29, passage 30, recess 69, passages 70 and the open valve 16.

Pressure in the auxiliary chamber 14 acts rearwardly on the boost piston 4, which therefore remains in its retracted position, and also on the output piston 7 which advances relative to the boost piston 4 to actuate the master cylinder to start pressurisation of the braking system. Initially the pressure in the auxiliary chamber 14 will be relatively low, but as the braking system pressurises after the lost-motion has been taken up a reaction is fed back through the output piston 7, which causes the pressure in the chamber 14 to increase.The forward end of the valve piston 65 is also subjected to this pressure and at a predetermined value, corresponding to the required low level of pressure in the braking system, the pressure acting on the piston overcomes the force in the spring 66, and piston 65 moves rearwardly, allowing the auxiliary valve 1 6 to close, and trapping fluid in the auxiliary chamber 14 which then acts as an hydraulic strut.

The closure of the auxiliary valve 16 causes the pressure of the fluid flowing through the control valve means 9 to increase, and at a second higher predetermined pressure the pressure acting on the rear end of the spool 41 overcomes the force in the spring 48, and the spool 41 moves forwardly, putting the port 53 into communication with the port 44 and allowing pressure fluid to flow into the boost chamber 8. Since the full area of the boost piston 4 is greater than that of the output piston 7, while the reaction load from the master cylinder is unchanged, the boost chamber 8 is supplied with fluid at a pressure lower than the pressure of the fluid trapped in chamber 14. Thus the valve 1 6 is subjected to a pressure differential which holds it closed.As the boost piston 4 is advanced due to pressurisation of the boost chamber 8 the inlet port 1 7 will move out of registry wil:h the port 43 to shut off communication between the boost chamber 8 and the inlet port, if the input piston 5 has not been moved past the initial stage. The booster is then in the "hold" or equilibrium position.

If the input piston 5 is moved further in a brakeapplying direction the piston 5 and the control valve mechanism 35 will move together to reestablish communication between the inlet port 17 and the intermediate sleeve port 43 to admit fluid to the boost chamber 8. If the force on the input piston is now held constant the booster will move into the equilibrium position described above, and the boost chamber pressure will act over the area of the input piston 5 to provide the reaction load or "feel" to the pedal. The pressure in the auxiliary chamber 14 will rise and fall with the pressure in the boost chamber 8, but will be at a higher level determined by the ratio of the areas of the pistons 4 and 7.Leakage of the higher pressure fluid out of chamber 14 past the valve 1 6 or the seal 68 will however only result in increased pedal travel and will not affect the operation of the booster adversely.

If the force on the input piston 5 is reduced, the input piston 5 and the control valve means 9 will move rearwardly until the port 43 of the sleeve 39 comes into registration with the ports 20 and 26, allowing fluid to flow from the boost chamber 8 to the outlet 11 via passage 27, ports 26, 43 and 20 and passages 21, 22, 23 and 24 and also via ports 44, 53 and 52. When the pressure in the boost chamber 8 has fallen to a level where the reaction it applies to the input piston 5 is below the force applied to the piston 5 from the pedal, the booster moves back into the equilibrium position.

If the force on the input piston 5 is relieved altogether, the boost chamber 8 will be connected to the outlet 11, as described above, until the spring 48 is able to urge the spool 41 rearwardly to close communication between the ports 44 and 53, but the boost chamber 8 will still be connected to the outlet 11 via passage 27 and ports 26, 43 and 20. The boost pressure will continue to decrease until the spring 66 moves the valve piston 65 to open the valve 16 against the spring 62 and the pressure in the chamber 14.

The fluid from chamber 14 then flows to the outlet via the control valve means 9.

If the source of hydraulic pressure fails, the output piston 7 will not move relative to the boost piston 4, and force from the pedal will be transmitted from the input piston 5 through the boost piston 4 to the output piston 7 to actuate the master cylinder.

Clearly, if a master cylinder circuit fails when the booster is in an inoperative position, or during initial movement of the input piston 5 the control valve mechanism 35 is operative to take up the extra lost-motion created. If a circuit fails when the booster is operative, then the decrease in the reaction force will cause a decrease in pressure in the auxiliary and boost chambers, which enables the auxiliary valve 16 to re-open, so that the control valve mechanism can operate to compensate for the extra lost-motion.

Pressurising the auxiliary chamber 14 to start pressurisation of the braking system ensures that the required low level of pressure in the braking system is substantially constant, as the forces which have to be overcome before the braking system starts to be pressurised depend only on the characteristics of the auxiliary valve 16, and the seals for the auxiliary chamber 14, which are substantially constant for a given booster.

Arranging for the output piston 7 to work in a bore in the boost piston means that the length of the booster can be kept to a minimum.

Figure 3 shows a modification of the booster of Figure 1 in which the booster is combined with a master cylinder 74, and the output piston 7 also acts as the piston of the master cylinder 74. The booster and the master cylinder share a common reservoir, from which a master cylinder pressure space 75 is supplied through a recuperation port 76. Initial movement of the piston 7 moves a seal 77, which also performs the function of seal 31 in Figure 1, to cut off communication between the space 75 and port 76 to start pressurising the braking system. The construction and operation of the embodiment of Figure 3 is otherwise the same as that of Figures 1 and 2.

Figure 4 also shows a modification of the booster of Figure 1 in which the arrangement of the spool and the auxiliary valve are modified, and corresponding reference numerals have been applied to corresponding parts.

In the modification of Figure 4 the reduced diameter bore portion 6 in which the input piston 5 works is formed by a separate annular cup member 78 located in the bore 2, and sealed by a seal 79. The seal 34 is retained in the cup member 78 by an annular retainer 80.

The inlet port 17 is disposed in the same axial plane as the port 29. There is only one port 29, and two ports 17 lying on a line perpendicular to the plane of the drawing, instead of three as in Figure 1. The exhaust arrangement is similar to that of Figure 1, except that inclined passages 22 and 24 are replaced by radial passages 81, 82. On the sleeve 39 the first port 42 controls communication with the ports 29 and 13, while the second port 43 controls communication between the ports 20 and 26. The third port 44 as before is in communication with the boost chamber 8. The sleeve 39 also controls a port 83 controlling communication between passage 30 and bore portion 25. The spool 41 itself is modified, and the three ports 51, 52 and 53 are all combined into an annular recess 84, which is not in communication with the blind bore 50.Instead the bore 50 is provided with a further radial port 85 which opens into the bore portion 25, so that the bore 50 is always in communication with the outlet 11. In this embodiment the spring 48 is omitted, and the spool 41 is instead located- at its forward end in a blind bore in the valve piston 65 by a pin 86.

The arrangement of the auxiliary valve 1 6 is also modified, with the valve piston 65 working in the bore 1 3 of the boost piston 4 instead of the bore 58 in the member 55. The valve piston 65 is biased into engagement with the valve member extension 64 by a spring 87 acting against a circlip 88 in the bore portion 25.

The connections of the booster inlet 10 to an accumulator 89 through a one-way valve 90, and of the outlet 11 to a reservoir 91 are also shown.

In the inoperative position shown all the parts are retracted, and the input piston 5 abuts against a circlip 92 on the boost piston 6. The boost chamber 8, the auxiliary chamber 14 and the bore portions 25 are all connected to the outlet 11.

When the brakes are to be applied the pedal actuates the input piston 5 through an input rod 93, and moves the sleeve 39, but not the spool 41. Initial movement cuts off the port 43 from the ports 20 and 26, cutting off the boost chamber 8 from the outlet 11. Further movement brings the port 42 into registry with the ports 1 7 and 29, allowing fluid to flow from the inlet port 1 7 along passage 30, and through the valve 16 to the auxiliary chamber 14. Pressure fluid will also flow into the annular recess 84 of the spool 41.As in Figure 1, when the pressure increases in the auxiliary chamber 14 the valve piston 65 moves rearwardly to allow the valve 1 6 to close, and this also moves the spool 41 rearwardly to put the inlet port 1 7 into communication with the boost chamber 8 through the port 42, recess 84 and port 44. The boost piston 4 will then move forward into the equilibrium position. The remainder of the operation of this embodiment is similar to that of Figure 1, with the spool 41 and the valve piston 65 returning to their retracted positions simultaneously once the boost piston 4 is fully retracted.

The embodiment of Figure 4 has the advantage that the spool 41 is operated by the valve piston 65 in response to pressure in the chamber 14, and not by fluid pressure in the bore 50 acting to overcome the force in a spring. This means that it is not possible to admit pressurised fluid into the boost chamber 8 until the pressure in chamber 14 has reached its predetermined level. The embodiment of Figure 4 has the further advantage that the bore 50 in the spool 41 is connected at all times to the reservoir, thus eliminating the reaction felt at the pedal which in the embodiment of Figure 1, is generated by the fluid pressure in the bore 50 during initial pressurisation of the braking system and is transmitted through the sleeve 39 and the input piston 5.In Figure 4 the only reaction generated during initial pressurisation is in chamber 14, and this is transmitted to the housing by the boost piston 4.

Figure 5 shows the booster of Figure 4 combined with a master cylinder 74. In this construction the booster and the master cylinder have separate supplies of fluid the booster has a mineral fluid, whereas the master cylinder uses conventional brake system vegetable or synthetic fluid. As it is important to keep the two fluids separate, a space 94 is provided between the seal 31 on the boost piston 4 and an extra stationary seal 95 in which the combined master cylinder piston and output piston 7 slides. The space 94 is provided with a drainage port 96, out of which fluids leaking into space 94 past seals 31, 68 or 95 can drain, so that the fluids do not contaminate each other.

Figure 6 shows another booster, in which the control valve means are arranged and operated in a different way. Corresponding reference numerals have been applied to corresponding parts.

Thus in the booster of Figure 6 the boost piston 4 is of stepped outline, and works in the bore 2 in the housing 1 , with a portion of the boost piston 4 projecting rearwardly outside the housing 1. The boost chamber 8 is defined round the boost piston between a seal 97 on the boost piston and seal 98 in a smaller diameter portion of the bore 2 at the rearward end of the housing 1. The input piston 5 works in a bore 99 in the boost piston 4, and the output piston 7 works in a further bore 100 in the boost piston 4.

The auxiliary chamber 14 is defined in the bore 98 and is permanently connected to the boost chamber 8 through an axial passage 101 and a radial port 102. The limiting means 15, which determines the distance through which the output piston 7 can move relative to the boost piston 4, comprises a stop member in the form of a circlip 103 mounted on the boost piston 4. The circlip 103 thus provides a stop limiting the relative movement of the output piston 7 and the boost piston 4 away from each other. A spring 104 acts between the circlip 103 and an abutment member 105 located in the bore 2 at the forward end of the housing 1 to bias the boost piston 4 rearwardly, so that in the inoperative position shown, a shoulder 106 at a step in diameter of the boost piston 4 engages a shoulder at a step in the bore 2.

The control valve means 9 comprise valve means 107 located in a transverse bore 108 in the housing 1. The valve means 107 is operated in response to pressure in a control chamber 109, pressurisation of which is controlled by relative movement of the input piston 5 and the boost piston 4. The control chamber 109 is defined in the forward port of the bore 99 and by an annular chamber 110 defined round the forward end of the boost piston 4 between seal 111 and the seal 97, the chamber 110 being connected to the bore 99 by a radial passage 112 in the boost piston 4.

The input piston 5 is of stepped outline, and is biased rearwardly by a spring 113 acting between the boost piston 4 and a shoulder 1 114 formed by a forward reduced diameter portion 11 5 on the input piston 5. A spherical head 11 6 on a pedaloperated input rod 11 7 is received in a recess 11 8 in the rearward end of the piston 5, and rearward movement of the input piston 5 and the rod 117 is limited by their engagement with an abutment member 11 9 retained in the bore 98 by a circlip 120.The abutment member 11 9 also retains a flanged annular member 121 in the bore 98, and a portion 1 22 of the input piston 5 of intermediate diameter works in the member 121, which also carries a seal 123.

A recuperation valve 1 24 for the control chamber 109 is formed by a seal 125, carried on a portion of the input piston 5 of greatest diameter, which cooperates with the radial port 102 in the boost piston 4, which provides communication between the control chamber 109 and the boost chamber 8. In the inoperative position shown the recuperation valve 124 is open, so that communication between the boost chamber 8 and the control chamber 109 is permitted. Movement of the input piston 5 in a brake-applying direction relative to the boost piston 4 moves the seal 125 to close the recuperation valve 124, trapping fluid in the control chamber 109 which is pressurised on further movement of the input piston 5.A second radial port 126 in the boost piston 3 provides permanent communication between the boost chamber 8 and an annular chamber 127 in the bore 99 round the portion 122 of the input piston 5. The pressure in the boost chamber 8 is also present in this annular chamber 127 and acts on the input piston 5 to urge it in a direction to close the recuperation valve 124.

The pressure in the control chamber 109, the control pressure, acts on the valve means 107 via a radial port 128 in the housing 1 connecting the bore 2 to the bore 108. The control chamber 109 can be bled by means of a bleed screw 129, connected to the control chamber 109 by the port 128, the bore 108 and an inclined passage 130 leading from the bore 108. The valve means 107 comprises a spool 131 of stepped outline working in the bore 108 and in a bore 1 32 of a stationary sleeve 133 located at the outer end of bore 108.

The sleeve 1 33 forms with the bore 9 an inlet recess 134 in communication with the inlet 1 0, and the sleeve 133 has inlet ports 135 providing communication between the recess 134 and the bore 132. Similarly an exhaust recess 136 is formed in the bore 108 in communication with the outlet 11, and the sleeve 133 has exhaust ports 137 providing communication between the recess 136 and the bore 132. Three spaced seals 138, 139, 140 and provided round the sleeve 133 to seal the inlet and exhaust recesses.

The spool 131 is biased towards the bore 2 by a spring 141 which is located round a portion 142 of the spool 131 of least diameter and acts between the spool 131 and the sleeve 133. The spool 131 also has an inner end 143 of greatest diameter which works in the inner end of the bore 108 and carries a seal 144 to seal the outer end of the bore 108 from the control chamber 109, and an outer end 145 of intermediate diameter which works in the sleeve bore 132. The outer end 145 is provided with an inwardly extending bore 146 leading to a further diametral port 147, which is in communication with a chamber 148 formed in the bores 108 and 132 round the spool portion 142.

The outer ends of the bores 108 and 132 are closed by a sealed plug 149 located in the bore 108 by a circlip 1 50. The chamber 148 communicates with the boost chamber 8 through an inclined passage 1 51.

Thus the control pressure acts on the inner end 143 of the spool 131 to urge it outwardly, against the loading in the spring 141, and boost pressure acting in the chamber 148 and a chamber between the spool 131 and the plug 149. These forces control the movement of the spool 131 to control communication of the ports 146 and 147 with the inlet and exhaust ports of the sleeve 133.

In the inoperative position shown all the parts are retracted, and the recuperation valve 1 24 is open so that the control chamber 109 is in communication with the boost chamber 8, and the boost chamber 8 is connected to the reservoir through the valve means 107 - that is through passage 151, chamber 148, port 147, bore 146, ports 147 and 137 and exhaust recess 136.

When the booster is to be operated, a force applied to a pedal (not shown) is transmitted to the input piston 5 through the input rod 11 7, causing movement of the input piston 5 against the loading in the spring 113 and relative to the boost piston 4, which moves the seal 1 25 past the port 1 32 to close the recuperation valve 124 trapping fluid in the control chamber 109. Further movement of the input piston 5 then starts to pressurise the fluid in the control chamber 109, which acts on the inner end 143 of the spool 131.

When the control pressure acting on the spool 1 31 produces a force sufficient to overcome the force in the spring 141 the spool 131 moves outwardly. Movement of the spool 1 31 first isolates the port 147 from the exhaust ports 1 37 to cut off communication between the boost chamber 8 and the reservoir, and then brings the chamber 148 into communication with the inlet ports 135 to open communication between the pressure source and the boost chamber 8.

Pressure fluid flows into the boost chamber 8 and thence into the auxiliary chamber 14 through the port 102 and passage 101, and into chamber 127 through the port 126. The areas of the boost piston 4 on which pressure fluid acts are arranged so that at this stage of actuation the forces acting on the boost piston 4 in a forward direction are less than or equal to the forces acting on the boost piston 4 in a rearward direction. In this embodiment this gives the condition that (ignoring spring forces and friction forces) the area of the boost chamber 8 times the boost pressure plus effective area of the control chamber 109 times the control pressure is less than or equal to the area of the auxiliary chamber 14 times the boost pressure plus the area of chamber 127 times the boost pressure.However, the boost pressure present in the auxiliary chamber 14 acts on the output piston 7 to advance it relative to the boost piston 4 to start pressurisation of the master cylinder to actuate the braking system. The output piston 7 then engages with the circlip 102, which prevents further movement of the output piston 7 relative to the boost piston 4, and the travel of the output piston 7 is arranged so that at this point the lost-motion in the braking system will have been taken up, and the system will be pressurised to a relatively low level, in response to a small initial movement of the input piston 5.

When the output piston 7 engages with the circlip 103 the force acting on the output piston 7 due to the boost pressure in the auxiliary chamber 14 is transmitted to the boost piston 4 to cancel out the rearwardly-acting force due to the boost pressure in the auxiliary chamber 14 acting directly on the boost piston 4. The force acting on the boost piston 4 in the rearward direction is therefore reduced, and the boost piston 4 and the output piston 7 advance together to augment the output force, in response to further movement of the input piston 5.

Once the recuperation valve 1 24 has closed movement of the boost piston 4 will be the same as movement of the input piston 5 to ensure that the recuperation valve 1 24 remains closed. The boost pressure is also present in the chamber 127, where it acts on the input piston 5 to urge it in a brake-applying direction, and also in the chamber 148 where it acts on the spool 1 31 in opposition to the control pressure. When the forces on the spool 1 31 due to the boost pressure and the spring 141 overcome the force due to the control pressure, the spool 131 will move into the equilibrium position, in which the boost chamber 8 is connected neither to the pressure source nor to the reservoir.The spool 131 and the spring 141 are arranged so that in the equilibrium position the control pressure is higher than the boost pressure, and the difference is conveniently 2 bars.

If the effort on the pedal is reduced, but not wholly removed, then the control pressure decreases to allow the valve spool 131 to move inwardly, re-opening communication between the boost chamber 8 and the reservoir until the forces on the spool 131 equalise and it moves back into the equilbirium position. The boost pressure in the chamber 127 urges the input piston 5 in a brakeapplying direction to ensure that the recuperation valve 124 remains closed, and the boost pressure in the auxiliary chamber 14 ensures that the output piston 7 does not move rearwardly relative to the boost piston 4.

If the effort on the pedal is removed altogether, the control pressure decreases to allow the spool 131 to open communication between the boost chamber 8 and the reservoir, and the boost piston 4 and the input piston 5 move back into their retracted positions. The recuperation valve 124 opens to connect the boost chamber 8 to the control chamber 109 only when the boost pressure has fallen to a level at which the spring 11 3 can overcome the effect of the boost pressure acting in the chamber 127. The output piston 7 then returns to its retracted position due to the force exerted on it by the master cylinder as the master cylinder returns to its retracted position.

Should the valve spool 131 stick in the open or equilibrium positions when the pedal effort is reduced, then the control pressure decreases, but the boost pressure does not. The boost pressure acting on the input piston 5 in the chamber 127 will ensure that the recuperation valve 124 remains closed as long as the valve is stuck, and then the boost pressure acting on the spool 1 31 will, in general, produce a force sufficient to move the spool 1 31 inwardly to connect the boost chamber 8 to the reservoir. It is important to ensure that the recuperation valve 124 remains closed in this situation, since if it opened fluid flowing from the boost chamber 8 to the control chamber 109 could balance the spool 131 with the booster operative, and it would then be impossible to control the booster in order to release the brakes.

As the auxiliary chamber 14 is permanently connected to the boost chamber 8 there will be a point at which the load in the output piston 7 cannot be supported by the pressure in the auxiliary chamber 1 4. The operation of the booster described above assumes that this limiting load has not been reached. When this load is reached the output piston 7 is moved relatively towards the boost piston 4 back to its retracted position, which increases the pedal travel necessary to operate the booster. In order to avoid this increase in pedal travel it is desirable to ensure that the limiting load in the output piston 7 is difficult to achieve. The value of the load is determined by the pressure which can be generated in the boost chamber 8 and the relative areas of the boost piston 4 and the output piston 7 on which pressures act.Thus, for example, if these areas are such that the condition stated above achieves equality, then the limiting load will be reached when the boost pressure reaches its maximum value, after which any further increase in output force must be supplied wholly by increased input force, that it at the 'knee point' on the graph of input force against output force. In most cases this load will be too low that is, it can be achieved relatively easily in an emergency, so it is desirable to make the area of the auxiliary chamber 14, and thus the output piston 7, larger so that the limiting load is not reached until after the knee point has been reached.

If the pressure source fails the output piston 7 will be unable to move relative to the boost piston 4, and the input piston 5 will engage with the boost piston 4, so that the input force is transmitted through the boost piston 4 to the output piston 7 to actuate the master cylinder.

This embodiment has the advantage of a simple construction as the auxiliary valve 1 6 is omitted.

However, the output piston 7 moves through a fixed distance relative to the boost piston to take up the lost-motion in the braking system rather than through a distance dependent on the pressure in the system, and so cannot compensate for failed brake circuits. Arranging the valve means 107 in the transverse bore 108 also reduces the length of the booster. In a modification (not shown) the control valve means could be arranged in a bore in the boost piston however.

In all the embodiments shown it will be noticed that those seals which do not need to be leakproof especially those e.g. 32, 33, 97 which seal the high pressure chambers on the boost piston 4, include a part of low-friction non-elastomeric material which reduces the sliding friction of the seals. Those seals which must be leakproof, for example 34 or 98 on the input piston 5, are of elastomeric material, such as rubber.

Claims

1. A booster of the kind set forth, in which the output member works in a bore in the boost piston, and is movable in response to pressurisation of an auxiliary chamber, and on an initial movement of the input member in a brakeapplying direction the control valve means is operative to admit pressurised fluid from the source to the auxiliary chamber such that the output member is advanced relative to the boost piston through a distance determined by a limiting means, without the boost piston being advanced in the bore, and on further movement of the input member the boost piston and the output member move together in response to pressurisation of the boost chamber.

2. A booster as claimed in claim 1, in which the auxiliary chamber is defined in the bore in the boost piston between the boost piston and the output member.

3. A booster as claimed in claim 1 or claim 2, in which the limiting means comprises an auxiliary valve means which is operative when the pressure supplied to the auxiliary chamber reaches a predetermined value to isolate the auxiliary chamber to prevent further movement of the output member relative to the boost piston.

4. A booster as claimed in claim 1 or claim 2, in which the limiting means comprises a stop member mounted on the boost piston with which the output member engages, to determine a fixed distance through which the output member moves relative to the boost piston.

5. A booster as claimed in claim 3, in which on initial movement of the input member the control valve means is operative to open communication between the auxiliary chamber and the inlet to supply fluid through the auxiliary valve means, which comprises an auxiliary valve which closes at a predetermined level of pressure supplied to the auxiliary chamber to isolate the auxiliary chamber, the control valve means then being operative to open communication between the inlet and the boost chamber.

6. A booster as claimed in claim 5, in which the auxiliary valve has a valve member which is adapted to close when a predetermined pressure is supplied to the auxiliary chamber.

7. A booster as claimed in claim 6, in which the valve member comprises a member biased to closure by a spring, but held open by a spring loaded auxiliary piston, the piston being responsive to the pressure supplied to the chamber to allow the valve to close when the predetermined pressure is reached.

8. A booster as claimed in any of claims 5 to 7, in which the control valve means comprises relatively movable parts including a bore and a valve mechanism working in the bore.

9. A booster as claimed in claim 8, in which the valve mechanism works in a bore in the boost piston, one part of the valve mechanism being movable relative to the input member and having a pressure-responsive face.

10. A booster as claimed in claim 9, in which closure of the auxiliary valve results in the increase in pressure of fluid applied to the pressure responsive face to move the part relative to the input member to place the inlet in communication with the boost chamber

11. A booster as claimed in claim 9, in which closure of the auxiliary valve acts directly on the part of the valve mechanism to move it in order to place the inlet in communication with the boost chamber.

12. A booster as claimed in claim 7 and claim 11, in which the auxiliary piston is coupled to the part, and movement of the piston in response to the predetermined level of pressure supplied to the auxiliary chamber to allow the auxiliary valve to close also moves the part relatively towards the input member to place the inlet in communication with the boost chamber.

13. A booster as claimed in claim 1, claim 2 or claim 4, in which permanent communication is provided between the boost chamber and the auxiliary chamber, and on initial movement of the input member the control valve means is operative to supply pressurised fluid both to the auxiliary chamber and the boost chamber.

14. A booster as claimed in claim 4 and claim 13, in which the relative areas of the output member and the boost piston are arranged so that the force acting on the boost piston in the forward direction is less than or equal to the force acting on the boost piston in the rearward direction until the output member has moved through the fixed distance relative to the boost piston.

1 5. A booster as claimed in claim 13 or claim 14, in which the control valve means comprises relatively movable parts including a bore and a valve mechanism working in the bore.

1 6. A booster as claimed in claim 1 5, in which the valve mechanism works in a bore in the boost piston.

17. A booster as claimed in claim 1 5, in which the valve mechanism works in a bore in the housing separate from the bore in which the boost piston works.

1 8. A booster as claimed in claim 17, in which operation of the control valve means is controlled by pressure in a control chamber, which is pressurised in response to relative movement of the input member and the boost piston.

19. A booster of the kind set forth substantially as herein described with reference to and as illustrated in Figures 1 and 2 of the accompanying drawings.

20. A booster of the kind set forth substantially as herein described with reference to and as illustrated in Figure 4 of the accompanying drawings.

21. A booster of the kind set forth substantially as herein described with reference to and as illustrated in Figure 6 of the accompanying drawings.

22. A booster of the kind set forth in combination with a master cylinder substantially as described herein with reference to and as illustrated in Figure 3 of the accompanying drawings.

23. A booster of the kind set forth in combination with a master cylinder substantially as described herein with reference to and as illustrated in Figure 5 of the accompanying drawings.