Actuator assemblies for hydraulic braking systems

Abstract

In an actuator assembly for an hydraulic braking system the effective volume of a chamber through which hydraulic fluid under pressure is supplied to a wheel brake is adapted to be varied between a minimum and a maximum value by movement of a piston assembly working in a bore in communication with the chamber. An actuator piston having first and second different faces of different areas normally holds the piston assembly in a position in which the effective volume of the chamber is at a minimum until equal hydraulic pressures are applied to the areas of the actuator piston to cause the actuator piston to retract and permit the piston assembly to be withdrawn whereby the effective volume of the chamber is increased.

We claim: 
     
       1. An actuator assembly for a vehicle hydraulic braking system comprising a housing, means defining at least one first chamber in said housing having a first inlet for connection to a fluid pressure source and an outlet for connection to a slave cylinder of a wheel brake, a piston assembly working in a first cylinder bore in said housing in communication with said chamber and movable between a first advanced position and a second retracted position, an actuator piston working in a second cylinder bore in said housing and normally acting on said piston assembly to urge said piston assembly into said first advanced position in which the effective volume of said first chamber is at a minimum value and said inlet and outlet connections are open, said actuator piston having a first area at an end of said actuator piston remote from said piston assembly and a second area opposed to said first area, a movable member .[.in said second cylinder bore.]. adjacent said first area of said actuator piston, a compression spring urging said movable member towards said actuator piston, means for applying hydraulic fluid under pressure, between said movable member and said first area of said actuator piston to urge said piston assembly into said first advanced position, and a second inlet connection in communication with said second cylinder bore through which hydraulic fluid under pressure is adapted to be admitted into said second bore when the deceleration of a braked wheel controlled by a supply of hydraulic fluid from said outlet exceeds a predetermined value, said supply of hydraulic fluid under pressure admitted to said second bore acting over said second area of said actuator piston to apply to said actuator piston a force to move said actuator piston in a direction away from said piston assembly and permit said piston assembly to be moved towards said second retracted position in which said first inlet connection is closed and the effective volume of said first chamber is increased to reduce pressure in a line between said outlet connection and said slave cylinder. 
     
     
       2. An actuator assembly as claimed in claim 1, incorporating a valve for cutting off communication between said first inlet connection and said chamber upon initial movement of said piston away from said first advanced position. 
     
     
       3. An actuator assembly as claimed in claim 1, wherein said actuator piston is of differential outline working in a stepped bore and engages at its end of smaller diameter with said piston assembly, said first area being located at the base of a recess in the end of said piston of greater diameter, and said second area being defined by a shoulder at a step in diameter between portions of said stepped piston of different diameters. 
     
     
       4. An actuator assembly as claimed in claim 3 wherein said shoulder on said actuator piston is spaced from a complementary shoulder at a step at the change in diameter of said stepped bore when said effective volume of the chamber is at said minimum value, and an annular space between the shoulders and surrounding a portion of said actuator piston of lesser diameter defines a second chamber into which hydraulic fluid under pressure is adapted to be admitted though through said second inlet connection. 
     
     
       5. An actuator assembly as claimed in claim 4, incorporating a metering device for delaying the release of fluid pressure from said second chamber when the supply of hydraulic fluid under pressure to that chamber is terminated. 
     
     
       6. An actuator assembly as claimed in claim 5, wherein said metering device comprising a cup-shaped piston located in said second chamber and having an axially extending skirt and a spring for urging said skirt into sealing engagement with said shoulder at said step at the change in diameter in said stepped bore and wherein said cup-shaped piston incorporates in a wall thereof at least one orifice providing a restricted return path to said second inlet connection for fluid actuator said second chamber after said actuator piston has moved towards said first chamber by a distance sufficient to enable said skirt to seat against said shoulder. 
     
     
       7. An actuator assembly as claimed in claim 6, wherein said cup-shaped piston is slidable guided in a portion of said differential piston of smaller diameter which projects into the portion of said stepped bore of greater diameter, and there is a clearance between an outer peripheral edge of said cup-shaped piston and the wall of said portion of said stepped bore of greater diameter. 
     
     
       8. An actuator assembly as claimed in claim 4, including a third inlet connection in said housing leading to said second chamber, wherein a one-way valve assembly is housed in each of said second and third connections, each one-way valve assembly incorporating a valve member, a seating surrounding an orifice and adapted to be engaged by said valve member and spring means for urging said valve member into engagement with said seating, said one-way valve assemblies being constructed and arranged to operate in opposite directions whereby high pressure fluid is adapted to be supplied to both connections but is applied to said actuator piston only through said one-way valve which opens in that direction, fluid from said actuator piston being returned from said second chamber upon re-application of a wheel brake through said other one-way valve and said connection in which that one-way valve is located, whereby the delay in the re-application of a wheel brake after the deceleration of the wheel has been restored to a predetermined value is controlled by the return of fluid from said actuator piston .[.to the control  valve.]. through said orifice in said one-way valve in the return connection. 
     
     
       9. An actuator assembly is claimed in claim 8 wherein each orifice is provided in a plate which is interchangeable with a series of similar plates having orifices of different diameters whereby the rate of flow fluid in each connection can be varied in accordance with a desired braking characteristic. 
     
     
       10. An actuator assembly as claimed in claim 3, wherein the movable member is a third piston working in said recess in said end of said actuator piston of greater diameter and having an inner end, wherein said inner end of said third piston defines with said base of said recess forming the first area a third chamber into which hydraulic fluid under pressure is admitted through an axial bore in said third piston. 
     
     
       11. An actuator assembly as claimed in claim 1 incorporating means for delaying re-application of a wheel brake following cut-off and relief of the supply of fluid to a slave cylinder from said outlet connection. 
     
     
       12. An actuator assembly as claimed in claim 1, wherein said piston assembly comprises a single expander piston working in a bore in said housing and controlled by a single actuator piston. 
     
     
       13. An actuator assembly as claimed in claim 1, wherein said piston assembly comprises at least two expander pistons working in separate parallel bores in said housing, said expander pistons being controlled by single actuator piston, and each expander piston being adapted to control the effective volume of a chamber provided with separate inlet and outlet connections and operation of a spring loaded valve means for controlling communication between said inlet connection and said chamber of which the effective volume is variable by that expander piston. 
     
     
       14. An actuator assembly as claimed in claim 1, wherein said piston assembly comprises a two part axially separable assembly adapted to regulate the effective volume of a first and second axially spaced chamber, each provided with an inlet and an outlet connection. 
     
     
       15. An actuator assembly as claimed in claim 14, wherein said first axially spaced chamber is located at the end of said bore in which works said expander piston and into which a free end portion of a first part of said separable assembly extends, and adjacent to the opposite end of said first part is formed an axially extending annular recess with which an adjacent end of a second part of said separable assembly defines said second axially spaced chamber of which the effective volume is variable upon relative axial movement between said first and second parts of separable assembly. 
     
     
       16. An actuator assembly as claimed in claim 15 wherein communication between said inlet and outlet connections with said first axially spaced chamber is controlled by a valve operable by a push-rod carried by the free end of said first part of said separable assembly, and communication between said inlet and outlet connections with said second axially spaced chamber is controlled by a spring loaded tipping valve which is normally held in an open position by the engagement of a stem of the tipping valve with a shoulder at the end of said recess adjacent to said second part of the separable assembly. .Iadd. 17. An actuator assembly for a vehicle hydraulic braking system comprising a housing having first and second opposite ends, means in said housing adjacent to said first end defining at least one chamber having a first inlet for connection to hydraulic fluid under pressure and an outlet for connection to a slave cylinder of a wheel brake, a valve controlling communication between said inlet and outlet, a piston assembly working in a cylinder bore in said housing in communication with said chamber and movable between an advanced position in which said valve is held open and the effective volume of said chamber is at a minimum value and a retracted position in which said valve is shut and the effective volume of said chamber is greater than the minimum, means in said housing for enabling fluid from a high pressure source to urge said piston assembly to said advanced position, means in said housing for enabling fluid to move said piston assembly to said retracted position when the deceleration of a braked wheel controlled by a supply of hydraulic fluid from said outlet exceeds a predetermined value, a movable member in said housing disposed between said piston assembly and said second end of said housing and movable between a first position in which said member permits said piston assembly to assume said retracted position and a second position in which said member maintains said piston assembly in said advanced position, resilient means for constantly biassing said movable member to said second position, and means in said housing for enabling the fluid from said high pressure source to hold said movable member in said first position in opposition to said resilient means, whereby, in the event of the pressure of said high pressure source falling, said resilient means can move said movable member to said second position to maintain said piston assembly in said advanced position. .Iaddend. .Iadd.18. An actuator assembly as in claim 17 wherein said piston assembly is adapted to be urged to said advanced position by hydraulic fluid under pressure and wherein said valve is separate from said piston assembly and movable away from a fixed valve seat in said housing by said piston assembly. .Iaddend..Iadd.19. An actuator assembly as in claim 118 wherein said piston assembly comprises an expander piston working in a first cyliner bore in the housing in communication with said chamber, and an actuator piston working in a second cylinder bore in said housing and acting on said expander piston, said movable member being adapted to maintain said expander piston in said advanced position. .Iaddend. .Iadd.20. An actuator assembly as in claim 17 wherein said housing includes a plurality of said chambers, and a plurality of said valves controlling communication between said inlet and said outlet of each chamber, said piston assembly actuating said valves. .Iaddend..Iadd.21. An actuator assembly as in claim 20 wherein said piston assembly is adapted to be urged to said advanced position by hydraulic fluid under pressure and wherein said valves are held open by a mechanical coupling between said piston assmbly and said valves. .Iaddend. .Iadd. 22. An actuator assembly as in claim 21 wherein said piston assembly comprises a plurality of expander pistons working in a plurality of first cylinder bores, each of said first cylinder bores being in communication with one of said chambers, and an actuator piston working in a second cylinder bore in said housing and acting on each of said expander pistons, said movable member being adapted to maintain all of said expander pistons in said advanced positions. .Iaddend..Iadd. 23. An actuator assembly for a vehicle hydralic braking system comprising a housing having first and second opposite ends, means in said housing adjacent to said first end defining at least one chamber having a first inlet for connection to hydraulic fluid under pressure and an outlet for connection to a slave cylinder of a wheel brake, a valve in said housing controlling said inlet, a piston assembly working in a cylinder bore in said housing in communication with said chamber and operating said valve, said piston assembly being movable between an advanced position in which said valve is held open and the effective volume of said chamber is at a minimum value, a first retracted position in which said valve is shut, and a second retracted position in which said valve is shut and the effective volume of said chamber is at a maximum value, retraction of said piston assembly first closing said valve and then progressively increasing the volume of said chamber, means in said housing for enabling fluid from a high pressure source to urge said piston assembly to said advanced position, means in said housing for enabling fluid to move said piston assembly to said retracted positions when the deceleration of a braked wheel controlled by a supply of hydraulic fluid from said outlet exceeds a predetermined value, a movable member in said housing disposed between said piston assembly and said second end of said housing and movable between a first position in which said member permits said piston assembly to assume said retracted positions and a second position in which said member maintains said piston assembly in said advanced position, resilient means for constantly biassing said movable member to said second position, and means in said housing for enabling the fluid from said high pressure source to hold said movable member in said first position in opposition to said resilient means, whereby in the event of the pressure of said high pressure source falling, said resilient means can move said movable member to said second position to maintain said piston assembly in said advanced position. .Iaddend..Iadd. 24. An actuator assembly as in claim 23 wherein said means for urging said piston assembly to said advanced position and said means for holding said movable member in said first position comprise an accumulator chamber defined between said piston assembly and said movable member, and an inlet leading into said accumulator chamber for connection to said high pressure source, and wherein said means for moving said piston assembly to said retracted position comprise a pressure chamber defined between said piston assembly, on the opposite side to said accumulator chamber, and said housing, and a port leading into said pressure chamber for connection to a supply of hydraulic fluid. .Iaddend..Iadd. 25. An actuator assembly for a vehicle hydraulic braking system comprising a housing, means defining at least one first chamber in said housing, said first chamber having a first inlet for connection to a fluid pressure source and an outlet for connection to a slave cylinder of a wheel brake, a piston assembly working in a first cylinder bore in said housing in communication with said chamber and movable between a first advanced position and a second retracted position, a valve for controlling communication between said first inlet connection and said chamber, said valve being open when said piston assembly is in said first advanced position and being closed upon initial movement of said piston assembly away from said first advanced position, an actuator piston working in a second cylinder bore in said housing and normally acting on said piston assembly to urge said piston assembly into said first advanced position in which the effective volume of said first chamber is at a minimum value and said inlet and outlet connections are open, said actuator piston having a first area at an end of said actuator piston remote from said piston assembly and a second area opposed to said first area, a movable member sealingly engaging said actuator piston to define a second chamber incorporating said first area of said actuator piston, a second inlet connection for admitting hydraulic fluid under pressure into said chamber to act on said first area of said actuator piston to urge said piston assembly into said first advanced position, a compression spring urging said movable member towards said actuator piston, and a third inlet connection in communication with said second cylinder bore through which hydraulic fluid under pressure is adapted to be admitted into said second bore when the deceleration of a braked wheel controlled by a supply of hydraulic fluid from said outlet exceeds a predetermined value, said supply of hydraulic fluid under pressure admitted to said second bore acting over said second area of said actuator piston to apply to said actuator piston a force to move said actuator piston in a direction away from said piston assembly and permit said piston assembly to be moved towards said second retracted position in which said first inlet connection is closed and the effective volume of said first chamber is increased to reduce pressure in a line between said outlet connection and said slave cylinder.