HYDRAULIC MASTER CYLINDER

Abstract: A master cylinder has a stepped bore in which a piston (13) separates a high pressure chamber (12) from a low pressure chamber (26). A primary seal (17) is slidable in a smaller diameter bore portion (14) and an atmospheric seal (18) is slidable in a larger diameter bore portion (15). Any air seeping past the atmospheric seal (18) is kept from the region of the primary seal (17) by an auxiliary seal (29) and escapes into a reservoir (27) through an inlet port (28). Passageways (31,32,33) in the piston allow air free fluid into the region behind the primary seal.

Description: "HYDRAULIC MASTER CYLINDER" This invention relates to hydraulic master cylinders particularly but not exclusively for vehicle hydraulic braking systems. A typical known hydraulic master cylinder has a bore divided into a high pressure chamber for connection to a main hydraulic system and a low pressure chamber by a piston reciprocable in the bore, a primary seal adjacent one end of the piston for pressurising hydraulic fluid in the high pressure chamber and an atmospheric seal adjacent to its other end for sealing the low pressure chamber from atmosphere, the low pressure chamber having an inlet port for connection to a reservoir of hydraulic fluid. The primary seal functions in one direction only such that when an operating load is applied to the piston by an input rod, the piston is moved to pressurise the hydraulic fluid in the high pressure chamber. If the pressure in the high pressure chamber drops below that in the low pressure chamber the primary seal allows hydraulic fluid to flow from the low pressure chamber to the high pressure chamber. This allows pressure bleeding of the hydraulic system and when used in a braking system having a trap valve, allows the brake pedal to be pumped in an emergency. A problem arises if air should leak past the atmospheric seal into the low pressure chamber. Air bubbles collect in the vicinity of the primary seal and may then be swept past the primary seal into the main hydraulic system. This causes sppginess of operation and in a braking system can lead to brake failure. The problem is found mainly in master cylinders which are installed with the axis of the master cylinder vertical, the high pressure chamber being above the low pressure chamber. It is the object of the present invention to alleviate the above problem. According to the present invention, a hydraulic master cylinder of the kind described is characterised in that: (i) the bore is stepped so that the primary seal operates in the smaller diameter bore portion and the atmospheric seal operates in the larger diameter bore portion; (ii) an auxiliary seal operates to seal the piston and the smaller diameter bore portion between the primary and atmospheric seals; (iii) the inlet port is in the bore between the auxiliary and atmospheric seals in the region of a shoulder between the larger and smaller diameter bore portions; and (iv) passageways connect the smaller diameter bore portion in the region between the primary and auxiliary seals with the larger diameter bore portion in the region between the inlet port and the atmospheric seal. The invention will be described by way of example and with reference to the accompanying drawing in which is illustrated a longitudinal section through part of a master cylinder according to this invention. The hydraulic master cylinder in the drawing is for the hydraulic braking system of a motor vehicle and is intended to be installed in the vehicle with its bore axis substantially vertical. The master cylinder has a body 11 with a stepped bore comprising a smaller diameter portion 14 and a larger diameter portion 15 with a shoulder 16 therebetween. A stepped piston 13 is reciprocable within the bore and has a primary seal 17 located on a smaller diameter portion 24 slidable in the smaller diameter bore portion 14 and an atmospheric seal 18 located on a larger diameter portion 25 slidable in the larger diameter bore portion 15. The piston 13 is biassed towards the mouth 19 of the bore by a spring 21 located in the smaller diameter bore portion 14, which forms a high pressure chamber 12. The piston 13 is retained in the bore by a circlip 22 acting against an abutment washer 23 which also serves to retain a driver operable input rod 20. The annular space 26 between the piston 13, the primary and atmospheric seals 17 and 18 and the larger diameter bore portion 15 constitutes a low pressure chamber and is connected to a hydraulic fluid reservoir 27 by an inlet port 28 in the region of the shoulder 16. An auxiliary seal 29 is located on the smaller diameter portion 24 of the piston to seal against the smaller diameter bore portion 14. Seal 29 is of the annular cup type and is orientated to prevent flow from the low pressure chamber to the bore portion between the primary seal 17 and the auxiliary seal 29 on movement of the piston 13 towards the mouth of the bore. The primary seal 17 is also an annular cup type and is orientated for pressurising hydraulic fluid in the high pressure chamber 12 on movement of the piston 13 into the bore when an operational load is applied at the input rod 23. The back face of the seal 17 (the face away from the high pressure chamber) is supported on the piston 13 and is connected to the low pressure chamber 26 by passageways 31, 32 and 33 through the piston. These passageways thereby connect the smaller diameter bore portion 14 in the region between the primary and auxiliary seals 17 and 29 with the larger diameter bore portion 15. when the piston 13 is in the 'at rest" position shown in the drawing, the high pressure chamber 12 is connected to the fluid reservoir 27 through a cut-off port 34. This allows the pressure in the hydraulic braking system to remain at atmospheric pressure when the master cylinder is not being operated. To effect operation of the brakes a load is applied through the input rod 23 to move the piston 13 into the bore as described above. When the load on the input rod 23 is relieved and the brakes are released the spring 21 moves the piston 13 back towards the mouth 19 of the bore. After prolonged use, the atmospheric seal 18 may allow a small quantity of air to leak past. Air that enters the low pressure chamber 26 tends to cling to the wall of the larger diameter bore portion 15, and on operation of the piston 13 to apply the brakes is swept by the piston 13 and the atmospheric seal 18 towards the shoulder 16 and hence out of the bore through the inlet port 28 and into the reservoir. The auxiliary seal 29 prevents air from reaching the region of the bore adjacent to the primary seal 17 and will tend to sweep air that enters the smaller diameter bore portion 14 on the piston pressurising stroke, back to the shoulder 16 on the piston return stroke. If the capacity of the hydraulic system varies during a brake application, for example by wear of the brakes or from temperature changes, then a partial vacuum may be set up in the high pressure chamber 12 on return movement of the piston 13. The passageways 33, 32 and 31 through the piston 13 and the collapse of the primary seal 17 under the reverse pressure differential then allow the hydraulic brake system to recuperate, je. hydraulic fluid will flow into the high pressure chamber 12 from the low pressure chamber 26. This also allows pressure bleeding and, when the braking system has a trap valve, allows the master cylinder to be pumped in an emergency or for bleeding purposes. The radial passageway 33 opens into the low pressure chamber 26 adjacent to the atmospheric seal so as to be remote from any accumulation of air bubbles near the shoulder 16. Although in the embodiment of the invention just described the seals 17, 18 and 29 are all carried by the piston 13, the invention is also applicable to master cylinders having some or all of the seals located in the master cylinder body, the piston having co-operating cylinder surfaces which slide through the seals.

Claims: Claims 1. A hydraulic master cylinder having a bore divided into a high pressure chamber (12) for connection to a main hydraulic system and a low pressure chamber (26) by a piston (13) reciprocable in the bore, a primary seal (17) adjacent to one end of the piston for pressurising hydraulic fluid in the high pressure chamber (12) and an atmospheric seal (18) adjacent to the other end for sealing the low pressure chamber (26) from atmosphere, the low pressure chamber (26) having an inlet port (28) for connection to a hydraulic fluid reservoir (27), characterised in that: : (i) the bore is stepped so that the primary seal (17) operates in the smaller diameter bore portion (14) and the atmospheric seal (18) operates in the larger diameter bore portion (15); (ii) an auxiliary seal (29) operates to seal the piston (13) and the smaller diameter bore portion (14) between the primary and atmospheric seals (17 and 18); (iii) the inlet port (28) is in the bore between the auxiliary and atmospheric seals (29 and 18) in the region of a shoulder (16) between the larger and smaller diameter bore portions (15 and 14); and (iv) passageways (31,32,33) connect the smaller diameter bore portion (14) in the region between the primary and auxiliary seals (17 and 29) with the larger diameter bore portion (15) in the region between the inlet port (28) and the atmospheric seal (18). 2. A master cylinder according to Claim 1, characterised in that the passageways (31,32,33) open into the low pressure chamber (26) adjacent to the atmospheric seal (18). 3. A master cylinder according to Claim 1 or Claim 2, when installed with its axis substantially vertical, the high pressure chamber (12) being above the low pressure chamber (26).

Inventors: YOUNG, ALASTAIR JOHN
Applicants: AUTOMOTIVE PRODUCTS PUBLIC LIMITED COMPANY
Application year: 1982
Main classification: B60T11
IPC codes: B60T11