Abstract:
A proportioning device for controlling the distribution of fluid from separate pressurizing chambers in a master cylinder to corresponding first and second wheel brakes of a vehicle. The proportioning device is adapted to respond to changes in the pressure from the separate pressurizing chambers to assure that the fluid pressure supplied to the individual wheel brakes is substantially equal at all times.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to twin proportioning valves including two correcting valves notably adapted to be separately connected to two independent hydraulic braking sub-circuits of a vehicle each hydraulic valve being of the type comprising a movable differential piston. Such twin valves may be used particularly in diagonal brake circuits to control the rear axle braking forces. 
     There are known twin valves comprising two hydraulic proportioning valves, wherein the differential pistons are mounted in separate housings and are operatively connected to resilient return means by a mechanical linkage. This separate housing structure presents two main drawbacks : in effect manufacturing is rather expensive and the adjustment of the mechanical linkage in order to obtain substantially equal knee-point pressures is rather difficult and furthermore can only occur after mounting of the two valve housings on the vehicle. This last problem is critical when the twin valve is used in a diagonal circuit due to the fact the kneepoint pressures and the characteristic slopes of the proportioning valves should be the same. 
     SUMMARY OF THE INVENTION 
     The invention proposes a twin proportioning valve of simple structure and permitting an easy adjustment of the characteristics of the hydraulic circuits associated therewith, such adjustment being made in manufacture. No further adjustment is required after mounting on the vehicle except the usual one, which is necessary only when the twin valve is of the load sensing type. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The preferred embodiment of invention is now described in reference to the sole FIGURE wherein a sectional view of a proportional valve made according to the invention and adapted to be used in a vehicle hydraulic diagonal braking circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     When considering the FIGURE a housing 10 is provided for retaining twin proportioning valves. The housing 10 is provided with a blind stepped bore having a larger diameter portion 12 and smaller diameter portion 14 in which are slidably mounted two correcting valves of the type including a differential piston. The differential pistons 20 and 22 are of the stepped type well known in conventional correcting valves and include rod and head portions of smaller and larger effective areas respectively. The head portions 16 and 18 of the differential pistons 20 and 22 are slidably mounted in tandem relationship within said stepped bore and separated by an intermediate piston 24 slidable within the smaller diameter piston 14. The ends 62 and 64 of pistons 24 have equal external diameters. The differential piston 22 which has its head portion adjacent to the closed end 60 of the stepped bore is hereinafter referred as the inner piston, whereas the other differential piston 20 is hereinafter referred as the outer piston. The end 64 of the intermediate piston 24 which is adjacent to the inner piston 22 is provided with a recess or blind stepped bore 29. End 31 of the rod portion 28 of the inner piston 22 extends into the blind stepped bore 29 to create a central chamber 26. The chamber 26 is operatively vented to the atmosphere through passages 30 and 32 provided in intermediate piston 24 and housing 10 respectively. A sleeve 36 is fixed to the housing 10 by a retainer ring 37. The rod portion 34 of the outer piston 20 projects outside of the housing 10 through the sleeve 36 and is engaged by resilient return means 38. The return means 38 consists of a spring, the free end of which abuts a spring retainer 40 secured to the sleeve 36. However, for some applications it may be desirable to connect the resilient return means 38 of the two correcting valves to an axle load sensing device through a lever arrangement to control the operation of the two correcting valves as a function of the load on the vehicle. 
     As usual in a proportioning valve, the housing 10 is provided with inlet ports 42 and 44 and outlet ports 46 and 48 adapted to be connected to two independent vehicle braking sub-circuits A and B. The inlet ports 42 and 44 for example are adapted to be connected to a tandem master-cylinder and the outlet ports 46 and 48 in case of a diagonal braking circuit to the brake actuators of the wheels located on either side of the rear axle of the vehicle. Furthermore, as shown on the drawing and as usual in proportioning valve technology, the differential pistons 20 and 22 are provided with seals and with passages in which are mounted spring-biased valve members 54, 56 adapted to sealingly cooperate with annular seats. Finally, a light spring 58 is mounted between the bore closed end 60 and the head 18. 
     The effective area A1 of the rod portion 34 of the outer piston 20 is equal to the effective area A5 of the end 62 of the intermediate piston 24. Also the differential piston ratios i.e. the ratio of the effective areas of the rod and head portions A1/A2 and A3/A4 respectively are substantially the same. While not compulsory for the operation of the device effective areas A4 and A5 are chosen preferably the same thereby simplify manufacturing of the device. 
     MODE OF OPERATION OF THE INVENTION 
     The twin proportioning valve is illustrated in brake release position wherein the return resilient means 38 biases the components of the device towards the right of the drawing in their rest position. More specifically the inner piston 22 abuts the closed end 60 of the stepped bore thereby opening the check-valve 56, the intermediate piston 24 abuts the rod portion 28 and finally the outer piston 20 abuts the adjacent end of the intermediate piston 24 thereby opening the check valve 54. Consequently, free fluid communication occurs between the inlet ports 42, 44 and their corresponding outlet ports 46 and 48. 
     During a full brake operation, assuming that both subcircuits A and B are in order, the increasing pressures at ports 42 and 44 are transmitted to ports 46 and 48. When the pressure over the rod area A1 develops a force more than the resistive load of the spring 38, the outer piston 34 moves to the left as shown in the drawing, while inner piston 22 moves under the influence of light spring 58 follows up and after a short travel valve member 56 engages its seat. There is now no direct connection between ports 44 and 48 and increasing pressure at port 44 works only on the annular area A4 - A3 so that the pressure at the outlet port 48 increases at a lower rate than that of the increasing pressure at port 44, as it occurs in usual proportioning valves. Due to differences of effective areas of the intermediate piston 24 the latter piston remains in contact with the rod portion 28 of the piston 22. Meanwhile outer piston 20 moves a little further and valve member 54 engages its seat. Once again the pressure in outlet port 46 will increase at a lesser rate than pressure at inlet port 42. 
     The knee-point in the characteristic curve is determined for sub-circuit B by a movement of piston 22 relative to the housing 10 of approximately 0.04 inch and for the circuit A by a movement of piston 20 relative to the housing of approximately 0.08 inch (0.04 + 0.04). Thus without adjustment after manufacture the pressures at the knee-points is substantially the same for both sub-circuits A and B and when not considering the preload of the light spring 58 are equal to the preload of spring 38 divided by area A1. The slope after the knee-point is determined for sub-circuit A by A2 -A1/A2 and for subcircuit B by A4 - A3/A4. As already stated these ratios are chosen to be equal. 
     When sub-circuit A fails and no pressures is developed in outlet port 46, the spring 38 must be overcome by pressure in inlet 44 acting on rod portion 28 of inner piston 22 and on the adjacent end 64 of the intermediate piston 24. To achieve the same knee-point in circuit B as before when both sub-circuits where intact the area A5 is chosen equal to the area A1. 
     When sub-circuit B fails pressure at the outlet 46 hold pistons 24 and 22 in their rest position against the force of light spring 58 and the pressure in the outlet port 46 is controlled exactly as before except that the knee-point is negligibly lower that with an intact device because the outer piston 20 is not followed by the intermediate piston and the check valve 54 therefore closes a little earlier. 
     One main advantage of the twin proportioning valve according to the invention is that the performance of each breaking sub-circuit A and B connected thereto remains substantially the same after the other sub-circuit has failed. 
     Assuming that the twin proportioning valve has a load sensing device connected to the resilient return means 38, the knee-point pressures is obtained by a single adjustment of the preload of the load sensing spring after mounting on the vehicle. However, thanks to the structure of this twin proportioning valve the equality of the knee-point pressure is obtained in both sub-circuits without adjustment after manufacture even if the valve is of the load sensing type. 
     This invention is not limited to a twin valve incorporating differential pistons of stepped type but also relates to differential pistons wherein the rod portion or the portion having the smaller effective area is of an annular shape. In another embodiment of the invention (not shown) the intermediate piston is stepped to define with the bore wall an annular vented recess in which projects the rod portion of an annular shape of the inner piston, the external diameter of which rod portion being equal to that of the end of the intermediate piston adjacent to the outer piston, the effective area of the latter end being equal to that of the rod portion of said outer piston. Shortly this structure is the converse of that hereinabove described and both devices operate in the same manner.