Abstract:
A master cylinder having a body ( 10 ) with a bore ( 12 ) therein in which slides at least a primary piston ( 20 ). The primary piston ( 20 ) has a compensation passage ( 58 ), which in a position of rest, places a pressure chamber ( 24 ) in communication with a reservoir ( 14 ). The communication between interrupted upon actuation of the master cylinder by a control rod ( 18 ) acting on a second piston ( 48 ) sliding in the bore ( 12 ). A first compression spring ( 28 ) and a second compression spring ( 42 ) urge the primary piston ( 20 ) and second spring ( 16 ) toward a rear position of rest. The preload force of the first compression spring ( 28 ) exceeding that of the second compression spring ( 16 ) such that the second piston ( 48 ) is the first to move in response to a force applied to the control rod ( 18 ). The rear position of rest for the primary piston ( 200  and second piston ( 48 ) is defined by a same stop ( 36 ) which is integral with the body ( 10 ) of the master cylinder.

Description:
The present invention relates to master cylinders, of the type with valves, and which are intended in particular to equip hydraulic braking circuits of motor vehicles. 
     BACKGROUND OF THE INVENTION 
     A valve-type master cylinder consists, in the conventional way, of a bore formed in a body and in which there slides at least one primary piston provided with a compensation passage capable of placing a pressure chamber of the master cylinder in communication with a reservoir of fluid at low pressure when the master cylinder is in its position of rest, its communication being interrupted by a second piston upon actuation of the master cylinder by a control rod acting on the second piston sliding in the bore of the master cylinder, a first compression spring urging the primary piston towards a rear position of rest, a second compression spring being arranged between the primary piston and the second piston and urging the latter towards a rear position of rest, the preload at rest of the first compression spring exceeding the preload at rest of the second compression spring. 
     A valve-type master cylinder of this kind is known, for example, from document GB-B-1,155,160. In this known master cylinder, the primary piston rests, in the position of rest, on a stop formed at the end of a screw interacting with a tapped bore formed in the body of the master cylinder, while the second piston rests, in the position of rest, on an annular stop kept at the rear end of the bore of the body of the master cylinder. 
     Upon actuation of such a master cylinder via the control rod, the second piston is lifted off its rear stop to move to seal the compensation passage so as to interrupt the communication between the pressure chamber of the master cylinder and the reservoir of fluid at low pressure, and so that the pressure in the pressure chamber can increase. 
     This travel of the control rod needed to achieve closure of the compensation passage is known in the field of braking by the name of master cylinder dead travel. Depending on the manufacturing tolerances on the various elements of which a master cylinder is composed, it will be understood then that the dead travel can vary across a production run. Furthermore, with the objective of obtaining a braking action as soon as possible after the beginning of the actuation of the master cylinder, it is very important on the one hand for its dead travel to be as small as possible and on the other hand for it to be possible for this travel to be set to this minimum value when the master cylinder is assembled and for it to keep this minimum value during use of the master cylinder. 
     In the master cylinder known from the aforementioned document, the dead travel is determined by the positions of rest of the primary piston and of the second piston are determined respectively by a stop screw and an annular stop, the relative positions of which can vary over the course of time or over the course of the operation of the master cylinder. [sic]. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is therefore to provide a master cylinder in which the dead travel can be set simply to a minimum value and which keeps this minimum value throughout the use of the master cylinder, it being necessary also for this master cylinder to be obtained using means which are simple and quick to employ in a reliable and inexpensive way. 
     According to the present invention, the rear positions of rest of the primary piston and of the input piston are defined by the same stop integral with the body of the master cylinder. 
     Other objectives, features and advantages of the present invention will emerge more clearly from the description which follows of one embodiment given by way of illustration with reference to the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view in longitudinal section of a master cylinder produced in accordance with the present invention, and 
     FIG. 2 is a view on a larger scale of the rear part of the master cylinder of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The figures depict a master cylinder produced in accordance with the invention. By convention, the direction in which the parts move upon actuation of the master cylinder is called the “front” and the direction in which the parts move to return to their position of rest is called the “rear”. In the figures, the front is therefore to the left and the rear is to the right. 
     The master cylinder depicted in the figures consists of a body  10  in which there is a blind bore  12 . A reservoir of brake fluid at low pressure (not depicted) is connected to this master cylinder via at least one communication port  14 . An actuating piston  16  is able to slide in this bore  12  under the action of a control rod  18  itself made to move by a brake pedal (not depicted) situated in the cockpit of the vehicle or by a pneumatic brake-booster (not depicted) actuated by this brake pedal. 
     A primary piston  20  is able to slide in leaktight fashion thanks to a cup  21  in the bore  12  and therein delimits, with the closed end of the blind bore  12  or, as has been depicted in the figures, with a secondary piston  22 , a primary working chamber  24 , the secondary piston  22  delimiting with the closed end of the blind bore  12  a secondary working chamber  26 . The primary chamber  24  is connected to a primary brake circuit (not depicted) by a primary outlet orifice (not depicted). Likewise, the secondary chamber  26  is connected to a secondary brake circuit (not depicted) by a secondary outlet orifice (not depicted). 
     The primary piston  20  is urged into its rear position of rest by a device of the spring-box type, consisting of a spring  28  resting on the one hand on a shoulder  30  of the primary piston  20  and, on the other hand, on a spring cup  32  whose maximum separation from the primary piston  20  is determined by the head of a screw  34  screwed onto the front end of the primary piston  20 . 
     In its position of rest, the primary piston  20  rests on the body  10  of the master cylinder or, as has been depicted, on a washer  36  integral with the body  10  of the master cylinder, for example arranged between a shoulder  38  of the body  10  and a device  40  that provides a seal between the inside of the master cylinder and the outside thereof, and especially between the body  10  of the master cylinder and the actuating piston  16 . 
     The actuating piston  16  is itself urged into its rear position of rest by a compression spring  42  arranged between it and the primary piston  20 . The spring  42  has a preload at rest which is less than that of the spring  28  which means that in its position of rest, the actuating piston  16  is itself also resting on the body  10  of the master cylinder or, as has been depicted, on the washer  36  integral with the body  10  of the master cylinder, via a shoulder  44  formed between a front part  46  of larger diameter and a rear part  48  of smaller diameter. 
     More precisely, the primary piston  20  rests on the body  10  of the master cylinder via fingers  50 , of which there are at least two, uniformly angularly spaced about the axis X-X′ of the master cylinder, these fingers  50  passing through axial slots  52  formed at the periphery of the part  46  of the actuating piston  16 , there being as many of these slots as there are fingers  50  and these slots being uniformly angularly spaced about the axis X-X′ of the master cylinder. 
     The fingers  50  are formed with an outer radial peripheral surface that has the same radius of curvature as that of the bore  12  so as to be able to slide in this bore  12  and they thus form a means of guiding the primary piston  20  in the bore  12 . In the same way, the front part  46  of the actuating piston  16  is formed, where the axial slots  52  are not, with an outer peripheral surface that has the same radius of curvature as that of the bore  12 , so as to be able to slide in this bore  12  and form a means of guiding the actuating piston  16  in the bore  12 . 
     The front part  46  of the actuating piston  16  is formed with a blind bore  54  in which a rear part  56  of the primary piston  20  can slide in non-leaktight fashion because, for example, of axial grooves  55 , SO as to guide this primary piston with respect to the actuating piston  16 . The axial grooves  55  may just as well be formed on the outer peripheral surface of the rear part  56  of the primary piston  20  as on the inner peripheral surface of the blind bore  54  of the actuating piston  16 . 
     The primary piston  20  is formed with a compensation passage  58  which, in the position of rest, allows communication between the primary working chamber  24  and the reservoir of fluid at low pressure connected to the communication  14 , via the axial grooves  55 , the axial slots  52 , and a drilling  60  formed in the body  10  of the master cylinder. 
     The primary piston  20  is also formed, between the rear part  56  and the fingers  50 , with an annular surface  62  perpendicular to the axis X-X′ and flat, forming a seat for an annular valve  64  made of an elastomeric material such as rubber arranged on the front face of the front part  46  of the actuating piston  16 . Of course, provision could be made for the annular valve to be arranged on the primary piston  20  and to interact with a valve seat formed at the front of the actuating piston  16 . 
     From the foregoing explanations it will have been understood how the present invention is employed. At rest, all the moving parts occupy their rear position depicted in the figures. In particular, the primary working chamber  24  communicates with the reservoir of fluid at low pressure via the compensation passage  58 , the axial grooves  55 , the axial slots  52  and the drilling  60 . 
     Upon actuation of the master cylinder by application of a force to the control rod  18 , the actuating piston  16  leaves its position of rest and moves away from the washer  36  or from the body  10  of the master cylinder, compressing the spring  42 . It continues to move in this way until the annular valve  64  comes into contact with the valve seat  62  formed on the primary piston  20 . When this contact occurs, the communication between the reservoir of fluid at low pressure connected to the pipe  60  and the primary working chamber  24  is interrupted. 
     The travel of the actuating piston  16  needed to close the communication between the reservoir of fluid at low pressure and the primary working chamber corresponds to what is known as master cylinder dead travel. 
     Once this dead travel has been covered, the actuating piston  16  then makes the primary piston  20  leave its position of rest and this piston in turn moves away from the washer  36  or from the body  10  of the master cylinder, compressing the spring  28 , and making the pressure in the primary working chamber  24  increase, this increase in pressure being transmitted to the wheel cylinders (not depicted) and possibly to the secondary piston  22  in the case of a tandem master cylinder. 
     It can therefore be clearly seen that, thanks to the present invention, the master cylinder dead travel is represented only by the distance D between the annular valve  64  borne by the actuating piston  16  and the annular surface  62  borne by the primary piston  20 . Since at rest these two pistons rest on the same piece  36 , it will be very easy to determine this distance D when manufacturing or assembling the master cylinder so as to give it any predetermined value, and in particular a value which is as small as desired. Furthermore, one can be sure that this distance D will be maintained over time, irrespective of the use of the master cylinder, and even if it has to be dismantled and reassembled for maintenance operations. 
     According to the invention, a master cylinder in which the dead travel can be fixed by construction in a simple way to any desired value, and preferably to a minimum value, and which keeps this minimum value throughout the use of the master cylinder has indeed been achieved. A minimum dead travel of this kind is furthermore obtained very simply, reliably and inexpensively. 
     Of course the invention is not restricted to the embodiment which has just been described, but can be modified in numerous ways which will be clear to a person skilled in the art and which fall within its scope. Thus, for example, the invention can be applied to single or to tandem master cylinders, it being possible for these master cylinders to be controlled directly by the brake pedal or by a pneumatic brake-booster, and it being possible for the secondary piston itself, in the case of a tandem master cylinder, to be of the type which has a valve, an expansion hole, or of any other type.