Servomotor for braking assistance with reduced stroke of actuation

Servomotor with reduced idle stroke of actuation without decrease in valve lift on release of the brake. According to the invention, the rest position of the actuating rod 36 is determined by movable stop 60 capable of withdrawing on release of the brake in order not to impede lift of the valve poppet 32 relative to the seat 40, which allows, in the rest position, these two elements to be brought closer with a short distance d, thus reducing the idle stroke of actuation. Application to power-assisted braking.

The invention concerns a servomotor for braking assistance with reduced 
idle stroke of actuation and more particularly an improvement in the 
actuating mechanism of this servomotor allowing such a reduction in idle 
stroke to be obtained without decreasing the valve lift on release of the 
brake. 
In a conventional servomotor for braking assistance, including two chambers 
separated from each other by a driving piston capable of actuating a brake 
master cylinder by displacement from a rest position, the driving piston 
comprises a three-way valve-forming mechanism actuated by an axial push 
rod connected to the brake pedal. One of the chambers is provided for 
connection to a vacuum source available on the vehicle (for example the 
vacuum in the intake manifold of the engine) and the role of the three-way 
valve consists in putting the other chamber in communication with the 
vacuum chamber or with the atmosphere. The valve is essentially composed 
of a movable annular valve poppet capable of coming alternately into 
abutment on the one hand against a movable seat integral with the push rod 
and on the other hand against an annular seat defined in the driving 
piston around the movable seat, the latter being constructed at one end of 
a plug mounted slidingly within the driving piston. In the rest position 
of the valve-forming device, the movable annular valve poppet is in 
abutment against the movable seat and this position establishes 
communication between the two above-mentioned chambers. On the other hand, 
axial displacement of the push rod induces retraction of the movable seat 
and the annular valve poppet then becomes engaged against the annular 
seat, which has the effect of cutting off the two chambers from each other 
and of putting in communication with the atmosphere the one which is not 
directly connected to the vacuum source; which induces displacement of the 
driving piston and actuation of the brake master cylinder. 
The rest position of the device is normally defined by mechanical contact 
of the sliding plug on which is defined the movable seat against a stop 
called a "locking key" integral with the driving piston. Axial sliding of 
the plug relative to the driving piston is limited over a certain axial 
distance calculated to guarantee an appropriate lift of the movable valve 
poppet on release of the pedal, ensuring good passage of air. This 
structure functions satisfactorily but imposes a relatively great idle 
stroke on the application of braking, which is represented by the axial 
distance separating the movable valve poppet from the annular seat, in the 
rest position of the system. The reduction of this idle stroke is a 
current preoccupation of engineers, notably by reason of the fact that it 
is multiplied by the ratio of the brake pedal and that, consequently, the 
least gain at the level of this idle stroke may be manifested by a notable 
gain in space in the occupants' compartment of the automobile. The 
invention answers this preoccupation while maintaining a suitable valve 
lift for good operation of the three-way valve-forming assembly. 
More precisely, the invention therefore concerns a servomotor for braking 
assistance of the vacuum type including a first chamber provided for 
connection to a vacuum source, a second chamber separated from the first 
by a driving piston capable by displacement from a rest position of 
actuating a brake master cylinder, said driving piston comprising a 
three-way valve actuated by a push rod for putting said second chamber in 
communication with said first chamber or with the atmosphere, which valve 
comprises a movable annular valve poppet capable of coming alternately 
into abutment on the one hand against a movable seat integral with said 
push rod and on the other hand against an annular seat defined in said 
driving piston around said movable seat, said movable seat being 
constructed at one end of a plug mounted axially slidingly within said 
driving piston while said valve poppet is in abutment against said movable 
seat in the rest position of the servomotor, characterized in that it 
comprises a movable stop extending approximately perpendicularly to the 
axis of said rod, maintained by a resilient element against a support 
which is fixed relative to said driving piston, in said rest position, and 
in that said stop cooperates with a shoulder of said actuating rod to 
maintain said movable valve poppet at a short distance from said annular 
seat, thereby reducing the idle stroke of actuation of the servomotor. 
On the other hand, on release of the pedal, the movable stop will be able 
to withdraw axially against the force of said resilient element and will 
not therefore be able to constitute an obstacle to lift of the movable 
valve poppet. With such an arrangement, the rest position of the valve 
components will no longer be defined by contact between a shoulder of the 
plug and the locking key.

Referring to FIG. 1, the conventional servomotor for braking assistance, as 
shown, comprises a housing 10 formed of two shells 12 and 14 assembled at 
their periphery so as to enclose the external flange 15 of a flexible 
diaphragm 16 constituting, in association with a rigid plate 18 and a 
central body 19, a driving piston assembly 20. The assembly edges of the 
shells 12 and 14 are shaped to be assembled by crimping, holding the 
flange 15 captive. In this way, the driving piston 20 separates the 
housing 10 into two chambers, respectively a first chamber or low pressure 
chamber 22 designed for permanent connection to a vacuum source via a 
connecting adapter 24 and a second chamber or high pressure chamber 26 
capable of being put in communication either with the chamber 22 or with 
the atmosphere, the latter via a filter 27. The central body 19 extends 
rearwards, that is, in the direction of the actuating members of the 
servomotor, so as to accommodate a device forming a control valve 30, of 
the three-way type. The latter consists essentially of a movable annular 
valve poppet 32, made of rubber, capable of coming alternately into 
abutment on the one hand against a movable seat 34 integral with an input 
push rod 36, connected to the brake actuating pedal (not shown) and, on 
the other hand, against an annular seat 40 defined in the central body 19 
around the movable seat 34. The central body 19 comprises an axial bore 37 
constituting a sliding bearing for a plug 50 at the end of which is 
constructed the movable seat 34. The sliding plug 50, articulated by a 
ball joint on the rod 36, ends in a stop surface 35 capable of becoming 
engaged with a reaction device 38 composed of a rubber washer. The 
opposite face of the reaction device 38 cooperates with a push rod 42 
which is capable of setting in operation the drive of a brake master 
cylinder (not shown). A spring 44 is supported on the reinforcing piece of 
the shell 12 and urges the driving piston 20, in its rest position as 
shown in the drawing, against the shell 14. When the servomotor is in this 
rest position, the movable valve poppet 32 is disengaged from the annular 
seat 40 so as to allow the two chambers 22 and 26 to communicate via 
passages such as 46, 48 and 49, through the body 19. More precisely, the 
passages 46 allow fluid communication between the chamber 22 and the 
opening of the annular seat 40 while the passages 48 and 49 allow fluid 
communication between the second chamber 26 and the opening of the annular 
seat 40. Thus, when the servomotor is in its rest position as shown, the 
two chambers 22 and 26 communicate effectively and the same level of 
partial pressure prevails in these two chambers, since the valve poppet 32 
which controls communication with the atmosphere via the filter 27 as in 
sealing tight abutment against the seat 34 by the action of the pressure 
difference and springs such as 39 and 41. The spring 39 is mounted under 
compression between a support collar 51 and a first shoulder 52 of the 
push rod 36 so that the movable seat 34 is normally urged in the direction 
of the annular valve poppet 32. The spring 41 is mounted under compression 
between the support collar 51 and the movable valve poppet 32 to urge it 
in the direction of the coaxial seats 34 and 40. The support collar 51 is 
itself in abutment against an annular base of the valve poppet 32, 
integral with the central body 19. Axial sliding of the plug 50 within the 
bore 37 is limited over a certain axial distance by cooperation of an 
annular groove 54, of predetermined width, made in the surface of the plug 
50, and a locking key 53, integral with the central body 19. Contact 
between one of the ends of the groove 54 and the key 53 determines the 
rest position of the valve mechanism, which is therefore provided, by 
construction, with an idle stroke of acutation D represented by the axial 
distance between the seat 40 and the movable valve poppet 32. The value of 
this idle stroke D corresponds to the valve lift necessary, at the end of 
a braking stage, to ensure a suitable response time of the servomotor, for 
return to the rest position. 
FIG. 2 shows the improvement according to the invention adapted to the 
conventional servomotor which has just been described. Consequently, the 
unmodified constituent parts of this servomotor bear the same reference 
figures and will not be described a second time. The valve mechanism is 
completed by a movable stop 60 extending approximately perpendicular to 
the axis of the push rod 36 and maintained by a resilient element, 
consisting of the spring 39, against the support collar 51. The stop 60 
cooperates with a second shoulder 61 of the actuating rod 36 to define a 
new rest position of the constituent parts of the three-way valve 30. The 
position of the shoulder 61 is therefore selected to maintain the valve 
poppet 32 at a short distance d from the annular seat 40, via the rest 
position of the movable seat 34. In this new rest position with reduced 
idle stroke, the relative positions of the different elements of the valve 
are therefore no longer determined by cooperation of the annular groove 54 
and the key 53. On the contrary, as can be seen clearly in FIG. 2, the 
edges of the groove 54 are maintained removed from the key 53 by 
positioning of the push rod (and of the plug 50) imposed by the movable 
stop 60. The role of the key 53 is therefore confined to fixing the limits 
of maximum stroke of the plug 50 within the bore 37 and the maximum value 
of valve lift on release of the brake. It is to be noted that the support 
collar 51 advantageously comprises at its radially outermost perimeter 
clip lugs 63 improving its fixing within the central body 19. In the 
example described, the movable stop advantageously has the form of a 
washer, apertured to allow the passage of air between the filter 27 and 
the chamber 26 and shaped with an approximately spherical profile in its 
central portion. This last detail allows angular deflection of the push 
rod around its ball joint connecting it to the plug 50, the shoulder 61 
being supported within this spherical portion. The shoulder 61 may be 
constructed by one of the ends of an annular groove made in the push rod 
36 and of which the width is sufficient not to impede actuating movement 
of the rod. 
The servomotor for braking assistance according to the invention operates 
in the following manner. In the absence of braking, the situation is as 
illustrated in FIG. 2, the chambers 22 and 26 are in communication and 
connected to the intake manifold. A low pressure therefore prevails in 
these two chambers. When the driver actuates the brake pedal and therefore 
the push rod 36, the plug 50 is driven back within the bore 37 so that 
after a relatively short idle stroke d, the chamber 26 is cut off from the 
chamber 22. Additional thrusting of the plug 50 puts the chamber 26 in 
communication with the atmosphere since the valve poppet 32 becomes 
separated from the movable seat 34. From this moment, the driving piston 
assembly 20 is displaced from right to left, considering FIG. 2, and 
actuates the master cylinder. On release of braking, the response time of 
the servomotor, that is, the time necessary for the driving piston to 
return to its rest position, depends notably on the lift of the valve 
poppet 32 relative to the annular seat 40. This may be as great as in a 
conventional servomotor, due to a slight withdrawal of the stop 60, caused 
by the return movement of the push rod 36. The return of the latter is due 
partly to the action of the pedal return spring and above all to the 
thrust applied to the rod 42 by hydraulic reaction of the master cylinder, 
transmitted through the reaction disc. In this phase, the spring 41 is 
compressed to allow increasing of the valve lift. At rest this spring 
returns to its initial position and the clearance "d" reappears between 
seat and valve poppet, hence a short idle stroke at the following 
application. The stop 60 returns rapidly into abutment on the collar 51 by 
the action of the spring 39. 
Naturally, the invention is not limited to the embodiment which has just 
been described but includes all technical equivalents of the means 
involved if these are within the scope of the claims which follow.