Hydraulic brake booster

The booster is installed downstream of a vehicle's power steering gear. The booster has a power piston and a valve seat piston reciprocably mounted in a bore of the booster housing. Hydraulic inlet pressure is introduced into the housing bore between these pistons and flows through a normally open poppet valve, which acts as a booster control valve. Movement of the booster input push rod restricts hydraulic fluid flow through the control valve and causes pressure to be built up in the power chamber between the pistons. The pressure acts on the power piston to move the booster output member. The pressure also holds the valve seat piston in position against the bore and wall so that it does not move during normal power boost operation. In the power mode brake pedal travel can be very low, permitting a high brake pedal ratio for the no-power operating mode.

The invention relates to a hydraulic brake booster for operating a master 
cylinder assembly in a brake system. It more particularly relates to such 
a booster which operates with a very small amount of brake pedal travel 
during power boost operation, and is capable of having a high brake pedal 
ratio so that manual actuation of the master cylinder assembly by pushing 
through the booster can be accomplished without requiring extremely heavy 
brake pedal force loads. 
Hydraulic brake boosters have been used for some years which have a valve 
seat forming a part of the control valve provided on the power piston or 
moving with that piston. The disclosure of U.S. Pat. No. 
3,699,680--Shellhouse is an example of this type of brake booster. Such a 
booster requires controlling movement of the brake pedal which is directly 
related to the amount of booster output member movement. Other boosters 
have been used which have a lever mechanism permitting the controls to 
move a proportionally smaller distance than the power piston during normal 
power operation, but yielding so that the control moves during manual 
operation at a distance substantially equal to that traveled by the 
piston. Such boosters are exemplified by a U.S. Pat. No. 
3,603,209--MacDuff and U.S. Pat. No. 3,625,112--Brown. Variable ratio 
brake pedal linkages are also well known, as exemplified by U.S. Pat. No. 
3,858,457--Mathues; U.S. Pat. No. 3,911,760--Elbers et al; and U.S. Pat. 
No. 4,069,722--Derrick. These linkages typically modify the effective 
ratio of the brake pedal lever during its stroke from a brake release 
position to a full brake apply position. Other brake mechanisms have been 
proposed which utilize a zero or minimum travel brake pedal which is 
relatively low to the floor of the vehicle for power booster operation, 
and which have some type of device which raises the brake pedal to a 
higher level so that additional brake pedal travel is available for manual 
brake operation when power is no longer available. Examples of such brake 
booster mechanisms are illustrated in U.S. Pat. Nos. 3,093,120--Ayres, 
Jr.; 3,246,473--McCotter et al; 3,250,183--Gephart; and 3,250,184--De 
Hoff. 
The mechanism embodying the invention has many of the advantages of a 
variable ratio mechanism as well as the zero brake pedal travel mechanism 
but is much simpler in concept and operation. A brake booster embodying 
the invention requires only nominal movement of the brake pedal during 
booster operation of the master cylinder assembly, and requires only 
slight movement to afford a complete closure of the control valve 
mechanism to thereafter provide a manual force transmitting path through 
the booster mechanism for manual operation of the master cylinder. Manual 
operation can occur either when there is no power pressure available at 
all, or can also occur when power runout has occured and additional 
actuating force on the master cylinder is required by the vehicle 
operator. In the latter situation, closure of the control valve operates 
to establish the pressurized hydraulic fluid in the power chamber as a 
hydraulic link through which additional manually induced brake master 
cylinder operational force is transmitted.

The system of FIG. 1 is schematically illustrated as including a power 
steering pump 10 which receives hydraulic fluid from a reservoir 12 
through conduit or passage 14 and delivers the hydraulic fluid through 
conduit 16 to the power steering gear 18. Gear 18 is of the well known 
open center type so that the hydraulic fluid flows through the gear 
without any substantial restriction when the gear is not operating. The 
fluid then passes through conduit 20 to the hydraulic brake booster 22. 
Conduit 20 is connected to the booster inlet 24 for this purpose. Booster 
outlet 26 delivers hydraulic fluid from the booster through conduit 28, 
which returns it to the reservoir 12. Thereservoir 12 may be a physical 
part of the power steering pump 10, as is common practice. 
The booster 22 is mounted on a suitable fixed portion 30 of the vehicle in 
which it is installed, as is the brake pedal 32. The brake pedal is 
connected to actuate the booster input push rod 34 in the usual manner. 
The output of the booster 22 is connected to the master cylinder assembly 
36, which when actuated pressurizes hydraulic brake fluid in brake 
circuits 38 and 40. Circuit 38 is schematically illustrated as including 
the vehicle front wheel brakes 42, and circuit 40 is schematically 
illustrated as including the vehicle rear brakes 44. Other split circuit 
arrangements may also be used. 
The hydraulic brake booster 22 embodying the invention is illustrated in 
detail in FIG. 2. It includes a housing 46 having a bore 48 formed 
therein. The front portion of bore 48 is closed by an end cover 50 through 
which the booster output member 52 extends. The after end of bore 48 is 
formed with an end wall 54 through which the booster input member 56 
sealingly and reciprocably extends. Input member 56 is suitably connected 
with the push rod 34 so as to be moved under control of the vehicle 
operator by operation of the brake pedal 32. A power wall 58, formed as a 
power piston, is reciprocably received in bore 48. The power wall 58 has a 
forwardly extending portion 60 which extends through the end cover 50 in 
sealing and reciprocal relationship. The booster output member 52 is 
received by and extends from portion 60. Output member 52 is operatively 
connected to a pressurizing piston in the master cylinder assembly 36, as 
is well known in the art. On the forward side of power wall 58 is a 
chamber 62 containing the power wall return spring 64. Spring 64 has one 
end acting on the forward face 66 of the power wall 58 and the other end 
acting on end cover 50. Spring 64 continually urges the power wall 58 
rearwardly to the released position. A seal 68 on the outer periphery of 
the power wall 58 seals the power wall relative to the surface of bore 48 
so that any pressure in the power chamber 72 will not leak into chamber 
62. The rear face 70 of the power wall 58 forms the forward wall of the 
power chamber 72. The center portion of power wall rear face 70 is 
slightly recessed to provide a first abutment surface 74 and a second 
abutment surface 76. 
A valve piston 78 is reciprocably received in bore 48 intermediate power 
wall 58 and the bore end wall 54. The forward face 80 of valve piston 78 
forms the rear wall of the power chamber 72. A seal 82 on the outer 
periphery of valve piston 78 seals the piston relative to the surface of 
bore 48 so that any pressure in power chamber 72 will not leak into 
exhaust chamber 98. An extension 84 of valve piston 78 extending from the 
forward face 80 thereof has a forward end 86 which is engageable with 
abutment surface 74 of the power wall 58. A passage 88 is formed through 
valve piston 78 and its extension 84. Adjacent the forward end 86 of 
extension 84 one or more cross passages 90 provide continuous 
substantially unrestricted fluid communication between power chamber 72 
and passage 88. The rear end of passage 88 is radially enlarged to some 
extent and the rear edge thereof defines a valve seat 92. Several legs 94 
extend rearwardly from the rear face 96 of valve piston 78 and normally 
abut the bore end wall 54 as shown in FIG. 2. Legs 94 are somewhat similar 
to castellations so that they provide axial spacing of the valve piston 78 
relative to the wall 54 while permitting relatively free hydraulic fluid 
passage between the valve piston rear face 96 and wall 54. This portion of 
bore 48 between the valve piston 78 and rear wall 54 defines the exhaust 
chamber 98 from which the booster outlet 26 delivers hydraulic fluid to 
conduit 28 as earlier described. 
The control valve 100 includes the valve seat 92 and a valve element 102 
secured to or formed as a part of the input member 56. Input member 56 
extends through the exhaust chamber 98 and supports valve element 102 in 
valving relation with valve seat 92 to define the control valve 100 as an 
annular poppet valve controlling hydraulic fluid flow from power chamber 
72 through passage 88 to the exhaust chamber 98. A spring 104 within 
passage 88 has its rear end acting on the valve element 102 to continually 
urge that element and input member 56 rearwardly. The forward end of 
spring 104 engages the abutment surface 76 of power wall 58. Suitable 
seals 106 and 108 in the rearward reduced diameter portion of bore 48 
cooperate with the outer surface of input member 56 to prevent loss of 
hydraulic fluid from exhaust chamber 98 through the rear portion of the 
bore. 
The brake booster inlet 24 communicates through a port 110 with the power 
chamber 72 through a check valve 112. Check valve 112 is arranged to open 
under very light hydraulic pressure so as to provide no substantial 
resistance to hydraulic flow. However, the check valve will close whenever 
the pressure in the power chamber 72 is substantially equal to or greater 
than the pressure being received from the power steering pump 10 through 
conduit 20 and inlet 24. A pressure limiting check valve 114 is positioned 
in a passage 116 formed through the valve piston 78 and is normally 
closed. Valve 114 is so positioned and arranged that it opens to limit to 
a predetermined desired maximum the pressure that can be introduced into 
power chamber 72 relative to the pressure in exhaust chamber 98. When the 
hydraulic pressure in chamber 72 exceeds the predetermined desired maximum 
value, valve 114 opens and bleeds off the pressure through passage 116 to 
the exhaust chamber 98 until the excess pressure no longer exists in the 
power chamber 72. 
When the booster is in the released position and the power steering pump 10 
is operating, hydraulic fluid is pumped with little pressure loss through 
the power steering gear 18 and the booster 22. When the vehicle operator 
operates the brake pedal 32 to actuate the vehicle brakes, slight movement 
of the brake pedal results in slight forward movement of push rod 34 and 
input member 56 so that valve element 102 approaches valve seat 92 and 
restricts or closes the control valve 100. This immediately causes a 
build-up of hydraulic pressure in power chamber 72, which acts on the 
forward face 80 of valve piston 78 and tends to hold the valve piston in 
the rearward position illustrated with the rear ends of legs 94 in 
engagement with bore rear end wall 54. The pressure also acts on the rear 
face 70 of the power wall 58, moving the power wall and therefore output 
member 52 forwardly against the force of piston return spring 54 to 
actuate the master cylinder assembly 36. The pressure also acts across the 
area of the forward end of input member 56 and valve element 102 to 
provide reaction to the vehicle operator. As power wall 58 moves 
forwardly, the end 86 of extension 84 is separated from the abutment 
surface 74 of the power wall since the valve piston 78 remains in rearward 
position. Thus the vehicle operator can control the amount of boost 
operation by controlling the amount of restriction established by control 
valve 100. This is done with minimal axial forward movement of valve 
element 102 and input member 56. The total amount of movement from the 
fully released position to the fully closed position of the control valve 
100 may be a relatively small amount, on the order of sixty to one 
hundred-thousandths of an inch, for example. Thus the vehicle operator is 
required to move brake pedal 32 a very small amount to obtain any desired 
power operation of the brake booster 22. 
If more master cylinder operational force is required than can be generated 
by the maximum determined pressure in power chamber 72, the vehicle 
operator only needs to move brake pedal 32 sufficiently to engage valve 
element 102 with valve seat 92. This will completely close control valve 
100 and will trap hydraulic fluid in the power chamber 72 at no more than 
the predetermined maximum pressure value. Further operation of the brake 
pedal in a brake applying direction will cause the manual force to be 
transmitted from input member 56 and valve element 102 to valve piston 78, 
moving the valve piston forwardly so that it acts on the hydraulic link 
established by the trapped pressure in power chamber 72 to move power wall 
58 forwardly to the same extent. This causes the booster output member 52 
to move an additional amount equal to the amount of movement of the input 
member 56. The brake pedal 32 may be provided with a relatively high pedal 
ratio so that this movement can occur without high brake pedal force 
required to be exerted by the vehicle operator. Similarly, if there is no 
hydraulic pressure available, the input member 56 may be initially moved 
until valve element 102 engages valve seat 92, and the manual force is 
then transmitted mechanically through valve piston 78 and its extension 84 
to the power wall 58 and therefore to the output member 52 to manually 
actuate the master cylinder assembly 36.