Tandem brake booster

A tandem brake booster (10) having a housing (12) with a cavity therein divided by a first wall (24) and a second wall (28) into first, second and third chambers (20, 26 and 30). The first and second chambers (20, 26) are connected to each other through a control valve (62) and the first and third chambers (20, 30) are connected to each other by a conduit (38). The second wall (28) is connected to the first wall through lever arrangements (100 and 102). When the first chamber (20) is connected to a source of vacuum, air is evacuated from the first, second and third chambers (20, 26 and 30). In response to an input force, the control valve (62) interrupts communication of vacuum to the second chamber (26) and allows air to be communicated to the second chamber (26). With air in chamber (26) and vacuum in the first and third chambers (20 and 30 ) a pressure differential is created across walls (24 and 28). This pressure differential simultaneously acts on the first wall (24) and second wall (28) by moving the first and second walls (24 and 28) in opposite directions to develop first and second output forces. The second output force is transmitted through the lever arrangement and added to the first output force to produce an operational force for activating a master cylinder.

This invention relates to a tandem brake booster for providing a master 
cylinder with an input force in response to an operator brake input. 
Tandem brake boosters such as disclosed in U.S. Pat. No. 3,096,689 have two 
substantially identical power producing movable walls located in a single 
housing. The movable walls are separated by a partition member which 
prevents fluid communication between a rear chamber associated with the 
first wall and a front chamber associated with the second wall. Thus, the 
first and second walls and partition divide the cavity within the housing 
into four distinct chambers which are required to operate such brake 
boosters. 
While a tandem brake booster does produce substantially twice the amount of 
output force of a single brake booster of the same diameter, the overall 
length of a tandem brake booster is also about double that of a single 
brake booster. In some vehicles currently in production, an increase in 
the length of such a brake booster requires the selection of a different 
type brake booster since the under hood space is limited. 
In a single brake booster disclosed in U.S. Pat. No. 3,188,920 it was 
discovered that the housing was divided into three chambers by two walls 
with the working chamber located between two reference chambers. One wall 
was connected to the output member while a second wall supplied the 
operator with a reactionary force corresponding to the output of the first 
wall produced by a pressure differential between the working chamber and 
reference chamber. However, the output force developed in the brake 
booster is entirely dependent on the action of the pressure differential 
on the first wall. 
SUMMARY OF THE INVENTION 
The tandem brake booster disclosed herein has a housing divided into three 
chambers by first and second walls. The first wall and housing define the 
first chamber, the first and second walls and housing define the second 
chamber, and the second wall and housing define the third chamber. The 
first wall is connected to an output member and responds to a pressure 
differential between the first and second chamber by moving toward the 
first chamber to develop a first output force. The second wall responds to 
the pressure differential between the third and second chamber by moving 
toward the third chamber and away from the first chamber to develop a 
second output force. A lever arrangement attached to the second wall 
pivots about a fulcrum to transfer the second output force into the output 
member to produce an operational force made up of the first and second 
output forces. 
One advantage of this invention occurs through the use of a common working 
chamber for the two reference chambers to provide a compact tandem brake 
booster. 
Another advantage of this invention occurs through the use of the lever 
arrangement to match the movement of the second wall with the first wall 
by locating the position of the fulcrum. 
It is an object of this invention to provide a brake booster with first and 
second movable walls that respond to a pressure differential by moving in 
substantially opposite directions to produce an additive output force and 
thereby develop an operational output to meet an input demand. 
These advantages and objects should be apparent from reading this 
specification while viewing the drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The tandem brake booster 10 shown in FIG. 1 has a housing 12 made of a 
front shell 14 and a rear shell 16 joined together by a band 18. The 
interior of the housing 12 is divided into a first chamber 20 whose limits 
are defined by the housing 12 and a first wall 24, a second chamber 26 
whose limits are defined by the first wall 24, housing 12 and a second 
wall 28, and a third chamber 30 whose limits are defined by the second 
wall 28, housing 12 and a cylindrical projection 32 extending from the 
rear shell 16. 
The first wall 24 includes a diaphragm 22 and hub member 34. Hub member 34 
has an annular backing plate 44 attached thereto that extends toward the 
side wall 46 of the housing. A first bead 21 on diaphragm 22 is retained 
by band 18 and a second bead 23 is located on a shoulder 48 on the hub 
member 34 to seal the second chamber 26 from the first chamber 24. The hub 
member 34 has an annular projection 38 that extends through opening 41 in 
shell 16. An axial bore 36 which extends through the annular projection 38 
and hub member 34 has an opening 40 into chamber 24 and an opening 42 into 
chamber 26. 
A reaction disc 50 located in bore 36 adjacent shoulder 52 forms a base for 
head 54 on push rod 56. Push rod 56 which extends through an opening 58 in 
shell 14 is connected to master cylinder 60. The master cylinder 60 is 
connected to the wheel brakes of a vehicle. 
A valve 62 located in bore 36 has a plunger 64 that engages the bearing 
surface 66 adjacent shoulder 52 and a poppet 68. A first end 70 of the 
poppet 68 is fixed to the annular projection 38 and a second end 72 is 
free to move within the bore 36. A first spring 74 located between the 
first end 70 of the poppet 68 and a retainer 76 on push rod 78 urges the 
plunger 64 into engagement with a key 80 to retain the valve 62 in bore 
36. A second spring 82 located between retainer 76 and poppet 68 urges the 
second end 72 into engagement with the end of plunger 64 to allow free 
connection between the first chamber 24 and second chamber 26. 
The second wall 28 includes a disc member 84 and a diaphragm 86. A seal 88 
located on rib 90 of the disc member 84 engages the peripheral surface 92 
on the cylindrical fulcrum 32. Diaphragm 86 has a first bead 94 that 
engages bead 21 on diaphragm 22 to seal chamber 26 from the surrounding 
environment and a second bead 96 that is located in a groove 98 on disc 84 
to seal the second chamber 26 from the third chamber 30. 
The second wall 28 is connected to the first wall 24 through lever 
arrangements 100 and 102. 
The lever arrangements 100 and 102 have arms 108, 110 and 108', 110' that 
are connected to each other by braces 112, 112', respectively. Pins 113, 
113' extend through flanges 104 and 106 on disc member 84 and the first 
ends of arms 108, 110 and 108', 110 to secure the lever arrangements 100 
and 102 to the second wall 28. The second ends of arms 108, 110 and 108', 
110' engage a rib 116 on hub 34. End 114 of the cylindrical member 32 
engages arms 108, 110 and 108', 110' to establish a fulcrum for lever 
arrangements 100 and 102. A spring 118 located on groove 120 on hub 34 
engages the front shell 114 and urges the hub 34 toward the rear shell 16 
until radial projection 122 on projection 38 engages stop 124. As hub 34 
moves toward shell 16, arms 108, 110 and 108', 110' pivot about fulcrum 
114 to move disc member 84 toward the second chamber 26. A seal 126 
attached to the radial projection 122 engages surface 93 on cylindrical 
member 32 to seal chamber 26 from the surrounding environment and to 
assist in holding the hub 34 in the center of opening 41 as wall 24 moves 
toward chamber 20. 
MODE OF OPERATION OF THE INVENTION 
Vacuum is produced at the intake manifold of internal combustion engines. 
In a vehicle having such an engine and equipped with a brake booster 10 
shown in FIG. 1, conduit 130 connects check valve 132 with the intake 
manifold. The check valve 132 which is fixed to shell 14 has a bore 134 
that is connected to chamber 20 and a passage 136 which is connected to 
chamber 30 by a conduit 138. 
With the first and second walls 24 and 28 and control valve 62 in the rest 
position shown in FIG. 1, vacuum present in bore 134 evacuates air 
directly from chambers 20 and 30 and from chamber 26 by way of passages 
42, bore 36 and passage 40. 
When an operator desires to effect a brake application, an input force is 
applied to pedal 140 causing lever 142 to pivot about fixture pin 144 and 
impart a linear input to push rod 78. 
Initial movement of push rod 78 allows spring 82 to move end 72 of poppet 
68 against vacuum seat 73 and interrupt fluid communication from chamber 
20 into bore 36 through passage 40. Further movement of push rod 78 moves 
plunger 64 away from end 72 of poppet 68 to allow air or any other 
operational fluid, such as compressed gas, to flow into chamber 26 by way 
of bore 36, and passage 42. With air in chamber 26 and vacuum present in 
chambers 20 and 30 a pressure differential is created across walls 24 and 
28. This pressure differential simultaneously acts on wall 24 to develop a 
first force that is transmitted into hub 34 and on wall 28 to develop a 
second force that is transmitted into hub 34 by the lever arrangement 100 
and 102. The first and second forces are combined in the hub 34 and 
transmitted to push rod 56 by way of reaction disc 50 to provide master 
cylinder 60 with an operational force corresponding to the input force. 
As best shown in FIG. 2, wall 24 moves toward chamber 20 and wall 28 moves 
in the opposite direction toward chamber 30. The diameter of cylindrical 
member 22 can be selected such that the fulcrum for the levers 108, 110 
and 108', 110' can be in the midpoint between pins 114, 114' and ends 115, 
117 and 115', 117'. In which case, the output force produced by the 
movement of the second wall 28 toward chamber 30 is directly transmitted 
into shoulder 116. However, under some circumstances it may be desirable 
to change the position of the fulcrum to modify the force transmitted into 
the shoulder or rub 116 by a ratio of the distance between the fulcrum and 
input and output ends of the levers 108, 110 and 108' 110'. 
When the input force on brake pedal 140 terminates, return spring 74 moves 
push rod 78 to initially bring plunger 64 into engagement with end 72 of 
poppet 68 to interrupt the communication of air from bore 36 to chamber 26 
through passage 42 and thereafter move end 72 of poppet 68 away from seat 
73 to allow vacuum present in chamber 20 to evacuate air from chamber 26. 
As air is evacuated from chamber 26, the pressure differential across wall 
24 and 28 is correspondingly reduced to allow spring 118 to act on hub 34 
and urge the first wall 24 toward the second chamber 26 until radial 
projection 122 engages stop 124 as shown in FIG. 1. 
The movement of the first wall 24 toward the second chamber 26 creates a 
force that causes levers 108, 110 and 108', 110' to pivot about the 
fulcrum and also move the second wall 28 toward the second chamber 26. 
When the radial projection 122 engages stop 124, the second wall 28 is in 
the rest position shown in FIG. 1.