A TANDEM BRAKE BOOSTER

Abstract: A tandem brake booster having first and second lever arms for transferring an output force from a second wall into an output member in response to an input signal being applied to a control member. A first cam surface on each lever arm engages an annular fulcrum member and a second cam surface engages the output member. The engagement of first and second cam surfaces on the fulcrum and output member direct said output force from said second wall into said control member along a force vector substantially parallel to the output member without the in­ troduction of rotational forces on a first wall.

Description: This invention relates to a tandem brake booster having lever arms with cam surfaces thereon through which an output force from a secondary wall is transferred into an output member without the introduction of rotational forces to a primary wall located therein. In evaluation of the tandem brake booster disclosed in U.S. -A- 4,387,626 and in an effort to simplify the construction thereof, it was observed that under some conditions the actual output force was less than the theoretical output force. On investigation it was found that the lever arms through which the output force from the secondary piston is transferred to the output member was introducing a rotative torque into the output member. A booster according to the preamble of claim 1 is known, e.g. from US-A-4475 439. It is an object of this invention to provide a tandem brake booster of the kind set forth with a simpler lever arrangement. The present invention provides a tandem brake booster having first and second walls that move in opposite directions within a cavity to produce first and second output forces in response to an input force applied to a control member and a lever arrangement having first and second arms each of which has a first and connected to the second wall and a second end connected to a hub which retains an output member, the first and second arms further engaging a cylindrical member to transfer the second output force into the output member where the second output force is combined with the first output force to create a joint output force, each of first and second arms has a first cam surface and a second cam surface, each of first cam surfaces engaging a contact point on curved surface of the hub to direct the second output force along a vector substantially parallel to the output member, the second cam surface engaging an apex of curved surfaces located in slot in the cylindrical member. According to the invention, the second cam surface is curved so that the second output force is always presented substantially through the axial center of the center of the output member, the engagement of the second cam surface with the curved surfaces (58, 60) modifying the lever ratio arms while the first cam surface continues to engage the contact point, the first and the second arms are parallel to each other and symmetric to the output member, this arrangement preventing the introduction of forces that may rotate the first wall and thereby reduce the effective value of the second output force. The tandem brake booster according to the invention, further includes a return spring that urges the first wall toward and into engagement with a first end of the cylindrical member in the absence of an input force to establish a rest position. According to an other characteristic of the invention, the second end on each of first and second arm means extends past the axial center of the output member, the second cam surfaces engaging the curved surface to maintain the transmission of the force vector into the output member in a vertical plane substantially parallel to the axial center. A preferred embodiment of the present invention will now be described more precisely with reference to the accompanying drawings wherein : Figure 1 is a sectional view of a tandem brake booster having lever means for transferring an output force from a movable wall along a vertical vector to an output member; and Figure 2 is an end view of the movable showing the lever arms attached thereto; Figure 3 is a sectional view of the lever arm of Figure 1 showing the relationship between the cam surfaces with the control valve in the rest position; and Figure 4 is a sectional view of the Figure 1 showing the relationship between the cam surfaces with an input force applied to the control valve. The tandem brake booster 10 shown in Figure 1 has a housing formed by joining as first shell 12 to a second shell 14 by a twist lock mechanism 16. A first wall 18 has a diaphragm 22 with a first bead 20 retained by the twist lock mechanism 16 and a second bead 24 located in a groove 26 in a hub mechanism 28. The first shell 12 and first wall 18 define a first chamber 30. The first chamber 30 is connected to a source of vacuum through check valve 32. Hub 28 has a projection 34 that extends from wall 18 through an opening 36 in shell 14. A bearing-seal arrangement 38 located in opening 36 engages projection 34 to hold the hub 28 in axial alignment within housing 10. A cylindrical member 40 which surrounds projection 34 has a first end 42 and a second end 44. In the rest position as shown, return spring 46 located in chamber 30 acts on hub 28 to position face 48 against the first end 42 and hold end 44 against end member 50 on shell 14. As seen in Figure 1 and 2, cylindrical member 40 has a first slot 52 and a second slot 54 located adjacent the first end 42. Each slot 52 and 54 has an apex 56 (only one shown) located at the bottom thereof that forms the fulcrum for a lever arrangement 62. Curved surfaces 58 and 60 which extend from apex 56 define the point of contact with the lever arrangement 62 after movement from the apex 56. The cylindrical member 40 has a series of slots 64 (only one is shown) that connect opening 66 in projection 34 with chamber 68. A second wall 70 has a first bead 73 of diaphragm 74 retained by the twist lock arrangement 16 and a second bead 71 located in a groove 78 in a disc member 80. The disc member 80 has a seal 82 located on the inner surface thereof which surrounds surface 87 on cylindrical body 40. Diaphragm 74, disc 80, cylindrical member 40 and shell 14 effectively define the limits of chamber 68. Walls 18 and 70 and cylindrical member 40 define a chamber 72 between chambers 30 and 68. Chamber 30 is connected to a bore 77 by an opening 76, chamber 72 is connected to bore 77 by an opening 79, shown in phantom, and chamber 68 is connected to bore 77 by way of passage or slots 64 and opening 66. As seen in Figure 3, a control valve 81 located in bore 77 has a plunger 82 connected to a push rod 84. Movement of plunger 84 by an imput applied to pedal 86 allows spring 88 to position face 90 on the flexible pleated section of poppet member 92 against a vacuum seat 94. At the same time sleeve 96 and bore 77 define a passage 98 which provides for continual connection between passage 76 and opening 66. A spring 100 attached to push rod 84 holds sleeve 96 against a stop 102 to assure that face 90 of poppet member 92 is positioned adjacent vacuum seat 94 and that passage 98 is aligned with opening 66 and passage 76. The lever arrangement 62 located in chamber 72 has a first arm 110 and a second arm 112. Each of the arms 110 and 112 has a first end 120 fixed to wall 70 by a pivot pin 116 located in a yoke 114 and a second end 122. The second end 122 has a cam surface 124 that located on a curved surface 126 in slots 128 on hub 28, only one of which is shown. The cam surface 124 has a contact point 121 that engages the apex 123 surface 126 in slot 128 that is on the vertical center of the output push rod 132. The tandem brake booster 10 operates in the following manner. When an internal combustion engine is operating, vacuum is produced at the intake manifold. This vacuum is communicated through check valve 32 to evacuate air from chamber 30. With vacuum in chamber 30, air in chamber 68 is evacuated by way of slots 64, opening 66, passages 98 and 76 while air in chamber 72 is evacuated by way of opening 79, bore 75 (that portion of bore 77 adjacent seat 94 that is connected to passage 79), and into passage 76. With vacuum in chambers 30, 68 and 72, return spring 46 acts on and holds hub 28 against the second end 42 of the cylindrical member 40. Contact point 121 engages cam surface 124 to position the second wall 70 in the manner shown in Figures 1 and 3. In response to an input force applied to pedal 86 push rod 84 moves plunger 83 and permits spring 88 to position face 90 on poppet member 92 on vacuum seat 94. Further movement of plunger 83 moves atmospheric seat 85 on plunger 8, away from face 90 to allow air present in bore 77 to enter chamber 72 by way of bore 75, and opening 79. With air in chamber 72, a pressure differential is created across the first and second wall 18 and 70. The pressure differential across the first wall 18 creates a force that is directly transferred into the output push rod 132 for operating the master cylinder rod. At the same time, the pressure differential acting on the second wall 70 is transferred by way of the lever arrangement 62 into hub 28 where it is combined with the force from the first wall 18 to produce an output force for operating the master cylinder 200 corresponding to the input force applied to brake pedal 86. As best seen in Figure 4 which illustrates an operational position of the first and second walls 18 and 70 in response to an input force, the contact point 121 for the cam surface 124 on lever 110 and 112, remain in a vertical plane that passes through the axial center of the output push rod 132. The relationship between the engagement of cam surfaces 124 on lever arms 110 and 112 with the curved surface 126 in the bottom of slots 128 and the engagement of cam surface 111 with the apex 56 located at the bottom of slots 52 and 54 hold the contact points 121 in substantially the vertical plane throughout the entire stroke of the walls 18 and 70 to assure that rotational forces are not introduced into hub 28. On termination of the input force, spring 100 moves atmosphere seat 85 on plunger 83 against face 90 to interrupt the communication of air into chamber 72. Further movement of the plunger 83 by spring 100 moves face 90 away from vacuum seat 94 to allow vacuum present in chamber 30 to evacuate air from chamber 72. As air is evacuated from chamber 72, the pressure differential across walls 18 and 70 is reduced and return spring 46 moves the walls 18 and 70 to the rest position shown in Figure 1.

Claims: A tandem brake booster (10) having first (18) and second (70) walls that move in opposite directions within a cavity to produce first and second output forces in response to an input force applied to a control member (81) and a lever arrangement (62) having first (110) and second (112) arms each of which has a first end (120) connected to the second wall (70) and a second end (122) connected to a hub (28) which retains an output member (132), said first and second arms further engaging a cylindrical member (40) to transfer said second output force into the output member (132) where said second output force is combined with the first output force to create a joint output force, each of said first (110) and second (112) arms having a first cam surface (124) and a second cam surface (111), each of said first cam surfaces (124) engaging a contact point (121) on curved surface (126) of said hub (28) to direct the second output force along a vector substantially parallel to said output member (132), said second cam surface (111) engaging an apex (56) of curved surfaces (58, 60) located in slots (52, 54) in said cylindrical member (40) characterized in that said second cam surface (111) is curved so that the second output force is always presented substantially through the axial center of the output member (132), said engagement of said second cam surface (111) with said curved surfaces (58, 60) modifying the lever ratio between the first (120) and the second (122) ends of said arms (110 and 112) while said first cam surface (124) continues to engage said contact point (12l) and in that said first (110) and second (112) arms are parallel to each other and symmetric to the output member (132), this arrangement preventing the introduction of forces that may rotate said first wall (18) and thereby reduce the effective value of the second output force. The tandem brake booster (10) according to claim 1 characterized in that it further includes a return spring (46) that urges said first wall (18) toward and into engagement with a first (42) end of said cylindrical member (40) in the absence of an input force to establish a rest position. The tandem brake booster (10), according to claim 1 or 2 characterized in that said second (122) end on each of said first (110) and second (112) arms extends past the axial center of said output member (132), said second cam surfaces (111) engaging said curved surfaces (58, 60) to maintain the transmission of said force vector into said output member (132) in a vertical plane substantially parallel to said axial center.

Inventors: WILSON, ROBERT K.
Applicants: ALLIED-SIGNAL INC. (A DELAWARE CORPORATION)
Application year: 1985
Main classification: B60T13
IPC codes: B60T13