Pneumatic booster output rod adjustment method

A method of adjusting the length of a vehicle brake booster output rod and assembling the same into the brake booster is provided, the output rod being a thermoplastic or thermoset material that is generally cylindrical in shape with a central longitudinal bore with an axial stop means, and the output rod having an insertable button having a head with an attached stem, the booster having a housing with a central opening surrounding the output rod with a surrounding first formed surface for mounting a master cylinder, the method including piloting a cutting tool by inserting an elongated member into the bore of the output rod, orienting the cutting tool by mating a fixture locationally associated with the cutting tool on the first formed surface of the vacuum booster housing, preloading the output rod with the elongated rod by contacting means with the elongated rod to simulate a spring load of a master cylinder, cutting the output rod with the cutting tool to the correct dimension, removing the elongated member from the output rod bore, and inserting the stem of the button into the output rod with a sheet material juxtaposed between the stem and the bore of the output rod causing an interference fit retaining the button into the output rod bore.

FIELD OF THE INVENTION 
The field of the present invention is that of methods of adjusting the 
lengths of output rods on vehicle brake boosters. Although the present 
invention can be used on a plurality of types of brake boosters, 
particularly pneumatic or vacuum-type boosters, in the present application 
the present invention will be explained in the embodiment of the vacuum 
powered brake booster. 
DISCLOSURE STATEMENT 
Engine manifold vacuum-actuated brake boosters are well known, having been 
used in automotive vehicles for many years. Essentially they comprise a 
booster housing having a power piston which is axially movable by 
differential pressure. The power piston is connected with at least one 
diaphragm. The diaphragm separates the booster housing into a vacuum 
chamber and variable pressure chamber. A power piston-mounted valve 
mechanism actuated by a rod operatively associated with a brake pedal 
allows the vehicle operator to selectively connect the variable pressure 
chamber with the vacuum chamber or the atmosphere to create the 
differential pressure to move the power piston. 
The booster has a rear end which is attached typically to a vehicle fire 
wall and a front end which is mated with a master cylinder. Booster 
housings are typically fabricated from stamping and, therefore, the length 
of the booster housing will vary from booster to booster. The force 
generated in the booster is transmitted to the master cylinder by an 
output rod which is captured between the power piston and the piston of 
the master cylinder. Since the housing dimensions vary, a method of 
adjustment in the length of the output rod must be provided. Too long of 
an output rod will cause the master cylinder to be shifted to an activated 
position when the vehicle brake pedal is not being operated by the vehicle 
operator. Too short of an output rod will bring upon a requirement for 
additional travel of the brake pedal before the master cylinder is 
activated, increasing what is commonly referred to as "dead time" travel, 
which makes the vehicle brakes take longer to operate. 
Prior to the present invention there existed several methods to adjust the 
length of the output rod. One method of adjustment was to measure the 
required length of the output rod using a surface of the booster housing 
which mates with the master cylinder as the datum point. A two-piece 
output rod was then provided having a standard cup member combined with 
selected insertable members (there being several such members available of 
various lengths) to provide an output rod of the required length. 
A second method was to provide a two-piece output rod wherein the 
insertable member was threadably inserted and, therefore, by rotation the 
threaded member would achieve the degree of insertion necessary to provide 
the output rod of the correct length. 
The first method typically required the output rod length adjustment to be 
made on the assembly line if it was desirable to ship the boosters and 
master cylinders separately to a vehicle assembly operation. 
If output rod adjustment was also undesirable at the assembly plant, the 
master cylinder and booster would have to be mated at their point of 
manufacture and thereafter shipped in the mated condition to the vehicle 
assembly operation. Shipment of the booster and master cylinder in the 
mated condition is typically more expensive. 
Furthermore, to meet the environmentally mandated goal of decreasing 
vehicle weight, more and more components of the booster have been 
converted to polymeric materials instead of metallic materials. Therefore, 
it is desirable to make output rods of a nonmetallic material. However, 
utilization of a nonmetallic output rod provided special problems in 
relation to the prior methods of output rod adjustment. 
SUMMARY OF THE INVENTION 
To overcome the above-noted problems and to provide an improvement of the 
environment resultant from decreased vehicle weight, the present invention 
is brought forth. The present invention involves a method of output rod 
length adjustment of a brake booster output rod which is manufactured from 
a thermoplastic material during the assembly process of the booster, 
before assembly into the vehicle. 
The present novel method of adjustment provides a thermoplastic output rod 
having a metallic button. The output rod is machined to the proper 
dimension after assembly into the booster, and thereafter the button is 
attached to the output rod utilizing a polymer sheet material juxtaposed 
therebetween to provide an interference to retain the button to the output 
rod. Therefore, shipment of the booster can be separate from shipment of 
the master cylinder, and premating of the booster and master cylinder is 
not required. Utilization of the novel inventive method of output rod 
adjustment allows the brake booster to be assembled and manufactured 
separately from the vehicle master cylinder, and therefore adjustment upon 
the vehicle assembly line is eliminated.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 and 2, there is illustrated a dual tandem vehicle 
brake vacuum booster 7. The booster illustrated is similar to that claimed 
and described in co-pending U.S. Ser. No. 07/692,888 filed Apr. 29, 1991, 
the disclosure of which is incorporated by reference herein. Mounted 
within the opening 16 of the booster 7 is an output rod 20. The output rod 
20 is generally cylindrical in shape with an enlarged diameter which fits 
within a cylindrical-shaped aperture 12 power piston 10. 
The output rod is an elongated member and has a central longitudinal bore 
22 with an axial stop means provided by a blind end 24 on an end of the 
output rod 20 toward the power piston 10. Additionally, the longitudinal 
bore 22 has a polygonal faced cylindrical wall 26 end toward the blind 
end. The output rod 20 is usually a thermoplastic or thermoset material 
which is capable of withstanding high pressures. 
The output rod protrudes through an opening 16 in a front housing 14 of the 
booster. The booster front housing 14 opening has a surrounding flat 
surface 18 generally perpendicular to the output rod for placement of an 
elastomeric seal 30 to seal the output rod 20. Radially outward and 
forward from the second surface 18 is a first mounting surface 40 for 
mounting of a master cylinder (not shown) which is typically generally 
perpendicular to the output rod 20. 
To effectuate the adjustment of the output rod's length, a rotating cutter 
spindle 50 is typically hung downwards. The front end of the booster 
housing is typically placed to face upward with a rear 15 portion of the 
booster housing being set on a locator plate 60. The rotating cutter 
spindle 50 has an elongated member 52 with a polygonal head 54 for 
insertion within the bore of the output rod. The elongated member is 
oriented by cutting tool 59 with a diamond insert 58 which is used to 
machine off the end of the output rod 20. The elongated member 52 by 
gravity places a 15-pound preload on the output rod to simulate a return 
spring of a master cylinder. The elongated member 52 pilots the output rod 
20 into position. 
The fixture 62 has an axially floating locator plate 64 which makes contact 
with the master cylinder mounting surface 40 of the booster front housing. 
The floating locator plate 64 is suspended (in a downward biased 
direction) by springs 65 along an axis parallel to the output rod 20. 
Along a line perpendicular to the longitudinal axis of the output rod, the 
floating 64 locator plate has rotary motion by virtue of spherical pins 
67. The floating locator plate 64 also has openings 69 to allow passage of 
housing bolts 13. A laser distance gauge 70 is provided which determines 
the distance traveled by the floating locator plate 64 to engage with the 
second surface 40. The laser distance gauge 70 then feeds the distance 
information to a controller (not shown) associated with the rotary cutter 
spindle 50 which is powered in an axial direction toward the output rod 20 
by a stepper motor 63. The output rod 20 is then machined in position. 
The polygonal head 54 of the elongated member mates with the output rod's 
bore polygonal wall 26 to prevent the output rod 20 from being rotated by 
the cutting head 59. Cuttings are confined by a dust cover 61, and a 
suitable vacuum system (not shown) removes the cuttings to prevent any 
contamination to the booster 7 by foreign material. The elongated rod is 
removed, and the booster is then removed from the fixture. 
Referring to FIG. 2, a two-piece punch die 80 cuts a circular blank 92 of 
polymer sheet material 90 typically of the same diameter of the reduced 
diameter portion of the output rod (the diameter adjacent to the 
elastomeric seal). The blanked sheet material, which is typically a 
polyester film such as Mylar, is placed over the output rod bore 22. A 
stem of a metallic button 93 is then inserted by a press rod 95 into the 
output rod bore, and the polymeric sheet material juxtaposed in between 
causes an interference which retains the button 93 within the output rod 
bore. The above retention is critical during the shipment of the booster 
fabrication to the assembly plant location for assembly to the vehicle. 
The output rod button is made to a tight tolerance utilizing a cold heading 
process or similar type operation which can hold tight tolerances. The 
button is common for all booster vehicle applications. The variations in 
the actual location in the booster/master cylinder interface is accounted 
for by the adjusted length of the output rod 20. 
While an embodiment of the present invention has been explained, it would 
be readily apparent to those skilled in the art of the various 
modifications which can be made to the present invention without departing 
from the spirit and scope of this modification as it is encompassed from 
the following claims.