Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to 2 hydraulic piston for
use in automotive hydraulic bra~e systems an~ comprising
a synthetic material.
Although an automotive hydraulic drum brake em-
bodiment of the invention is disclosed, one skilled in the
automotive brake art may well adapt the invention to other
types of hydraulic or air-actuated brake systems.
Conventional hydraulic drum brakes for vehicles
typically include two arcuate brake shoes slidably mounted
in an end-to-end orientation on a staticnary backing plate
affixed to the vehicle. The brake shoes have friction
material affixed to their outer surfaces for engaging the
inner surface of a rotatable drum upon which the vehicle
wheel is generally mounted. ~rhe brake shoes have upper
ends abutting opposed pistons of a hydraulic cylinder
mounted on the backing plate. Typically, return springs
keep the brake shoes in const~nt abutment with the pistons.
When hydraulic pressure is applied to the wheel
cylinder, the pistons extend in opposite directions to urge
the brake shoes outwardly against the inner surface of the
brake drum. The return springs retract the brake shoes
when the hydraulic pressure i, released, thereby freeing
the drum to rotate.
Conventional wheel cylinders generally have one-
piece metal pistons or, alternatively, two-piece pistons
having a piston body with an abutment plug inserted into
the outboard end (see Fig. 1) The abutment plug, which
is generally hardened metal, serves as a bearing surface
for engaging the upper ends oi the brake shoes. Metal
pistons are relatively heavy, expensive to manufacture and
subject to high rates of wear between the pistons and their
mating surfaces. The need has thus arisen for a hydraulic
brake piston that may be inexpensively manufactured from
a lightweight material having a low coefficient of friction,
that is compatible with commonly-used brake fluids, and
that is capable of withstanding the high stresses and tem-
peratures of brake operation while maintaining its dimen-
sional integrity.
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In accordance with the pres~nt invention, there
is provided a fluid dynamic piston which comprises an outer
cylinder having an open forward end and a closed rearward
end, a post coaxially disposed within the outer cylinder
and having an axial cavity in its forward end, the post
extending axially from the outer cylinder's closed rearward
end through at least a portic,n of the outer cylinder's axial
length, a plurality of circumferentially-spaced ribs extend-
ing radially between the post and the outer cylinder and
extending axially from the outer cylinder's closed rearward
end through at least a portion of the axial length of the
annular space, the outer cylinder, the post and the ribs
comprising synthetic material, the ribs having forward ends
intersecting the post rearwardly of the outer cylinder's
open forward end, and abutment means attached to the post's
open forward end and comprising a pin having forward and
rearwa~d ends and a flange on the pin's forward end, the
rearward end of the pin extending into the cavity to a depth
no further rearward than the intersection of the ribs and
the post.
Fig. 1 is an overall view of a typical vehicle
drum brake assembly with the wheel cylinder partially cut
away to show a typical hydraulic piston known in the prior
art.
Fig. 2 is a cross-sectional view of a typical
hydraulic drum brake wheel cylinder assembly having a piston
embodying the present invention.
Fig. 3 is an exploded cross-sectional elevation
view of a preferred hydraulic piston embodying the present
invention taken along line 3-3 of Fig. 4.
Fig. 4 is a forward end view of the preferred
hydraulic piston embodying thle invention.
Figs. 5 through 10 are radial cross-sectional
views of exemplary alternate embodiments of the present
invention.
Referring to the drawings, where like elements
are designated by like numeraLs, Fig. 1 illustrates a
typical vehicle drum brake assembly 9 of the priOr art for
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a vehicle wheel normally rotating in the direction indica-
ted by arrow 10. Drum brake assembly 9 includes braking
plate 11 onto which leading and trailing arcuate brake shoes
12]L and 12T respectively are slidably mounted. The brake
shoes 12L and 12T have webs 13L and 13T supporting arcuate
rims 14L and 14T, respectively, to which friction material
sections 15L and 15T are affixed by any known means.
A prior art hydraulic wheel cylinder assembly
18 of the prior art is shown in Fig. 1 secured to backing
plate 11 between upper brake shoe ends l9L and l9T. Return
spring 20 includes hooks 21L and 21T engaging upper web
apertures 22L and 22T, respectively, biasing brake shoes
12L and 12T toward wheel cylinder assembly 18. Anchor block
25 is rigidly attached to backing plate 11 between lower
brake shoe ends 26L and 26T. Retaining spring 27 includes
hooks 28L and 28T engaging lower shoe apertures 29L and
29T, respectively, urging lower brake shoe ends 26L and
26T toward each other in constant abutment with opposite
sides of anchor block 25.
In operation, as hy~raulic pressure is applied
to wheel cylinder assembly 1~, piston 16 extends to slidably
urge leading brake shoe 12L outwardly. Lower shoe end 26L
rotates against anchor block 25, and friction material
section 15L frictionally engages the inner surface of brake
drum 30, thereby retarding the rotation of brake drum 30
and the vehicle wheel. The trailing brake shoe assembly
operates similarly and simultaneously. Upon release of
the hydraulic pressure, pistons 16 retract under the in-
wardly-directed biasing force of return spring 20 which
also urges brake shoes 12L and l~T to their original posi-
tions, thereby freeing brake drum 30, and thus the vehicle
wheel, to rotate.
Fig. 2 presents a cross-sectional view of wheel
cylinder assembly 18, incorporating two pistons 32 which
represent the preferred embod.;ment of the present invention.
Pistons 32 are positioned for opposed slidable movement
within cylinder 31, forming hydraulic chamber 33 there-
between. Elastomeric cups 34 act as hydraulic seals and
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abut the rearward surface of pis~ons 32, and cup expanders
35 abut the rearward sides of cups 34 and receive biasing
spring 36. Dust boots 38 engage external grooves 39 of
wheel cylinder assembly 18 and extend into cylin~er 31 to
sealingly engage actuating pistons 32.
Piston 32 includes a cylindrical outer sleeve
45 having a closed rearward wall 46, an open forward end
47, inner surface 48, and outer surface 49. Post 50 is
coaxially disposed within outer sleeve 45 and includes an
outer cylindrical surface 51, internal cavity 52, cavity
bottom 53, cavity opening 54, which is preferably chamfered,
and cavity inner surface 55. Post 50 protrudes in a forward
axial direction from rearward wall 46 and may, depending
upon the particular brake mechanism design, extend beyond
outer sleeve forward end 47. Post 50 has an external dia-
meter that is smaller than the internal diameter of outersleeve 45 thereby forming annular space 56 therebetween.
Post outer surface 51 diverges at angle B, and
cavity inner surface 55 converges at angle C, to form a
frustoconical annular base portion 70 adjacent rearward
wall 46. Base portion 70 allows the axial forces encounter-
ed during brake operation to be more evenly distributed
across rearward wall 46. Angle B lies within the range
of 5 to 25 degrees, and is preferably 15 degrees. Angle
C lies within the range of 30 to 90 degrees and is preferably
60 degrees.
Ribs 57 are circumferentially spaced within an-
nular space 56 and extend between the inner surface 48 of
outer sleeve 45 and the outer surface 51 of post 50. Ribs
57 preferably extend from piston rearward wall 46 along
at least a portion of the axial length of post 50 to a point
rearward of outer sleeve forward end 47. The forward ends
59 ~f ribs 57 preferably slope forwardly and outwardly from
post 50 to forward end 47 of o~ter sleeve 45 and inter-
secting the post 50 rearwardly of the open forward end 47
of the outer sleeve 45 as shown in Fig. 3, thus leaving
a portion of annular space 56 open to form expansion space
58 between post 50 and the outer sleeve.
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Since existing synthetic resins of the day do
not exhibit adequate surface load bearing characteristics,
a metal abutment plug or cap is preferably included in the
piston assembly to act as a bearing surface for the brake
shoe ends 19L and l9T. Should resins be developed in the
future that have such characteristics, the abutment plugs
or caps in the preferred and alternate embodiments disclosed
herein may be eliminated.
Abutment plug 37 in the preferred embodiment in-
cludes flange 61 and pin 62. Pin 62 diverges in the rear-
ward direction at angle A so as to provide a tight fit wheninserted into post cavity 52. Angle A normally lies within
the range of one-half to two degrees, and is preferably
one degree. The rearward end 63 of pin 62 may be provided
with chamfer 64 for ease of insertion into cavity opening
54. Preferably, abutment plug 37 is pressed into post cavity
52 during assembly until its abutment flange 61 seats
against the forward end 66 of post 50. Pin 62 projects
into cavity 52 only to an axial depth equal to, or less
than, the axial length of open expansion space 58. Thus
any diametric expansion of post 50 caused by the force-
fitting of abutment plug 37 into cavity 52 is not imparted
to outer sleeve 45. Ribs 57 therefore provide radial
structural reinforcement between post 50 and cylindrical
outer sleeve 45 without causing an increase in the outside
diameter of piston 32. Expansion space 58 also serves as
an area for dust boot 38 (see Fig. 2) to sealingly engage
post 50 and outer sleeve 45.
~ variation on the preferred embodiment of the
invention may employ an alternate abutment plug having a
pin of uniform diameter that does not cause expansion of
the inner sleeve. The plug may be secured to the piston
with any known adhesive to keep it in place during brake
maintenance. Since such an abutment plug pin causes no
expansion of the inner sleeve, the strengthening ribs may
extend the full axial length of the post wi-thout resulting
in expansion of the outer sleeve.
A further variation on the preferred embodiment
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has an abutment cap that fits over the forward end of the
post rather than an abutment plug that fits into a post
cavity. Such a cap may be secured to the post with an ad-
hesive as discussed above.
Piston 32 may be composed of any of several known
thermoplastic or thermosetting resins that are compatible
with commonly used hydraulic brake fluids and that exhibit
suitable dimensional stability under the temperatures
expected in the piston environment during brake operation.
Such temperatures may range from ambient to as high as 250F.
The resin may be reinforced with glass fibers, glass spheres,
talc, carbon, or any other known organic or inorganic
materials for added strength and dimensional stability.
Examples of a suitable material is glass-filled 66 nylon
(such as FIBERFILL G 10/40) or glass-filled 612 nylon (such
as FIBERFILL G 4/34 or G 4/45).
` The preferred embodiment as described herein has
been successfully reduced to practice for a drum brake wheel
cylinder with a nominal bore size of 17 millimeters. The
piston is composed of glass-filled nylon and is compatible
with the glycol-based hydraulic brake fluid currently used
in the United States. After 250,000 hydraulic pressure
cycles simulating braking applications, the pistons and
cylinders showed appreciably less scuffing, abrasion and
wear than did aluminum pistons of the prior art. Such re-
duced wear is especially advantageous because such pistonsmay be used in aluminum wheel cylinders without costly
anodizing of the cylinders.
Other synthetic materials, such a phenolic or
reinforced epoxy resins, which are thermosetting resins,
also provide the necessary compatibility with glycol-based
brake fluid and may be expected to have adequate mechanical
properties to meet the above~mentioned criteria. The use
of silicone or mineral based brake fluids common in Europe
may require the use of other synthetic resins that are com-
patible with those fluids.
Since pistons embodying the present inventionmay be injection molded, one is given an infinite choice
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of piston configurations and :Ls not ~ound by the cost and
process limitations associated with the machining and fabrica-
tion of other materials such as metals. Some of these
choices are illustrated in Figs. 5 through 10 and represent
examples of alternate embodiments of the present invention.
After examining these examples, further alternate embodi-
ments of the present invention will undoubtedly occur to
one skilled in the art.
In Fig. 5, piston 7:L has an inner member 72 co-
axial with outer sleeve 45. In contrast with the cylin-
drical post 50 of the preferred embodiment, inner member72 is "star-shaped" and includes a plurality of apexes or
corners 73 circumferentially ,paced about its periphery.
The apexes 73 extend to join outer sleeve inner surface
48 and provide radial structural reinforcement between inner
member 72 and the outer sleeve.
Piston 71 is shown in Fig. 5 having a coaxial
cavity 74 for receiving an abutment plug. However, as with
the preferred embodiment, piston 71 and any of the other
alternate embodiments in Figs. 6 through 10 may have solid
inner members or posts. In such cases, an abutment cap
may be provided to fit over the forward end of the inner
member or post, or alternatively an abutment disc may be
mounted on the end of the inner member or post. Such an
abutment disc may be recessed in an opening in the forward
end or may be mounted such that its thickness axially pro-
trudes from forward end.
Fig. 6 illustrates another example of an alternate
embodiment of the present invention in which piston 76 in-
cludes an inner post 77 having a rectangular cross-section.
Inner post 77 intersects with, and is attached to, outer
sleeve inner surface 48 at its corners for radial reinforce-
ment of the outer sleeve 45.
In Fig. 7, piston 7~ has an inner post 80 that
is also rectangular in cross-section however, unlike that
35 of piston 76 in Fig. 6, inner post 80 intersects with, and
attaches to, outer sleeve inner surface 48 along its entire
width ~w) to provide radial reinforcement for outer sleeve 45.
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Fig. 8 illustrates piston 82 which has post mem-
ber 83 that is '`cross-shaped". Post 83 has integral radial
reinforcement extensions 84 which intersect and join with
out:er sleeve inner surface 48 along their entire width (w).
Piston 82 is shown in Fig. 8 with adjacent radial extensions
84 preferably oriented substantially perpendicular to each
other. However, adjacent radial extensions 84 may be
oriented at angles other than 90 if such other angles are
desirable or advantageous.
Figs. 9 and lO illustrate pistons 86 and 86'
respectively having posts 87 and 87' respectively. Post
87 and 87' are both hexagonal in cross-section and have
radial reinforcement spokes 88 and 88' respectively for
radially reinforcing outer sleeve 45. Spokes 88 and 88'
are circumferentially spaced about the periphery of p~s~t
87 and 87' and extend radially to intersect and join
the outer sleeve inner surface 48 of pistons 86 and
86' respectively. In Fig. 9, spokes 88 radially extend
from the intersections of the sides of the hexagonal cross-
section of inner member 87, while spokes 88' in Fig. lO
radially extend from the flat portions of the sides of inner
member 87'. Although Figs. 9 and lO show pistons with posts
having a hexagonal cross-section, the piston of the present
invention may have posts of other cross-sectional shapes
if desirable or advantageous.
The foregoing descriptions represent merely exem-
plary embodiments of the present invention. Various changes
may be made in the arrangement and details of production
of the embodiments shown and described without departing
from the spirit and scope of 1he present invention.
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