Fluid actuator including a composite piston rod

Fluid actuator includes a piston rod comprised of a wound filamentary composite structure to provide an extremely light weight, relatively low cost, envelope efficient design. Attached to the piston rod are one or more non-integral piston heads which are preloaded against one or more ramp surfaces on the exterior of said piston rod. A pair of oppositely facing ramp surfaces may be formed on the piston rod by interspersing filler means between the piston rod fiber layers. Another ramp area may be formed by providing an external groove in the piston rod which is engaged by longitudinally split support shoes that provide an attachment surface for a detachable piston head. An internal ramp may also be provided on the inner diameter of the piston rod adjacent the axial outer end thereof for mating engagement by a conically ramped exterior surface on an internal metal stud used to attach a rod end assembly to the axial outer end of the piston rod. The internal ramp may be formed by radially displacing the layers of piston rod fibers radially inward by interspersing circumferential fiber windings therebetween.

BACKGROUND OF THE INVENTION 
This invention relates generally as indicated to a fluid actuator, and, 
more particularly, to a fluid actuator including a composite piston rod 
which incorporates methods of attachment of one or more non-integral 
piston heads in order to efficiently utilize the directional composite 
material, and to simplify fabrication. 
In copending U.S. patent applications Ser. Nos. 642,539, now U.S. Pat. No. 
4,697,499, dated Oct. 6, 1987, and 642,540, both filed Aug. 20, 1984, such 
latter application having been abandonded in favor of a continuation 
application Ser. No. 834,501, filed Feb. 28, 1986, now U.S. Pat. No. 
4,685,384, dated Aug. 11, 1987, and assigned to the same assignee as the 
present application, there are shown several different fluid actuator 
designs intended for use in flight controls of aircraft and other high 
pressure applications in which the cylinder walls are made of relatively 
high strength to weight composite materials in order to obtain a 
substantial reduction in the weight of the cylinders without sacrificing 
strength. 
By the same token, it would be desirable to make the piston rod out of 
similar type composite materials so that the weight of the rod, like that 
of the cylinder, could also be substantially reduced. Moreover, it would 
be desirable to provide for the attachment of one or more non-integral 
piston heads to the composite rod structure in order to efficiently 
utilize the directional composite material, and to simplify fabrication. 
Also, it would be desirable to provide such a composite piston rod with 
the requisite sealing and rod end attachments along with increased fatigue 
resistance, damage tolerance, and ballistic tolerance. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, the piston rod is 
comprised of a wound filamentary composite structure to provide an 
extremely light weight, relatively low cost, envelope efficient design. 
Also in accordance with the invention, the composite rod structure provides 
for fatigue resistance, damage tolerance, and ballistic tolerance. 
Further in accordance with the invention, the composite rod structure 
provides for the attachment of one or more non-integral piston heads to 
efficiently utilize the directional composite material and to simplify 
fabrication. 
Still further in accordance with the invention, the composite rod provides 
for the requisite sealing and rod end attachments. 
To the accomplishment of the foregoing and related ends, the invention, 
then, comprises the features hereinafter fully described and particularly 
pointed out in the claims, the following description and the annexed 
drawings setting forth in detail a certain illustrative embodiment of the 
invention, this being indicative, however, of but one of the various ways 
in which the principles of the invention may be employed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now in detail to the drawings, and initially to FIG. 1, there is 
shown one form of fluid actuator 1 in accordance with this invention 
including a cylinder 2 containing a piston rod 3 extending through an end 
wall or gland 4 at the outboard end of the cylinder. At the outermost end 
of the rod is a rod end assembly 5 which is adapted to be attached to a 
movable part to be actuated. At the inboard end of the cylinder is a 
suitable mount (not shown) for attachment to the stationary part of the 
device to be actuated. 
The fluid cylinder 2 shown is a dual tandem cylinder containing a pair of 
fluid chambers 10, 12 in series and having respective pistons 14, 16 
connected to a common ram output rod 3 for common movement therewith. In 
service, the two chambers may be used in tandem or independently, to 
extend or retract the rod, or provide a compressive or tension load. The 
particular structure of the cylinder itself forms no part of the present 
invention and therefore will not be described in detail. Suffice it to say 
that the cylinder 2 is desirably constructed of relatively high strength 
to weight longitudinal and/or biased and circumferential hoop fibers 17, 
18 of suitable composite materials in order to obtain a substantial 
reduction in the weight of the cylinder without sacrificing strength. 
Examples of such a cylinder construction are disclosed in the 
aforementioned U.S. Pat. Nos. 4,697,499 and 4,685,384 which are 
incorporated herein by reference. 
As shown, the two fluid chambers 10, 12 (hereinafter referred to as the 
head end chamber and rod end chamber, respectively) are separated by a 
center gland or dam 19 having a central opening through which the piston 
rod 3 extends, with suitable seals therebetween. 
In accordance with the present invention, the piston rod 3, rather than 
being made of metal as is the usual case, consists of a plurality of 
layers of longitudinal (or nearly longitudinal) and circumferential fibers 
20 and 21 forming a composite structure throughout the length of the 
piston rod. Such longitudinal and circumferential fibers may be made of a 
suitable composite material having the required high tensile strength such 
as high modulus graphite filament wound epoxy impregnated fibers which may 
be protected from handling damage by a thin layer of fiberglass reinforced 
composite. Also, a suitable coating, plating or shell 22 may be provided 
on both the outer diameter (O.D.) and inner diameter (I.D.) of the piston 
rod in order to provide a sealing surface against fluid intrusion into the 
piston rod composite material. 
The longitudinal fiber layers 20 run lengthwise of the rod to react the 
axial tension loads, whereas the circumferential fiber layers 21 are 
desirably wrapped around the I.D. and O.D. and may also be wrapped in 
between the longitudinal fiber layers to provide resistance to radial 
pressure and react axial compression loads through the rod. Preferably, 
approximately one-half of the normal cross-sectional area of the piston 
rod is comprised of longitudinal fibers, whereas the circumferential 
fibers are distributed more or less according to their intended purpose 
along the length of the rod as described hereafter. 
Throughout the major portion of the length of the piston rod, the 
circumferential fibers 21 comprise the other half of the cross-sectional 
area of the piston rod 3. However, at certain points along the length of 
the piston rod there may be more or less circumferential fibers depending 
on the rod configuration. In the preferred embodiment shown herein, one 
such area is located intermediate the length of that portion of the piston 
rod 3 extending through the head end chamber 10 wherein there is a 
relatively short, radially outwardly extending built-up area 25 extending 
circumferentially around the rod for a purpose to be subsequently 
described. The built-up area 25 may be formed by progressively wrapping 
additional circumferential or hoop stress windings 21 between the 
longitudinal fibers 20 from opposite ends of the built-up area toward the 
center to cause outward deflection of the outermost layers of longitudinal 
fibers and/or circumferential fibers beyond the normal outer dimension of 
the rod. As best seen in FIG. 4, the layers of fibers 20, 21 are 
progressively built up toward the center to form two oppositely facing 
ramp surfaces 26, 27 in the composite material on the O.D. of the rod for 
mating engagement by correspondingly shaped ramps 28, 29 of a two-part 
piston head 14. The piston head parts 31, 32 may be preloaded into the 
ramps 26, 27 as by bolting or threading the two parts together to 
eliminate possible free play. 
The piston head 14, in addition to carrying the usual external piston rod 
seal 35, also desirably incorporates an internal seal 36 for sealing 
engagement with the exterior surface of the piston rod built-up area 25 to 
prevent across-the-head leakage. 
Adjacent the inner end of the piston rod 3 an external groove 40 may be 
provided for engagement by a pair of longitudinally split support shoes 41 
held together as by pins 42 (see FIGS. 2 and 3). The longitudinal tension 
fibers 20 are brought closer together, by eliminating circumferential 
fibers 21 where necessary, to permit the longitudinal fibers to pass 
radially inwardly beneath the external groove 40. Adjacent the opposite 
(inner) side of the groove, the longitudinal fibers 20 are turned outward 
by interspersing additional circumferential supporting hoop fibers 21 
between the longitudinal fibers to form a ramped area 43 along one side of 
the groove 40 for interface with the longitudinally split support shoes 
41. Additional circumferential fibers 21 (or other similar filler 
material) may be included where necessary to fill out the normal rod 
thickness adjacent the other side of the external groove 40 as shown. 
The purpose of the external groove 40 and associated support shoes 41 is to 
hold in place on the inner end of the rod 3 another detachable piston head 
16 which may have a threaded counterbore 46 in one end for threaded 
engagement onto the support shoes. At the inner end of the piston head 16 
opposite the counterbore 46 is an inturned flange 47 which engages the 
innermost end of the piston rod 3 during tightening of the piston head 
onto the support shoes to provide a preload force on the piston head. 
Wrenching slots 48 may be provided in the inner end of the piston head 16 
to facilitate assembly and removal of the piston head from the rod. 
At the outer end of the rod 3 is the rod end assembly 5 which may be made 
of metal. To attach the metal rod end assembly 5 to the composite rod 
structure, an internal ramp 50 (see FIGS. 1 and 5) is desirably formed on 
the I.D. of the piston rod adjacent the outer end thereof by radially 
displacing the longitudinal windings 20 inward as by interspersing 
additional circumferential windings or other suitable filler material 21 
therebetween. An internal metal stud 51 is either wound in or inserted 
within the hollow I.D. of the piston rod 3, and has a conically ramped 
exterior surface 52 adjacent the inner end thereof for mating engagment 
with the internal ramp 50 on the piston rod. The outer end of the metal 
stud 51 protrudes axially outwardly beyond the outer end of the piston rod 
and is externally threaded at 53 for threaded engagement by internal 
threads 54 on the metal rod end assembly 5 to impart tension loads through 
the threads into the stud and then into the longitudinal fibers 20 through 
the mating ramp surfaces 50, 52. 
A compression load from the rod end assembly 5 may be reacted into the 
outer end of the composite rod 3 by providing a stepped shoulder 55 at the 
outer end of the composite rod which is engaged by a correspondingly 
shaped end of a metal washer 56 (see FIG. 5). The other (outer) end of the 
metal washer 56 may be flat for flat engagement by a flat washer 57. 
Locking of the rod end assembly 5 to the stud 51 to prevent relative 
rotation therebetween may be accomplished by providing a keyway 58 on the 
stud 51 which is engaged by a key (tab) 59 on the I.D. of the washer 57 
and bending deformable tabs 60 on the washer 57 into slots 61 on the rod 
end assembly 5 (see FIG. 1). 
To preload the stud 51 so that the stud does not fail out due to cycling 
fatigue, the rod end assembly 5 may include a spiral washer 65 between the 
flat washer 57 and the inner end of the rod end 66. As shown in FIG. 1, 
the adjacent ends of the spiral washer and rod end have a matching spiral 
(cam) shape, whereby rotation of the rod end 66 relative to the spiral 
washer 65 will cause the rod end to move axially away from the stud to 
apply a preload force to the stud. 
The opposite (inner) end of the spiral washer 65 has the circumferentially 
spaced slots 61 therein for engagement by the tabs 60 on the flat washer 
57 to prevent relative rotation therebetween. On the O.D. of the flat 
washer 57 are a plurality of circumferentially spaced fingers 67 which may 
be engaged by a suitable tool to resist turning of the spiral washer 65 
during relative rotation of the rod end 66 to place the stud 51 in 
tension. Thereafter, a C-washer 70 having tabs 71, 72 on opposite sides 
thereof may be placed over the rod end assembly 5 and its tabs 71, 72 bent 
into respective slots in the adjacent ends of the rod end 66 and spiral 
washer 65 to lock the rod end against further rotation relative to the 
spiral washer. 
To form the composite piston rod 3, the composite fibers 20, 21 (including 
the filler material) may be wound on a mandrel or other suitable device 
and then placed in an oven and heated to a temperature sufficient to cause 
the composite fibers to bond together. Thereafter, the non-integral piston 
heads 14, 16 may be attached to the piston rod 3 and the metal rod end 
assembly 5 connected to the rod end in the manner previously described. 
Also, if desired, a linear variable differential transformer (LVDT) 75 may 
be installed internally within the hollow piston rod 3 with its movable 
core 76 attached to the internal metal stud 51 as further shown in FIG. 1 
for position sensing. 
From the foregoing, it will now be apparent that both tension and 
compression loads may be introduced into the composite piston rod of the 
present invention which includes not only longitudinal or biased fibers 
but also circumferential hoop fibers that not only fill the voids between 
the longitudinal fibers, but also effect the desired radial displacement 
of the longitudinal fibers where necessary and stabilize the longitudinal 
fibers in order to react compression loads. Also, one or more metal piston 
heads may be attached to the composite piston rod to form a single or dual 
stage fluid cylinder in which the loads generated by the fluid pessure 
acting on the attached piston heads is reacted through the composite 
piston rod. 
Although the invention has been shown and described with respect to a 
certain preferred embodiment, it is obvious that equivalent alterations 
and modifications will occur to others skilled in the art upon the reading 
and understanding of the specification. The present invention includes all 
such equivalent alterations and modifications, and is limited only by the 
scope of the claims.