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Description  |
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BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to tubular shaft members and in particular, relates
to tubular members having high stiffness to weight ratios which are
ideally suited for power transmission in aircraft such as the drive shaft
for a helicopter tail rotor or a remotely mounted propeller.
2. Description of the Prior Art
A number of aircraft have been developed which require the transmission of
power from an aircraft engine to a remotely driven member such as the tail
rotor of an aircraft or a remotely mounted propeller driven from a midship
mounted engine. In such installations it is necessary to achieve minimum
weight of the shaft and bearing supports while maintaining sufficient
strength and stiffness in the tubular drive member to resist torsional and
lateral oscillations. The problem is particularly acute in drive shafts
for the helicopter tail rotors which are separated from the drive engine
by a substantial length of fuselage which flexes during normal maneuvers
of the helicopter or in helicopters having tandem, dual rotors which are
driven through drive shafts which extend from a centrally located engine
station.
Heretofore, the drive shafts employed for such aircraft installations have
been fabricated of tubular metal members which bear radial end flanges for
their interconnections to each other and to the engine drive or working
member such as the tail rotor. Typically, the end flanges are
approximately the same diameter as the tubular section of the member and
are carried at the ends of the frustoconical sections which provide access
to the radial faces of the flanges for mounting interconnecting fasteners
between adjoining flanges. The radial end flanges are connected at bearing
stations to a flexible coupling that generally comprises a series of
stacked plates which permit some flexing between the coupling member and
the tubular shaft member. The bearing stations are required to permit a
limited amount of flexing of the tubular drive shaft to accommodate
flexing of the supporting fuselage and to permit joining of the tubular
sections which are limited in length to avoid development of resonating
oscillations at critical natural frequencies in the tubular shaft members.
It is desirable to minimize the number of the bearing stations in any
installation because of the associated weight of the components of the
bearing station. Since most metals have approximately equal stiffness to
weight ratios, the distance between bearing stations for metalic tubular
members is generally only a function of the diameter of the tubular
members and is independent of the metal identity. It is, therefore,
desirable to employ tubular members as drive shaft components which are
formed of materials having stiffness to weight ratios superior to those of
metallic members.
With many helicopter applications, particularly for military aircraft, the
metallic drive shaft members are susceptible to catastrophic failure from
relatively minor damage. Metallic members, when subjected to torsional
stress, are highly notch sensitive so that if struck by a bullet or other
fragment, the entire member will fail, resulting in loss of the aircraft.
It is therefore desirable to provide a drive shaft formed of a material
which is less stress tear sensitive than metallic members.
BRIEF DESCRIPTION OF THE INVENTION
This invention provides tubular members which are ideally suited for use as
drive shafts in aircraft applications. These members have stiffness to
weight characteristics superior to those of metallic members. The members
also have superior resistance to stress tearing than do metallic members.
The tubular members of this invention are formed by a continuous filament
winding of fibrous reinforcing material, typically of fiberglass or
graphite fibers and the like. The tubular members are formed with
frusto-conical end portions which terminate in radial flanges which bear
means for interconnection of the tubular members or for their connection
to a power source or working member. The filaments of the fibrous
reinforcement material are wound on a mandrel which is formed of a tubular
center section by a flexible membrane that extends between opposite
frustoconical end sections which bear end return rings for the winding
process. The mandrel is inflated with a positive differential fluid
pressure to maintain its shape during the winding process. The filaments
are applied on the mandrel at helix angles over the frusto-conical
sections which are greater than the helix angles on the center tubular
section whereby the filaments follow the general contour of the winding
mandrel. The filaments in the resultant sheathing are embedded with a
bonding resin, typically an epoxide or a polyester resin, and the mandrel
with its sheathing is placed in a mold and inflated with a positive
differential fluid pressure to expand the sheathing outwardly into contour
conforming contact with the die faces of the mold. The bonding resin is
thereupon cured to cement the sheathing into a rigid tubular member which
has a stiffness to weight ratio superior to that of metallic members. The
ends of the sheathing which are wrapped about the end return rings are
then cut about the periphery of the ring to form radial, circular flanges
at the ends of the tubular members.
In a preferred embodiment of the invention, the inboard faces of the return
ring members have a raised pattern surface to impart a relieved pattern of
predetermined shape and dimensions to the radial flange of the tubular
member whereby the flanges can be readily interconnected in torsional
engagement. In the preferred embodiments, the frusto-conical end sections
of the winding member are formed of a thermoplastic material whereby these
end sections and the associated flexible tubular section of the winding
mandrel can be deformed and withdrawn from the tubular member after its
fabrication.
In other embodiments, the winding mandrel can be formed of a destructible
material such as a water disintegratable material, a low melting point
metal alloy or plaster.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by reference to the drawings of which:
FIG. 1 illustrates a portion of a typical drive shaft formed with the
tubular members of the invention;
FIG. 2 illustrates the winding process for the manufacture of the tubular
members of the drive shaft;
FIG. 3 is a partial sectional view of the winding mandrel and a typical
sheathing formed thereon;
FIG. 4 illustrates the forming of the sheathing into a cured and rigid
shaft tubular member; and
FIG. 5 illustrates a flange member useful to impart a desired mating
pattern on an end flange of a tubular member.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, there is illustrated an assembly of tubular
members 10 and 12 of the invention. Each tubular member is formed with a
center cylindrical section 14 and opposite frusto-conical end sections 16
and 18, each of which bears radial ring flanges 20.
Tubular members 10 and 12 are illustrated as joined by a bushing member 22
which has a center, cylindrical sleeve portion 24 that is mounted in the
bearing 26 of a conventional pillow block bearing member 28. The opposite
ends of sleeve 24 are connected to frusto-conical end sections 30 and 32
which bear radial ring flanges such as 34. The flanges 34 of the sleeve
member 24 are interconnected to flanges 20 of the tubular members by
suitable fastening and joint means. As illustrated, each of the mating
faces of flanges 34 and 20 bear mating patterns 36 which are surface
relieved thereon. As illustrated, pattern 36 can comprise a plurality of
radially disposed teeth 38 on each of the flanges 20 and 34 which
intermesh when the flanges are butted together in the illustrated manner.
The joint is completed by a suitable clamping means such as the band clamp
40 which has annular lips 42 at either side thereof to engage about the
peripheral portions of the flanges. The clamp 40 is secured by a suitable
fastener such as 44 which extends through ends 46 and 48 of band 40. Other
suitable means can be employed for effecting the interconnection of
flanges 20 and 34, e.g., conventional bolt fasteners, rivets and the like
could be placed through the butting surfaces of these flanges to effect
such interconnection.
One of the advantages of the tubular members of the invention, which are
formed by continuous filament winding of fibrous reinforcement material,
is that the flanges 20 are resiliently carried on the ends of
frusto-conical sections 16 and 18. The flexibility of these flanges on the
tubular members is sufficient such that in most installations the
conventional flexible coupling of stacked plate members necessary for
interconnection of metallic tubular members is not required. Instead, the
tubular shaft members of this invention provide a limited flexing between
bearing stations which will be sufficient to accommodate the deflections
normally experienced in a helicopter tail rotor installation.
FIG. 2 illustrates the manufacture of the tubular sections in accordance
with the invention. As previously mentioned, the tubular sections are
formed by a continuous filament winding process using filaments of a
fibrous reinforcement material. These filaments are wound about a mandrel
of a desired configuration such as that shown in FIG. 2. The mandrel 50 is
mounted on shaft means 52 which, as illustrated, can be a continuous shaft
extending the length of the mandrel and resting on suitable bearing
supports 54 or can be stub shafts supporting opposite ends of the mandrel
50. The mandrel 50 is formed with a generally cylindrical center section
56 and opposite, frusto-conical end sections 58 and 60. The small diameter
end of sections 58 and 60 bear circular end plates 62 and 64 which serve
as end return members for the winding process. Shaft 52 is mounted for a
driven, rotational movement as indicated by arrow 66.
The continuous filament winding apparatus employed includes a reciprocally
driven carriage generally indicated at 68 which comprises a vessel 70
bearing a plurality of tensioning rollers 72 carried on standards 74. The
winding apparatus also includes guide ring 76 which, together with
carriage 68, is adapted for reciprocal driven movement parallel to the
axis of rotation of mandrel 50.
The vessel 70 contains a bath of the bonding resin indicated at 78 and the
filaments 80 of the fibrous reinforcement material are guided into the
bath, over the tensioning rollers 72, through guide ring 76 and then
applied to the mandrel 50 in a predetermined pattern.
The filaments 80 are applied to mandrel 50 in a helical path thereon by
guiding the filaments in a mandrel end-to-end traversing path as the
mandrel is rotated to apply a plurality of layers of helically wound
filaments on the mandrel with helix angles along the tubular cylindrical
section 56 of from 5.degree. to about 75.degree. and, on the
frusto-conical sections, at greater helix angles, typically from
45.degree. to about 90.degree., which are sufficient to form the resulting
sheathing about the mandrel in conformity to the frusto-conical end
portions 60 and 58.
The desired relationship between the helix angle of the windings of the
frusto-conical sections and the helix angle of the windings on the
cylindrical section can be mathematically expressed. This relationship is
as follows:
##EQU1##
wherein: .alpha..sub.n is the helix angle at any given point along the
frusto-conical section;
.alpha..sub.o is the helix angle along the cylindrical section;
R.sub.n is the radius of the given point about the mandrel centerline; and
R.sub.o is the radius of the cylindrical section about the mandrel
centerline.
The filaments are deflected into geodesic paths in a spoke-like pattern in
chordal paths about the inboard face such as 82 of each end return member
62 and 64. The filaments are applied to the cylindrical portions of the
end return member 86 at helix angles similar to those used for the center
cylindrical section 56. The filaments are then applied across the end
faces such as 84 of the mandrel to reverse the windings for the return
path of the carriage 68.
This winding process results in the application of layers of continuous
filaments of the fibrous reinforcement material with each layer comprising
two adjacent thicknesses of filaments, with the filaments being applied in
parallel alignment in each thickness and in diagonal orientation between
thicknesses at twice the value of the aforerecited helix angles.
Preferably center section 56 of the mandrel comprises a flexible member
which is maintained in the desired configuration by a positive internal
differential fluid pressure. Preferably gas pressure is maintained within
the winding mandrel by a supply means such as hollow shaft 52 which is
provided with suitable valve means 88 for controlling the gas pressure
within the mandrel and suitable indicating means such as pressure gage 90
for registering the magnitude of the differential pressure maintained
within the winding mandrel.
In another embodiment, mandrel 50 can be formed of a destructible material
which can be formed after the resin has been cured. Examples of suitable
destructible materials include sand and similar granular material which
can be cast and bonded into the desired mandrel configuration using from 3
to about 10 weight percent of a water soluble resin such as a partially
hydrolyzed polyvinyl acetate. After the resin has been cured, the mandrel
can be removed by immersing the mandrel and surrounding tubular member in
water. Another suitable material is plaster, e.g., plaster of Paris which
can be broken from the interior of the tubular member after its cure
hardening. Eutectic alloys can also be used, such as the commercially
available Cerrobend products. These alloys can be obtained as lead and
antimony alloys which have melting points from about 117.degree. to about
480.degree. F. and which comprise binary, ternary, quaternary and quinary
eutectic alloys of lead, antimony, bismuth, cadmium and indium. An example
of such is the quaternary alloy of bismuth, lead, antimony and cadmium
having a melting point of 158.degree. F. and commonly referred to as
Wood's metal.
Referring now to FIG. 3, the constructional features of the winding mandrel
and the sheathing formed thereon will be described. As shown in FIG. 3,
shaft 52 supports each end of the winding mandrel. The winding mandrel is
defined by the cylindrical center section 56 that is formed of a suitable
flexible membrane and that is attached to the frusto-conical end sections
such as 60. The small diameter end of the frusto-conical end section 60
bears an inwardly turned annular lip 61. Shaft 52 bears a ring retainer
member 92 to which is mounted circular flange 94 by suitable means such as
a plurality of bolt fasteners 96 which extend through radially disposed
bores in ring 92 and into threaded engagement with tapped bores 98 of
flange 94. Flange 94 bears an upstanding boss 100 on its inboard face
which is bored at 102 and counterbored at 104. Counterbore 104 is threaded
with female threads to receive a threaded plug member 106. Plug 106 has an
annular flange 108 which bears against the peripheral lip 61 of
frusto-conical member 60 whereby the latter member can be removably secure
to flange 94.
Flange 94 also supports the end return ring member 110 which is a generally
ring shaped member secured to the outboard face of flange 94 by suitable
fastening means such as bolts 112 which are seated in radially disposed
bores of ring 110 and which extend into threaded engagement with mating,
tapped bores on the outboard face of flange 94. Return ring 110 and flange
94 are assembled in a manner providing a smooth transitional surface
between these members. Accordingly, when the return ring 110 is of larger
diameter than flange 94, the flange 94 can be countersunk into the mating
surface of ring 110 by providing a circular depression 114 in the inboard
surface of return ring 110.
The mandrel 50 can be secured in a rotationally driven interconnection to
shaft 52 by suitable means such as bracket 116 that is rigidly secured to
shaft 52 and that is removably secured to end return member 110 by
suitable means such as screw 118 that extends through a bore in the radial
portion of bracket 116.
The end return member 110 can be formed of any suitable material such as
plastics, wood, metal and the like. Flange 94 can be constructed of
similar material, however, this flange is preferably metallic. The
frusto-conical members such as 60 are preferably formed of thermoplastics
which can be readily molded into the frusto-conical shape and which can be
removed from the completed tubular member in a manner hereinafter
described. The tubular center section 56 of the mandrel is formed of a
suitable flexible material such as a sheet or film of plastic material,
e.g., polyethylene, polypropylene, polybutadiene and co-polymers thereof
with acrylonitrile, styrene, etc. A convenient fabrication of the flexible
cylindrical section 56 is achieved by thermal deformation of an extruded
conduit of thermoplastic material by heating the conduit and applying a
positive differential pressure to the interior thereof to expand the
conduit into a larger diameter and thinner walled sheet such as 56.
As previously mentioned, the interior of the winding mandrel is maintained
under a positive differential fluid pressure. Preferably gas pressure and,
most preferably, compressed air is used which is introduced through the
interior of shaft 52. The interior of the mandrel is sealed by suitable
means such as by O-ring 120 which is mounted in annular groove 122 in the
central bore of flange 94. Shaft 52, of course, is provided with suitable
apertures or fluid passageways 124 along its length within the mandrel 50
whereby the gas pressure within shaft 52 can be applied to the interior of
the mandrel. The amount of differential pressure applied to the interior
of the mandrel can be from 0.5 to about 125 p.s.i. The pressure can be
gradually increased as the winding progresses and in a typical application
could be increased from about 0.5 p.s.i. initial to about 25 p.s.i. at the
completion of the winding. The filaments are supplied from one or more
rovings on spools or bobbins and are passed through one or more baths of
liquid, such as that contained in vessel 70, and then through guide means
76 and onto the mandrel as the latter is rotated about its axis. The
mandrel can be rotated at a speed from 1 to about 250, typically from 15
to about 50 revolutions per minute.
The filaments are applied at a predetermined tension typically from about
0.1 to 10, preferably from 0.5 to about 5 pounds per roving. This is
accomplished by passing the filaments through tensioning means such as
rollers 72. The carriage 68 is moved in a longitudinal direction in a
mandrel traversing path as shown by the double-headed arrow. When the
assembly reaches the end of the mandrel, its direction of movement is
reversed to apply the remaining half layer of the filaments. The velocity
of the carriage can be from about 0.5 to 5 feet per second, typically
about 2 feet per second. This velocity is usually limited in a wet winding
process by the tendency of the fiber to sling off the resin at higher
velocities. When pre-preg or post-preg applications are used, i.e., when
the resin is applied before or after the winding rather than onto the
fiber, higher carriage speeds could, of course, be used. Sufficient resin
can be wetted onto and retained on the surface of the filaments when the
resin viscosity is maintained between about 1,000 and 6,000, preferably
between about 2,000 and 3,000, centipoises, at the application
temperature. Resins of greater viscosities at ambient temperature can be
used simply by warming or heating the resin in vessel 70, care being
taken, of course, to limit the residence time of the resin in the vessel
to avoid initiating the cure hardening of the resin.
The outer sheathing formed of the fibrous reinforcement material which is
wound in the aforedescribed pattern on the exterior of the mandrel and the
bonding resin therefor is illustrated in FIG. 3 as layer 126. This layer
is shown as having a relatively uniform thickness throughout portion 128
which overlies the cylindrical center section of the mandrel and an
increasing thickness of the frusto-conical portion 130 which overlies the
frusto-conical end former 60 and flange 94. The increased thickness, as
shown, may not be necessary, depending on the type of interconnection
which is made between the tubular members. Accordingly, this construction
is an optional variation of the invention. The increasing thickness of the
fibrous reinforcement material can be obtained in the continuous filament
winding process, if desired, by applying sheets or strips of fabric of the
fibrous reinforcement material during the winding process, such strips or
sheets being applied between adjacent layers or thicknesses of the
filaments of the fibrous reinforcement material. When the filleted area is
increased in thickness, as illustrated, this increased thickness is also
preferably extended across the inboard face 132 of flange 94 to provide a
ring flange on the tubular member of reinforced thickness.
After the filaments have been applied onto the mandrel in the
aforedescribed pattern and sufficient layers of such filaments have been
applied to achieve the desired thickness of the continuous sheathing about
the surface of the mandrel, the sheathing can be consolidated into a rigid
and resilient tubular member. Generally from one to about 50, preferably
three to about 10, and most preferably, about six or seven, layers of
filaments are applied to the mandrel to achieve the maximum, desired
stiffness to weight ratio of the final tubular member.
As shown in FIG. 4, the assembly of the mandrel 50 and the sheathing 126
carried thereon is placed in a suitable die which is formed of a center
split mold 134 which has a cylindrical mold cavity of the desired exterior
dimension of the finished tubular member. The mold 134 is mated to end
molds 136 and 138 at opposite ends thereof and these end molds have
generally frusto-conical cavities 140 and 142 which impart the desired
exterior configuration to the frusto-conical portion of the tubular
member. Molds 136 and 138 are, as mold 134, formed of split halves which
are joined along the centerline of the tubular member. The end molds are
drawn into butting contact with the inboard faces 144 and 146 of the
sheeting disposed on the mandrel return end members 62 and 64. This
contacting of the molds in the fillets between the frusto-conical
portions, and the inboard faces 144 and 146 can be facilitated by the use
of end plates 148 which are provided with a number of radially disposed
bores through which are extended fasteners such as bolts 150 that project
into tapped bores in the upward faces of the molds 136 and 138.
The sheathing 126 on the exterior surface of the mandrel is expanded into
contour conforming contact with the interior die faces of molds 134, 136
and 138 by the application of fluid pressure through valve 88 and shaft 52
in the previously described manner. The bonding resin is thereupon cured
thermally or catalytically and to this end, various internal passages can
be provided in molds 134, 136 and 138 through which a heat exchange medium
can be circulated to heat the sheathing and its bonding resin to the
requisite curing temperature. Suitable resin curing temperatures that can
be used include temperatures from ambient, e.g., about 75.degree.F, to
about 400.degree.F., preferably from 100.degree. to 225.degree.F. The
application of this heat is continued for a sufficient time for resin
curing, e.g., from 10 minutes to about 7 days, preferably from 60 to about
300 minutes as required for the particular resin and fibrous reinforcement
system employed in the fabrication of the article.
After the resin has cured to a hardened condition, the molds 134, 136 and
138 can be opened and the formed tubular member can be removed therefrom.
The sheathing about the return end pieces 62 and 64 can be cut about a
peripheral portion thereof to form the generally circular flanges 20 on
opposite ends of the tubular member.
The mandrel used in the fabrication of the tubular member is then
disassembled. This disassembly is achieved by removing the return end
members 62 and 64 and the associated flanges 94 from the assembly. As
previously mentioned, the preferred material for use as the frusto-conical
section of the mandrel is a thermoplastic. Accordingly, the tubing member,
when in heated condition, can be freed from its internal mandrel by
deforming the frusto-conical end portions and withdrawing them with the
associated center flexible mandrel section 56 through the central aperture
on the tubular member. To permit the facile removal of the mandrel from
its outer tubular member, the mandrel should be coated with a suitable
parting agent prior to application of the sheathing and bonding resin.
Various parting agents can be employed for this purpose such as the
conventional mold release agents for epoxies or polyester resins.
As previously mentioned, the outboard faces of the cylindrical end flanges
on opposite ends of the tubular portions can be formed with a suitable
pattern of surface relief. An example of a suitable surface relief pattern
comprises the radial teeth 34 which are shown in FIG. 1. Other
configurations which can be employed would be to provide a scalloped or
wavy shape to the outer surface of these ring members. FIG. 5 illustrates
a suitable flange 94 which has an inboard face 132 that bears an intaglio
pattern of the surface relief desired on the outboard ends of circular end
flanges 20 of the tubular member. As illustrated, this pattern comprises
regularly disposed and raised, generally wedged shaped teeth 152, which
project from surface 132 of the flange. The spaces between these teeth 152
are undercut to provide an inclined and recessed surface 154 of the same
dimensions as the raised surfaces 152, whereby mating patterns will be
formed on surfaces that are formed against the inboard face 132 of flange
94.
The following illustrates a typical application of the invention and
demonstrates the superiority of the tubular sections of the invention over
conventional metal sections. The tubular sections are designed for
installation as the drive shaft for a rotor of a military helicopter. The
sheathing is formed with graphite filaments, coated with an epoxy resin
and molded to prepare tubular members having an outside diameter of 4.25
inches. This diameter of tubular sections could be provided with lengths
of 69 inches for the particular installation without exceeding the
torsional and bending stresses encountered in the application. In
contrast, a conventionally formed tubular member of aluminum had an
outside diameter of 3 inches and maximum length of 57.5 inches. The
invention can therefore be seen to have provided about 20% greater spacing
between the necessary bearing supports for the tubular shaft assembly.
Other installations have shown as great as about 50% increased spacing
over metallic members between the bearing stations, thereby greatly
reducing the weight of the assembled shaft and bearing supports.
SUITABLE FIBROUS MATERIALS
Any available fiber can be used as a reinforcement for resins used in the
process. Examples of available fibers include fibers of rayon, cotton,
silk, polyesters, etc. Most desirably, however, fibers are employed which
have relatively high tensile strength such as glass, boron or graphite
fibers, the latter being preferred for the excellent stiffness to weight
ratios they achieve in the finished tubular member. Type S glass fibers
can be used and are preferred for their greater strength over Type E glass
fibers. Typically the Type S fibers have tensile strength from 3 to
7.times.10.sup.5 p.s.i. and Young's Modulus of about 12 to
13.times.10.sup.6 p.s.i. The Type S fibers are obtained from a glass melt
of silica, alumina and magnesia.
In their manufacture, the glass filaments are usually sized immediately
upon formation to avoid any mechanical damage. A textile sizing, which
consists of a dextrinised starch and emulsified vegetable oil, is
sometimes applied. More commonly, a plastics sizing is applied which
comprises a polyvinyl acetate base, a plasticizer and a resin coupling
agent. When the textile sizing is present, it is necessary to remove the
sizing from the filaments before their application since the textile
sizing is generally not compatible with the bonding resins. When the
plastics sizing is employed, the filaments can be directly applied and
embedded with the bonding resin since they are compatible with the
commonly used bonding resins.
When necessary, the yarn can be de-sized by carmelization which comprises a
heat treatment to volatilize the sizing and carbonize the starch, thereby
reducing the residual sizing content to about 0.6%. The sizing can also be
removed by passing the filaments through a scouring bath to remove the
organic material and reduce the residual sizing to below 0.3%. A
combination of both treatments can also be used where the filaments are
passed through the aqueous bath and then passed through an oven at a
temperature of about 300.degree. - 350.degree. C.
The filaments of glass fibers have diameters which range from about 0.0045
to about 0.015 millimeters in diameter. As described herein, the term
filaments has been used generally to refer to a single filament or to
yarns or rovings of a plurality of filaments. The filaments can be used as
yarns which are formed from a plurality of filament strands by twisting
and plying the strands, or as rovings which are bands of untwisted
strands. The latter are preferred. The yarn is commonly designated by
count which is the weight of the yarn per unit length and typical yarn
counts are from 2.75 to 135 grams per kilometer of yarn length. The number
of filaments which are combined into a strand of yarn or into a roving
range from about 50 to about 250 filaments. The yarns or rovings are
supplied on a spool or bobbin with from 1 to about 300 ends which are
wound into a cheese or cone shape. The rovings unwind during their
application as a band of parallel, multiple filaments which are applied to
the mandrel as a band.
Carbon base fibers can also be employed and are preferred because of the
high strength and stiffness which they impart to the tubular members. The
carbon base fibers are prepared from filaments of carbonaceous materials
which are heated to high temperatures under carefully controlled
conditions to convert the carbonaceous material into substantially pure
carbon. Rayon is one of the most commonly used carbonaceous material for
preparation of the carbon and graphite fibers. The fibers are commonly
referred to as partially carbonized, carbonized or graphitized, depending
upon the severity of the heat treatment. The partially carbonized fibers
are obtained by treatment of fibers at temperatures from 1300.degree. to
1700.degree. F. and have a carbon content up to about 90 weight percent.
Fibers having carbon contents above 90 weight percent are obtained by
carbonization at slightly higher temperatures and are generally referred
to as carbonized fibers, while fibers which have been heated to
graphitizing temperatures, i.e., temperatures from 4900.degree. to
5400.degree. F. are referred to as graphitized fibers. The commercially
available carbon or graphite fibers have tensile strengths of about 1 to
about 5.times.10.sup.5 p.s.i. and Young's Modulus of 6 to
100.times.10.sup.6 p.s.i. The fibers have densities of about 1.4 grams per
cubic centimeters, although fibers having a high content of graphitic
structure will have densities up to about 2 grams per cubic centimeter.
The carbon base fibers are available as rovings similar to those described
for the glass fibers.
The boron fibers are produced by drawing a tungsten wire having a diameter
of about 0.5 mil through a reactor containing a boron-containing gas. The
wire is electrically heated to a sufficient temperature to decompose the
gas and deposit a coating of boron on the wire. Typically, the coated
product has a diameter of about 0.003 to 0.005 inch.
The strength of the boron fibers is intermediate that of graphite and glass
fibers. The following table lists typical strengths and densities of these
fibers:
TABLE
______________________________________
Density Tensile Modulus Modulus
Material lb/in.sup.3
psi.times.10.sup.3
psi.times.10.sup.6
density.times.10.sup.6
______________________________________
Graphite 0.05 400 50 910
Boron 0.09 500 60 666 | | |
