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Description  |
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FIELD OF THE INVENTION
This invention relates to extendible structures, for example booms and
towers that can be stored at lesser lengths and extended to greater
lengths.
BACKGROUND OF THE INVENTION
Extendible structures are extensively used as portable towers and booms.
They are collapsible to be stored in envelopes of lesser height or length,
and can be extended (deployed, erected) to a greater height or length.
Such structures most frequently require cables or other axially-operated
systems to extend them to their full length, and locking means to hold
them in their extended condition. Especially for airborne and space
vehicles, where such structures find frequent employment, these are not
only additional complications subject to malfunction, but represent
undesirable weight. For land-based use, such as for towers to support
antennae, they represent undesirable ground weight and resistance to the
wind.
It is an object of this invention to provide an extendible structure which
is inherently biased towards its extended configuration, and which
requires no locking elements to hold it extended.
Furthermore, the integrity of such structures when damaged or disabled is a
serious matter, especially when the structure is deployed where it can be
repaired only with difficulty, or where it may not be repairable at all.
It is an object of this invention to provide an extendible structure which
can be damaged or fractured to a substantial extent, and still retain
substantial integrity.
BRIEF DESCRIPTION OF THE INVENTION
An extendible structure according to this invention has an axis of
extension. The structure comprises a plurality of axially spaced-apart
station members, each of which is rigid, and which lies in a plane normal
to said axis. At least three longeron members, each comprising two rigid
longeron elements join adjacent station members together. First joint
means join first ends of the elements of each pair to one another for
foldable movement relative to each other, and second joint means join the
second ends of said longeron elements to respective station members for
foldable movement relative to said station member.
A buckling spring which buckles springily under end load compression exerts
an endwise expansive force adjacent first longeron joints, whereby to bias
said joints away from each other, with a resultant force on each first
joint means directed outwardly away from said axis. The station members
and longeron members, when all joint means respective to each pair of
joined longeron elements are aligned, form a respective side panel.
A pair of first stay members, and a pair of second stay members for each
said panel, have respective maximum lengths when fully stretched by
tension forces. Each of the first stay members extends from a respective
second joint means of one longeron member to the first joint means of an
adjacent longeron member, converging in the direction of folding of said
longeron elements. Each of the second stay members extends from a
respective second joint means of the said adjacent longeron member to the
first joint means of said first mentioned longeron member. At least the
second stay members are non-rigid in compression. All stay members have
maximum length when fully stretched by tension forces.
According to a preferred but optional feature of the invention, the
assembly can be elongated by using the top station member of one structure
for the lower station of the next structure.
According to still another preferred but optional feature of the invention,
all stay members may be chains, cables or otherwise flexible members.
The above and other features of this invention will be fully understood
from the following detailed description and the accompanying drawings, in
which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing the presently preferred embodiment of
the invention fully extended;
FIG. 2 is a perspective view of the structure of FIG. 1 partly collapsed;
FIG. 3 is a perspective view of the structure of FIG. 1 fully collapsed;
FIG. 4 is a top schematic view showing the basic geometry of the structure
when fully extended;
FIG. 5 is a side schematic view taken at line 5-5 in FIG. 4;
FIG. 6 is a top schematic view showing the basic geometry of the fully
collapsed structure; and
FIG. 7 is a side schematic view taken at line 7-7 in FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION
The presently preferred embodiment of the invention is shown in its fully
extended condition in FIG. 1. In this figure an extendible structure 10 is
shown which has an axis of extension 11. It has a first station member 12
and a second station member 13. These station members are rigid and they
preferably comprise a rod-like framework. They could instead be solid
bulkhead members if preferred. But because this device will generally be
used where lightness is a preferred feature, rod-like elements will
generally be used. The term "rod-like" is used generically to describe
linearly extending objects such as rods, tubes, L-shaped angles, and the
like.
Station member 12 has elements 15, 16, 17, 18 and station member 13 has
elements 19, 20, 21 and 22. These are joined at their corners by fastening
means in a quadrilateral array. They may be directly attached to one
another or can be attached to one another by corner brackets of which
bracket 23 is a characteristic example. In any event, the resulting
structure will be a polygon. In the illustrated embodiment the polygon is
a rectangular quadrilateral. It is equally possible for there to be
three-sided structures or for the station members to have different
shapes. As far as the collapsable and extendible feature is concerned, the
section is determined by apices as defined by the intersection with the
station members of a plurality of longeron members 25, 26, 27 and 28.
Longeron member 25 is comprised of a pair of longeron elements 29 and 30.
Longeron member 26 is comprised of a pair of longeron elements 31 and 32.
Longeron member 27 is comprised of a pair of longeron elements 33 and 34.
Longeron member 28 is comprised of a pair of longeron elements 35 and 36.
The pairs of longeron elements of longeron members 25-28 are jointed
together at and by first joint means 40, 41, 42 and 43 respectively. These
joint means join the first ends of the longeron elements in pairs relative
to longeron members 25-28 as follows: 43, 44; 45, 46; 47, 48; 49, 50.
The first joint means enable the pairs of longeron elements to pivot
relative to one another so that they can fold toward one another.
Each longeron member also has a pair of second joint means as follows. As
to longeron members 25, 26, 27, 28; the second joint means are as follows:
55, 56; 57, 58; 59, 60; 61, 62. The second joint means connect the second
ends of the longeron elements to respective station members. The second
ends relative to longeron members 25, 26, 27 and 28 are as follows: 65,
66, 67, 68, 69, 70, 71, 72.
Four buckling springs 75, 76, 77, 78 extend respectively between adjacent
first joint means. These buckling springs buckle resiliently under end
load compression, behaving as an Euler column. They are connected to and
extend between each pair of adjacent first joint means and bias the first
joint means away from each other and thereby also provide a resultant
force on each first joint means directed outwardly away from the axis of
extension.
The material of which the buckling springs are made will be determined by
the forces which it must exert, and the environment to which it will be
exposed. For atmospheric and space applications, fiberglass epoxy
constructions are quite suitable. For example, laid-down fiberglass fibers
along the length of the buckling spring in an epoxy matrix is a very
suitable material .The ends of each of the springs are fitted with eyelets
of which a typical eyelet 79 is shown. These are spindled to a post (not
shown) that may or may not form part of the first joint means, and which
enables the exertion of the resultant forces on the longeron members. In
the fully extended condition, the buckling springs will still be somewhat
flexed so as to continue to exert the resultant force described.
While the first joint means are shown comprised of two separate hinges, and
means for attaching the springs, it is emphasized that only a single hinge
is needed between them. Furthermore, the relative rotation of the spring
end relative to the first joint is not necessary, because the strains can
be accommodated by the springiness of the spring material. However most
frequently the spring ends will be attached rotatably as shown. The two
hinges shown for the first joints are treated herein as a single hinge
action around the center of the joint means. Also, the springs need not
exert their forces precisely on the axis of the longeron elements.
Instead, attachment means may be attached radially displaced inboard or
outboard thereof.
The buckling spring is preferably a flat blade construction in
cross-section and longitudinal shape, although it may have any other
desired cross-section and characteristic which the designer might wish to
use, so long as when buckled it exerts an endwise force as illustrated
when the construction is extended, and which can be additionally buckled
resiliently, remaining able to exert an extending force between its two
ends.
When all joint means respective to each longeron member are aligned, they,
together with the station members form a respective rectangular side
panel. There are four of these side panels 80, 81, 82 and 83. If there
were other than four longeron members (there must be at least three), the
panels would still be rectangular, but there would be a number of them
equal to the number of longeron members.
All of the panels are identical, so that only one of them will be described
in detail and this will be panel 80. A pair of first stay members 85, 86
extends between first joint means 43 and second joint means 55 and 56.
They converge toward the first joint means 43 which as it transpires will
be the leading joint means in folding, as will later be described. A pair
of second stay members 87, 88 extends from first joint means 40 to second
joint means 61, 62 and converge toward first joint means 40.
All stay members have a maximum length when fully stretched by tension
forces whereby to limit the separation of their ends. Conveniently they
may be made as chains or cables or some other flexible type of device. The
first stay members may if desired be rigid instead of flexible, because in
the operation of this device the distance between their two ends will not
change. The second stay members, however, must allow the distance between
their two ends to change. Such a result is most conveniently accomplished
by the use of flexible devices such as chains or cables. However
telescopic arrangements or even spring-like arrangements are also useful.
When the device changes its configuration the spacing between the
"trailing joint means" in this case joint means 40 and the second joint
means to which it is attached will change.
These stay means will be repeated for each one of the panels, and like
numbers are used when they are referred to.
The device shown in the drawings is a demonstration device intended to show
maximum flexibility in design and construction. As shown, the joint means
all have at least one degree of rotational freedom, i.e., that of their
hinge pin which enables the swinging action to occur between the longeron
elements and their next assembly. In this device, a second degree of
rotational freedom is also shown which is normal to the station elements
and which enables rotation of the joint means to occur.
However this is not a necessary feature because the folding action of the
longeron members will occur in a single plane as will be described.
However there may be some constructions in which the rotation of the
longeron element or some other joint means would be desirable. If not,
then the hinges will be fixed, with the axes of the three hinges
respective to each longeron member parallel to one another, and normal to
the plane in which the folding action occurs. It is desirable for the
eyelets adjoining the buckling springs to enable relative angular movement
of the springs to occur, and this is provided for by passing pin members
as part of the joint means through the eyelets to join the buckling
springs to the longeron members.
While the device has been shown four sided, triangular structures are
equally useful and also other polygons of other numbers can be
manufactured.
It should also be understood that the device can be multiplied in the axial
direction by duplicating the already described structure using the upper
station member as the lower station member of the next assembly.
The structure can also be multiplied laterally by coupling two or more
structures together by fastener or bracket means. In this case, however,
each of the lateral devices must in itself be complete as shown. It cannot
share parts with its neighbor.
The operation of the device will be evident from FIGS. 1-3. FIG. 1 shows
the device fully extended with its shape fully defined by the stay
members, which are all drawn taut. Under these circumstances, the
longerons can be in no position other than the aligned one where their
respective three joint means are in linear alignment and are held in that
position against the stay members by the bias force of the buckling
springs which are attempting to return to a straight or at least to a less
curved condition. It will now be seen that the longeron elements provide
strong columnar strength and that the station members provide good lateral
strength.
To collapse the device, the station members are held against rotation
relative to one another and all of the first joints are moved in the same
clockwise or counterclockwise sense. As shown in FIGS. 2 and 3 looking
downwardly, they are moved in the clockwise sense. Under these
circumstances, there is a group of rigid triangular structures formed. For
example the first stay members remain fully taut. The movement of the
longeron elements 29 and 30 will be in the plane of panel 80. This is
because there is a rigid right triangle defined by elements 16, 29 and
first stay member 90, and rotation of the longeron elements is in a plane
normal to the adjacent station element.
All of the first stay members 85, 86 will remain taut as shown in FIG. 2
and all second stay members will become slack. This condition persists in
the folded condition shown in FIG. 3 from which most of the stay members
have been deleted in order to simplify the drawings for purpose of
illustration. Suffice it to say that these right triangles remain fully
defined, and only the first one, bounded on one of its sides by stay
member 85 is shown in FIG. 3 for purposes of illustration.
It will, however, be noted that the buckling springs have been more acutely
bent, and their ends are moved closely together. Thus, the radius of
curvature will have been decreased, and the energy it took to cause this
change of shape is stored in the buckling spring which exerts a bias force
tending to enlarge the radius of curvature and spread the ends farther
apart. This situation is shown in FIGS. 4-7. In FIG. 4, the buckling
springs are shown in the deployed condition. When in the stored condition
it is as schematically shown in FIG. 6. In effect, this shows that the
ends of the buckling springs are on the circumscribed circle when the
structure is extended, and on the inscribed circle when collapsed. Because
the arc length of the buckling spring does not change, it is more acutely
bent when stored, thereby exerting a strong expansive force tending to
return the structure to its extended condition.
The theory of the right triangles referred to above is shown in FIGS. 5 and
7 which schematically show the side views of the extended and stored
device relative to longeron station element and a first stay.
Because the buckling spring remains in compression, it continues to exert
its force for extension at all times. For this reason the device must be
restrained in its stored condition and can readily be extended simply by
releasing the station members, preferably one at a time, and controlling
the rate at which the station members are permitted to move apart from one
another. Of course when one or more of these structures is provided they
may be deployed one at a time.
It will now be seen that this device is an elegantly simple construction
which derives its erected structural integrity from the limitation of the
stays and by the bias force of the buckling springs to maintain the
longeron elements in a condition in which the stays are maintained fully
taut.
The device is reliable and will have substantial residual strength even if
substantially damaged. For example, destruction of any of the stays will
affect only one of the panels. Destruction of any of the first joint means
will damage one of the longerons but not the others. Therefore substantial
columnar strength will be maintained even in the event of what would
ordinarily be considered as a very serious damage.
This invention thereby provides an elegantly simple, reliable and
long-lived structure which does not require means to extend it, and which
provides substantial strength from minimal structural weight.
This invention is not to be limited by the embodiments shown in the
drawings and described in the description which are given by way of
example and not of limitation, but only in accordance with the scope of
the appended claims.
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