FIELD OF THE INVENTION
The present invention relates generally to flight control
actuators, and more particularly to an output shaft for a flight control actuation
unit. Even more particularly, the present invention relates to a removable output
shaft for a missile fin actuator in a missile control actuation unit.
BACKGROUND OF THE INVENTION
Missile control fins are commonly positioned by actuators
mounted within the missile body. Each control fin is usually coupled to a corresponding
actuator by means of a cylindrical output shaft. The actuator, via the output shaft,
exerts appropriate rotational torque and control on the fin so that missile control
is achieved. In general, a high degree of torsional and bending stiffness is required
of the actuator and its output shaft. Actuators may be electrically, pneumatically,
or hydraulically powered, as is known.
A conventional actuator has a large outer bearing and significantly
smaller inner bearing. The bearings are spaced apart at a distance D along the actuator
output shaft. These bearings allow the shaft to rotate freely, and react to loads
imposed by the output shaft as a result of aerodynamic loads on the fin. In conventional
actuators, the distance D is the minimum bearing spacing compatible with maximum
allowable bearing loads. These bearing loads are transferred to the actuator housing
and reacted to by the missile body to achieve the desired missile control (e.g.,
One shortcoming of such a conventional actuator design
is that the bending stiffness needed to meet flutter requirements is limited by
the small diameter of the inner bearing. As noted above, the diameter of the inner
bearing is significantly smaller than the diameter of the outer bearing. The limited
bending stiffness results, for example, in the onset of aerodynamic instability
(flutter) at reduced airspeeds.
Another shortcoming of such a conventional actuator design
is that the resultant reduced bending load capability reduces the manoeuvrability
of the missile by reducing the loads which can be transferred to the bearings via
the output shaft.
Yet another shortcoming of the conventional actuator design
is the requirement for high manufacturing tolerances. Such high manufacturing tolerances
are due, in part, to the unitary design of the actuation unit housing.
The conventional housing is usually a solid piece of aluminium
in which several holes are bored out to receive each actuator unit and corresponding
output shaft. A counter bore is required to produce a retaining shoulder to hold
the smaller inner bearing. In the past, a counter bore of this nature could not
be done automatically by a computer programmed boring machine. It had to be done
manually, which drove up manufacturing costs. In addition, the unitary design made
it difficult to service the output shafts in the field. This required that the entire
missile be shipped back to the manufacture for servicing.
Further, the unitary design of the actuation unit housing
makes it difficult to test actuators in the assembled units as required before delivery
to a customer. Additionally, the unitary design made it difficult to test new missile
fin attachment stub designs. In each of these sorts of tests, the missile actuation
unit has to be taken substantially apart
US 5,255,882 discloses a setting device for the control
surface of a projectile, which includes a drivable spindle or actuated screw which
is screwed through the intermediary of a nut which is secured against rotation.
As a consequence, a need exists for improvement in actuation
unit construction to reduce the cost associated with using, manufacturing, testing
and servicing the output shaft of a missile control actuation unit and thereby eliminate
costly corrective measures required to be taken as a result thereof.
SUMMARY OF THE INVENTION
The present invention provides an output shaft for a missile
fin actuator in a missile control actuation unit designed to satisfy the aforementioned
Accordingly, the present invention provides a shaft assembly
for coupling a control fin to a missile, the shaft assembly comprising:
the inner shaft portion includes a fin attachment stub along an axis of the shaft,
- a shaft wherein the shaft includes:
- an inner shaft portion; and
- an outer shaft portion coupled to the inner shaft portion; and characterised
- a pair of preload nuts threadedly engaged with the shaft at respective opposite
ends of the shaft, wherein
the pair of preload nuts are axially adjustable relative to the fin attachment stub.
The outer and inner shaft portions may be detachably coupled
together, allowing removal of one of the shaft portions while the other shaft portion
remains in the missile. In an exemplary embodiment, the inner shaft portion, to
which the control fin is coupled, is removable. The pair of preload nuts serve to
adjust the position of the control fin relative to the skin of the missile, as well
as to provide necessary bearing preload. The preload nuts may for example be engaged
on opposite threaded ends of the outer shaft portion.
In an embodiment of the invention, the shaft portions are
coupled together by means of a fastener.
The shaft assembly may include a pair of bearings, the
bearing closer to the center of the missile having approximately the same diameter
as the bearing farther from the center of the missile.
In an embodiment of the invention, the shaft includes an
inner shaft portion coupled to an outer shaft portion, and the preload nuts are
threadedly engaged with the outer shaft portion.
The outer shaft may be detachably coupled to the inner
shaft, and a pair of bearings may be coupled to the outer shaft for enabling rotation
of the outer shaft relative to the missile.
The following description and the annexed drawings set
forth in detail certain illustrative embodiments of the invention. These embodiments
are indicative, however, of but a few of the various ways in which the principles
of the invention may be employed. Other objects, advantages and novel features of
the invention will become apparent from the following detailed description of the
invention when considered in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the annexed drawings:
DETAILED DESCRIPTION OF THE INVENTION
- FIG. 1 is a perspective view of the flight control actuation unit of a missile,
including four missile fin control surfaces that manoeuvre the missile by use of
shaft assemblies in accordance with the present invention;
- FIG. 2 is an exploded perspective view of a shaft assembly of FIG. 1; and
- FIGS. 3 and 4 are sectional perspective views of the shaft assembly of FIG.1
installed in a housing of a missile.
Referring initially to FIG. 1, a missile control actuation
unit 6 is shown which is a part of or is mounted on a portion of a missile body
8. The missile control actuation unit 6 may be at an end or at an intermediate location
on the missile body 8. The actuation unit 6 secures and provides means to rotate
a number of control fins F1-F3 of the missile. Although only the three control fins
F1-F3 are visible in FIG. 1, it will be appreciated that the control fins will in
general be evenly spaced around the circumference of the missile body 8, and that
therefore the missile control actuation unit 6 includes a fourth control fin F4
which is hidden from view For each of the control fins F1-F4, the control actuation
unit 6 has a shaft assembly, such as the shaft assembly 10, to which the fin is
coupled (or integral with) and which is partially inserted into, and which extends
outwardly from, a missile control actuation unit body 12. The shaft assembly 10
is mounted in the body 12 in a suitable manner to allow rotation about an axis 14
of the shaft assembly. As shown, the axis 14 may extend substantially normally from
a skin 16 of the body 12. The shaft assembly 10 may have a fin attachment stub 18
for receiving and securing one of the control fins F1-F4. Alternatively, as mentioned
above, one of the control fins may be integrally formed with the shaft assembly.
As explained in greater detail below, the shaft assembly
10 has multiple shaft parts that facilitate assembly and disassembly of the control
actuation unit 6, in particular facilitating easy removal and replacement of all
or part of the shaft assembly from the control actuation unit. Further, the multiple
shaft parts of the shaft assembly 10 are coupled together by a pin, which advantageously
provides low output backlash. In addition, the shaft assembly 10 includes adjustment
means (a pair of bearing preload nuts) to adjust the position of the fin attachment
stub 18 and the fin relative to the skin 16 of the body 12. The preload nuts may
provide a "back to back" preload on the bearings, which is advantageous in terms
of increasing the bending stiffness of the output shaft.
Each of the shaft assemblies rotates as commanded by a
guidance control computer, to thereby rotate its corresponding control fin, enabling
the missile to maneuver. In the exemplary embodiment, four missile fins are substantially
symmetrically arrayed about the circumference of the missile 8. The guidance control
computer may be used to provide a torque command for rotation of each fin separately
in accordance with known tracking algorithms. Alternatively or in addition, the
guidance control computer may be used to provide one torque command for common rotation
of a pair of opposed fins.
Referring to FIGS. 2-4, details of the shaft assembly 10
are shown. The shaft assembly 10 includes a shaft 19 made up of an outer shaft portion
20 (also referred to as an outer shaft) and an inner shaft portion 22 (also referred
to as an inner shaft). As is described more fully below, the inner shaft 22 is received
by and detachably connected to the outer shaft 20.
In the following description, the terms "outer" and "inner"
are generally used in reference to relative distance from the axis 14 of the shaft
assembly 10. By contrast, the terms "distal" and "proximal" are generally used in
reference to relative distance from the axis of the missile body 8.
The fin attachment stub 18 is fixedly mounted to the inner
shaft 22 for receiving and securing a control fin along the axis 14. It will be
appreciated that the fin attachment stub 18 may either be an integral part of the
inner shaft 22, or may be a separate part which is attached or otherwise coupled
to the inner shaft. Alternatively, as mentioned above, a fin and the inner shaft
may be integrally formed.
The inner shaft 22 has a cylindrical portion 24 which has
an open, non-accessible axial end 26. The open end 26 is within the body 12, inside
the skin 16, when the inner shaft 22 is detachably connected to the outer shaft
20 and the shaft assembly 10 is installed in the body. The open end 26 may include
a circular opening about the axis 14 of shaft assembly 10. The cylindrical portion
24 is an external cylindrical surface, which is substantially coaxial with and slidable
within an internal cylindrical surface 28 of the outer shaft 20. The cylindrical
portion 24 of the inner shaft 22 and the internal cylindrical surface 28 of the
outer shaft 20 are aligned therewith along the axis 14 when the inner shaft is detachably
connected to outer shaft.
The cylindrical portion 24 of the inner shaft 22 has pair
of diametrically-opposed holes 30 and 32 (also referred to as "bores"). The hole
30 is internally threaded. As described below, the holes 30 and 32 are used in detachably
securing the outer shaft 20 to the inner shaft 22.
A distal preload nut 34 has a central, generally circular,
opening 36 which is somewhat larger in diameter than the cylindrical portion 24
of the inner shaft 22. The cylindrical portion 24 extends centrally through the
distal nut 34 without engagement therewith when the inner shaft 22 is inserted through
the distal nut 34 and into the outer shaft 20. The distal nut 34 has an internal
threaded surface 38 which engages an external threaded end 40 of the outer shaft
20. The engagement of the distal nut 34 and the threaded end 40 is used in adjusting
the position of the fin attachment stub 18, and more particulary the fin, relative
to the skin 16. This adjustment process is described in greater detail below.
The shaft assembly 10 has a distal bearing 44 which includes
an inner race 46, an outer race 48, and a plurality of balls 50 between the races
to allow the races to rotate relative to one another. The inner race 46 is associated
with and rotates with a cylindrical outer bearing shoulder surface 52 of outer shaft
20. The outer race 48 is associated with and is stationary relative to a cylindrical
surface 54 of a distal housing portion 56 into which the shaft assembly 10 assembly
is inserted. The distal bearing 44 is thus disposed radially between the cylindrical
surface 52 and the cylindrical surface 54, and allows low friction rotation of the
outer shaft 20 relative to the distal housing portion 56.
The outer shaft 20 defines the generally circular socket
58, coincident axially with the axis 14, into which the inner shaft 22 is inserted.
The inner shaft 22 extends centrally into and is circumscribed by the socket 58,
thus juxtaposing both the inner shaft 22 and the outer shaft 20 coaxially along
the axis 14.
The socket 58 includes a cylindrical interior surface 28
which has open ends 60 and 62 at respective opposite sides of the outer shaft 20.
The open end 60 is disposed in a distal direction from the axis of the body 12.
The open end 60 is accessible when the inner shaft 22 is detached and removed from
the outer shaft 20, and it is into the open end 60 where the inner shaft is inserted
for assembly of the shaft assembly 10. The open end 62 is proximal relative to the
open end 60.
The outer shaft 20 has an externally threaded end 66 about
the open end 60, the external threaded end 66 abutting a cylindrical surface 68.
The cylindrical surface 68 is larger in diameter than open end 62, and extends therefrom
toward the open end 60 a substantial portion of the distance therebetween. Between
the cylindrical surfaces 52 and 68, the outer shaft 20 has an integrally formed
partial gear 70. When the shaft assembly 10 is installed, the partial gear 70 is
operatively coupled to other gearing and components for rotating the shaft assembly.
In an exemplary embodiment, the partial gear 70 has an angular spread of approximately
The partial gear 70 has a receiving hole or bore 72, and
a through-hole or bore 74 which is diametrically opposed to the receiving hole 72.
The through-hole 74 is counterbored at the exterior of the outer shaft 20. The holes
72 and 74, in conjunction with the holes 30 and 32 of the inner shaft 22, are used
to detachably couple the inner shaft to the outer shaft 20. A fastener such as a
bolt or tension element 76 is slidably received through the holes 30, 32, 72, and
74. The bolt 76 has an externally-threaded portion 78 which engages the threads
of the internally-threaded hole 30. The bolt 76 has a bolt head 80 disposed outwardly
of the hole 74. The bolt head 80 is larger in diameter than the balance of the bolt
76, and may be configured in a manner for engagement by a suitable corresponding
driving tool, for rotational manipulation of the bolt. The bolt 76 may be secured
in place by use of a securing device such as a locking ring 82 which is received
in a suitable annular groove in the bolt, adjacent to the hole 32 of the inner shaft
22. This locking ring 82 serves to retain the bolt 76, thereby maintaining the outer
shaft 20 and the inner shaft 22 coupled together.
It will be appreciated that a wide variety of other known
suitable means for detachably coupling the shafts 20 and 22 alternatively may be
A proximal bearing 84 allows low-friction rotation of the
outer shaft 20 relative a proximal housing portion 85. The proximal bearing 84 includes
an inner race 86, an outer race 88, and a plurality of balls 90 mounting the races
for rotation relative to one another. The inner race 86 is associated with and rotates
with the cylindrical outer bearing shoulder surface 68. The proximal bearing 84
is disposed radially between cylindrical surface 68 and a cylindrical inner surface
92 of the proximal housing portion 85. An interior surface 93 of the inner race
86, and the exterior surface 68 of the outer shaft 20, conform and are engaged with
each other. Similarly, an exterior surface 95 of the outer race 88 and the inner
surface 92 of the proximal housing portion 84 conform and are engaged with each
The proximal bearing 84 has a diameter similar to that
of the distal bearing 44, although it will be appreciated that alternatively other
configurations may be employed.
A proximal preload nut 96 has a central opening 98, which
in turn has an internal threaded surface 100. The threaded surface 100 engages the
external threaded end 66 of the outer shaft 20. The proximal preload nut 96 bears
against the inner race 86 of the proximal bearing 84. Similarly, the distal preload
nut 34 bears against the inner race 46 of the distal bearing 44. Thus the position
of the preload nuts 34 and 96 relative to (along) the outer shaft 20 positions the
shaft assembly 10 relative to the skin 16. By adjusting the position of the preload
nuts 34 and 96 along the outer shaft 20, the position of the fin attachment stub
18, and thus the corresponding fin, relative to the skin 16, may be controlled.
It will be appreciated that the adjustment of the position of the preload nuts 34
and 96 along the outer shaft 20 may be effected by rotation of the nuts and/or rotation
of the outer shaft. Therefore desired adjustment in fin placement relative to the
skin 16 may easily be made. Such adjustment may be desirable, for example, to vary
performance or to compensate for the inevitable non-zero manufacturing tolerances.
It will be appreciated that the easy adjustment of the fin position increases flexibility
in use of the missile, and may allow increased manufacturing tolerances, thereby
providing a means of facilitating fabrication and reducing manufacturing costs.
The assembly and operation of shaft assembly 10 is briefly
reviewed at this point with reference to FIGS. 1-4. Prior to insertion of the inner
shaft 22, the bearings 44 and 84 are secured relative to the outer shaft 20 by use
of the preload nuts 34 and 96. In doing so, the distal preload nut 34 is rotated
in the appropriate direction to threadedly engage the internal threaded surface
38 with the external threaded end 40 until proper engagement is made with the inner
race 46. Next the proximal preload nut 96 is rotated in the appropriate direction
to threadedly engage its internal threaded surface 100 with the external threaded
end 66 until proper engagement with the inner race 86. The proximal preload nut
96 is tightened with a suitable torque to provide a desired "back to back" bearing
Then, with one of the fins F1 detached from body 12 of
missile control actuation unit 6, the corresponding inner shaft 22 mounted on the
fin is aligned with the corresponding axis 14 and positioned thereabout so that
the axis of bores 72 and 74 of the outer shaft will be aligned with the bores 30
and 32 of the inner shaft 22 when the inner shaft 22 is received in the socket 58.
The inner shaft 22 is then inserted into the socket 58 until the bores 30 and 32
are aligned with the bores 70 and 72. The bolt 76 is then aligned with the corresponding
axes of the holes 72 and 74 of the outer shaft 22 and with holes 30 and 32 of the
inner shaft 22. The bolt 76 is then inserted through the holes until the threaded
portion 78 reaches the threads of the threaded hole 30. The bolt 76 is then rotated
in the appropriate direction to threadedly engage its portion 78 with the threaded
hole 30 and draw the bolt head 80 into engagement with the surface 64. It is apparent
that the holes 72, 74, 30, and 32 are engagable by the bolt portion 78 only when
outer shaft 20 and inner shaft 22 are positioned so as to align the holes. Further
rotation of the bolt 76 securely fixes the outer shaft 20 and the inner shaft 22
so that the fin attachment stub 18 and the fin F1 thereon are fixedly positioned
along and angularly about the axis 14 in relation to the outer shaft 20.
It is apparent that the distal preload nut 34 and the proximal
preload nut 96 are selectively engagable with the outer shaft 20 and rotatable to
move the fin F1 distally and proximally relative to the skin 16 of the body 12.
When it is desired to detach one of the fins F1-F4, the
corresponding bolt 76 is rotated in the opposite direction from the direction for
attaching the fin and removed from the holes 30, 32, 72, and 74. The fin attachment
stub 18 and the inner shaft 22 are thus released from the outer shaft 20 so that
the inner shaft 22 may be removed from the socket 58. When the inner shaft 22 is
removed from the socket 58, the bearings 44 and 84 may then be conveniently removed
from socket 26 by rotating the preload nuts 34 and 96 in the opposite direction
from the direction for attaching the nuts to the outer shaft 20.
Although the invention has been shown and described with
respect to a certain embodiment or embodiments, it is obvious that equivalent alterations
and modifications will occur to others skilled in the art upon the reading and understanding
of this specification and the annexed drawings.