U.S. patent application number 16/258565 was filed with the patent office on 2019-06-13 for fixed axle compound crossbow and method for operating a crossbow.
The applicant listed for this patent is AVAILABLE TECHNOLOGIES COMPANY INC.. Invention is credited to MARK LAURENCE.
Application Number | 20190178603 16/258565 |
Document ID | / |
Family ID | 66734666 |
Filed Date | 2019-06-13 |
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United States Patent
Application |
20190178603 |
Kind Code |
A1 |
LAURENCE; MARK |
June 13, 2019 |
FIXED AXLE COMPOUND CROSSBOW AND METHOD FOR OPERATING A
CROSSBOW
Abstract
A crossbow includes two rotatable cam assemblies mounted on a
rigid cam support structure, the rotation axes of which are fixed
relative to and arranged at a forward end of a stock. Limbs are
coupled to provide limb tips rearward from the cam support
structure. The limbs couple to the cam assemblies via power cables
arranged generally parallel to the stock. The cam assemblies
provide travel distance multiplication to a bowstring relative to
the travel distance of the limb tips.
Inventors: |
LAURENCE; MARK; (BOTHELL,
WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AVAILABLE TECHNOLOGIES COMPANY INC. |
BOTHELL |
WA |
US |
|
|
Family ID: |
66734666 |
Appl. No.: |
16/258565 |
Filed: |
January 26, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15816904 |
Nov 17, 2017 |
10190841 |
|
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16258565 |
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62423922 |
Nov 18, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F41B 5/123 20130101 |
International
Class: |
F41B 5/12 20060101
F41B005/12 |
Claims
1. A compound crossbow, comprising: a stock assembly having a front
end, back end, left, right, top, and bottom sides; left and right
flexible limbs extending laterally respectively from the left and
right sides of the stock assembly to respective limb tips; a rigid
cam support structure disposed at or near the front end of the
stock assembly; left and right cam assemblies rotationally coupled
to the rigid cam support structure on axles such that rotational
axes of the left and right cam assemblies are fixed relative to the
stock assembly; left and right power cables respectively directly
coupled between the left limb and a cam surface of the left cam
assembly and between the right limb and a cam surface of the right
cam assembly; and a bowstring operatively coupled to the left and
right cam assemblies, wherein a tension force applied to the
bowstring causes transmission of corresponding strain to the left
and right limbs by way of the left and right cam assemblies.
2. The compound crossbow of claim 1, wherein the left and right
power cables do not pass over any intermediate guide between the
respective limb and cam assembly.
3. The compound crossbow of claim 1, further comprising: a
synchronizing cable operatively coupled to the left and right cam
assemblies to cause the left and right cam assemblies to counter
rotate synchronously.
4. The compound crossbow of claim 3, wherein the synchronizing
cable comprises two synchronizing cables.
5. The compound crossbow of claim 3, wherein the synchronizing
cable comprises a cogged belt, and wherein a synchronizing cable
path in each of the cam assemblies comprises a complementary and
meshing cog pattern with the cogged belt.
6. The compound crossbow of claim 1, wherein the left and right
flexible limbs are each characterized by a spring constant having a
value within 5% of its complement flexible limb.
7. The compound crossbow of claim 6, wherein the left and right
flexible limbs are each characterized by a spring constant having a
value within 1% of its complement flexible limb.
8. The compound crossbow of claim 7, wherein the left and right
flexible limbs are each characterized by a spring constant having a
value within 0.3% of its complement flexible limb.
9. The compound crossbow of claim 1, wherein the left and right
flexible limbs extend laterally at an angle between 15 degrees and
165 degrees from the stock assembly respectively to the left limb
tip and a right limb tip.
10. The compound crossbow of claim 9, wherein the left and right
flexible limbs extend laterally at an angle between 30 degrees and
150 degrees from the stock assembly respectively to the left limb
tip and a right limb tip.
11. The compound crossbow of claim 10, wherein the left and right
flexible limbs extend laterally at an angle between 70 degrees and
110 degrees from the stock assembly respectively to the left limb
tip and a right limb tip.
12. The compound crossbow of claim 1, wherein the cam assemblies,
the synchronizing cable, and the power cables are configured to
cooperate to apply a respective selected tension from the limbs to
the bowstring at all respective points along a bowstring path of
travel.
13. The compound crossbow of claim 1, wherein the left and right
limbs extend laterally from locations rearward from the front end
of the stock.
14. The compound crossbow of claim 1, further comprising a force
transfer bracket operatively coupled to the front of the stock and
operatively coupled to or continuous with the rigid cam support
structures.
15. The compound crossbow of claim 14, wherein the left and right
limbs are directly coupled to or continuous with the force transfer
bracket; and wherein the left and right limbs extend laterally from
the force transfer bracket at respective rearward angles away from
the stock.
16. The compound crossbow of claim 1, wherein the stock has a
length L between the front end and the back end; and wherein the
left and right limbs extend from locations at least 25% of L
rearward from the front end of the stock.
17. The compound crossbow of claim 16, wherein the left and right
limbs extend from locations about half way L/2 between the front
end and the back end of the stock.
18. The compound crossbow of claim 17, wherein the left and right
limbs extend from locations greater than 50% of L rearward from the
front end of the stock.
19. The compound crossbow of claim 1, wherein the power cables pull
the limb tips in a forward direction relative to the stock when the
bowstring is pulled rearward.
20. The compound crossbow of claim 1, wherein the top surface of
the stock defines an arrow groove configured to hold an arrow
before release of the bowstring.
21. The compound crossbow of claim 1, wherein the left and right
limbs join to the stock sufficiently far below the top surface of
the stock to prevent vanes of the arrow from contacting the limbs
during propulsion of the arrow.
22. The compound crossbow of claim 1, wherein the rigid cam support
structure is positioned below the top surface of the stock
sufficiently to prevent vanes of the arrow from contacting the
rigid cam support structure during propulsion of the arrow.
23. The compound crossbow of claim 1, wherein the rigid cam support
structure supports the axles of the left and right cam assemblies
forward of the front end of the stock.
24. The compound crossbow of claim 1, wherein the rigid cam support
structure supports the rotational axes of the left and right cam
assemblies a locations perpendicular to the front end of the
stock.
25. The compound crossbow of claim 1, wherein the rigid cam support
structure is formed integrally with the stock.
26. A method for operation of a crossbow, comprising the steps of:
supporting, from a stock assembly having front, back, left, right,
top, and bottom sides, left and right flexible limbs to extend
laterally respectively from the left and right sides of the stock
assembly to respective limb tips; supporting, from the stock
assembly, a rigid cam support structure at or near the front of the
stock assembly; supporting, from the rigid cam support structure,
left and right cam assemblies for rotational motion around
respective locationally fixed axles; coupling the left limb tip to
the left cam assembly with a left power cable; coupling the right
limb tip to the right cam assembly with a right power cable;
receiving tensile forces from a bowstring through the left and
right cam assemblies and through the power cables to cause a
reactive spring force in the respective left and right limb tips;
storing the spring force in the limbs; receiving a user input to
release the bowstring; and driving the bowstring in tension,
through the cam assemblies and the power cables by releasing the
stored spring force in the limbs; whereby driving the bowstring in
tension causes the bowstring to propel an arrow or bolt in flight
forward from the front of the stock.
27. The method for operation of a crossbow of claim 26, wherein the
left and right power cables do not pass over any intermediate guide
between the respective limb and cam assembly.
28. The method for operation of a crossbow of claim 26, further
comprising: synchronizing rotation of the cam assemblies through at
least one synchronizing cable coupled between the left and right
cam assemblies.
29. The method for operation of a crossbow of claim 28, wherein
synchronizing rotation of the cam assemblies through at least one
synchronizing cable coupled between the left and right cam
assemblies comprises synchronizing the rotation through two
synchronizing cables.
30. The method for operation of a crossbow of claim 28, wherein
synchronizing rotation of the cam assemblies through at least one
synchronizing cable coupled between the left and right cam
assemblies comprises synchronizing the rotation through a cogged
belt to a cam surface having a complementary and meshing cog
pattern with the cogged belt.
31. The method for operation of a crossbow of claim 26, wherein
driving the bowstring in tension comprises applying force from the
limbs to the cam assemblies in pure tension along straight power
cables.
32. The method for operation of a crossbow of claim 26, wherein
driving the bowstring in tension comprises applying force from the
limbs wherein each limb is characterized by a spring constant
having a value within 5% of its complement flexible limb.
33. The method for operation of a crossbow of claim 26, wherein
driving the bowstring in tension comprises applying force from the
limbs wherein each limb is characterized by a spring constant
having a value within 1% of its complement flexible limb.
34. The method for operation of a crossbow of claim 26, wherein
driving the bowstring in tension comprises applying force from the
limbs wherein each limb is characterized by a spring constant
having a value within 0.3% of its complement flexible limb.
35. The method for operation of a crossbow of claim 34, wherein
supporting the left and right flexible limbs includes supporting
limbs that extend laterally at an angle between 30 degrees and 150
degrees from the stock assembly respectively to the left limb tip
and a right limb tip.
36. The method for operation of a crossbow of claim 35, wherein
supporting the left and right flexible limbs includes supporting
limbs that extend laterally at an angle between 70 degrees and 110
degrees from the stock assembly respectively to the left limb tip
and a right limb tip.
37. The method for operation of a crossbow of claim 26, wherein
driving the bowstring in tension, through the cam assemblies and
the power cables by releasing the stored spring force in the limbs
comprises providing motion in the cam assemblies and the power
cables to apply a respective selected tension to the bowstring at
all respective points along a bowstring path of travel to cause
acceleration of the arrow or bolt according to a smooth third
derivative of position.
38. The method for operation of a crossbow of claim 26, wherein
driving the bowstring in tension, through the cam assemblies and
the power cables by releasing the stored spring force in the limbs
comprises providing motion in the cam assemblies and the power
cables to apply a respective selected tension to the bowstring at
all respective points along a bowstring path of travel to cause
acceleration of the arrow or bolt according to a linear third
derivative of position.
39. The method for operation of a crossbow of claim 26, wherein
supporting the left and right flexible limbs includes supporting
limbs that extend laterally from locations rearward from the front
end of the stock.
40. The method for operation of a crossbow of claim 26, wherein
supporting the left and right flexible limbs includes supporting
the left and right flexible limbs from the rigid cam support
structures or from a force transfer bracket operatively coupled to
the front of the stock and operatively coupled to or continuous
with the rigid cam support structures.
41. The method for operation of a crossbow of claim 26, wherein
supporting the left and right flexible limbs includes supporting
the left and right limbs extending from locations at least 25% of L
rearward from the front end of the stock.
42. The method for operation of a crossbow of claim 26, wherein
supporting the left and right flexible limbs includes supporting
the left and right limbs extending from locations about half way
L/2 between the front end and the back end of the stock.
43. The method for operation of a crossbow of claim 26, wherein
supporting the left and right flexible limbs includes supporting
the left and right limbs extending from locations greater than 50%
of L rearward from the front end of the stock.
44. The method for operation of a crossbow of claim 26, wherein
receiving tensile forces from a bowstring through the left and
right cam assemblies and through the power cables to cause a
reactive spring force in the respective left and right limb tips
causes the power cables pull the limb tips in a forward direction
relative to the stock when the bowstring is pulled rearward.
45. The method for operation of a crossbow of claim 26, further
comprising: defining an arrow groove in the top surface of the
stock to hold an arrow before release of the bowstring.
46. The method for operation of a crossbow of claim 26, wherein
supporting, from the stock assembly, a rigid cam support structure
at or near the front of the stock assembly includes providing the
rigid cam support structure formed integrally with the stock.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a Continuation-in-Part of
co-pending U.S. patent application Ser. No. 15/816,904, entitled
"FIXED AXLE COMPOUND CROSSBOW," filed Nov. 17, 2017 (docket number
3033-002-03). U.S. patent application No. 15/816,904 claims
priority benefit from U.S. Provisional Patent Application No.
62/423,922, entitled "FIXED-AXIS AXLE CROSSBOWS," filed Nov. 18,
2016 (docket number 3033-002-02), now expired. Each of the
foregoing applications, to the extent not inconsistent with the
disclosure herein, is incorporated by reference.
SUMMARY
[0002] According to an embodiment, a compound crossbow includes a
stock assembly having a front end, back end, left, right, top, and
bottom sides; left and right flexible limbs extending laterally
respectively from the left and right sides of the stock assembly to
respective limb tips; and a rigid cam support structure disposed at
or near the front end of the stock assembly. Left and right cam
assemblies are rotationally coupled to the rigid cam support
structure on axles such that rotational axes of the left and right
cam assemblies are fixed relative to the stock assembly. Left and
right power cables are respectively directly coupled between the
left limb and a cam surface of the left cam assembly and between
the right limb and a cam surface of the right cam assembly. A
bowstring operatively coupled to the left and right cam assemblies
and a tension force applied to the bowstring causes transmission of
corresponding strain to the left and right limbs by way of the left
and right cam assemblies.
[0003] A synchronizing cable may optionally be operatively coupled
to the left and right cam assemblies to cause the left and right
cam assemblies to counter rotate synchronously.
[0004] According to an embodiment, a method for operation of a
crossbow, includes the steps of supporting; from a stock assembly
having front, back, left, right, top, and bottom sides; left and
right flexible limbs to extend laterally respectively from the left
and right sides of the stock assembly to respective limb tips. The
method includes supporting, from the stock assembly, a rigid cam
support structure at or near the front of the stock assembly and
supporting, from the rigid cam support structure, left and right
cam assemblies for rotational motion around respective positionally
fixed axles. The method includes coupling the left limb tip to the
left cam assembly with a left power cable and coupling the right
limb tip to the right cam assembly with a right power cable. The
method includes receiving tensile forces from a bowstring through
the left and right cam assemblies and through the power cables to
cause a reactive spring force in the respective left and right limb
tips, storing the spring force in the limbs, and receiving a user
input to release the bowstring. Upon release of the bowstring, the
method includes driving the bowstring in tension, through the cam
assemblies and the power cables, by releasing the stored spring
force in the limbs. Driving the bowstring in tension can cause the
bowstring to propel an arrow or bolt in flight forward from the
front of the stock.
[0005] According to an embodiment, a crossbow includes a stock
assembly defining an arrow track configured to be in contact with
an arrow prior to release, the stock assembly having a front end
and a back end, the stock assembly being characterized by a length
L between the front end and back end. Left and right flexible limbs
extend laterally from the stock assembly respectively to a left
limb tip and a right limb tip. A left cam assembly and a
mirror-image right cam assembly are each rotatably mounted on a
respective end of a rigid cam support structure. Each one of the
left and right cam assemblies includes a respective bowstring cam,
a synchronizing wheel, and a power cable cam. The rigid cam support
structure is disposed adjacent and perpendicular to the front end
of the stock assembly and fixed rigidly thereto. The cam support
structure can be disposed to place a top surface in a first
horizontal plane . The first and second limbs may be disposed to
have respective limb centerlines lying along a common second
horizontal plane parallel to the first horizontal plane and
extending left and right from the stock. A left bearing and a right
bearing are disposed in the cam support structure and configured to
rotatably support the left and right cam assemblies on respective
fixed axes of rotation. A left limb cable is coupled between the
left limb tip and the left cam assembly. A right limb cable is
coupled between the right limb tip and the right cam assembly. A
synchronizing cable is disposed between a left synchronizing wheel
and the right synchronizing wheel, and is configured to cross
itself in a central medial portion for synchronizing motion of the
left and right cam assemblies in opposition to one another. A
bowstring extends between a left bowstring cam or wheel and a right
bowstring cam or wheel, the bowstring being anchored to both. The
bowstring, limb cables, and synchronizing cable are arranged so as
to exert force only on an outer circumferential portion or cam
surface of any cam or wheel. The synchronizing cable extends freely
in space between the left and right cam/wheel units. Force is
exerted between any cable or string and any other cable or string
only by way of axial torque exerted by the cam/wheel units.
[0006] According to an embodiment, for a crossbow including a
trigger actuating a bowstring hold-and-release mechanism to release
a bowstring after the crossbow has been cocked, and a stock, the
stock further including a projectile track, a method of stiffening
the stock of the crossbow includes deploying a strut substantially
parallel to the stock and at distance below the stock and fastening
the stock to the strut to the stock at selected places along the
length of the stock with fasteners.
[0007] According to embodiments, a compound crossbow achieves
rigidity and low mechanical play for precision arrow flight.
[0008] According to an embodiment, in a compound crossbow, all
cams, wheels, and other rotating parts are rigid and fixed, except
for their ability to rotate. None of the rotating parts are
translatable, nor do their axes of rotation change direction.
[0009] According to another embodiment, in a compound crossbow, the
rotating parts are rotatably coupled to a rigid cam support
structure, which in turn is rigidly coupled to the front end of the
stock. This creates a framework with minimal play or looseness. In
an embodiment, braces or trusses prevent the cam support structure
from rotating relative to the stock, especially in the horizontal
plane that is parallel to the bowstring and the stock.
[0010] The cam support structure can be made additionally rigid, in
relation to the stock, by connecting these two frame parts with
tensile stays, braces, trusses, etc.
[0011] According to an embodiment, a compound crossbow keeps arrow
fletches or vanes out of contact with other parts of the crossbow,
especially tensile members such as the bowstring and limb cables.
An arrow is fletched with vanes to keep it flying straight.
Typically, the vanes protrude farther from the arrow shaft in the
case of hunting arrows as compared to target arrows. If the vanes
hit any part of the crossbow, including a limb cable or a figure-8
synchronizing cable, the arrow or bolt will be deflected
somewhat.
[0012] In an embodiment, a strut may be disposed above or below the
stock on which the bolt slides. The strut may be, along with the
stock, part of the crossbow frame. The strut can have a similar
layout as the stock and its rigid cam support structure. The strut
can include a second bearing for each cam/wheel axle.
[0013] In an embodiment, a synchronizing cable extends between left
and right cam/wheel units.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1A is a plan view of a crossbow stock, according to an
embodiment.
[0015] FIG. 1B is a side view of the crossbow stock with cams,
according to an embodiment.
[0016] FIG. 2A is a plan view of a crossbow, according to an
embodiment.
[0017] FIG. 2B is a side view of the crossbow of FIG. 2A, according
to an embodiment.
[0018] FIG. 3 is a detailed elevational view of the crossbow of
FIGS. 2A and 2B, according to an embodiment.
DETAILED DESCRIPTION
[0019] In the following detailed description, reference is made to
the accompanying drawings, which form a part hereof. In the
drawings, similar symbols typically identify similar components,
unless context dictates otherwise. Other embodiments may be used
and/or other changes may be made without departing from the spirit
or scope of the disclosure.
[0020] As used herein, the terms bolt and arrow are used
interchangeably and are considered synonymous. As used herein the
terms limb cable and power cable are used interchangeably and are
considered synonymous, referring to a cable that couples a limb or
other spring device to a cam assembly.
[0021] A compound crossbow includes fixed axles and bearings
configured to support cams. The fixed axles are coupled by a
synchronization mechanism configured to equalize motion between the
cams. According to embodiments, the bearings, axles, and cams are
disposed at or near a front end of a crossbow stock to maximize
length of travel of the crossbow string while in contact with the
knock of the arrow.
[0022] An object of embodiments is perfect horizontal and vertical
nock travel, which requires a rigid crossbow structure. Rigidity,
in the stock especially, is important for the following reason: at
rest, during the draw stroke and at full draw, tensions in the
bowstring and other tensile members are exerted between points that
are offset from the centerline of the stock, which tends to bend
the stock; when the bolt is shot, these bending forces dissipate,
changing the bending moment on the stock during the launch of the
bolt, and causing it to change shape as the bowstring and other
tensile members relax. Thus, the stock will change shape during
discharge of the arrow, the direction of the bolt will change, and
the aim of the shot will be thrown off. The shot can also be thrown
off by looseness between parts.
[0023] FIG. 1A is a simplified depiction of a crossbow assembly
100, according to an embodiment. The crossbow assembly includes a
stock 110, a cam support 140, and a truss 130, all of which are
fastened together or made as a unit so as to achieve a high
rigidity. According to embodiments, a separate truss 130 may be
omitted if the cam support 140 is sufficiently stiff. The
illustrated left and right trusses 130 triangulate the frame and
increase the resistance to relative rotations of the stock 110 and
the cam support 140. The trusses 130 are optional and may have
different forms, such as triangulated beams, tensile members,
etc.
[0024] The cam support 140 may define bearing holes 141 and 142 on
the left and right sides respectively, which accept optional
bearings (see 180, 182 in FIG. 3) and axles and support the axles
against translational and inclination movements, as discussed
below.
[0025] The stock 110 defines an arrow guide or projectile track
112, which may be of conventional design, and a space 117 that can
accept a bowstring hold-and-release mechanism (not shown in FIG.
1A), which may also be of conventional design.
[0026] FIG. 1B is a side view of the embodiment of FIG. 1A, with
additional elements shown: a strut 120, a limb 150, and a cam
assembly 160. The strut 120 may parallel the stock 110 over at
least a part of the length L of the stock 110, and if fastened to
the stock 110 in selected locations, will greatly stiffen the stock
110 against bending. One place where the stock 110 and strut 120
may be fastened together is around the limb 150, to which both may
be fastened (limb 150 is depicted in cross section in FIG. 1B). The
strut 120 and stock 110 may be fastened at other places to stiffen
the frame, and in general the more places they are fastened the
stiffer the structure will be. According to an embodiment, the
entire space between the stock 110 and the strut 120 may be filled
with a layer of crush-resistant material (not shown) adhered to the
stock 110 and strut 120 to create a "stressed-skin" structure.
[0027] On the right side of FIG. 1B is shown a cam assembly 160,
which is illustrated in detail in FIG. 3.
[0028] FIGS. 2A and 2B are plan and elevation views of a crossbow
including the frame of FIG. 1A, and showing the limbs 150. The tips
of the illustrated limbs 150, which are resilient and bendable, are
attached to respective left and right power cables 191 and 192, for
example by direct attachment or by way of a rotatable pin, eye,
etc. The power cables do not exert any substantial rotational
moment on the ends of the limbs 150, and convey only tension force.
The distal end of the left power cable 191 is wrapped around the
outer circumference of a left power cable cam 163 (obscured in FIG.
2A), and the right power cable 192 is wrapped around outer
circumference of a right power cable cam 164 (obscured FIG. 2A).
FIG. 3 provides a better elevation view of the right cam assembly
160.
[0029] In FIGS. 2A and 2B, the space 117 houses a bowstring
hold-and-release mechanism 118 which is actuated by a trigger 116.
Pulling the trigger 116 releases a bowstring 196 and an arrow (not
shown). The bowstring 196 is wrapped around bowstring wheels 165,
166, which are the uppermost cam or wheel on the cam units 160;
although shown as wheels (circular, round, or arc-segment cams),
they can comprise cams of any shape needed to adjust the force
function.
[0030] A synchronizing cable 194, seen in FIG. 2A, is wrapped
around in figure-8 configuration around the circumference of a
synchronizing wheel or wheels 162, as seen in FIG. 3 (left-hand
synchronizing wheel(s) 161 is behind wheel 162 in FIG. 3). This
arrangement reduces any unevenness in the forces or motions of the
two sides of the crossbow.
[0031] FIG. 3 also illustrates an upper bearing 180 and a lower
bearing 182, mounted respectively in the stock 110 and the strut
120, which hold the axle 169 of the cam unit 160 on either side.
The strut 120 and the lower bearing 182 are optional. According to
many embodiments, the cam units 160 are supported on a single
bearing 180. The bearing(s) 180, 182 can be of various types,
including simple holes in the frame, but should be precise enough
to prevent the axis of the cam unit 160 from changing direction
relative to the frame. According to embodiments, the bearings 180,
182 are bronze, sintered, and/or polymer, such as
polytetrafluoroethylene. The strut bearing 182 can further
immobilize the rotation axis of each cam unit axle, as compared to
the use of just an upper bearing, which will more firmly fix the
axes of the cam units 160.
[0032] In the illustrated embodiments, the strut 120 is on the
lower side of the stock 110, leaving the upper side of the stock
110 open for emplacing an arrow (not shown) in the arrow guide 112.
Alternatively, the strut 120 can be placed above the stock 110,
with a slot or other arrangement to permit an arrow to be placed
into arrow guide 112 from above. Furthermore, the limbs can be
doubled (with one strut above the stock and another below) if
double cams are used to transmit the limb forces; this would result
in a more symmetrical frame and could lead to reduced bending.
[0033] Whether one strut is used, or two or more, an external strut
will increase the stiffness of the frame and resist any bending of
the stock which would throw off the aim. Considering a cross
section of the frame taken perpendicular to the arrow, the stock
itself will have a certain moment of inertia, I (a measure of
resistance to bending, or stiffness), which is proportional to the
square of the thickness of the stock. If a strut is added the stock
effectively becomes thicker, the moment of inertia is greatly
increased because of the factor of the square of the thickness;
this is true even if the strut is fastened to the stock only at
certain points, such as near the ends. (If there were no connection
between the two, the stiffness of the stock would not be increased
at all.)
[0034] The structure on the left and right sides of a center plane
can be symmetrical and the motions synchronized, so that any forces
tending to bend the stock left or right are incidental. In
contrast, the forces exerted on the stock that tend to bend it in a
vertical plane are generally not as symmetrical. The vertical plane
is substantially perpendicular to the horizontal plane (or
planes).
[0035] In an embodiment, the limb or limbs 150 are fastened between
the stock 110 and a strut 120, and can act as one stiffener
connecting the stock to the lower strut. As noted above, such
connections make the stock stiffer in the vertical direction.
[0036] If the limb or limbs 150 are centered between the stock and
a strut, and so are the limb cables 191, 192, and the limb cables
are symmetrical in the vertical direction (illustrated in FIG. 3),
then the force transmitted from the limbs by the limb cables will
be exerted along a line halfway between the stock and the strut,
assuming that the cams to which they attach are also centered. If
so, then the limb cables will not exert any bending force on the
combination of the stock and strut. The height of the cables 191,
192 can alternatively be adjusted downward to compensate for the
tension in the bowstring 196, and by proper design known to those
skilled in the mechanical arts, can result in a negligible bending
force on the strut 120.
[0037] Referring to FIGS. 1A, 1B, 2A, 2B, and 3, according to an
embodiment, a compound crossbow 100 includes a stock assembly 110
having a front end, back end, left, right, top, and bottom sides.
Left and right flexible limbs 150 extend laterally respectively
from the left and right sides of the stock assembly 110 to
respective limb tips. A rigid cam support structure 140 is disposed
at or near the front end of the stock assembly 110. Left and right
cam assemblies 160 are rotationally coupled to the rigid cam
support structure 140 such that rotational axes of the left and
right cam assemblies 160 are fixed relative to the stock assembly
110. Left and right power cables 191, 192 are respectively directly
coupled between the left limb 150 and a cam surface of the left cam
assembly 160 and between the right limb 150 and a cam surface of
the right cam assembly 160. A bowstring 196 is operatively coupled
to the left and right cam assemblies 160, wherein a tension force
applied to the bowstring 196 causes transmission of corresponding
strain to the left and right limbs 150 by way of the left and right
cam assemblies 160.
[0038] According to an embodiment, the left and right power cables
191, 192 do not pass over any intermediate guide between the
respective limb 150 and cam assembly 160.
[0039] One or more synchronizing devices, referred to herein as a
synchronizing cable, may optionally be provided to help to maintain
equal rotation and equal tensile stress across the two cam
assemblies 160, through the power cable 191, 192, and through each
respective limb 150 to the stock 110. Among other utilities, the
synchronizing cable may be used to accommodate tolerance variations
between left and right sides of the crossbow 100 (e.g., to
accommodate tolerances between the left and right limbs).
[0040] According to an embodiment, the compound crossbow 100
further includes a synchronizing cable 194 operatively coupled to
the left and right cam assemblies 160 to cause the left and right
cam assemblies 160 to counter rotate synchronously. In one
embodiment, the synchronizing cable 194 includes two synchronizing
cables. In another embodiment, the synchronizing cable 194 includes
a belt and/or a cogged belt. A synchronizing cable 194 path in each
of the cam assemblies 160 may include a complementary and meshing
cog pattern with the cogged belt.
[0041] In an embodiment, the crossbow 100 may include only a single
flexible limb 150 configured to provide force to both cam
assemblies 160. For example, two power cables 191, 192 may
respectively operatively couple the single limb 150 to both cam
assemblies 160. Alternatively, one or more limbs 150 may provide
power, via power cables 191, 192, to a single powered cam assembly
160. A synchronizing cable or cables may then transmit power and
rotational motion between the powered cam assembly and a driven cam
assembly. The use of a cogged synchronizing cable 194, in addition
to conventional uses, may help to maintain predictable power
transmission from the powered cam assembly 160 to the driven and
opposing cam assembly 160. In an embodiment, a cam in at least one
of the power cam assembly 160 or driven cam assembly 160 may be
characterized by a diameter selected to compensate for mechanical
strain between the powered cam assembly 160, across the stock 110,
and through the driven cam assembly 160 in the crossbow 100.
[0042] According to an embodiment, the left and right flexible
limbs 150 are each characterized by a spring constant having a
value within 5% of its complement flexible limb 150. In one
embodiment, the left and right flexible limbs 150 are each
characterized by a spring constant having a value within 1% of its
complement flexible limb 150. In another embodiment, the left and
right flexible limbs 150 are each characterized by a spring
constant having a value within 0.3% of its complement flexible limb
150.
[0043] According to an embodiment, the left and right flexible
limbs 150 extend laterally at an angle between 15 degrees and 165
degrees from the stock assembly 110 respectively to the left limb
tip and a right limb tip. In one embodiment, the left and right
flexible limbs 150 extend laterally at an angle between 30 degrees
and 150 degrees from the stock assembly 110 respectively to the
left limb tip and a right limb tip. In another embodiment, the left
and right flexible limbs 150 extend laterally at an angle between
70 degrees and 110 degrees from the stock assembly 110 respectively
to the left limb tip and a right limb tip. The stock may optionally
include respective limb support structures (not shown) disposed to
support the limbs 150 away from the stock 110.
[0044] The cam assemblies 160, the synchronizing cable 194, and the
power cables 191, 192 are configured to cooperate to apply a
respective selected tension from the limbs 150 to the bowstring 196
at all respective points along a bowstring 196 path of travel.
[0045] The left and right limbs 150 may extend laterally from
locations rearward from the front end of the stock 110.
[0046] According to an embodiment, the compound crossbow 100
further includes a force transfer bracket operatively coupled to
the front of the stock 110 and operatively coupled to or continuous
with the rigid cam support structures 140. In an embodiment, the
left and right limbs 150 are directly coupled to or continuous with
the force transfer bracket, and the left and right limbs 150 extend
laterally from the force transfer bracket at respective rearward
angles away from the stock 110.
[0047] According to an embodiment, the stock 110 has a length L
between the front end and the back end, and the left and right
limbs 150 extend from locations at least 25% of L rearward from the
front end of the stock 110. For example, the left and right limbs
150 may extend from locations about half way (L/2) between the
front end and the back end of the stock 110. As depicted in FIGS.
1B, 2A and 2B, the left and right limbs 150 can extend from
locations greater than 50% of L rearward from the front end of the
stock 110.
[0048] When the bowstring 196 is pulled rearward to cock the
crossbow 100, the power cables 191, 192 pull the limbs 150 toward
the front end of the stock 110 into respective stored energy
positions. The power cables 191, 192 may each remain within a 30
degree angle of parallel to the long axis of the stock 110
throughout the range of the bowstring pull. According to an
embodiment, the power cables 191, 192 each remain within a 15
degree angle of parallel to the long axis of the stock 110
throughout the range of the bowstring pull. According to an
embodiment, the power cables 191, 192 each remain within a 5 degree
angle of parallel to the long axis of the stock 110 throughout the
range of the bowstring pull. According to embodiments, each of the
power cables 191, 192 remains at an equal and opposite angle to the
stock 110 compared to the other power cable, to a precision of
within 1 degree of angle.
[0049] The synchronizing cable 194 can be disposed on wheels below
the bottom of the stock 110, the synchronizing cable wheels being
coupled to axles 169 of the left and right cam assemblies 160 to
rotate synchronously with other portions of the cam assemblies. The
top surface of the stock 110 may define an arrow groove 112
configured to hold an arrow before release of the bowstring 196.
The arrow groove 112 is formed coincident with the longitudinal
axis of the stock 110 on or adjacent to the top surface of the
stock. In an embodiment, the left and right limbs 150 join to the
stock 110 sufficiently far below the top surface of the stock 110
and the arrow groove 112 to prevent vanes of the arrow (not shown)
from contacting the limbs 150 during propulsion of the arrow (i.e.,
release of the bowstring 196). Similarly, the rigid cam support
structure 140 may be positioned below the top surface of the stock
110 sufficiently to prevent vanes of the arrow from contacting the
rigid cam support structure 140 during propulsion of the arrow.
[0050] In one embodiment, the rigid cam support structure 140
supports axles 169 of the left and right cam assemblies 160 forward
of the front end of the stock 110. In another embodiment, the rigid
cam support structure 140 supports bearings that rotatably support
cam axles for rotation about respective rotational axes in the left
and right cam assemblies 160. In another embodiment, the rigid cam
support structure 140 supports the axles of the left and right cam
assemblies 160 at locations perpendicular to and within the length
of the stock assembly 110, such as one to two inches rearward of
the front end of the stock 110.
[0051] In an embodiment, the rigid cam support structure 140 is
formed integrally with the stock 110.
[0052] Referring to the preceding FIGS. 1A, 1B, 2A, 2B, and 3, a
method for operation of a crossbow 100 may include the steps of
supporting; from a stock assembly 110 having front, back, left,
right, top, and bottom sides; left and right flexible limbs 150 to
extend laterally from the left and right sides of the stock
assembly 110 to respective limb tips. The stock assembly 110
supports a rigid cam support structure 140 at or near the front of
the stock assembly 110. The rigid cam support structure 140
supports left and right cam assemblies 160 for rotational motion
around respective positionally fixed axles 169. The left limb tip
is coupled to the left cam assembly 160 with a left power cable
191. The right limb tip is coupled to the right cam assembly 160
with a right power cable 192.
[0053] In use, tensile forces are received from a bowstring 196
(when cocking the crossbow) through the left and right cam
assemblies 160 and through the power cables 191, 192 to cause a
reactive spring force in the respective left and right limb tips.
Spring force is stored in the limbs 150 while the crossbow is
cocked. When the crossbow is fired, user input is received to
release the bowstring 196. The bowstring 196 is driven in tension,
through the cam assemblies 160 and the power cables 191, 192 by
releasing the stored spring force in the limbs 150. Driving the
bowstring 196 in tension causes the bowstring 196 to propel an
arrow or bolt in flight forward from the front of the stock
110.
[0054] According to an embodiment, the left and right power cables
191, 192 do not pass over any intermediate guide between the
respective limb 150 and the cam assembly 160.
[0055] According to an embodiment, the method for operation of a
crossbow 100 may further include the step of synchronizing rotation
of the cam assemblies 160 through at least one synchronizing cable
194 coupled between the left and right cam assemblies 160. In one
embodiment, the step of synchronizing rotation of the cam
assemblies 160 through at least one synchronizing cable 194 coupled
between the left and right cam assemblies 160 includes
synchronizing the rotation through two synchronizing cables. In
another embodiment, the step of synchronizing rotation of the cam
assemblies 160 through at least one synchronizing cable 194 coupled
between the left and right cam assemblies 160 includes
synchronizing the rotation through a belt or with a cogged belt to
a cam surface having a complementary and meshing cog pattern with
the cogged belt.
[0056] According to an embodiment, the step of driving the
bowstring 196 in tension includes applying force from the limbs 150
to the cam assemblies 160 in pure tension along straight power
cables 191, 192. In one embodiment, the step of driving the
bowstring 196 in tension includes applying force from the limbs
150, wherein each limb 150 is characterized by a spring constant
having a value within 5% of its complement (opposing) flexible limb
150. In another embodiment, the step of driving the bowstring 196
in tension includes applying force from the limbs 150, wherein each
limb 150 is characterized by a spring constant having a value
within 1% of its complement flexible limb 150. Additionally or
alternatively, the step of driving the bowstring 196 in tension
includes applying force from the limbs 150, wherein each limb 150
is characterized by a spring constant having a value within 0.3% of
its complement flexible limb 150.
[0057] According to an embodiment, the step of supporting the left
and right flexible limbs 150 includes supporting limbs 150 that
extend laterally at an angle between 30 degrees and 150 degrees
from the stock assembly 110 respectively to the left limb tip and a
right limb tip. In one embodiment, the step of supporting the left
and right flexible limbs 150 includes supporting limbs 150 that
extend laterally at an angle between 70 degrees and 110 degrees
from the stock assembly 110 respectively to the left limb tip and a
right limb tip.
[0058] According to an embodiment, the step of driving the
bowstring 196 in tension, through the cam assemblies 160 and the
power cables 191, 192 by releasing the stored spring force in the
limbs 150 includes providing motion in the cam assemblies 160 and
the power cables 191, 192 to apply a respective selected tension to
the bowstring 196 at all respective points along a bowstring 196
path of travel to cause acceleration of the arrow or bolt according
to a smoothly changing third derivative of position. Additionally
or alternatively, the step of driving the bowstring 196 in tension,
through the cam assemblies 160 and the power cables 191, 192 by
releasing the stored spring force in the limbs 150 includes
providing motion in the cam assemblies 160 and the power cables
191, 192 to apply a respective selected tension to the bowstring
196 at all respective points along a bowstring 196 path of travel
to cause acceleration of the arrow or bolt according to a linear
third derivative of position.
[0059] According to an embodiment, the step of supporting the left
and right flexible limbs 150 includes supporting the left and right
flexible limbs 150 from the rigid cam support structures 140 or
from a force transfer bracket operatively coupled to the front of
the stock 110 and operatively coupled to or continuous with the
rigid cam support structures 140. In one embodiment, the step of
supporting the left and right flexible limbs 150 includes
supporting the left and right limbs 150 extending from locations at
least 25% of L rearward from the front end of the stock 110. In
another embodiment, the step of supporting the left and right
flexible limbs 150 includes supporting the left and right limbs 150
extending from locations about half way L/2 between the front end
and the back end of the stock 110. Additionally or alternatively,
the step of supporting the left and right flexible limbs 150
includes supporting the left and right limbs 150 extending from
locations greater than 50% of L rearward from the front end of the
stock 110.
[0060] According to an embodiment, the step of receiving tensile
forces from a bowstring 196 through the left and right cam
assemblies 160 and through the power cables 191, 192 to cause a
reactive spring force in the respective left and right limb tips
causes the power cables 191, 192 to pull the limb tips in a forward
direction relative to the stock 110 when the bowstring 196 is
pulled rearward.
[0061] According to an embodiment, the method for operation of a
crossbow 100 may further include the step of defining an arrow
groove 112 in the top surface of the stock 110 to hold an arrow
before release of the bowstring 196.
[0062] According to an embodiment, the step of supporting, from the
stock assembly 110, the rigid cam support structure 140 at or near
the front of the stock assembly 110, includes providing the rigid
cam support structure 140 formed integrally with the stock 110.
[0063] The resilient limb 150 on either side of the stock 110 may
be half of a single limb passing between the stock and strut, which
can double as a fastener (as shown), or may be an individual piece
mounted on either side, for example on a plate attached to the side
of the strut and stock.
[0064] The cam units 160, which can alternatively be referred to as
"cam/wheel units," "cam assemblies," "cam/wheel assemblies," and
the like, need not be integral in the sense that they are made from
a single piece of material, or are otherwise inseparable into
parts, or have no parts; they can be integrally constructed, for
example with different planar cams or wheels riveted, welded,
adhered, or otherwise fastened together, but such assemblies are
referred to in the claims as "cam/wheel units." Assemblies are
suitable for the invention if they are rigid enough to act as a
unit, or as one piece.
[0065] The phrase "cam/wheel" is redundant in the sense that a
"wheel" is merely a circular "cam," and thus "cam" is broader than
"wheel." Here, "wheel" means a cam having a substantially circular
shape and/or a constant radius through at least some angle around
the axis of the cam. The phrase "cam/wheel," though redundant, is
used for clarity.
[0066] Above, and in the following claims, "substantially" means a
factor of 0.9; 0.99; 0.999; and so on.
[0067] It will be understood that terms such as "lower" and "above"
are used for convenience and refer to the usual shooting position
for a crossbow. They do not limit the crossbow to any
orientation.
[0068] In the following claims, "limb" covers any arm-or leg-like
extension such as the traditional bow, but also covers other
devices for storing mechanical potential energy, including but not
limited to coil springs, compressible gases, elastomers, etc. that
lie at least partially outside the stock on the left or right. For
example, the traditional elastic bow can be modified to include an
elastic tensile member (coil spring, elastic cable, etc.) and a
more-rigid arm (limb).
[0069] As used herein, the term "synchronizing cable" can consist
of one continuous cable fixed to a left and right cam assembly, a
cogged cable, a cogged belt, or a chain, etc., or two cables with
ends fixed in their respective tracks.
[0070] While various aspects and embodiments have been disclosed
herein, other aspects and embodiments are contemplated. The various
aspects and embodiments disclosed herein are for purposes of
illustration and are not intended to be limiting, with the true
scope and spirit being indicated by the following claims.
* * * * *