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.

What is claimed is: 
     
         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.