Crossbow Power Cable Helical Journal Design
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Solution Overview
Problem
Conventional crossbows have a limited power stroke due to the design of string guides and power cables, which restrict the rotation of cams to about 270 degrees, limiting the length of the power stroke and the kinetic energy that can be generated.
Innovation Solution
A crossbow design featuring flexible limbs attached to a center rail with a string carrier that moves from a released to a drawn configuration, utilizing power cables that wrap around helical journals to increase the power stroke without increasing the diameter of the string guides, allowing for continuous draw weight increase and extended rotation of the string guides beyond 360 degrees.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If power cables are used to load the bow limbs and synchronize cam rotation, then the mechanical advantage is increased, but the cam rotation is limited to about 270 degrees which reduces the power stroke length
Solution Approach 1:
The power cable system is segmented into multiple independent cables (typically three) that can wrap around the cam in different configurations. This segmentation allows each cable to follow a different path and wrap angle, enabling the cam to rotate beyond 270 degrees while maintaining mechanical advantage through the distributed cable arrangement.
Solution Approach 2:
The power cables are routed in three-dimensional space around the cam, utilizing vertical and radial dimensions rather than just planar rotation. The cables wrap around the cam circumference and can extend along the cam axis, creating a multi-dimensional cable arrangement that increases the effective wrap angle and power stroke length without increasing cam diameter.
2Length of moving object
If the diameter of pulleys is increased to increase the length of the power stroke, then the power stroke length is increased, but the bow becomes larger and less usable
Solution Approach 1:
The cam diameter is kept small and fixed, but the power stroke length is increased dynamically through multi-loop cable arrangements. The cables wrap around the small cam multiple times (e.g., 1.5 to 2.5 loops), creating a long power stroke path within the compact cam circumference. This dynamic cable routing allows long power stroke without increasing cam or bow size.
Solution Approach 2:
The power cables are nested in multiple loops around the cam, with inner loops wrapping tighter and outer loops providing additional rotation range. This nested cable arrangement packs multiple turns of the cable around the small cam diameter, effectively multiplying the power stroke length within the limited radial space of a compact cam.
3Length of moving object
If the draw string is positioned on the down-range side of the string guides, then the power stroke is increased, but the power cables limit the rotation of the cams
Solution Approach 1:
The cable wrap angle parameter is changed from the conventional single-loop configuration to multiple loops around the cam (e.g., 1.5 to 2.5 loops). This parameter change increases the cam rotation range from 270 degrees to potentially 540 degrees or more, allowing the draw string to travel further along the down-range side while the cables maintain their load-bearing function throughout the extended rotation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design enhances the power stroke by up to 40% and increases kinetic energy generation to over 70 ft.-lbs. with a power stroke of 8-15 inches, enabling crossbows to produce greater than 125 ft.-lbs. of energy with a power stroke of 10-15 inches, while maintaining a compact and accurate firing mechanism.
Implementation Method 1
utilizing power cables that wrap around helical journals to increase the power stroke without increasing the diameter of the string guides
Implementation Method 2
First and second flexible limbs are attached to a center rail. A draw string extends across the center rail that translates between a released configuration and a drawn configuration.
Data Source
AI summary
A crossbow including first and second flexible limbs are attached to a center rail. A draw string extends across the center rail that translates between a released configuration and a drawn configuration. A string carrier including a catch is movable between a closed position that engages the draw string and an open position that releases the draw string. The string carrier slides along the center rail to engage with the draw string in the released configuration and slides to a retracted position that locates the draw string in the drawn configuration. A retaining mechanism retains the string carrier in the retracted position and the draw string in the drawn configuration. A trigger moves the catch from the closed position to the open position to fire the crossbow when the string carrier is in the retracted position.