Crossbow Cam Layout for Longer Power Stroke Without Larger Pulleys
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Solution Overview
Problem
Conventional bows and crossbows have limited power strokes due to the design of power cables and pulleys, which restrict the rotation of cams to about 270 degrees, limiting the length of the power stroke and requiring larger pulleys that reduce usability.
Innovation Solution
A crossbow design with pulleys rotatably attached to the center rail or riser, where power cables connect to the pulleys, allowing the draw string to translate between configurations while the power cables wrap onto power cable take-up journals, enabling cams to rotate beyond 270 degrees without increasing pulley size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the draw string is positioned on the down-range side of the string guides so that the draw string unrolls between the string guides toward the user as the bow is drawn, then the power stroke is increased, but the power cables limit the rotation of the cams to about 270 degrees
Solution Approach 1:
The patent repositions the power cable attachment points from the limbs to the string guides, and routes the power cables to wrap around pulleys mounted on the center rail. This spatial reconfiguration allows the power cables to follow a different path that does not interfere with cam rotation, enabling the cam to rotate beyond 270 degrees while maintaining the extended power stroke achieved by the down-range string positioning.
2Length of moving object
If the diameter of the pulleys is increased to increase the length of the power stroke, then the power stroke is extended, but the bow becomes larger and less usable
Solution Approach 1:
The patent employs a dynamic power cable routing system where the power cables wrap around pulleys mounted on the center rail rather than using fixed attachment points. This dynamic configuration allows the power stroke length to be extended through increased pulley rotation angle (beyond 270 degrees) rather than increased pulley diameter, thereby achieving a longer power stroke without increasing the overall bow size and maintaining usability.
3Power
If power cables are used to synchronize rotation of the cams, then the mechanical advantage is increased, but the power cables limit the rotation of the cams to about 270 degrees
Solution Approach 1:
The patent introduces pulleys mounted on the center rail as intermediary elements between the power cables and the cams. The power cables wrap around these pulleys, which act as mediators to transmit the synchronizing force to the cams without the cables directly contacting the cams. This intermediary mechanism allows the cams to rotate beyond 270 degrees while the power cables maintain their synchronizing function and mechanical advantage.
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
The design extends the power stroke by 25% to 40% without increasing pulley size, enabling crossbows to generate kinetic energy of over 70 ft.-lbs. with a power stroke of 8 to 15 inches, and achieves high accuracy and repeatability through a string carrier system.
Implementation Method 1
Power cables connect the limbs to the pulleys such that as the crossbow is drawn from the released configuration to the drawn configuration the power cables wrap onto the respective power cable take-up journals
Data Source
AI summary
A crossbow including a frame with a riser and a center rail. First and second flexible limbs are attached to the riser. A draw string is received in string guide journals in first and second cams rotatably attached to the frame. The draw string unwinds from the string guide journals as it translates between a released configuration and a drawn configuration. The first and second cams include at least first and second power cable take-up journals, respectively. At least first and second power cables are attached to the first and second limbs and received in the first and second power cable take-up journals, respectively. As the crossbow is drawn from the released configuration to the drawn configuration the first and second power cables wrap onto the respective first and second power cable take-up journals.