Spiral-Wound Crossbow CAMs for High Speed With Lower Draw Force
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Solution Overview
Problem
Existing crossbows face challenges in achieving high projectile firing speeds while maintaining a compact form factor and ease of drawstring loading, often requiring significant force to draw and unload flexible limbs.
Innovation Solution
The crossbow incorporates pulleys and cams with a mechanical advantage system, utilizing power cables and drawstrings that reduce the draw weight needed to load flexible limbs, allowing faster projectile firing with a more compact design.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the force required to load the flexible limbs is increased to achieve faster projectile speeds, then the projectile firing speed is improved, but the draw weight required to draw the drawstring increases
Solution Approach 1:
A cam mechanism acts as an intermediary between the drawstring and the flexible limbs. The cam converts the drawing motion into a mechanical advantage system that loads the limbs more efficiently, reducing the draw weight required while maintaining high projectile speeds. The cam's asymmetric profile allows for optimized force distribution during the loading cycle.
Solution Approach 2:
The system changes the temporal distribution of force application through the cam mechanism. By varying the rate at which force is applied to the limbs during the draw cycle, the system achieves higher peak limb loading with lower average draw weight, thereby improving projectile speed without proportionally increasing drawing effort.
2Productivity
If the force required to load the flexible limbs is increased to achieve faster projectile speeds, then the unloading speed of the flexible limbs is improved, but the force required to draw the drawstring increases
Solution Approach 1:
The cam mechanism serves as a mediator that decouples the drawing force from the limb loading rate. It allows the limbs to be loaded rapidly during a portion of the draw cycle while the user applies reduced force, and then releases this stored energy quickly during unloading, achieving high productivity without excessive drawing force.
Solution Approach 2:
The cam mechanism creates periodic variations in the mechanical advantage during the draw cycle. There are phases of high force multiplication during limb loading and phases of lower resistance during the return stroke, allowing the system to achieve rapid limb cycling with manageable draw weights through rhythmic, periodic action.
3Volume of moving object
If a compact form factor is maintained, then the portability and handling are improved, but the mechanism complexity increases to achieve the mechanical advantage
Solution Approach 1:
The cam mechanism is integrated within the existing crossbow structure, with the cam profile incorporated into the trigger assembly or limb mounting area. The power cables are routed through guides that are part of the frame, nesting multiple functions within compact spatial arrangements that minimize overall crossbow size while maintaining the mechanical advantage system.
Solution Approach 2:
The cam mechanism performs multiple functions: it provides mechanical advantage during drawing, controls the timing of limb loading, guides power cable motion, and assists in the return stroke. By consolidating these functions into a single integrated component rather than separate mechanisms, the system achieves complex functionality without proportionally increasing device complexity or size.
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 mechanical advantage system enables faster projectile speeds with reduced draw force, improving efficiency, balance, safety, and accuracy, while allowing for a more manageable drawstring operation.
Implementation Method 1
The cam is configured to rotate about a cam axis that extends within a second plane. The second plane is substantially parallel to the projectile plane. The cam includes a first portion configured to engage the drawstring and a second portion configured to engage the power cable.
Data Source
AI summary
A crossbow includes a frame, first and second limbs, first and second cams, a drawstring, and first and second power cables. The frame includes a first side and a second side. The frame defines a projectile axis. The first limb is coupled to the first side and the second limb coupled to the second side. The first cam is coupled to the first side and configured to rotate about a first cam axis that is substantially parallel to the projectile axis. The second cam is coupled to the second side and configured to rotate about a second cam axis that is substantially parallel to the projectile axis. The drawstring is engaged with the first cam and the second cam. The first power cable is engaged with the first cam and the first limb. The second power cable engaged with the second cam and the second limb.