Reverse-Draw Crossbow Layout for Longer Power Stroke
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Solution Overview
Problem
Conventional bows and crossbows are limited by a power stroke of about 270 degrees due to the design of power cables, which restrict the rotation of cams, and increasing pulley size results in a larger and less usable bow.
Innovation Solution
A crossbow design with a reverse draw configuration using helical power cable take-up journals that allow the string guides to counter-rotate more than 360 degrees, extending the power stroke without increasing the size of the pulleys, and incorporating a string carrier system for enhanced accuracy and repeatability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the diameter of pulleys is increased to increase the power stroke, then the power stroke length is improved, but the bow size becomes larger and less usable
Solution Approach 1:
The patent inverts the conventional cam rotation direction by using a reverse draw configuration where the cam rotates in the opposite direction, allowing the power cable to wrap around the cam in reverse. This inversion enables greater rotation angles (300-360 degrees) without increasing pulley diameter, thus achieving longer power stroke while maintaining compact bow size.
Solution Approach 2:
The patent changes the rotation angle parameter of the cam from the conventional 270 degrees to 300-360 degrees by modifying the power cable attachment point and wrap configuration. This parameter change allows extended power stroke length without increasing the physical dimensions of the bow components.
2Length of moving object
If the power cable wrap configuration is modified to increase cam rotation, then the power stroke is extended, but the device complexity increases
Solution Approach 1:
The reverse draw configuration serves multiple functions simultaneously: it extends the power stroke length, maintains compact bow size, and provides a systematic method for power cable routing. The same cam and power cable components achieve both the extended rotation angle and the power transfer function, reducing the need for additional specialized parts.
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 power stroke is increased by at least 25%, generating kinetic energy of greater than 70 ft.-lbs. with a power stroke of 8 to 15 inches, and the string carrier system ensures high accuracy and repeatability, improving the performance and usability of the crossbow.
Implementation Method 1
the power cables 20 are wrapped around respective power cable take-up journal of the string guides 22, which in turn bends the limbs toward each other to store the energy needed for the bow to fire the arrow
Data Source
AI summary
A crossbow including a center rail including a bottom portion, a proximal end, and a distal end positioned opposite the proximal end, the center rail at least partially defining a crossbow length and a midpoint plane intersecting a horizontal midpoint of the crossbow, a riser extending horizontally from the distal end, one or more limbs coupled to the riser, and a trigger extending from the bottom portion of the center rail and positioned within ten percent of the crossbow length from the midpoint plane.