Crossbow Cam Rotation via Stationary Axis Pulleys

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Solution Overview

Problem

Conventional crossbows have a limited power stroke due to the design of pulleys and power cables, which restrict the rotation of cams to about 270 degrees, limiting the length of the power stroke and kinetic energy output.

Innovation Solution

The crossbow design features pulleys that rotate around stationary axes fixed relative to the center rail and riser, with power cables connecting the limbs to these pulleys, allowing the draw string to wrap onto power cable take-up journals, enabling increased rotation of the cams between 270 to 360 degrees or more, thereby extending the power stroke without increasing pulley size.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Duration of action 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

Engineering Contradiction:

Improvepower stroke lengthVSAvoidcam rotation range

Core Design Contradiction:

Duration of action of moving objectVSEase of operation

Solution Approach 1:

Instead of having the draw string unroll between the string guides as in conventional designs, this patent inverts the arrangement by having the power cables unroll between the string guides while the draw string remains positioned differently. This inversion allows the cams to rotate beyond 270 degrees while still increasing the power stroke, resolving the contradiction between power stroke length and cam rotation range.

Inventive Principle:

Principle #13The other way round (Inversion)

2Duration of action 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

Engineering Contradiction:

Improvepower stroke lengthVSAvoidbow size

Core Design Contradiction:

Duration of action of moving objectVSVolume of moving object

Solution Approach 1:

Instead of increasing pulley diameter in the radial dimension to extend power stroke, this patent changes the approach by utilizing the angular dimension through extended cam rotation (beyond 270 degrees). The power stroke is extended by increasing the rotation angle rather than the pulley size, thereby avoiding an increase in overall bow volume while still achieving a longer power stroke.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

3Device complexity

If the power cables are wrapped around the power cable take-up journals, then the rotation of the cams is limited to about 270 degrees, but this design simplifies the power cable routing

Engineering Contradiction:

Improvepower cable routingVSAvoidpower stroke length

Core Design Contradiction:

Device complexityVSDuration of action of moving object

Solution Approach 1:

This patent applies dynamics by making the power cable routing adaptable during the draw cycle. The power cables are routed to unroll between the string guides during the power stroke, allowing the cam rotation to exceed 270 degrees. This dynamic routing approach maintains simplicity while enabling extended power stroke through increased rotation range.

Inventive Principle:

Principle #15Dynamics

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 output to over 70 ft.-lbs. with a power stroke of 8 to 15 inches, allowing for more efficient energy storage and arrow propulsion.

Implementation Method 1

the power cables wrap onto the respective power cable take-up journals

Methodology Applied

Scientific EffectFriction: Friction

Implementation Method 2

pulleys that rotate around stationary axes fixed relative to the center rail and the riser

Methodology Applied

Scientific EffectMechanical Advantage: Mechanical Advantage

Implementation Method 3

First and second flexible limbs are attached to the riser

Methodology Applied

Scientific EffectElasticity: Elasticity

Data Source

PatentUS10209026B2Crossbow with pulleys that rotate around stationary axes

Publication Date: 2019.02.19 RAVIN CROSSBOWS LLC

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.