During any winding or unwinding action, a tether may experience unexpected changes in tension. For example, the load being wound could become stuck and stop moving, causing the tension in the line to suddenly become extremely high. Alternatively, the load could slip, causing a lack of tension, or slack, in the line. Both of these scenarios can be harmful to neatly winding a spool in a tightly packed pattern. Excessive tension can cause one layer of line to become lodged in a layer below it and jammed. Too low of tension can cause the line on the spool to partially unravel and cause disorganized spooling. A novel mechanism utilizes the tension in the line and springs to hold a pulley between friction pads and a ratcheting mechanism to keep the tension at the spool within a particular window.

When the tension of the spool line is within the allowable window, both of the spring-loaded pulleys are in neutral positions and able to rotate freely.
When winding a spool, it is important to maintain tension in the line being wound to the spool. Slack in the line can come from two places: from the spool itself unwinding and creating slack immediately at the spool, or the load at the end of the line relieving the tension. The mechanism below handles both of these cases.

In the first case, where slack is the result of the spool unwinding without adequate tension at the end of the line, to maintain tension at the line, the spring-loaded pulley at the spool will relax and engage the ratcheting stop to prevent the spool from continuing to unwind. The spool will not be able to unwind again until tension in the line (from increased tension at either the spool or the end of the line) resumes enough to pull the ratchet lock out of the spool to allow it to continue to unwind. If the spool is trying to wind line onto the spool, the ratcheting mechanism will not impede this motion.

Alternatively, if slack forms at the end of the line, the friction pad on the idler pulley will maintain some tension (adjustable based on the spring preload) within the system. When the tension at the end of the line is too high, the pulley closest to the end of the line will be pulled down away from the friction pad and onto the ratcheting catch to prevent it from rotating. Wrapping the line several times around that pulley will prevent the line from slipping on the locked pulley similar to a sailboat winch. Finally, if the tension is within the allowable window, both of the spring-loaded pulleys will be in neutral positions and able to freely rotate (see figure).

This mechanism takes advantage of several simple mechanical principles to passively keep the tension of the line at the spool within an acceptable window to allow proper line spooling. There is currently no prior art capable of achieving this task in a purely passive, mechanical manner.

This work was done by Charles M. Dandino of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49387


This Brief includes a Technical Support Package (TSP).
Passive Mechanism for Maintaining Tension at a Spool to Within a Given Window

(reference NPO-49387) is currently available for download from the TSP library.

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This article first appeared in the January, 2016 issue of NASA Tech Briefs Magazine.

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