Lemur Pulleys Redesign 2

Project Summary

Improved the strength and durability of existing plastic pulleys for the Lemur Center. 

Project Details

The Lemur Center's systems for raising and lowering some of the doors at their facility involve using pulleys. The original installed pulleys were poorly designed, so rather than replacing them with similar parts at significant expense, Lemur Center staff looked to DesignHub to design and manufacture a stronger, cheaper version. See https://make.duke.edu/web/nk124/projects/lemur-20center-20door-20pulley for the first redesign.

The redesigned pulleys were an improvement, but some of them broke after being in use for some time, so the Lemur Center asked for another redesign. In particular, they noticed that the first printed pulleys fractured at the corners of one of the cost-saving cutouts. 

Three changes were made to strengthen the area around the cutouts. First, the cutouts themselves were removed. These trapezoidal holes in the part reduced the amount of material in the pulley and thus the cost, but replacing an entire broken pulley costs much more than the few grams that were saved. The shape of the spokes in the flange was also changed; more material was used at the base of the spoke, which is where they usually snapped. The holes in the flange were also oriented to be farther from the corners of the central square hole.

A fourth measure, filleting the edge at which the flange meets the body of the pulley, was also tried. Finite element analysis found that this strengthened the part considerably, but when printed and tested it was found that the fillet prevented the cable from spooling properly on the pulley - the cable shifted too easily out from between the flanges. The final version of the pulley does not have a fillet at the base of the flange. 

Solidworks part files and STLs for the redesigned pulley with and without the fillet are included among the assets. 

Although the Lemur Center was reluctant to print pulleys in stainless steel, DesignHub decided to experiment with making a steel pulley with the idea that the increase in the durability of a metal part would outweigh the increase in cost. Several designs were considered and tested by printing them in PLA and drop testing them. At the time of writing, one steel pulley had been printed but not fully post-processed. DesignHub administrators agreed to cover the cost of the first metal pulley for the Lemur Center in hope of demonstrating its superiority to printed plastic parts. 

A Solidworks part file and STL for the printed version of the steel pulley are included among the assets. 

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