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Parachute Cord and Popsicle Crane Analysis

Face View of the Spring Scale

I spent some time analyzing the good the bad and the ugly of the popsicle crane boom test and there was some of all three.  The main problem was the parachute cord stretching but I also did not have the angles I thought I did.  First, the issue of the parachute cord.  The wikipedia article on parachute cord does say the cord will stretch, but it does not give an idea of how much.  I hung up a length of cord and applied a force to it.  A 15 inch section of cord stretched to 15 7/8 inches with 25 lbs force and 16.5 inches with 50 lbs force.  This is a 10% elongation for 50 lbs.  Significant!

The Crane Test Setup.

To get an idea of how much stretch I experienced during the test. I created diagrams of the actual boom and the cable arrangement.  The first thing I noticed in the diagram, there is no way I actually had a 10 degree difference in the boom angle and the cable although I measured it carefully before starting the test.  The source of error there was the pulley I had in the cables to share the load.   The weight of the pulley caused a sag in the cable.  In the schematically drawn 2nd picture I show two lines for the cables.  The bottom line is the 10 degree angle, the top line is the actual anchor point for the cable.   The significance of this is that once the cables started getting tension on them the boom would drop because the pulley weight would be lifted reducing the sag in the cable.   However, this did not account for all of the length increase.

25 degree boom angle diagram

I drew 4 more diagrams,  one at 35 deg boom angle, 25 deg, 15 deg and 5 deg.  From each of these diagrams I was able to get the angle of the cable to the boom as well as the length of the cable.

Ideally the cable length would be 62.33 with no load at 45 degrees and was 66.9″ when at 5 degrees. This about 4.6″ of stretch, which is less than 10%.  However, remember throughout most of that length the load was split between two cables because of the pulley arrangement I used.

Next I inputted the angles into a spreadsheet to calculate the forces on the cables and the boom at the various angles.   With a 10 lb load hanging on the crane and with the cables tied like I did for the test and with the boom dropping to a 5 deg angle I stressed the boom to 262 Lbs compression.   The boom passed the test!

Spreadsheet calculations of Force on the Boom and the Cable.

In summary, if paracord is used in any projects, be aware that it is designed to have a lot of stretch when under load.  This would be a very good thing when a parachute snaps open.  It is a bad thing when depending upon the length for alignment like I was doing for the crane test.   I have bought some steel rope to try for a future test, but I am not very happy with the method used to tie those to the boom.    In addition there are a few other things I want to think about with the future of this project and an announcement will be coming out later this week or the first of next week concerning this.

I consider the popsicle construction a very good success and possibly even overbuilt although I am not sure where I could cut much from it.


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