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The Popsicle Crane boom – Test 1.

Picture 1: All of the loads acting on the boom.

Over this long 3 day weekend I finally got the initial test tower completed and did some testing. In many ways it was my testing that failed and not the tower. There is a story often told in math class about the frog dying of thirst. The poor frog is getting weak and as he jumps toward the drinking pond he can only jump 1/2 as far as the previous jump. On his first jump he gets 1/2 the way to the pond, The 2nd jump is an additional 1/4 and so on. Will the frog ever make it?

Sometimes learning something new works exactly the same way.  We can afford to do that with this silly project that does not put anyone’s health and happiness (and finances) in danger.  If we were building a real crane carrying real loads over people’s heads, the project would be a lot more intense.

I built a test tower and tested it as shown in the video.  It successfully withstood 50 lbs of compressive load although one stick did come unglued.  Although the test never completed to destruction I did learn quite a few things.   First, I am going to have to be much better at quality during construction.  I have several bugs to work out during that phase.  The biggest problem is the clips use for clamping the pieces together while the glue dries get in the way of making good measurements of squareness.

The second issue is it is necessary to glue on the “skinny side” of the sticks.  This results in a very weak joint.  I have a couple of ideas to address that.  One may be to use string or thread to tie the sticks together before the glue sets and then apply more glue over the thread.  Another may be to put short pieces of popsicle stick between the others to provide more area to glue.

After doing the testing I decided to do some quick calculations.   Assuming the boom is at an angle of 45 degrees.  Each pound of load with have a perpendicular force to the boom of .707 lbs.   The only force that counter that is the rope pulling the boom upward.  If that rope is at an angle of 10 degrees above the boom, that rope will have to have pull will 4.07 lbs of tension and will also be adding 4.01 lbs of compression on the boom.

Adding all the compressive forces together including the hoist rope running down the middle of the boom yields that for each pound of load, there will be 5.7 lbs of compressive force.  If the boom can only stand 57 pounds of compression we will only be able to lift 10 pounds load.  I need to test at a greater load!

The actual test video is below.

 

Oh yeah… one very important thing learned: 50 pound test fishing line is not strong enough!

Gary

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