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Bungee Tool DeLuxe

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Aug 12, 2012
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I made some improvements on the basic tool used to install landing gear shock cord bungee loops on landing gear and since this improved unit seems to work pretty well for me, I decided to model it in SolidWorks and provide some drawings so others might benefit. As Dave Baxter has said, the desire is to help others not come to grief either physically or financially.

Bungee Tool Screen Shot.png


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There have been numerous previous posts on this bungee cord installation subject with a lot of folks, including myself, struggling with what should be a routine and easy job. A common thread in many of these problem stories seems to involve the make-it-yourself bungee installation tool described in the May '77 issue of the Skybolt newsletters: https://cdn.groupbuilder.com/biplaneforum/NLpdfs/May-77.pdf

Here is a photo showing results of my first attempt using a tool I made in accordance with the Skybolt newsletter:

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In this example, the inner shaft had too much flexure which allowed the new bungee cord to get pinched between the inner and outer slides, requiring the new bungee to be cut off. I concluded from this that I should have made the solid steel 3/4 in diameter acme thread shaft to extend the full length of the inner shaft to stiffen it in bending, whereas in this case I only had three inches of overlap of the lead screw shaft into the inner tube. Unfortunately, there are no length dimensions for the overlap of the acme thread shaft into the inner tube given on the May '77 Skybolt news sketch. I also struggled to interpret the design intent and details of this sketch. At this point I decided, why not address some of the design problems and missing information by preparing some better drawing definition for this tool.

I am aware that there are at least two bungee tools offered for sale that are based on the same design as the tool depicted in the Skybolt newsletter, one by Steen Aero Lab and the other by Wag Aero. I note that a number of folks have posted favorable assessments of these tools on this site. So I wish to emphasize that I have not personally seen either of these products and I don't mean to call into question their quality or suitability. But I do have some issues with the design depicted in the Skybolt newsletter and I have attempted to address these issues by incorporating design improvements. A summary of the design problems and design corrections are as follows:

1. Problem: Missing dimensions and confusing drawing. Solution: Provide dimensioned drawings for all parts in SolidWorks drawing format. Add a step by step assembly guide.

2. Problem: Stroke not sufficient to easily engage hanger head on bungee truss when using 1286HD bungees on Skybolt bungee truss. Solution: Add 2 inches to stroke of tool.

3. Problem: The 7/8 in inner square tube will not slide into the 1.0 in outer square tube because the weld flashing on the seam welded tubing projects from the inner surface of the 1.0 in tube, causing interference. Without a broaching type cutting tool and a reciprocating machine tool to drive it, this weld flashing cannot be removed from inside the tube. Grinding a groove on the 7/8 tube to clear the weld flashing is tedious work and weakens the 7/8 tube. Upgrading to seamless tubing for the outer housing would increase cost and would make tubing harder to find. Solution: Increase the outer housing to 1.25 X .125W square tube to gain clearance for internal weld flashing and incorporate short sections of 1.25 X .188W tube as bearing tubes as part of the outer housing. Weld flashing for the short bearing tubes can easily be removed with a flat file.

4. Problem: The Acme screw rod is attached to the inner square tube sleeve by a lap joint weld at one end of the tube. The rod is about ten times the thickness of the tube wall which makes it difficult to achieve a satisfactory weld using OA gas welding. I was not able to add any satisfactory rosette welds to the middle of the tube either because of this problem of welding a thin sheet to a massive bar. Perhaps TIG welding would have produced better results, but my conclusion was that this differential thickness characteristic makes for poor design practice for a welded joint. In any case, integrity of this joint is critical to safety of the tool because if this weld fails under load the shaft will be propelled downward with great force as the spring energy of the bungee cord is released. Solution: Mechanically fasten the Acme screw rod to the inner sleeve using roll pins.

The attachment design information consists of 12 SolidWorks drawings, each on a single A size (8.5 X11 sheet), a separate parts list and an assembly guide, all on PDF format that can be printed on an ordinary desktop printer. The parts list includes product numbers from McMaster-Carr for everything but the 7/8 inner square tube, which they don't list on their web site. You can probably find steel cheaper in your local area, but even getting all the listed parts from McMaster-Carr, I came up with a materials cost of $106.70. Dave Baxter has provided some extensive photos of a bungee tool in action at the following:

Note: the Separate Parts List and Assembly Guide are provided on a subsequent post due to reaching the site limit for posted files.

Mark
 

Attachments

  • -1 Tool, Bungee.pdf
    67.5 KB · Views: 157
  • -3 PUSHER ASSY.pdf
    53 KB · Views: 83
  • -5 HANGER ASSY.pdf
    65.8 KB · Views: 68
  • -13 Plate, Inner.pdf
    48.9 KB · Views: 56
  • -15 Plate, Outer.pdf
    48.7 KB · Views: 48
  • -17 Strap, Pusher.pdf
    49.2 KB · Views: 52
  • -19 Tube, Hanger.pdf
    50.8 KB · Views: 56
  • -21 Plate, Hanger.pdf
    48.4 KB · Views: 46
  • -23 Lip, Hanger.pdf
    50.2 KB · Views: 42
  • -25 Strap, Hanger.pdf
    49.8 KB · Views: 45
  • -27 Rod, Threaded.pdf
    55.3 KB · Views: 50
  • -29 Weldment, Hanger.pdf
    51.1 KB · Views: 58

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