Some testing by Doug La Rock:
Fourm Thread Crossrefrence click HERE
There has been some question about internal and external ferrules (alternate spelling Ferrel )and the optimum length of spar engagement, so I decided to do some destructive testing. Our friend Steve F. was kind enough to donate some spars for my project.
For external ferrules I made a sleeve I could slide the spar into and for internal ferrules I turned a steel pin to slide the spar on. To make a comparison across number of sizes I slid the spar into or onto the ferrule as a multiple of the spar outside diameter.
Example: a 0.265” diameter spar 1dia. = 0.265”, 2dia. = 0.530”, 3dia. = 0.795, 4dia = 1.06”, 5dia = 1.325”.
As you can see from the accompanying pictures I used a machine to hold the ferrule and spar and then pulled it down onto a scale. The scale was fixed at 12” from the end of the ferrule. I lowered the machine head down until the spar put pressure on the scale. This continued until the spar broke and I recorded the force.
In the tests I wasn’t trying to generate very accurate numbers about spar breakage or deflection but a comparison of spar strength in relationship to length of ferrule engagement.
When I started this project I made the assumption that external ferrules would prove to be superior. I was surprised to see how well internal ferrules performed. Spars that are made of material that is prone to splitting can be reinforced with an external sleeve, thread wrapping?, or a couple of layers of filament tape. The down side to internal ferrules is it’s difficult to get a ferrule with enough strength in a small diameter to match the spar strength. As you can see from my results in most cases anything over 4 diameters of engagement doesn’t seem to add much strength. Longer ferrules while not adding much strength will make a more stable joint especially were the spar may fit loosely. While this is not a complete list I hope it can be a guide in your next kite project.
|Avia sport 0.180” O.D.|| || || |
|Avia sport 0.265” O.D.|| || || |
|5D||9||2.7||4.5 wrapped with filament tape, bent pin|
|Pultruded carbon 0.5” O.D.|| || || |
|5D||9||16||wrapped with filament tape|
|7/16″ wood dowel|| || || |
|Glasform G50 0.320” O.D.|| || || |
|Glasform K75 0.352” O.D.|| || || |
|Sky Shark 2P|| || || |
|5D||3.4|| || |
|Sky Shark P400|| || || |
|5D||7.6|| || |
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