Ferrule Length Destructive Testing




Ferrule Length Destructive Testing

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.

 ExternalInternalNotes
Avia sport 0.180” O.D.   
1D1.5NA 
2D2.5NA 
3D2.8NA 
4D3.6NA 
5D3.7NA 
Avia sport 0.265” O.D.   
1D4.10.7 
2D7.41 
3D8.71.9 
4D8.92.8 
5D92.74.5 wrapped with filament tape, bent pin
Pultruded carbon 0.5” O.D.   
1D91.4 
2D143 
3D233.7 
4D295.9 
5D916wrapped with filament tape
7/16″ wood dowel   
1D7N/A 
2D11.5N/A 
3D12.1N/A 
4D12.5N/A 
Glasform G50 0.320” O.D.   
1D3.4N/A 
2D5.8N/A 
3D7N/A 
4D7.6N/A 
5D7.7N/A 
Glasform K75 0.352” O.D.   
1D3.92.7 
2D6.36.6 
3D7.99.1 
4D9.810.2 
5D10.511 
Sky Shark 2P   
1D1.70.8 
2D3.42.3 
3D42.1Splits
4D42.7 
5D3.4  
Sky Shark P400   
1D3.23 
2D5.96.9 
3D78.5 
4D7.410.3bent pin
5D7.6  

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