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.