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Old 7th January 2008, 05:32 PM   #11
Expander
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Join Date: Sep 2006
Location: Santa Croce lake, Italy
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Some times ago in Neil Pryde Windsurfing Forum I made a brief analysis about a severe design fault in a 2005 NP UXT aluminium extension.

At that time I used this kind of extension for my racing sails (from 10 to 12); as everybody knows, down-haul tension is formidable in these sails (maybe over 600 kgs) and this factor should force any serious racer to buy aluminium extensions rather than carbon ones.

Anyway, NP UXT extensions was, at that time, a good choice, at least till 2005 series; in fact, as you can see from following photos, I was able to broke an UXT red (purple) series extension bought in 2005.




At a first sight, extension plastic base was broken inside aluminium tube, certainly due to extreme down-haul tension... at a more accurate sight, plastic base was broken at level of front inox rivet: or better, there was an evidence of an original old fissure that has been extended till structural collapse.

For this extension (2005 model), NP has placed two rivets, one in front side, the other on back; previous NP aluminium extension instead, always UXT series but bought in 2004 (yellow series), had a different rivets location: in 2004 series they were placed sideways when in 2005 series they were placed one frontally, the other in the back.




At a deeper analysis, what I've highlighted was how these rivets worked:




In 2004 (yellow) extension, rivets that were placed on side of aluminium tube, worked like they has been on a rotation axes, allowing main bending force to be distribuited on base plastic top collar; in 2005 (red) extension instead, rivets that were on front and back of aluminium tube, were directly subjected by bending force and shearing stress that caused their breakage or a direct damage of plastic base (with following break-down of base structure as shown above).

What I've highlighted was that in this extension, under high loads, both rivets worked under a not calculated shearing stress: how to explain all this ? Evidently, like a design fault.

Now I want to make some consideration: NP makes a lot of very good products like, for example, X9 booms and, of course, their beautiful sails... however other products are less good like, for example, X9 masts (the damnation of all RS5-RS6 sailors), X6 booms (a boom with a ridiculous stiffness), old carbon and alu extensions, etc...

But now I have to admit the truth: new MXT alu extension is a very good product, heavy duty, with a formidable pulleys system and a long life Dynema rope, really designed taking care of lesson from previous errors in extensions production (see above)... I've bought this extension in 48 and 34 cm sizes using them from large Formula sails till small Slalom ones (from 11 to 7 m) and till now I had no problems.

So, I have to render unto Caesar that which is Caesar's: sometimes NP cocks things up but, with MXT extension, they have made a good product.


- Expander.

Last edited by Expander; 10th January 2008 at 09:43 PM.
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Old 9th January 2008, 12:02 AM   #12
rethanaish
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Join Date: Sep 2006
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Thumbs up Thank you !!

Expander, very good analysis !!
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