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Helicoils in Al struts

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faklord
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Helicoils in Al struts

Postby faklord » Sat Dec 31, 2016 5:18 pm

Just wondered if anybody knew why it isn't standard practice to install helicoils in Al struts (and any other Al components) to mate with SS bolts?
The use of these is standard practice in the aviation industry and higher end marine industry.

Helicoils should significantly reduce galvanic corrosion problems and and also wear from assembly/disassembly cycles.
They are not expensive (<£0.25 each much cheaper than Tefgel!!) and are easy to fit.

So why not used?

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Re: Helicoils in Al struts

Postby Blackrat » Sat Dec 31, 2016 5:52 pm

What are the helicoils made from that they don't react ?

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Re: Helicoils in Al struts

Postby faklord » Sat Dec 31, 2016 7:00 pm

^
There are a number of different types. Most commonly they are a form of stainless steel.
Sometimes plated to reduce the galvanic interaction with Al. Often they are installed with a loctite type barrier/adhesive which also reduces the likely hood of galvanic corrosion.
A potential problem is that running a SS bolt in a SS nut can be prone to 'pickup' and seizure (a form of friction welding). Using different grades of SS (for the nut & bolt) helps prevent this, as can plating & lubrication.

Bottom line is that they work and are subjected to rigorous qualification testing (eg salt fog test) in military & naval applications.

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Re: Helicoils in Al struts

Postby Kamikuza » Mon Jan 02, 2017 6:49 am

Length? I know you can stack two, but could the three or four necessary for the long bolts be done?

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Re: Helicoils in Al struts

Postby zfennell » Mon Jan 02, 2017 2:56 pm

Odds are that only 3 or 4 threads from the helicoil will be needed provide enough shear area to match the root area of the screw.

The details of the assembly should certainly be the last word. But typically the screw will fail first.

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Re: Helicoils in Al struts

Postby faklord » Mon Jan 02, 2017 4:52 pm

zfennell wrote:Odds are that only 3 or 4 threads from the helicoil will provide enough shear area to match the root area of the screw.
Yes..it also occurred to me that a single helicoil should, by design, be more than long enough for its bolt size. Maybe there is some advantage in spreading the load over a longer portion of the strut?? I suppose using a longer than needed bolt helps compensate for Al thread wear from assembly cycles in a nasty gritty environment.

Also if you can stack two helicoils. I can't see any reason you couldn't stack more.

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Re: Helicoils in Al struts

Postby zfennell » Mon Jan 02, 2017 5:18 pm

They come in standard length equivalent to 1dia or 2dia of the screw in question.
As mentioned, that should be sufficient to allow the screw to become weak link in the assy.
Any more than necessary usually means more work to clean up the mess when failure occurs.
Having enough room for thread depth equivalent to 4 Dia. might suggest the helicoil would be a good backup if good corrosion protection techniques were not effective.

However, lot of folks besides the surf industry pay little attention to corrosion issues created by soaking aluminum in salt water. The outboard motor industry is notorious for that. And they are a lot more expensive to repair once fasteners become frozen in aluminum.

The grownups that use helicoil in aluminum for marine application would like to anodize thread surface if possible, but usually not practical.
So internal threads are often passivated with solution like "aladyne" (sp)
Electrically isolating the threads when inserting helicoil with loctite is good practice.
Adding grease or thread sealant like permatex (sp) to screw is also good practice to minimize gauling and corrosion

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Re: Helicoils in Al struts

Postby plummet » Mon Jan 02, 2017 9:19 pm

To install a helicoil you need to drill a bigger hole. Its essentially a thread within a thread. Having the mast as thin as practical is a high priority. There may not be enough meat on the standard mast and a need to use a thicker mast section if you are to install a helicoil. That would reduce performance.

Also it adds expense and the standard threaded aluminium is unlikely to fail during the warrantee period. If it does foil within warrantee the manufacturer will site poor maintenance as the fault not poor design.

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Re: Helicoils in Al struts

Postby faklord » Mon Jan 02, 2017 10:11 pm

plummet wrote:To install a helicoil you need to drill a bigger hole. Its essentially a thread within a thread. Having the mast as thin as practical is a high priority. There may not be enough meat on the standard mast and a need to use a thicker mast section if you are to install a helicoil. That would reduce performance.

Also it adds expense and the standard threaded aluminium is unlikely to fail during the warrantee period. If it does foil within warrantee the manufacturer will site poor maintenance as the fault not poor design.

Good points!
Looking at the Manta strut it could get a bit thin particularly around the front hole. Can't see any reason why this couldn't have been positioned a bit further back, where there is more thickness. The increase in expense of installing helicoils should be very minimal and IMHO the increase in quality would be significant.
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Re: Helicoils in Al struts

Postby plummet » Mon Jan 02, 2017 10:41 pm

Extrapolate the cost of helicoils and labour to install them plus add margin for manufacturer, distributor, reseller on top of the cost increase would lead to a reasonable hike in price.


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