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Old 17-03-2018, 23:30   #1
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Deformed Crosstree Fitting Problem

I hope there is a small diagram below illustrating my problem; at some time, probably during a knockdown, my crosstree mount on the mainmast has been deformed – I was replacing a couple of backstays last week and I noticed it on my way past – a couple of the rivets are gone too and when I came back for a closer look to replace them I noticed the deformation – there is a bulge creating a 4-5mm gap between mast and fitting. I'm looking for any advice as to how I may be able to give it a bit of a squeeze and bring it back in nearer to the mast before I put in some new rivets. Gave it a measure from the soft hammer – minimal effect. Any thoughts/ ideas/ mad schemes welcome for consideration.
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Old 17-03-2018, 23:56   #2
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Re: Deformed Crosstree Fitting Problem

Spanish windlass or clamps might do the job. Maybe need to put extra rivets in for security.
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Old 18-03-2018, 00:35   #3
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Re: Deformed Crosstree Fitting Problem

I logged in to lend a word of caution. As an ex-home builder and having been involved in the construction of kit aircraft, I caution that 'extra rivets for good measure' may be worse.

From home building, too many times I would be called to repair a mistake made by someone else. Upon investigation it was revealed that too many nails were used. The thinking is, 'if 4 nails are good, 8 are twice as good'. WRONG! It makes the wood nail sick, subject to failure. Structural engineers supply a nailing schedule for a reason.

I've seen that same kind of thinking among builders of kit aircraft. I've known at least four builders have to scrap new spars because they went on bad advice.

Depending upon the metallurgy you might try applying heat to the crosstree then using clamps to pull it into place. However, I would consult with a naval arch first. It is difficult to give any accurate advice based on a single drawing. Such advice may jeopardize your rig.
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Old 18-03-2018, 02:14   #4
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Re: Deformed Crosstree Fitting Problem

Looks like a classic inversion failure, but it also could also be from sail loads on the spreader or a knockdown. Obviously clamping it is the go, but prehaps also tapping a thread in the mast and using a metal thread to pull it in might work, rather than trying to pull it in the last bit with a rivet.

If there is a small gap due to distortion somewhere you can bed it on epoxy as a crude but effective fix.

Another thought is to use the spreader as a lever to help force the base back into position. Make sure at the end the spreaders are both on the correct angle, ie 90 degrees to the mast axis in your case, and that the mast wall isn't deformed in that area.

A few long bolts passing right through the mast to the opposite side of the spreader base would really reinforce the whole lot. This is a typical weakness of this style of fitting.
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Old 18-03-2018, 03:46   #5
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Re: Deformed Crosstree Fitting Problem

I am not aware of the fitting or the loads etc but there are some amazing rivets in the market now, such as Orlock they are a structural rivet with incredible shear and tension figures. Maybe clamp it, rivet it and remove the clamp. I believe you can add an/some additional rivets provided you don't weaken or create a fault line in the parent metal.
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Old 18-03-2018, 14:41   #6
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Re: Deformed Crosstree Fitting Problem

thanks for your thoughts - spanish clamp was my first thought too ( although i didnt know it was called that - i was just thinking 'loop of rope with some stick-twisty - thanks mr google...). It wont need extra rivets because it has a dirty big 12mm bolt through the bottom that also carries the shrouds - that does all the necessary structural work, any pressure on the part thats deformed should actually be inward from the main shrouds. The main lurking danger is the possibility of damaging the mast whilst trying to fix a smallish not very important problem with the crosstree mount...
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Old 18-03-2018, 14:48   #7
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Re: Deformed Crosstree Fitting Problem

i'll also look into the idea of structural rivets - might be sufficient to pack and rivet it as is - i got a couple of aluminium rivets through it but because of the gap i dont think they're going to do much of job - so thanks for that also
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Old 20-03-2018, 14:19   #8
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Re: Deformed Crosstree Fitting Problem

thanks for your suggestions and advice - all good stuff and i am proceeding in consideration thereof. I wonder if anyone has anything not already mentioned to add? Its a tough old bit of aluminium and not so far very accommodating of my attempts to move it...
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Old 20-03-2018, 23:54   #9
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Re: Deformed Crosstree Fitting Problem

Quote:
Originally Posted by charliehows View Post
thanks for your suggestions and advice - all good stuff and i am proceeding in consideration thereof. I wonder if anyone has anything not already mentioned to add? Its a tough old bit of aluminium and not so far very accommodating of my attempts to move it...
I was thinking of a triangular jig with three points of contact. The jig fits around the mast.
The jig is constructed of welded steel. The apex of the triangle would be forward the mast and the two legs running port & stbd and tailing a bit aft of the mast. The two legs would be connected to one another aft of the mast. The three points of contact would be forward, port, stbd. These contact points would be screws fit into nuts welded to the jig that press in on the cross bar. (Think of adjustable leveling screws such as found on office furniture or floor standing kitchen appliances.)

Of course required would be a longer threaded shaft to allow the application of enough force.

The two contact points at the forward and stbd are only for support of the the jig and resistance of the force to be applied on the port side where you want to reform the cross beam. So these two points can be set in place once. 'set and forget'

Then gradually apply the force to bring in the deformed section of the cross bar.

Here I hesitate to again mention the application of heat. Even if I knew more about metallurgy than I do, I would want to physically inspect the components (cross bar, mast, rivets) and probably perform some testing such as hardness test to positively ID the material of the cross bar and to take accurate measurements, especially of it's cross section at various points. The application of heat must be gradual and only to a specific maximum temperature. Then there is the subject of quenching and annealing.

So...why not seek out a welding shop to discuss the problem and see what can be gotten.


Oh, another thing; fasteners, whether they be rivets, bolts, screws, nails, should not be used to pull a part into place. The part is formed to fit, the fasteners secure it in place. They are not designed for the stress and strain of pulling members into place. I imagine there are some that do but those are unique specialty items.
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