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Home Made Strut Clearances

Old 12-18-2009, 06:18 PM
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tasesq
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Default Home Made Strut Clearances

Hi All,

I am in the process of desiging a set of MLG for a future scale project. I am attempting to design a strut to take the landing loads in a trailing link arrangement.

The materials are:
Outer Aluminium Sleeve T6 12.7mm (0.5") OD, 10.2108mm (0.402") ID
Inner Aluminium Sleeve T6 9.525mm (0.375") OD, 7.0358 (0.277") ID
Stainless Steel Rod (oleo) 321 6.35mm (0.25") OD

This leaves the following clearances between the rod and the inner sleeve: 0.3429mm (0.0135")
and leave the following clearance between the inner and outer sleeve: 0.3429mm (0.0135")

My question:
Is this clearance enough? Too large? Will there be slop?
When it gets hot, will the Stainless expand more than the alu and get stuck?
How do you diecide how strong a spring to use? Trial and error?
There are a lot of questions marks aren't there?

Sorry if this is a convoluted question - i have not got a clue really, and i don't want to go and buy the materials at this stage just to test the fit!

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Old 12-18-2009, 07:44 PM
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R8893
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Default RE: Home Made Strut Clearances

I recently made some struts for a Val dive bomber. They are a straight strut so may not have the same characteristics as your trailing arm. I used .500" OD steel tubing for the outer strut, .029 wall thickness, hence ID .442". Inner strut is .375" steel tubing with brass bushings soldered on. I then turned the brass on my lathe aiming for .440". Couldn't hold that close with my cheap Chinese mini-lathe and missed by a couple thou under size on some (made two sets). Even with this tolerance they feel somewhat sloppy, although less than some Century Jet struts I have. Made some struts before and got the fit by polishing the brass bushings with emery cloth until they would just slide into place. They have worked well.

For spring rate I went for about 3 or 4 times the estimated weight of the model at full deflection1" in my case. Half the weight carried on each strut.

I doubt the expansion at the temperatures we see will have any effect.

Chuck
Old 12-18-2009, 07:59 PM
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Edwin
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Default RE: Home Made Strut Clearances

Those are actually very good questions. I dont really know the answer's, I'm not a mechanical engineer. I just stayed at a Holiday Inn Express last night. <g>. Like you mentioned, I tried the trial and error method on some struts thinking .004" would be acceptable. Boy was I wrong. The slop wasnt horrible, just fairly bad. And definately not good enough for any endurance. So I will be back at it again in a little while and get it down to about .001". If too tight, I can turn it down another thousandth until its right. Your wall thickness's might be a bit thin for long term endurance. Just my opinion, I could be wrong. I think mine is around .050 to .062" wall thickness. As for springs, havent even gotten to that point yet. I have some in there but its just for fit. I'll have to experiment later on with different tensions. Mine will be struts for a TF giant P-47. I figure worst case weight to be in the 27 to 30 lb range.
A machinist buddy told me there are two trains of thought for tolerance. Bushinged and non-bushinged. I have some of both types and cant decide if one is better than the other.
So there, probably no help at all. For me, the fun is in experimenting.
Edwin
Old 12-18-2009, 08:18 PM
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tasesq
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Default RE: Home Made Strut Clearances

Thanks Guys,

R8893 - so you are saying that 0.002" felt sloppy? Wow! They are close tolerances. I don't have a lathe so was hoping to find tubes that would telescope well enough not to worry about it. Maybe i'll have to find someone with a lathe!

here is a video for you all http://www.youtube.com/watch?v=PDZ1ACo0Fhc

any mechanical engineers out there to shed some light?

Thanks!
Old 12-21-2009, 05:57 PM
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BobH
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Default RE: Home Made Strut Clearances

I would make the struts from stainless and not alu. You'll most likely bend the alu in short order. Be aware that the two air cylinders need to work at different rates or you'll get binding at the top one. That's according to your animation. The lower cylinder needs to move first to allow the top to slip down in the slot and exert the correct force or otherwise it'll bind.

As far as clearences, the closer the better without binding. Don't mix materials if you don't have to. If you use steel and alu, the alu will most likely gall sooner than later.

You can check for web sites selling stainless tube to see if you can get a friction fit off the shelf. You'll need thick wall stainless tube so it doesn't kink under load.
Old 12-22-2009, 12:11 AM
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tasesq
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Default RE: Home Made Strut Clearances

Thanks Bob,

I had not thought of that regarding the cylinders. I see what you mean. The larger cylinder has to travel around 2 inches, while the smaller has to travel around 1 inch - so therefore the smaller one would have to move at half the speed?

How to do make the smaller cylinder travel slower? air line restrictors?

I was carfeul to make sure that the cylinders were always 'over centre' as such, so they will push only in the correct direction. The cylinders never get to 90 deg with the item they are pushing on, only about 88 deg.
See pic. What do you think?

Old 12-22-2009, 12:16 AM
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tasesq
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Default RE: Home Made Strut Clearances

oops here are the pics!


Also - I have searched and searched for tube suppliers here in Australia - can't seem to find any that have satisfactory 'telescoping' tube sizes. Can anyone help?

Specifically i have tried:
www.smallparts.com.au
www.airportmetals.com.au
www.aircraftspruce.com.au

The idea for Alu tubes for the gear came from Pat McCurry's archived Do335 build thread where he builds the gear from T6. I take your point though - i was planning on painting the gear white, so i guess if i can use SS it would also work (would have to take a look at the weight differences of course).

Thanks for your help.
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