Tube compressive strength

wolfy

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Does anyone know of a formula for calculating the column strength of 5/8 x 065 4130 tube?
I am wanting to use 5/8 x 065 for my control rods.

Thanks wolfy
 
I switched from round tube to square thin wall aluminum tube for my control rods. I think I'm using im using 1" x .065 square tube now. Seems to be working OK for me so far. My control rods are fairly long and it vibrates less than round. I also have added dampers too.
 
When the tube working in compression is long, the Euler's formula should be used in which it is the modulus of elasticity E that is critical. The breaking stress is not critical.
 
how long will your controls rods be ?

compression don't play an important role in the control rods in my opinion , as a mater of fat the rotor want to get flat and you have to pull the stick ant the rods to increase the rotor backward angle.

when you put your stick right the it pulls down the right rod , ok, it pushes the left rod but it is marginal.
I will have to measure them but they won't be long because of bell crank near the middle.
I agree most of the stress is in tension where where even a 1/4" tube is strong enough, but the weak limit is in compression like when the pre spinner wacks in, or taxiing over rough ground, rotor flapping etc.

wolfy
 
When the tube working in compression is long, the Euler's formula should be used in which it is the modulus of elasticity E that is critical. The breaking stress is not critical.
Thanks Jc I will look it up.
Do you think 5/8 x 065" will be strong enough for a medium weight single seater?

wolfy
 
I switched from round tube to square thin wall aluminum tube for my control rods. I think I'm using im using 1" x .065 square tube now. Seems to be working OK for me so far. My control rods are fairly long and it vibrates less than round. I also have added dampers too.
Thats a fair idea John, as apposed to round the square section has the added rigidity. Not good for mast's but use full in that respect.

wolfy
 
if your rods are in 2 parts, they will be strong enough,
the worst enemy are the vibrations ...

if you give me your flight rrpm and the exact length of the half bars I can ask my mate to calculate the 2/rev vibes modes to verify they are not destructive

on my monoseater there are vibes but not in flight , they vibrate with on the ground when the engine is idle ... it is impressive...
once the tandem is in action I will get back my gyro in my garage and overhaul it and change the rod bars, build a lighter engine mount (it will make JC smile hahaha) etc
Thanks mate I should have that info after I start making them this weekend, I will let you know the lengths it will be interesting.

wolfy
 
Hi wolfy,

I did a simulation on fusion 360 simulation module applying a 1000N force ( 100 "kg" approx) to the top of the tube ( steel 4130 at your sizes), tube length 1.2 meter

the tube is constraint at the bottom in all axes (as it if was dug into concrete) .

doing so the displacement is only appearing at the top of the rod and is negligible (0.05 mm !) , on the picture the displacement is exagerated by the computer to make it possible for users to see it (otherwise it would be too small to be visible).

the computer states a safety factor of 15 up to elastic limit ( not up to rupture)

I hope I have correctly entered all parameters in the module but all of this seems OK.

stress distribution is even (see the second picture it is oranger everywhere)

400 N is the german command resistance requirements for the dulv (certifyin body in germany) when DTA certified his gyro dta



View attachment 1149080View attachment 1149081View attachment 1149082
Excellent thanks mate looks perfect, I was advised I could use even smaller tubes (both diameter and wall) by some one very knowlegdable and that I trust but I just needed some re assurance as they just look small.
Thanks to all, one other thing I would like to check.
My threaded inserts are also 4130 and will be welded in, but given that my rod ends have 1/4" bore but 3/8" thread I only have 1.7 mm wall thickness from the peak of the thread to the necked down portion of the insert. Being 4130 I am sure they are more than strong enough but would like to be certain, any thoughts?

wolfy
 
Perfect thanks mate, my threads are 3/8 UNF.
What was the stress program you used to test the tubes I searched for days online and came up with nothing.

wolfy
 
autodesk fusion360, they have added this module in 2016, they have purchased the system, it is really reliable, you can spot the weel areas and and fillets chanfrains, put more metal, remove some when it is not necessary ...
I think that the fre version does not permit the access to this module (to be checked)

I am using it to design and to generate the G code to drive my cnc ...

I am making a bigger cnc at the time ... an other game
that is how I made all my CNC equipment. Started small while still working on my degree in CNC R&D Machine Development.

Each one got larger and more accurate.

My largest now in the shop is 5x12ft with a Z height of 18in, Repeatable accuracy of .001, used 35mm linear guide on all axis, manual tool changes but with automatic tool length measured after tool change but before continuing program, with both vacumn table and T slot tracks for hold down.
My go to is machine is 24x48in, Z height of 20in, Repeatable accuracy of .0005, 25mm linear guide on all axis, 12 tool ATC, flood coolant.

I also have 10" and 14" CNC lathes I made from modifying existing lathes, replace all cast guideways with 25mm guide rails. Also made and installing 8 tool turret tool holders

I am running closed loop steppers on all machines on all axes, been working fine for years, and just can see the need for changing to closed loop servos yet. I am being cheap as long as I can.
 
The most important test I use for control rods is after it is assembled, I put the stick in each extreme quadrant, full forward and full left, full forward and full right, full back and full left, and full back and full right. When it is in each of those positions, I make sure that both control tubes can swivel on the rod end. If it can't do that, the rod end is probably in a bind and could see very high bending forces. All the rod ends I've seen that have had problems were from bending loads at an extreme stick position.
 
Thanks Jc I will look it up.
Do you think 5/8 x 065" will be strong enough for a medium weight single seater?
Sorry, I am not familiar with English units.
Are you talking about a tube of 15.9 mm inside diameter (5/8 ") and 16.5 mm outside diameter (0.65") ?
 
Usually there are ball cranks on the upper and lower end of the push rods, which means that both ends are free to rotate. In that case the admissible load is half the one that jm calculated. The formula is

K=E*I*pi²/s²

Where
-E is the modulus of Elasticity
-I is the second moment of Inertia of the cross section
-s is the buckling length which depends on the type of support at each end.
-l length of the part (see blow)

If one end is clamped (as jm calculated) s=0.7*l with both ends free to rotate s simply is the length of the part. The admissible load for the case "one end clamped" is twice that of "both ends free to rotate" since 1/0.7² is about 2.0

K=E*I*pi²/(0.7*l)² = 2.048*E*I*pi²/l² for the case of "one end clamped".
K=E*I*pi²/l² = E*I*pi²/l² for the case of "both ends free".


Using square tubes for the push rods means you sacrifice some strength since the moment of inertia of the square tubes is smaller for the axes that go through the sides of the cross section compared to those passing through the corners. The best cross section for a compression column is a circle.

Cheers,

Juergen
 
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Sorry, I am not familiar with English units.
Are you talking about a tube of 15.9 mm inside diameter (5/8 ") and 16.5 mm outside diameter (0.65") ?
No Jc 15.9 mm is the outside diameter and about 12.7 mm for the inside diameter. 0.065" (about 1.6 mm) wall.

wolfy
 
Wolfy,
With hinge joints at each end, the critical load before buckling collapse of this 1.2 meter long steel tube is 2500 N. (880 N if aluminum alloy).
Juergen ?

Like shows the formula, collapse occurs well before the maximum breaking stress. Is only E that counts.
The best steel does not do better here than donkey-shoeing steel.
 
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Using square tubes for the push rods means you sacrifice some strength since the moment of inertia of the square tubes is smaller for the axes that go through the sides of the cross section compared to those passing through the corners. The best cross section for a compression column is a circle.
The square tubing sacrifices a small amount of MOI for an increase of inplane stiffness. Well worth it in my opinion, I doubt I'll go back to round tubing. The simple compressive calculations don't account for the round tubings greater flex in real use vibrations. Both sections can do the job and the only compressive failure for any shape on a gyro control rod I'm aware of was with someone who bent the tubing itself for clearance to avoid splitting the control tube into 2 sections. It wasn't a catastrophic failure, they noticed the stick position had changed and found that the bent tubing had bent even more.
 
If the cross sectional area is equal the square has a greater inplane stiffness for the usual range of wall thickness i.e. wall thickness is about 10% of the side of the rectangle. For a very small wall thickness the stiffness of the round tube becomes greater but that is something you'd probably only calculate for the design of an F-1 racing car....;-)
 
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