Re: Aluminium tube bending and resistance

o.parafuso@xxxxxxxxx <o.parafuso@xxxxxxxxx> wrote:

1) There is 50 coming from? Shouldn't it be 10 (force in N)?

That is the "force" (it ain't force, no nickpicking) of 5kg = 50N

2) 0.7E5 (70000) ? Shouldn't it be 0.007?

No, 0.7 * 10^5. Thats 70000. You can understand that number like a
spring rate. Quite a stiff spring out of solid.
0.7E5 is the engineering notation for 0.7 * 10^5. Maybe that confused
you.


0.7*Force^5/Length^2

Huh!? I didn't write that.

f = F * l^3 / (3 * E * I)
f = 50 * 1000^3 / (3 * 7000 * 11300)
= 21 [mm]

Sorry, I misstyped something. Don't know where.
Check that by yourself, I cant find my beloved pocket calculator and
have to use the crap on the PC.


Checking the units:
N * mm^3 / (N / mm^2 * mm^4)
= N * mm^3 / (N * mm^2)
= mm (looks good :-))

Of course, the result according to my calculations is 4.2Km J

4.2 Km What is that? Did you mean 4.2 km? And J? Where do the Joule come
from?


While I'm at it, I'll have a cup of coffe and calculate the stress:

s = M / W
s: stress in [N/mm^2]
M: moment in [Nmm]
W: don't know how you call that (Resistance moment?) in [mm^3]

M = F * l
F: Force (prev. posting)
l: length (prev. posting)

W = pi / 32 * (D^4 - d^4) / D
pi, D, d: prev. posting

Now with the real numbers (as I used in the prev. posting):
W = 3.14 / 32 * (25^4 - 20^4) / 25
= 0.098 * 9225
= 905 [mm^3]

M = 1000 * 50
= 50000 [Nmm]

s = 50000 / 905
= 55 [N/mm^2]

Now I know that you don't want that. I'm sorry for my wrong guestimate
in the previous posting, but 0.2mm deflection is quite different to the
21mm.

Now to the pdf about technical data Ned posted for the 6061-T6.
Its elastic modulus (E) is 0.97E5. Quite different ('bout 30% off for
the better) to the 0.7E5 that I assumed. Recalc with that number. You'll
get a different deflection.
The allowable stress for 6061-T6 is only 30N/mm^2 with just 1000 load
cycles. **CRACK**
With 10,000,000 (10Mio) about 15 N/mm^2 etc. see page 5, Fig 5. Thanks
Ned for finding that.

An other comment from Newshound is also really important. The clamping.
Your bar will break there. Don't use a tube at that place, but a solid.
Make radii smooth and make a security factor of 2.


Remember that I still used that stupid calculator!

Nick
--
The modular DRO
<http://www.yadro.de>
Engine models
<http://www.motor-manufaktur.de>
.

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