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 Post subject: Steel comparison
PostPosted: Thu Jan 15, 2009 12:53 pm 
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I need to remake the body mounts on the CJ and also make some struts that extend from the frame 8 - 14" so I can have mounting points for the cage.
I'm looking at using 2 x 3 rectangular tubing.

I can get the tubing in 2 x 3 with 3/16 wall or 2 x 3 with 1/8 wall. Aside from weight is there any difference structurally for what I plan on doing?

When I remade the back half the frame I used 3/16 thick material, but for mounting arms for the cage would the 1/8 wall be sturdy enough?

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PostPosted: Thu Jan 15, 2009 2:11 pm 
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I like to kind of overkill stuff when I build it, that way I dont have to worry about it as much, so I would use 3/16"

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PostPosted: Thu Jan 15, 2009 2:24 pm 
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I'm going to go with the 3/16... it's a $35 difference between the two from my metal supplier and I guess the weight is really negligible for this application....


Rummaging through some books at work I've found the 3 x 2 w/1/8 wall weighs 3.9 lbs/ft while the 3/16 is 5.59 lbs/ft.

Does anyone have any specs as to how the 1/8 vs 3/16 would react under loads? It'd be interesting to know something like that...

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2003 Sport - 'Fire Engine' Red
Detours KJ Backbone
Skids
Boulder Bars
JBA 4.5 A Arms
245/75/16 Wrangler Duratracs's on Cragar Soft 8’s
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Superwinch LP 8500
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PostPosted: Thu Jan 15, 2009 3:31 pm 
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it would depend on the metal whether hot or cold rolled.

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PostPosted: Fri Jan 16, 2009 10:51 am 
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I checked with a friend of mine who does industrial piping and a whole lot of metal fabrication, and he said,

"I would think that the type of steel - the chemical composition - would need to be considered. Other than that, maybe the steel supplier have tables on stregnth, etc. "

:D

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PostPosted: Wed Feb 11, 2009 10:32 pm 
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I'm not sure if you still are deciding here, but here goes.

How are these tubes being loaded? I'm mildly unsure, but I think they will be loaded at one end with the other end welded to a "fixed" point, like a cantilevered beam?

I assume by the size that these are A500 tubes. That may not be true, but that is the most readily available to me at my work. A500 has an average yeild strength of 46,000 psi. I would assume that they are a minimum of 30,000 psi no matter what they are made of.

Now to find how much force you can safely apply perpendicular to the beam at the end:

3 x 2 x 3/16 tubes, neglecting effects of corner radii, have a Section Modulus:

S = 1.37 in^3 in the hard bending direction, and
S = 1.06 in^3 in the easy bending direction.

Assuming a 14" long cantilevered beam and a safety factor of 3:

Bending Stress = Moment / Section Modulus

Bending Stress allowable = 30,000 psi / 3 (safety factor) = 10,000 psi
Moment = Design Load "F" x 14" = 14F in-lb
S=1.06 in^3 in easy bend direction
S=1.37 in^3 in hard bend direction

10,000 lb/in^2 = 14F in*lb / S in^3

F lb = 10,000 lb/in^2 * 1.06 in^3 / 14 in = 757 lbs. in easy bend direction
F lb = 10,000 lb/in^2 * 1.37 in^3 / 14 in = 978 lbs. in hard bend direction

There are likely other forces present, but if your load is like I think it is, the bend stress is the primary load. Keep in mind that if you do have A500 tubing you can multiply these loads by about 1.5. Also if you want a lower safety factor you can adjust them proportionally too. There is some generalization here, but I think this might help a little if I understand the loading right.

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 Post subject:
PostPosted: Wed Feb 11, 2009 10:44 pm 
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Oops...if you decide on 1/8" material, it has a section modulus of 0.98 in the hard direction and 0.77 in the easy, so it will be roughly 2/3 as strong.

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PostPosted: Thu Feb 12, 2009 2:16 am 
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Boiler wrote:
I'm not sure if you still are deciding here, but here goes.

How are these tubes being loaded? I'm mildly unsure, but I think they will be loaded at one end with the other end welded to a "fixed" point, like a cantilevered beam?

I assume by the size that these are A500 tubes. That may not be true, but that is the most readily available to me at my work. A500 has an average yeild strength of 46,000 psi. I would assume that they are a minimum of 30,000 psi no matter what they are made of.

Now to find how much force you can safely apply perpendicular to the beam at the end:

3 x 2 x 3/16 tubes, neglecting effects of corner radii, have a Section Modulus:

S = 1.37 in^3 in the hard bending direction, and
S = 1.06 in^3 in the easy bending direction.

Assuming a 14" long cantilevered beam and a safety factor of 3:

Bending Stress = Moment / Section Modulus

Bending Stress allowable = 30,000 psi / 3 (safety factor) = 10,000 psi
Moment = Design Load "F" x 14" = 14F in-lb
S=1.06 in^3 in easy bend direction
S=1.37 in^3 in hard bend direction

10,000 lb/in^2 = 14F in*lb / S in^3

F lb = 10,000 lb/in^2 * 1.06 in^3 / 14 in = 757 lbs. in easy bend direction
F lb = 10,000 lb/in^2 * 1.37 in^3 / 14 in = 978 lbs. in hard bend direction

There are likely other forces present, but if your load is like I think it is, the bend stress is the primary load. Keep in mind that if you do have A500 tubing you can multiply these loads by about 1.5. Also if you want a lower safety factor you can adjust them proportionally too. There is some generalization here, but I think this might help a little if I understand the loading right.



That is exactly what I was going to say! :lol: :lol:

Terry

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PostPosted: Thu Feb 12, 2009 11:42 am 
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Boiler wrote:
I'm not sure if you still are deciding here, but here goes.

How are these tubes being loaded? I'm mildly unsure, but I think they will be loaded at one end with the other end welded to a "fixed" point, like a cantilevered beam?

I assume by the size that these are A500 tubes. That may not be true, but that is the most readily available to me at my work. A500 has an average yeild strength of 46,000 psi. I would assume that they are a minimum of 30,000 psi no matter what they are made of.

Now to find how much force you can safely apply perpendicular to the beam at the end:

3 x 2 x 3/16 tubes, neglecting effects of corner radii, have a Section Modulus:

S = 1.37 in^3 in the hard bending direction, and
S = 1.06 in^3 in the easy bending direction.

Assuming a 14" long cantilevered beam and a safety factor of 3:

Bending Stress = Moment / Section Modulus

Bending Stress allowable = 30,000 psi / 3 (safety factor) = 10,000 psi
Moment = Design Load "F" x 14" = 14F in-lb
S=1.06 in^3 in easy bend direction
S=1.37 in^3 in hard bend direction

10,000 lb/in^2 = 14F in*lb / S in^3

F lb = 10,000 lb/in^2 * 1.06 in^3 / 14 in = 757 lbs. in easy bend direction
F lb = 10,000 lb/in^2 * 1.37 in^3 / 14 in = 978 lbs. in hard bend direction

There are likely other forces present, but if your load is like I think it is, the bend stress is the primary load. Keep in mind that if you do have A500 tubing you can multiply these loads by about 1.5. Also if you want a lower safety factor you can adjust them proportionally too. There is some generalization here, but I think this might help a little if I understand the loading right.



Yes :D That's a real answer
If you did that off the top of your head - I'd be really really impressed -
I'd have to dig out a design handbook I haven't seen in a long time

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 Post subject:
PostPosted: Thu Feb 12, 2009 12:33 pm 
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Well, I did pull up my spreadsheet that calculates section properties for tubes. I had to make it because I size tubing about every 4 hours at work.

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 Post subject:
PostPosted: Thu Feb 12, 2009 1:54 pm 
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Excellent! That's what I was needing :D

Thanks!

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1986 CJ7
Lots of custom stuff...

Gone, But not Forgotten...
2003 Sport - 'Fire Engine' Red
Detours KJ Backbone
Skids
Boulder Bars
JBA 4.5 A Arms
245/75/16 Wrangler Duratracs's on Cragar Soft 8’s
Spidertrax 1.25" Spacers
Superwinch LP 8500
Frankenlifted


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