Question for a engineer

[Josh_K]

O.K. here is an engineering question for a real engineer.

The part is an inverted "U" or "C" shaped rocker for a Ford crossover steering that I am making. The tubing that is taking all the torsion twist force is a 4130 1.75" o.d. .500" wall.

Now, most every one know that a tube is stronger that a solid bar. But my exact question is at what point does the wall get to thin or thick and when does strength become a loss.

Another way of asking this is; what is the optimum wall thickness?

Tosional forces only affect this part and the center I.D. is 1".

Thanks

Josh

[ntsqd]

Generically when in torsion the middle third is useless.

Stated another way, If you compare 1.5" bar to 1.5" x .5" wall tube, in torsion they are very close to being the same. That is not necessarliy true of bending or other loadings.

[Kritter]

More like a question for a shaft designer or an engineering student still fresh on the equations!

Im sure there are "rule of thumbs" or some simplified equation that someone who designs shafts can answer this pretty close from what you have given...otherwise its a 30 minute calculation solving for ID and also assumptions have to be made for K factors...

I referenced one of my text books and it would take me an hour to refresh and prob an hour to do the calc since I havent done these kind of calcs in 4 years..


How does the ID equal 1 inch if its a 1.75 OD with wall thickness of .5? Seems to me that would be a .75in ID...

"The tubing that is taking all the torsion twist force is a 4130 1.75" o.d. .500" wall...Tosional forces only affect this part and the center I.D. is 1"."

.500 wall seems pretty THICK to me unless its a LONG inch tube, but like non engineers always say..."when in doubt, build it stout"

[ntsqd]

This is from a spreadsheet I made a LONG time ago and have not fully verified the results. The bending deflection looks too great to me. So the numbers could be off, but they will be off by the same amount for all cases.
Beyond 1/2" wall looks pretty pointless to me.

OD---Wall thkns---lbs/ft---Bending(*)---Torsion(@)

1.750---0.1200------2.065-----5.80-------0.0300
1.750---0.2500------3.958-----3.52-------0.0181
1.750---0.5000------6.597-----2.70-------0.0138
1.750---BAR---------8.082-----2.61-------0.0134


(*) Simply supported 12" beam, 1000 lbs midspan load; Amount of deflection at midspan.

(@) Torsion arm = 12", Load = 1000lbs; Amount of deflection at end of arm.

[GOAT1]

I have heard the misconception of tubing being stronger than a solid of the same OD and I am curius as what makes people think that.

If you go by the numbers; looking at torsional shear stress, the 1.75 x .500 wall is only about 4% weaker than a 1.75" solid. You have to go just under 1.75 x .156 wall to loose 1/2 the torsional strength of 1.75" solid.

[GOAT1]

How's this, I dug up an old spread sheet I had

[BradM]

It is not that tubing is stronger in torsion, just more efficient in many applications. Why design for solid bar when you could find a nearly equivalent load carrying capacity from tubing?

Thom's original "rule of thumb" is pretty accurate and one that I have used in the past. Your chart also shows that to a reasonable percentage, the center of a solid section in torsion provides very little additional load carrying capacity.

[Josh_K]

Kritter, you caught me. Just after I posted I realized that I messed up on the center dia number. The I.D. is .750 and that how I got a .500 wall.

Thom thanks for the quick rule of thumb. I hate getting way too involved with k factors and allowances.

I think Rod Hall said it best, "I have never seen a truck built too strong on the side of the trail".

Josh

[Donahoe]

Dalton.... I would not look so far into the issue and just take Rod's Advice.....Build it overkill... Besides your going to blow the bushing out of the steering arm long before the arm breaks.

[SLORunner]

If you were mass producing this part, and wanted to use as little material as possible, I could see a reason for attempting to figure it out. But if its a one off part, build it stout. Donahoe has a good point too.

[Stephen]

I haven't been able to find anything supporting the idea that tube is stronger than solid bar given a similar OD. More efficient, heck yeah! Stronger, no.
My tubing chart for bending (conveniently at work while I'm home now) just shows "i", which lets me compare relative bending stiffness for different sizes. The same info would help with torsion also.