Yield strength test for the ME's (and good info for the rest of us).

[Zebaru]

Well, I went and corrected them, and still got the numbers screwed up.

The round is correct, the 2X2 should be .911 in ^3, and the 2x3 should be 1.698 in^3.

This doesn't change much else I wrote though.

Good thing I dont do this stuff for a living...

Travis

 

[Phil]

Instead I'm relying on ME students that drink too much.

Who, me?

I've got an idea, but I need to spend a little quality time with my mechanical design text.

What are your constraints?
-mild steel, for cost, availability, and ease of fabrication
-nonlinear links, arcing up towards the underside of the Jeep
-50% better resistance to bending than current link materials

What else?

 

[XJ_ranger]

these are in compression under acceleration and tension under braking at high speeds too...
also controling the torque of the axle housing rotating about the axle shafts...

I am thinking on this, but for some reason think that I am leaving factors out that will be an issue...

things like - for the shape of material used, when they are in compression, they are acting as an axialy loaded collumn, and you have to compute the radius of gyration from the neutral axis FOR ALL 3 axies! and choose the WEAKEST one of them to find the critical load....

hmmmm - too much brain work - time to go take a final... ill think on it more while im in Clayton this weekend...

 

[CRASH]

I'm no ME, so maybe someone will show up and prove me all wrong, but this is what I have:

It is the section modulus that you need to consider when designing for bending

Section modulus for a .25" wall material:

2" round = 0.537in^3
2" square = 0.771in^3
2"x3" rectangle = 1.438in^3

The maximum allowable moment is then a function of this value and the yield strength. So, for a given yield strength, the allowable moment for a 2x3" rectangle is about 2.5 times greater that of a 2" round tube...

If you are looking for the strongest shape to fit within your 2X3 parameters, then a 2x3 rectangle would be it. You could get away with significantly thinner walls and still have a 50% larger moment carrying capacity than your 2" round tube, but at some point buckling and denting becomes a very real concern.

Travis

I'm looking for the minimum material thickness in all 4 walls of a rectangular shape. So, for instance, to combat denting, I could use 1/4" for the bottom, and only 1/8" on top. The sides of the rectangle, are, of course the determining factor in combating a strain encountered when I land on a rock at 60 mph, or am powering into a large boulder. So, my inclination is that I need to be at .188" on the side material, which I think plays out in the section modulus calc.

 

[CRASH]

Who, me?

I've got an idea, but I need to spend a little quality time with my mechanical design text.

What are your constraints?
-mild steel, for cost, availability, and ease of fabrication
-nonlinear links, arcing up towards the underside of the Jeep
-50% better resistance to bending than current link materials

What else?

The links will be bowed in a large radius (my initial calcs show this at about 48" radius), so no point-specific loads. I can use different material for all 4 sides of the rectangle, as all sides will be cut and shaped independantly. This means I could even incorporate a third vertical member into the rectangle section if it meant saving weight with thinner material, while gaining yield.

 

[Dirk Pitt]

The sides of the rectangle, are, of course the determining factor in combating a strain encountered when I land on a rock at 60 mph, or am powering into a large boulder. So, my inclination is that I need to be at .188" on the side material, which I think plays out in the section modulus calc.

I think I understand you saying here that the sides are more impotrant in bending than the top/bottom. Is this right?

If that is in fact what you are stating, it's not right.

You want the most material the farthest distance from the centroidal axis that runs perpindicular to the force the rock is placing on the link.

For example, a rectangular link that has a top/bottom thickness of 0.25 in. with 0.125 vertical sides has much more resistance to bending than a section with top/bottom 0.125 in., and vertical sides of 0.250, when the load is placed from the bottom.

This is a good topic...

 

[Paul S]

Seems awfully complicated.
I took a quick drive down to Deaver last week & ordered a set of custom links with built in springs. Super simple.
Don't know the yeild strenth, but it's almost impossible to exceed it.

:confused1

Paul

 

[XJ_ranger]

I think I understand you saying here that the sides are more impotrant in bending than the top/bottom. Is this right?

If that is in fact what you are stating, it's not right.

You want the most material the farthest distance from the centroidal axis that runs perpindicular to the force the rock is placing on the link.

For example, a rectangular link that has a top/bottom thickness of 0.25 in. with 0.125 vertical sides has much more resistance to bending than a section with top/bottom 0.125 in., and vertical sides of 0.250, when the load is placed from the bottom.

This is a good topic...

so an I beam is the lightest, strongest best way to do this?

 

[SCW]

I hated my structural classes with a passion, but yes- you want as much steel away from the center as you can get it to resist bending.

An I-beam does a good job of putting steel away from center, but you end up with one strong axis and one weak axis. If you use the 2x3" HSS you do not have any unsupported flange sections and get more strenght for your cross-sectional surface area of steel. Increasing the amount of steel increases the load-carrying capacity, even if you are using the same yeild strength because the capacity is a combination of area and yield strength.

Round tube is also very strong in tension and compression, but less resistive to bending the way the HSS will be. I wouldn't build an I-beam out of it for 2 reasons-

1- it would be difficult for the gains
2-HSS is more supportive in weak axis and I'm not convinced the member won't experience and non-perfectly axial force.

 

[Dirk Pitt]

I hated my structural classes with a passion, but yes- you want as much steel away from the center as you can get it to resist bending.

An I-beam does a good job of putting steel away from center, but you end up with one strong axis and one weak axis. If you use the 2x3" HSS you do not have any unsupported flange sections and get more strenght for your cross-sectional surface area of steel. Increasing the amount of steel increases the load-carrying capacity, even if you are using the same yeild strength because the capacity is a combination of area and yield strength.

Round tube is also very strong in tension and compression, but less resistive to bending the way the HSS will be. I wouldn't build an I-beam out of it for 2 reasons-

1- it would be difficult for the gains
2-HSS is more supportive in weak axis and I'm not convinced the member won't experience and non-perfectly axial force.

Very well said.

 

[Weasel]

trick question, no matter what thickenss you use the yield strength isn't going to change. It will yield at 50-60ksi. Now if you mean you want to reduce the stress in the part by 50%, we can do that.

 

[What Rd]

I'm no engineer, but I know logic. Your goal of a 50% improvement is totally arbitrary with no stated support for the assumption that this level will satisfy your strength needs.
You first need to figure out how strong your wishbone needs to be, before you can work out a design to meet that goal.:dunno:

 

[SCW]

trick question, no matter what thickenss you use the yield strength isn't going to change. It will yield at 50-60ksi. Now if you mean you want to reduce the stress in the part by 50%, we can do that.

Yield strength is pretty near irrelevant imo. If you don't think you have enough SHEAR CAPACITY, increase the AREA of STEEL. Strength is the yield strength multiplied by the cross-sectional area of the steel. Think about the units, shear at 36ksi is 36 kips per SQUARE INCH. Increase the area and it taks that much more force to cause bending.

Think of rebar- they are all 60ksi yield strength, but if you need more capacity for your application you increase the area of steel. Now think of an I-beam. If you need more shear capacity you need to increase the area of steel in the web. 90% of structural steel is 55ksi steel, you get what you can and adjust the area of steel to meet required forces.

That said, this application should probably be more concerned with the ultimate moment capacity because as I said before, I doubt the forces will be completely axial. If you want good moment resistance use a steel with a high moment of inertia and plenty of steel (cross-sectional area). You can only get HSS in one yield strength anyway, and that really isn't all that high, I think it's 36ksi. Who cares- increase the area.

 

[Weasel]

Good grief, since when did engineers forget their sence of humor.

Yield strength is still important, although it's not the only number to use. It's only function is to give you an idea of where your stress's are in relation to the material propeties. As you said the Ixx and Iyy for the section propertites are key.

No the force are not axial but there will be bending forces and perhaps even some torsional forces on these members.

A wishbone member isn't exactly ideal as it subjects the members to loads in multiple directions, requiring extra material and weight. But we don't also get to work with ideal situations.

And as was said before to do this right, you really should have an idea of how much more is needed instead of just 50%.

 

[CRASH]

And as was said before to do this right, you really should have an idea of how much more is needed instead of just 50%.

If someone can show me a good way to calculate the forces experienced by a rear lower control arm in high speed and crawling application in a 5,000 lb Cherokee, I'd love to see it. I don't believe anyone can, as there are too many variables in that application.

So, we design around what we know, namely, that 2" x .25" wall is not quite strong enough. We know we want to be somewhat stronger. 50% stronger may be overkill, or it may not be. We'll have to see what kind of weight penalty there will be for various increases in strength.

I have only seen one post with calcs thus far! Seems to me we need to know the yield strength of a 2" x .25" round tube before we go much farther, right? So, how do we calculate the force necessary to deflect it past the point of plasticity?

 

[Zebaru]

I agree that bending isn't all that is important here, but considering that these links are typically going to fail when subject to a bending force, it is a good enough place to start.

As everyone had already pointed out, there are a number of things to take into consideration when determining the deflection in a given loading situation, but for simplicity, we can start with the most basic and very simple loading configuration -simply supported and only a single point load. I think that the simply supported assumption would be pretty good if this was one link of a four link setup, but is less appropriate for a wishbone. Without a real design, we can only make guesses at that anyway, so this is a good starting point.

You say 38" long, so we will go with that.

If we use a yield strength of 36ksi, and a section modulus of .536 in^3 for 2" round 0.25 wall tube - that gives us a maximum moment capacity of 1610 ft-lbs to yield. On a simply supported beam with a point load, the maximum moment for a point load will be at the center when the load is at the center. The moment generated by the load is (load *length) / 4. So the load is 4*moment/length or about 2000 lbs.

Travis

 

[Gary E]

Umm dude are you have some regrets about not getting an ME minor?

What I am envisioning sounds like alot of work. I guess if somebody would make it work I guess it would be you.

As far as the engineering throw out you calculator and get access to a good kinematic program, I think working model was one that was in play back in the school days. It will analyze forces through diffrent planes at each angle of movement instantlly. Each iteration of movement is about 3-4 pages of difficult math for each calculation.

I was really POed when I took that class slaved away for a semester on four page problems and the last day they took me to the computer lab and They showed us how to do our semesters work of problems in one hour. Heaven forbid they show us how to use the computer program and I would be able to do that stuff today.

 

[Jump This]

Leave me not suggest I have any ME background (I'm spending $20,000 a year for my son to get it for me ) BUTin the olden days when we couldn't find the slide rule we would use oval tubing in any place we didn't want any failure from impacts...(think swing arms on dirt bikes....)
Rick R
P.S. use 4130 you tight wad!

 

[Mr.OverKill]

Umm dude are you have some regrets about not getting an ME minor?

What I am envisioning sounds like alot of work.

and to think i was envisioning going back to school to get my real on paper degree :read: instead of the real life experiance title of structural field engineer ( hell i dont even know if i spelled it right :shiver: ) should have gotten the education when i was younger and didnt have a family :dunce:

 

[Dirk Pitt]

I agree that bending isn't all that is important here, but considering that these links are typically going to fail when subject to a bending force, it is a good enough place to start.

Please explain this. If bending is the failure mode expected, why on earth would it not be the important factor.

Crash, you mentioned placing an airbag at some location on top of the arm. This is going to change the load conditions...