Repeated Bending Stress - Proper Material? 1

[gccmech]

We have an process where we use a beam (3"x 3"x 48") to push a forming tool. Do to the geometry of the part we are forming we press on the ends of the beam and the forming tool is located in the center of the beam. The beam needs to be rigid enough to transfer the force applied on the ends, but it also needs to be flexible so that the beam does not deform or fracture. If anyone has some ideas I would appreciate it.

 

[desertfox]

If you provided the force and the materials involved you might get some answers, if in fact a sketch could be provided even better.

 

[1gibson]

So you don't press "on" the ends, you press "at" the ends, perpendicular to axis of the beam.

In addition to what desertfox stated, you have yet to identify what the problem is with your current situation.

 

[gccmech]

The forming is currently being done in a 2500 ton hydraulic press. I have attached a diagram of how we are using the beam. As far as materials I believe in the past we have just ordered steel bar stock. The part we have been using worked for roughly a year with occasional use. I am trying to decide if there is a better material we could use or given the nature of the process we should just buy a new bar every year. I hope this clarifies some.

 

[EdStainless]

There are two parts to this, the shape of the beam and the material.
For strength and stiffness you need a larger dimension in the loaded direction.
This is why they use "I" beams in buildings. They can be made tall in the load direction without adding a lot of metal, and the flanges on the ends help prevent twisting.
Maybe an "I", "H", of box beam would be better for you than just a bar.

All steels (if the parts are the same size and shape) will flex the same amount under load. It is just that stronger ones will flex farther before they become bent.
It is fairly straight forward to make a shaped beam out of an alloy steel (4130 for example) and then have the whole thing heat treated to get higher strength.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[drawoh]

gccmech,

Is the beam loaded in an impact?

If there is no energy to dissipate, then flexibility is irrelevant. You want your beam to be as strong as possible. Rigidity is good.

--
JHG

 

[1gibson]

Please identify the mode of failure. Does it bend, does it crack, does it turn into a pumpkin at midnight on it's one year anniversary?

 

[gccmech]

EdStainless: That was my initial thought but I wasn't sure if an I-beam would twist too much.
Drawoh: It is not in impact, that is a good point.
1gibson: The beam cracks in the middle.

 

[drawoh]

gccmech,

Go back and look at your beam theory.

You have a beam 3" thick.

Stress [σ] = Mc/I, where c is the centroid to the outermost fibre.

If the beam is symmetrical, c = t/2, there t=3in.

[σ] = Mt/2I.

With a fixed moment due to your forming force, and a fixed thickness, you get minimum stress at maximum moment of inertia. You that beam to be as stiff as you can make it.

Consider increasing the three inches, as well.

--
JHG

 

[Tmoose]

Hi qccmech ,
How many cycles did the original bar endure?
So the recent one failed after a few hundred stress cycles?
Sounds like low cycle fatigue due to loading close to the yield point.

Is the steel bar well finished an painter? Or severely rusted? Or was the cracked surface finished with a snaggin ggrinder?

Bad geometrical details can create strss concentrations that will murder an otherwise acceptable design.

"Cracks in the middle" doesn't tell much of the story.
Which side did the crack(s) start from? Did it originate from a surface defect, a weld strike, or at the sharp edge of the forming tool attachment saddle"

http://collections.infocollections.org/ukedu/collect/ukedu/index/assoc/gtz006e/p038b.gif

A plate welded to the bottom of the new beam might be all it takes.

Or even just smoothing the surface.

 

[desertfox]

Hi gccmech

I guess the cracks are appearing on the top of the beam as the beam when loaded is hogging.
The one thing you haven't given us at this stage is the force required to form the part, all we know is that it's a 2500ton capacity but a 3" x 3" beam wouldn't withstand full capacity without failing on the first operation.
To take full machine capacity your looking at a 8" square beam very approximately.
Twisting of an I beam shouldn't occur provided it is vertically loaded through its centroid.

If you can calculate,estimate or measure the forming force and inform us of this, we can probably help you further but the starting point is the forming force.

 

[tbuelna]

For sure a 3"x3"x48"L steel bar with a 2.5 million lb load applied to each end and simply supported at its center would easily deform. Regardless of what happens to the beam, all of the force applied by the press would still be transferred to the form tool. One benefit of a stiffer beam would be more precise control of press travel for a given applied force.