Simple stress analysis of a lifting bail

[Rickruss84]

Dear Engineer Friends,
I have a problem that I need to figure out. This is “simple stress analysis” even though it has caused me a bit of frustration. Please see diagram attached to see the conditions I am under, the bail is ½” round bar. I have analyzed the following successfully: Double Sheer through the top of the Lift Bail, Tensile Stress on side leg of the Lift Bail, Sheer stress on the weld on the left, and Sheer stress on the weld on the right. Now the problem is what else needs to be considered? Bending stress on the bail and possibly combined stresses. I have looked at superposition methods equating this to a straight beam and the results are not favorable. Any ideas would be helpful.

 

[rb1957]

see if i understand your sketch correctly ...

you're applying 625 lbs to a bent piece of plate, which is supported on both sides (identified by the welds).

the near weld will react something like 2/3 of the applied, whatever you can make a free body of the bent plate.

i guess you could analyze the bend as a hook, or as a curved beam; but i think you can get close enough looking at radial sections and making free bodies.

 

[dvd]

There is always load testing.

 

[dhengr]

Rickruss:
You have shown something which can be drawn with a CAD program of some sort, but given no rhyme or reason for the various shapes or dimensions. What is applying the 625lb. force and what is its shape, i.e. a crane hook, clevis? It would be best if the top 2" rad. shape of the bail matched that lifting device shape and radius. It would be an improvement if you could move that line of force closer to the right weld and support. What is that right support, how rigid and strong is it? What is the 2.5" round bar on the left and how is it supported? That will have a lot to do with how much load that left sloped leg of the bail will pick up. How do you make the .75" weld to thin air on the left? If you haven’t already addressed these questions, you haven’t done any design on this bail yet. All you’ve done is some conceptual drafting.

 

[Rickruss84]

rb1957: I don't think free body diagrams will get me out of the problem i have.

dvd: Load testing will prove that at the 625# this passes without yeilding but that is not the criteria I need to fulfil.

dhengr: This lift bail was drawin in 2D in a CAD program. This is the lifting bail on a sysytem that my company has had for years this bail was introduced in 1994. For a current customer we needed to provide lifting calculations that show that this design is adequite. I have been the engineer since 2006 and did not reevaulate the design as I assumed the previous engineer did design calculations when the system was made. The 625 lbs can be supported by a crane hook, shackle of any applicable size or a small grapple hook. The geometry needs to remain the same as it attaches to different members of the housing. I understand that if the load was distributed differently the forces could be changed. I need to evaulate a current design and change only minimal things. THe support on the left is a 2" pipe header and on the right is an 8" pipe that ties to the baseplate. The weld on the left is to the pipe and the .75 is a minimum length from the drawing. Again I am not the designer at this point I am just evaluating someone elses design. The designer thoughts I have will come out after I determine that this attempt to prove the design sufficient is in vain. The designer idea is to for this application to place a waterjet stiffner 1/4" - 3/8" thick to fit the raduis of the bail at the top and place a hole in the stiffner at the top to allow a shackle to hook. This is ok with the current customer. Finally I need to determine the most versitile design for all future bails for this system.

 

[rb1957]

do you agree with my overall reactions ?

why do you think free bodies won't help you (understand the internal loads) ?

 

[JStephen]

One thing to consider is what constitutes "failure". If an overstress condition is considered failure, then you may calculate a fairly weak item. If dropping the load is considered "failure", you may have a considerably different result. In particular, if the bent bar is reasonably ductile, it could wrap it self around whatever was lifting it (hook or whatever) and yield considerably without having a failure.

 

[Engdoitbetter]

I would undoubtedly start dealing with free bodies. I think my first assumption would be to approximate the welds as simple supports, though I'm not sure which direction I would assign to the weld reaction on the left; acting this way may be simplistic but surely makes the problem statically determinate, so you don't have to deal, at least during a first attempt, with hyperstatic unknowns.
Anyway, starting from left or right you could analyze the reactions on the different sections (bending, axial, shear). The hook may be modeled as a curved beam, while the rest is obviously a straight beam.
Regarding outstanding stresses, at a first glance welds undergo shear, while bending, axial load and shear are the main ones for the beams.
In the area near the point where the load is applied, I would verify local compression and shear, but this depends on the device which is expected to transmit the load.

hope it helps.

Stefano

 

[flash3780]

So, if beam superposition shows that you have unfavorable stresses, why don't you believe that you have unfavorable stresses? What are you using as a failure criteria?

Perhaps you should post what you've done so far so that we can look it over. And for the record, I would never recommend approaching such a problem without first drawing a free-body diagram. Even if you feel that you have a good understanding of the loads, you'll likely have to explain them to others at some point.