Lifting with straps

[ptc-380107]

Dear ProM's

I have a beam (tubing) with two endplates that I will be using for lifiting
some items. You will notice the endplates have lifting eyelets to put
straps through that will be lifted using an overhead crane.

My question is, in ProMechanica, what would be the best way to simulate the
lifting straps ( best way of constraining the beam from a Mechanica point
of view) that suspend the beam and the parts that are attached to the beam?

Here is a pic of the beam:

(See attached file: gas_newpg_liftingassy2.jpg)

I appreciate all your suggestions,


Greg Saiter

ABB Inc., PIPM
ATPA / INAPM CMC SERVICES


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[lgreene]

Hi,

The question of constraints seems to always be a good one . . .

You'd like to apply a "bearing" like constraint but alas, there is none.

I've used a rigidly fixed round bar surrounded by a thin wrap inside the
"eyelet". I made up a material with a Poisson ratio that was weak in shear
for the wrap (I used .9). That way the normal load was transferred but the
shear was not . . . .

Please forward the other responses you get.

Bye now,

Leo Greene
Principle
E-Cognition Inc, Engineering Services, Video tutorials

[jrich]

Greg,

We have done this sort of thing hundreds of times over a period of 15 years.

The only real way to do this is to model eight thick-walled pins in Pro/E and assemble them into the eight holes. Fix the ID of the eight pins in X, Y, and Z. Don't fix the OD of the pins, because that would cause their OD stiffnesses to be infinite. It's okay if the pin ID's have infinite stiffness, as those surfaces are 'some distance' from your assembly.

Ensure that the lengths of the pins are identical to the thickness of your assembly's mounting flanges. This way the element vertices of the pins and flange holes will line up better. Or, if the pin/hole clearance is line-to-line, you wont end up with a tiny ring of .003-sized elements if the pins protrude .003 inch out of the flange surfaces.

Assuming your loads are only normal to the "axis", fix (only) one point on the extreme end (either end) of your model in the "axial" direction only. This allows the other end to "breathe" axially. You can use a "point" constraint in this case because there are no net axial loads.

If your assembly is bolted at each end (fixed-fixed), rather than pinned, then modeling it as pinned-pinned will be conservative. If your design passes stress/fatigue in this case, then all the better. This may result in a slightly heavier design. But if your customer doesn't care, all the better!!

Ensure that your pin diameters reflect the actual pins that will be used in service. You may want to model them worst-case, that is at their minimum diameters.

Are the flanges welded to the green tubing? If so, you'll need to model a fillet, or whatever shaped transition between the flange and tubing to avoid infinite stress in the transition.

Also, what are the number of loading cycles? You must consider fatigue as well as one-time stress, if this is the case. A good reference for this is "Mechanical Engineering Design", 5th Edition, Shigley, McGraw Hill.

"Bearing" loads and "Rigid connections" have only given us results that are utter nonsense over the past 15 years.


Jeff