Lifting lug stress analysis 1

[Spoonful]

Hi all,

i am trying to do a stress analysis on a lifting lug.

the lifting lug will be made of SS316 flat bar, a hole will be drilled(dia D1)

a rod(dia D2) will fit in the hole to fiting the whole thing.

i rekon the weakest part on the lifting lug during lifting will be where the rod in contact of the lifting lug.

assmue theres no offset angle between the contact face of the rod and the interal face of the hole, the contact case can be treated as a cylinder inside of socket.


during lifting there will be deflaction of the lifting lug and the rod, so there will be a rectangle shope contact face inbetween.

assume the lifting lug have a thickness of L. in order to know the contact area, the question becomes what the width of the contact rectangle. in another word, how much deflection of each material?



i am trying to use stress=P/A

so which ever the smallest yelid stress= P max/ A conatct


i didnt plan to go into the combined stress or pricinpole stress details, i think just based on the normal stress will give me a reasonable Pmax can be applied to the lifting lug. am i right on this?

 

[Ron]

Shear stress parallel to the load application will control the thickness (t) of the plate. Assuming the lug is welded to something else, shear stress in the welds will control the amount of weld needed.

I don't mean to be unkind, but I hope your analysis is more careful than your spelling and attention to details in you posting.

 

[Spoonful]

thanks for the reply Ron,

i am not trying to see how much weld is need, i wanna to know at the contact point, within the yelid stress, what is the max load can be applied,

yeah, the spelling, blame the forum, theres no spelling check here, haha, just kidding.

 

[GregLocock]

Shifts key broken as well is it? Look up Hertzian contact stress.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Ussuri]

I always design lifting points to have some offlead (out of plane line of force at an angle thus generating minor bending on the lift point) on them. No offlead is the absolute best you can achieve so I would suggest allowing for something should your load swing once lifted.

Check at hole level: bearing contact stress, shear pull out, bending about hole due to offlead force. If this causes problems you can add cheek plates.

Check at base level: biaxial bending, tension, shear followed by checks on weld group capacity

Check on structure attachment: check the structure you are lifting is strong enough to transfer the load at the attachment point. No point designing a padeye for 50 tonnes and then welding it to a 4mm plate.

 

[rb1957]

IMHO i think you're over-thinking this, worrying about the curvature of the bar in the lug ...

assuming that even if the bar fits well in the hole there will always be some clearance (slding fit?). then "theoretically" the contact is a line (thru the thickness of the hole, where the two circles contact). this'll lead to infinite hertzian stress, as the contact area is zero. so the answer to your post is zero load (can be sustained without yielding).

but in reality local yielding on both parts will create a contact aera, and i expect that this will be of small scale so that when the load is removed both parts will return to their original size (the small zone of permanent strain will be reacted by compressive stresses in the surrounding material).

and, like Ron posts above, the welds are probably the critical loadpath ... so why so interested in a non-critical component. assuming that you're got a reasonable clearance between the pin and the hole, use Dbar*t as the contact area. btw, there are several other ways for lugs to fail (bearing, shear tear-out, pin bending, ...)

and, btw, you're responsible for your spelling; it's a bit "Gen Y" to blame the forum.

haha

 

[SnTMan]

Recommend ASME BTH-1, "Design of Below-The-Hook Lifting Devices", available from the same. The small cost is easily paid back in design time and confidence.

Regards,

Mike