Repads for Lift lugs

[dab2010]

Just looking for some opinions from the community,
Specifically lift lugs with repads, but also repads in general:

- MechE #1 has rule of thumb “repads should not be thicker than shell/head they are attached to. This is to avoid local stiff areas and stress risers”
- MechE #2 says it doesn’t matter
- To make it worse to this EN13445-3 specifically states that repads for lugs should be thicker than the shell/head they are attached to. (we surmise this to be to ensure lug/repad act as rigid unit)

Seems a decided lack of consistency, comments?

 

[unclesyd]

Not the total answer though it might give you a little guidance on the subject..
Checkout the "calculations" at the end of the paper.

http://ar.inel.gov/images/pdf/200509/2005092100687JMO.pdf

 

[MJCronin]

I think that it is important to point out the reasons for so-called "repads" on lifting lugs.

"Repads" are necessary in order to (1) get enough weld length on the item lifted or (2) distribute the load on the shell.

Reason #1 governs in most of the LL designs.

A review of many of the LL calculations and methodologies shows that the lug itself can be made strong simply by making the lug bigger and thicker......but without a repad sized to gain enough weld "footprint", the rated load would be limited.

 

[dab2010]

Thanks for the information so far, though it is still isn't very clear to me whether or not the repad for the lifting lug should be constrained to be thinner or thicker than the thickness of the shell.

 

[racookpe1978]

I think you're missing the point of the original answers.

There ISN'T a thumbrule or "always by the book" answer - which is what I think you are looking for.

You (yourself!) have to completely design and the whole thing - lug AND vessel walls - based on first principles looking at four separate loads. You have to work from the outside in; then, from that answer, then reverse and work back out.

1. The lug itself. What is the load on that lug? and How big does the lug have to be to withstand that load?

2. The weld itself. What is the stress on the weld (if no pad were used) around the right-sized lug, and can the weld hold that stress? If not, what size repad and welds are needed to hold the lug?

3. Now that you know the ideal lug, weld and repad, what does that repad do to the vessel wall under all points of thermal and pressure loading? Good? Bad? Ugly? Indifferent?

OK. If Ugly or Bad, change the repad and weld (usually by making it more flexible or thinner) and work back out to the lug to make sure the lug is still good.

Worse case, you have to add lugs to keep the flexibility: Your original intent of only using 3 means you need 4. Your original 4 have to be replaced with 6.

 

[chicopee]

Think about the purpose of a repad. To reinforce the shell because of high stresses at the welded area of the lugs, buckling or indentation of the shell, tear of shell metal. Thickness does not seem an issue but the size and preparation and welding of the repad are important.

 

[unclesyd]

The reinforcing pas for a lifting pad thickness has to be thicker than the shell, if not you would have the same conditions as welding to the shell. I looked at several vessel prints that have repads at the lifting lugs and all are at least 1.5 times shell thickness. It is the size of the reinforcing plate need ed distributed load, not to big or small.

If you look at the calculation of the lug base plate in this paper the pad 3 times (1.5") the shell thickness of 1/2".

http://ar.inel.gov/images/pdf/200509/2005092100687JMO.pdf

 

[unclesyd]

If you look at the design page you will find programs that will speed up your calculation with and with out base plates. There is also a program for up ending calculations.
these programs are not a crutch, you still understand the basics of material and design.

http://www.maximumreach.com/index.htm

 

[r6155]

In general, lugs should be avoided for horizontal vessel.
Please, see "Pressure Vessel Design Manual" by Dennis R. Moss -typical sling arrangement, not lugs-

Rgards
r6155

 

[unclesyd]

There is a program on the Maximum Reach site I posted that covers the subject along with diagrams.

 

[dab2010]

Thank you for the information, this is very helpful