Lifting Lug Analysis/Design 5

[EWFZQ]

Hello, I could use some help. I am a Systems engineering intern this summer. My boss has asked me to analyze some lifting lugs for our hoppers. He told me that the goal is to standardize the sizes of our lifting lugs, and to find the maximum force and then work my way backwards. I haven't done a lot of this, and I haven't completed a lot of relevant course work. If anyone has any tips or can point me to some good resources, that would be great.

 

[BAretired]

This may be a starting point.

https://www.pdhonline.com/courses/s106/s106.pdf

BA

 

[phamENG]

Do a search - the topic has been covered ad nauseam.

You're going to want ASME's Below the Hook Lifting Device design guide. That will give you what you need for the lift points, though it doesn't really break it out step by step. There's an online calculator suite called MechanicCalc...never used it, don't endorse it...that has some background literature and shows how they apply BTH to lifting lugs. It's valuable, but NOT all inclusive. For instance, I don't think they apply the load at an angle, which can be common depending on the rigging equipment used.

You have to consider how it will be lifted, how often, etc. as these play into requirements for fatigue resistance and other issues. Since this is for a single site, you may also want to consider the skill of your crane operators. Are they professionals, or do they just get on it once or twice a month to move something around? If they're lifting day in and day out and/or are required to pass strict qual requirements then you can probably count on a fairly smooth operation. If the company doesn't put much into their crane training and/or the guys doing it don't have much practice, you can get some jerky lifts that will result in greater dynamic loads on the lift points. And, of course, if you're picking these up with a crawler crane and moving them over rough terrain, you'll absolutely have dynamic loads. Search for "velocity factors." I think there's a thread on here with a velocity factor table - I don't have mine handy at the moment.

Of course this goes without saying, but I'll say it just in case - make sure your boss is aware of your limitations. Do your research, make your best attempt, but when you give it to him/her make sure they're aware that it's coming from a novice. They should know, but I was surprised how many times my supervisors seemed to forget that when I first started out.

 

[r13]

As your boss told you, it could be a reverse engineering process, as each of the existing was determined by different loading condition, and having different loading scheme/mechanism. Your task then is determine individual strength of the lugs, then back check the mechanism that causes the forces. Carefully study the materials provided by others in above comments, and ask questions. Good luck.

 

[dhengr]

EWFZQ:
This is partly (maybe) a testing of you, and how you approach an engineering problem, if your boss is a real engineer and worth his salt. He shouldn’t (will not) assume that anyone who can add and subtract is an engineer in training. Get yourself some good Engineering Mechanics and Strength of Materials (Theory of Elasticity) text books; Blodgett’s books “Design of Weldments” and “Design of Welded Structures” are very good, and very reasonably priced; The ASME Std. BTH-1, “Design of Below-the-Hook Lifting Devices” is important for your problem. These are all important refs. which you will use throughout your engineering career. See if you can find/borrow some texts on lifting/rigging. BA’s ref. is particularly good, in that it spells out most failure mechanisms one at a time. You can do plenty of study, and ref. reading, on each of those failure mechanisms and understanding of them. PhamENG’s post is very good, particularly the last two para’s. which suggest some important considerations, do search ‘lifting lugs’ here on E-Tips. You should find a bunch of threads.

As a young engineer or intern, don’t be afraid to ask intelligent questions of your superiors. Don’t have to ask the same question twice, out of laziness or inattention, and come armed with some study on your own, and some possible ideas or possible solutions. The actual/original question (by your boss) may have been asked in kinda a round-about-way, in that what he really meant was how can we standardize our lifting process with the equipment, crane and operators we have. Many times the exact lug design will depend on the lifting methods and equipment used, but it is also highly dependent on the equipment it is fixed to. So, maybe the real question is…, given the equipment we have, how can we standardize our lifting process; or what minor modifications, or added equipment would help standardize our process. There are hundreds of variations of lifting beams, slings, shackles, etc. etc., and you want to standardize to the extent that you don’t need all of them. Have a serious discussion about this after you have done some study, on your own, so you have some understanding on what you are talking about.

 

[human909]

Guessing here... But it would not surprise me if a many of those previous lifting lugs haven't been engineered, just drawn up by a draftsman.... At least that is the sort of stuff I see done all the time in hopper design I've seen. At least at the smaller end of things it seems that engineering anything is considered a waste of time. (Which sometimes it almost is. But sometimes things get too big and too heavy not to give it some thought.

 

[kingnero]

Be vary wary about the loading.
A vessel with 4 lifting eyes - do not assume 25% on each lifting point.
It's all about the assumptions - the calcs are cookbook, see BTH, Blodgett, EN 13155, ...