Lifting a large circular part 1

[jerry1423]

I have a large round cast part, with a finished weight of approx 4000 lbs.
It basically is a conical circular ring with the CG in the center.
The part is oriented in the machine (that it is a component of) the same way as it sits on the floor, so it simply gets lifted and placed into it.

For lifting I added bosses with tapped holes for three equally spaced lift hooks around the outside of the casting.

I was told by the main engineer on this project to add another lift hook so that there are four equally spaced ones instead of three.
With a part as symetrical as this one I think there may be problems, as theoretically all the weight could be balanced on two of the four slings.
I am just a peon here (temp project employee) so trying to explain something like this to him is virtually impossible; he cites that "that's the way we've always done it here".

How can I explain to this guy so that it seems like I know what I am talking about when I explain to him that three lift locations is better than four?
I am by no means an expert at this type of thing, but sometimes it's just hard explaining something that is such common sense to me.
Is there any publication out there that I can cite to support my point ?
If I am wrong at this, let me know that too. There are no issues here with sling or lift hook lift linits.

 

[btrueblood]

I'd think you could easily arrange to lift the part safely with two lift points. Balancing the load on the lifting slings is a job for the guy rigging the lift, it's the job of the designer to make that as easy as possible for the rigger. Balancing a 3-point lift is more difficult than a mulitple of 2 points, as not many slings/bridles are made with three ends.

 

[jerry1423]

I think you are right.

What I did not mention is that I am afraid that the calculations may be based on four slings each supporting the weight equally, which probably won't be true.

 

[boo1]

Design the four point lift, as he wants. Load rate each point as W/3, assuming one lift leg will be slack.

 

[Denial]

Except that (assuming symmetrical placement of lift points) ONE leg will not be slack. You will have ZERO or TWO slack legs.

 

[jlnsol]

I agree with you Jerry three is better than four because:
1. with three the system forces are statically be fully determined in other words every hook must carry one third of the weight
2. with four the rope forces are not statically be determined, you one cannot forcast the forces on the hooks and in the ropes. It may even be possible that two hooks opposite of each other carry all the weight.

 

[PNachtwey]

I agree, the load will probably not be evenly distributed with a 4 point lift.
I would stick to the 3 point lift. It isn't a matter of balance if the piece is truly symmetrical. Even if the load isn't symmetrical you will have better position control with a 3 point lift. Two point lifts will tilt unless the COG is in-line with the hooks. A tripod is always stable and doesn't rock.

An example of lifting heavy loads with 3 lift points.

http://deltamotion.com/peter/Pictures/NewBayBridge01.png

This is a picture of a span to the new bay bridge. It weighs 860 tons. The section was lifted at 3 points because the section had to be oriented just so to align with the previous section. Two points would not work if the 2 lift point didn't align with the COG and 4 would not work if two opposite lift points had most of the load so that the section would rock back and forth.



Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[MiketheEngineer]

Design each lug for 1/2 the load and then add 2 more so your boss will be happy

 

[chicopee]

I agree with Denial because you boss may think of what happens if one of your three lifting slings breaks, then you would have a real problem. the fourth sling is redundant sure but it should be viewed as a backup.

 

[zekeman]

There is nothing inherently wrong with 4 points vs 3 points.In fact,it is superior.
If the ropes are equal in length, then they should take an almost equal distribution of force . In case of one rope failure, the 4 point would fare better, since the plane formed by the 2 remaining ropes and the CM , there would be vertical with no net moment to rotate the ring which is quasi-stable ; then with little effort it could be guided ; in the case of the 3 point method, the ring would rotate 90 degrees from the horizontal plane to the vertical because the mass is offset with respect to the remaining line between the tie points, not a good outcome for 4000 lb of mass.
Accordingly,I strongly vote with the boss for the reason above plus the fact that he has field experience to support his approach.

 

[1gibson]

Good thing you are a temp there. If I heard the "main engineer" explain something with "that's the way we have always done it" then I would run. That is NOT engineering.

 

[fegenbush]

chicopee,

Assume loading at 45° and 3 supports: (1.414)(4000/3)=1890 lbs per sling. A well equipped shop should have plenty of "endless" slings, and if not, they are easy to order. A 1" single ply endless sling (the smallest that I can find readily available) in the choker configuration has a load limit of 1900 lbs. If your part is 10 ft in diameter, the length of sling needed is (1.414)(5 ft)(1.25)=8 ft 10 in (say 10 ft). The cost of each of those slings brand new is $20.80 (USD).

Why would you be concerned about a sling breaking with such a small load? If you are concerned about wear, why not buy new slings?

3 points is the most safe, efficient, and economical. The fourth point can lead to a false sense of security which is a dangerous proposition.

 

[KENAT]

Is the C of G of the part above or below the lifting hooks?

Assuming below then I'm tempted to agree with those that suggest 4 lifting points but sized to allow the load to shared by only 2 without failure. Of course you should have significant safety factors such that any one lifting point could probably handle the mass by itself in steady state at least.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Jboggs]

You are not in a position to second guess the "main engineer". You also don't know things he knows. for example, maybe he knows that three slings are all that's needed, but he also knows the workers on the floor would complain loudly if there wasn't a fourth sling, and he knows its not worth fighting that battle.

Which is my advice to you: this is not a battle worth fighting. Size everything so that a SINGLE sling could hold the entire load plus a safety factor, maybe 25%. Then it won't matter how many are actually used. You will be doing the job your boss wants, and you won't be the source of internal strife.

Like it or not, experienced workers do not trust young engineers. And the younger they are, the less they trust them. It has nothing to do with competence or knowledge. It has to do with gray hair.

 

[zekeman]

"3 points is the most safe....."


Who says so.
Read my post. If one rope breaks you have the ring rotating into the vertical plane because the CM is offset to the remaining tie points. That's not safe.
The 4 point system ends up with 2 remaining supporting ropes and the CM in the same vertical plane and would be easier to manage a safe lifting.

 

[racookpe1978]

Stop.

Put down your PC, mouse, pencil, and calculator.

Put on your hardhat, safety glasses, and steel-toed shoes.

Walk into the plant. Talk to the foreman, rigger, OR craft-labor-supervisor/supervisor.

Explain that you are specifying (NOT "designing") the rigging for the "new ring" and want their recommendations about "best practices": 3 lift points or four, size of shackles now used for each weight of ring, diameter and threads of the existing rigging gear (DON'T require somebody to buy new lifting gear if old ones are still good and are still available, best rigging practices THEY want to use, and determine what the limits are on THEIR gear.

DO NOT assume lifting gear (wire ropes, fabric loops, or anything else) is the same length or will stretch the same same distance under the same loads! I would assume, but will call my lifting experts later tonight, that most crew supervisors would prefer one sling going between two points, PLUS two different chainfalls to adjust the level of the load and the actual weight on the other two lift points. In the field, I do not often see a three point lift, but more often a four point lift with two legs controlled independently - NEVER a four point lift with four different wire ropes or web/nylon slings that ASSUMES each of the four slings is the same length.

 

[kwb74]

Depending on how critical to fly the load flat would drive me to pick 3 v. 4 legged sling.

If you have very little clearance I would probably opt for 3. If I wanted to be able to manually "tweak" how part is flying 4 will be easier to manipulate.

Personally I prefer 3 legged rigging as it is faster to setup, load is better controlled, and if it wasn't at the low end of sling/chain/lifting components ratings the math is more in line with real loads to insure that you are keeping to the 5:1 F.o.S... and with that said - the points above on failure potential either case if you had the odd occurance of a sling failure either two or three remaining slings still are capable of carrying the load.

But as a political move keeping the shop guys happy and making them feel involved in the process for somthing this small I would give them whatever they wanted, as far as the boss goes I have little reservation on argument of technical mertit details with them neither answer in this case is very likely to be wrong.

 

[chicopee]

The reply for fegenbush is that you may have damaged fittings or plate attachments that could fail. If the lifted plate is valuable and I am sure it would be, don't take the chance, otherwise, I agree with your statement. As a matter of fact any literature that deals with suspended loads within the construction industry will recommend a four leg over a three leg spreader.

 

[hvacpiper]

I would suggest reading up on EPRI rigging standards.Previous described methods for computing sling loads are dangerously wrong,be it a 2,3 or 4 legged bridal,each leg should be sized for 1/2 the load.Unless the load is headblocked,slings should be sized to give a min 60 degree angle of attachment.

 

[boo1]

Was the lift for nuclear lifting devices? Nuclear lifts are regulated from NUREG-0612, and ANSI N14.6.

Structural and mechanical lifting devices shall be designed, fabricated , labeled and assigned a design category according to the provisions of ASME B30.20 and ASME BTH-1.