Lift Lug Punched or Machined?? 1

[mechmine]

Can you punch the center hole on lifting lugs, or must they be machined (ex: with a MagDrill)??

I want to speed up production and reduce cost, so punching would the obvious choice, but I am concerned with the metallurgical changes that occur during punching. Would punching greatly reduce the structural integrity of the lug?

MAterial is 1/2" M.S. Plate.
Center diameter hole is 1.75", with 1.25" of material from edge of hole to edge of plate all around.

Thanks in advance.

 

[dhengr]

“Metallurgical changes” isn’t really the right terminology. Punching can give you some cross grain tearing or lamalar tearing, generally a scruffy hole finish in this size hole. You are trying to prevent stress raisers of any kind, more than causing any metallurgical changes. Burning the holes would cause metallurgical changes in the stl. pl. I would line bore a bunch of these plates at one time to save some money. And, I would then chamfer both edges of each hole to clean it up and soften it a bit. You could punch to 1.25 or 1.375" dia., then machine to the finished dia. You want a good fit btwn. the plate hole and the lifting pin, as this dictates the bearing stress level btwn. the lifting lug and the lifting pin. Look up Hertz stresses. The .5" pl. with a 1.75" hole looks a bit ill proportioned in terms of pl. thickness. And, for goodness sakes, don’t full pen. weld these plates to the object if you are trying to save some money. The pl. preparation and welding will cost you many times the drilling.

 

[geesamand]

In my workplace we'd laser or waterjet cut those lugs. Is that an option?

 

[EngineerTex]

I agree with most everything dhengr said, except for the chamfer, which reduces bearing area of the pin in the hole.

I would not laser or waterjet the hole, because of fatigue issues caused by having the direction of the machining marks in the wrong orientation. You will want to have any microscopic grooves left by machining to be tangential, as opposed to axial.

Some of this was in a post a few months back:

http://www.eng-tips.com/viewthread.cfm?qid=261911&page=1

Engineering is not the science behind building. It is the science behind not building.

 

[racookpe1978]

What's the diameter of the lifting lug/pin?

 

[dhengr]

I would allow laser or waterjet cutting of the outside shape of the lug and the hole, leaving about .125" of the radius to be cleaned up by line boring the hole to final dia. I would clean up the outside edge as needed by grinding, and at the same time I would grind outside corners (edge to face of pl.) with about 1/16" radius. I already commented on the pl. thickness being minimal and really didn’t mean to steal much in the way of pin bearing length. The corners of the pin holes would be chamferred about 1/32" to deburr them and to prevent nicking on pin application.

I have the same aversion any cutting nicks in the hole on the axis of the pin as EngineerTex does. The pl. thickness, and thus the pin bearing length and bearing stress, and the stresses in the pl. are still outstanding, as are the welding details.

 

[mechmine]

The end user normally uses a Crosby Shackle (3 1/4 TON - 4 3/4 TON). Pin diameters range from .75" to 1" roughly.

The lifting lugs are welded 1/4" fillet all around.

We do have access to a waterjet table, but the waterjet is expensive to run and cutting 1/2" steel would take some time, and use more consumables. Thats why I was thinking punching would be much faster.

Line bore sounds like it would be easy, fast and cost effective. That or cut them on the waterjet and allow for some left over material, and clean up the hole afterwards. like dhengr said.

 

[mfgenggear]

I don't understand why water jet is expensive to run,
it is one of the easiest & cheapest manufacturing methods.
it would be cheaper than milling/boring.

with out a drawing & it's requirements it hard to discuss what is a viable manufacturing method would be.

punching would be cheaper if the the material thickness to punch diameter ratio will allow it. however halve of the hole lenth is deformed and not cut.

if the hole is critical the boring is an option as said before.

 

[SnTMan]

We burn (plasma cut) them.

Regards,

Mike

 

[Tmoose]

Are the lugs used so frequently fatigue will be an issue?

 

[mechmine]

There normally just used for the initial unloading and placement of equipment. After that there no longer used.

 

[MikeHalloran]

It may depend on the customer's specs.

My friends in the GOM oil patch insist on bored and reamed holes and full penetration welds on all lifting lugs.



Mike Halloran
Pembroke Pines, FL, USA

 

[dhengr]

O.K., we may not be talking about fatigue here. In fact, neither EngineeringTex or I were talking only about fatigue (maybe a bad word selection), but we were talking about stress raisers, hard spots at high stress points, and potential fracture. I’ll say ‘ill proportioned’ one more time and then quit beating my head against a brick wall. You should not be using a 1.75" dia. hole for a 7/8" shackle pin, the bearing stresses will be quite high. Use a 1" dia. hole and save a bunch of machining cost, but still check the pin bearing stresses. And, you should not be using a 1/2" pl. with those shackles, because the pins are intended to be loaded in double shear, not bending. Finally, without some special corner prep. at the four corners on the lifting plate, about 1/2" high, at the piece of equip., your 1/4" fillet weld all around, will likely leave stress raisers at each of the four corners. Mike usually gives darn good advice, but he keeps bad company. For some reason M.E’s think every weld should be full pen., at great cost, and for little benefit in many cases. To convince them otherwise seems to be a tough sell. Although, rough use, abuse, fatigue and fracture are all applicable terms for the oil patch, and we don’t always control the purchaser’s spec. so he may be right.

You may get by with this design because it’s only used once, or a few times in its life; or the actual load is fairly light and those are the only shackles the end user has. But, that doesn’t make this good design, or indicate a real understanding of the problem.

 

[MikeHalloran]

I suppose the lift environment makes a difference.

Drop a skid in the dirt next to a landborne rig, and you get to dust it off and maybe unbend a few things.

Drop it next to a drilling platform, and somebody pays millions a day while you fish it out of the water or build a replacement. I'm guessing that something like that happened once a long time ago, and everyone decided that full-pen welds are cheaper than delay.

Not my rule. To tell the truth, my (now former) fab crew just flat refused to build skids any other way. ... and had finished building my first skids before I had finished reading the specs we were to meet. I'll miss those guys.




Mike Halloran
Pembroke Pines, FL, USA

 

[boo1]

ASME BTH-1–2005 Design of Below-the-Hook Lifting Devices

3-3.3.3 Fatigue Loading
"Pin holes in connections designed for Service Classes
1 through 4 shall be drilled, reamed, or otherwise finished
to provide a maximum surface roughness of 500 micro-in. (12.5 micro-m) around the inside surface of the hole."
Commentary:
"Pin holes in lifting devices used in construction
(Service Class 0) are at times flame cut. Experience
shows that this is acceptable practice for devices not
subject to cyclic loading. Connections in devices
designed for Service Classes 1 through 4 shall be
machined as required to avoid the notches that result
from flame cutting."

3-3.3.5 Pin-to-Hole Clearance.
when the diameter of the pin hole is greater than 110% of the diameter of the, the effect of the clearance shall be taken into account when determining the strength of the connection.