Lift Lug code question 2

[mschro]

We build a lot of heavy equipment for which we design lift lugs. I have been told that the hole that a shackle goes through for lifting should be 'drilled' versus 'burned out/plasma' cut. Is there any code that supports this, or is a plasma cut hole acceptable?

 

[dhengr]

Mschro:
I don’t know about the latest code reqr’mts., but just plain good practice has always dictated or recommended drilling/machining as the final step for hole sizing. Burning methods and accuracy have improved quite dramatically in recent years, so maybe this deserves some rethinking. But, until I was proven to be over doing it, I’d still opt for drilling or burning the hole 1/8-3/16" dia. small and then drilling to final size, and chamfering the edges of the hole. The thinking always was; burning was too rough, it left stress raisers in exactly the wrong orientation, it left a hardened layer at the hole surface, it was not accurate enough. The hole surface is pretty highly stressed and you want it to be well fitted with the pin and strong, but yielding, not hard, brittle and with many small notches perpendicular to the stress fields. From 11-1 o’clock you have very high bearing stresses btwn. the pin and hole in the lug, plus tensile and shear stresses, and at about 3&9 o’clock you have high tensile stresses due to axial loading and plate bending combined. All of these stresses get higher when the hole is too much larger than the pin dia. being used. If the lugs are only being used a few times to transport and install the equipment you might rationalize working to a smaller factor of safety in your design, at your own risk. Not so if this is a piece of lifting equipment which will be used many times and abused in the process. In this latter case you want a high FoS to tolerate field use and abuse.

 

[chicopee]

dhengr's answer pretty much covers it.

 

[JStephen]

One source that covers some of this design is ASME BTH and the corresponding B30 standard for below-the-hook lifting devices. I don't recall if it requires drilling, right offhand.

What I find is that if a lug is designed per this standard, even with a generous safety factor, the lug itself will calculate very small and the actual lugs are considerably oversize, in which case, some of the details aren't quite so important.

 

[racookpe1978]

But JStephen. It isn't the "entire lug" that is seeing the peak stress values due to irregular burning of the hole.

It is the tiny "point" of every burned irregularlity that is carrying the entire force of the entire machine. If 4x lugs, each lug carries 1/4 of the machine. The only surface across the entire top arc of that lug that touches the pin (the strap!) is the 1/16 of one inch "point" inside the hole.

 

[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.

 

[JStephen]

That's okay, racookpe1978, experience shows that this is acceptable practice for devices not subject to cyclic loading, which is up to 20,000 lifts. But yes, if you're going to lift your heavy equipment more than 20,000 times, you should then drill the holes.