When is a pin not a pin?

[canwesteng]

I'm designing some rigging, and I'm debating whether to use the equations for a pin or to check shear and bending together for the pin below the crane hook. The contact surface should be flat, and there won't be much gap between the ends of the pin and the hook, but the hook itself is 2.5" wide. I usually use pin equations for stuff at most half that thick. Anyone have any insight, maybe some knowledge on how shackles are designed?

 

[TehMightyEngineer]

Analyze the pin as being simple supported and check shear and bending. Here's a great video showing how a shackle deflects under excessive load:

https://www.youtube.com/watch?v=N6I9-VkqAQA

In short, the shackle sides deflect until it's an ovoid shape, this allows the pin to bend in the middle similar to a point loaded simple span beam. Ideally the bending of the shackle should always control over shear in the pin (ductile vs nonductile failure modes).

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TehMightyEngineer]

Incidentally, why are you designing a shackle? Shackles are pretty much off the shelf items. Anything not I believe must be load tested per OSHA (if you're in the US).

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[canwesteng]

Not designing a shackle, designing a spreader beam. The desire is to connect the crane hook direct to the beam.

 

[TehMightyEngineer]

I've only once tried something similar and it was a huge hassle. I now always default to a lifting lug plate with an oversized hole for a shackle now. Very easy to design and fabricate and offers much more versatility so the owner can just use a standard shackle to connect hooks, chains, straps, etc. to the beam.

If you're backed into a corner and have to design something I'd make a FEA model of the setup to evaluate the load that causes yielding of the parts. I'd the ensure that the device had enough strength so that it doesn't yield even under 5x the rated load. Watch out for fatigue and wearing items, specify some sort of a hardened steel for the pin (mild steel will get worn down by the harder hook). Ideally I would use a large grade 8 or A490 bolt as the pin. That way it's easily replaceable with an equivalent pin rather than some unmarked steel rod.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[canwesteng]

It's a one and done spreader beam - installs equipment that will be in place 50 odd years. Headroom is an issue so we're trying to the get the hook as low to the beam as possible. Already using a factor of safety of 5, even though ASME uses much less for below the hook devices.

 

[TehMightyEngineer]

How large is the load?

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[canwesteng]

25 tons

 

[TehMightyEngineer]

Hmmmm, hoist rings or eye bolts with a shackle probably wont save you much space then as you're at the high end of their capacities.

What about arranging the lifting beam like so:

I also like the bolted D-ring detail. No idea if those are off-the-shelf items but I imagine something similar could be created.

To go back to your original question, it might help to know what the pin connects to, as the flexibility of the pins supports will likely control how the pin behaves.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[UcfSE]

I would design for the bending plus shear. The deformation will allow the pin to bend a little. I usually save shear-only to bolts in a connection that is tight.

 

[dhengr]

Canwesteng:
The ASME (other regulators tend to follow this too) FoS of 5 is usually reserved for rigging items which are reused many times, such as pins, shackles, slings, hooks, etc. etc., which tend to be abused in usage, neglected in storage, tend not to be inspected regularly, etc. Maybe a lifting device, lifting fixture, spreader beam, etc., which is used dozens of times a day starts to fit this group too. However, many lifting components which are fully engineered, not used very often, and are inspected and managed during each lift are only designed to a FoS of 2 or 3. Then, they are proof tested periodically to 1.25 or 1.5 times their design cap’y. There is a good deal of engineering experience and judgement involved in this area of the design. The FoS of 5 is generally to failure, and the lower FoS’s are more likely to something not much higher than material yielding. I do agree with TME that you do want failures to be yielding failures (progressive), not brittle failures (sudden, catastrophic), “(ductile vs non-ductile failure modes).”

You say the hook is 2.5" wide, that’s probably a 15 or 25 ton hook. Its load bearing area will have a fairly generous radiused shape, maybe even slightly flat, in part from wear and deformation. You will always get some yielding in the bearing areas. Take a look at Hertz bearing stresses and deformations; Roark’s book has a good section on this, as do many good Advanced Strength of Materials texts. Your ‘pin?’ might be bar stock 2" wide and 3-3.5" high, acting as a short beam btwn. two pin/lug pls or the webs of your lifting frame members. This ‘pin?’ would fit into holes in the pin pls./webs, which are spaced only about 3" apart, and be welded all around, both sides. Shape the bottom, the bearing surface of your ‘pin?’ to match/fit the/a hook shape for best bearing area match. You are kinda working in an undefined area of our everyday design activity. Our beams are usually long w.r.t. their depth and bending some distance from the supports governs our design. We certainly don’t ignore shear, but it is usually less critical. The lifting pin, lifting lug, shackle treatment is usually more analogous to the faying surfaces and a bolt through three plates, in tension. Here we think in terms of pure shear of the pin/bolt. Your problem is somewhere in between these two ideal situations, and combined shear and bending should be looked into.

 

[canwesteng]

dhengre - agreed w.r.t to typical FOS for single use lifting devices. Strictly using ASME BTH-1 you end up somewhere around 2.2-2.6 for this application depending on failure mode. W.r.t to hertzian stresses I don't think it's feasible not to have the pin yield on the hook - it's single use and the pin can't rotate however so I'm not concerned. I'm going to imagine it as a simply supported beam and combine average shear stress with max normal stress and compare to von mises failure criterion. Should be a conservative lower bound unless I'm missing something,

 

[TehMightyEngineer]

Canwesteng:

Make sure your supports can take the bending from the pin. For that geometry the pin will transfer moments into it's supports. They'll likely be not anywhere a stiff as the pin so I agree the pin should be designed as simply supported but the pins supports likely are going to see pin end moments.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/