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Pin and Lug design and Bending of a pin 1

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bsimms89

bsimms89

Structural
Dec 8, 2014
7
US
Hi, I am currently analyzing a lug and pin combination that is to be used in lifting an 80K load. I am analyzing them using ASME BTH-1 and using Design and Construction of lifting beams by Ricker as a guide. Since this is a controlled lift and all the loads are known and will be moved delicately it is being design as a catagory A lifter.

I have already analyzed the lugs which are a pair of 2" plates with a 3 1/16" pin hole in the middle with a spread of 4" on the inside (6" center to center). I have come up with a capacity of 117K between the two ears (58.9K to each ear) with the applied factor of safety of 5 to 1 to ultimate as in Ricker.

My question pertains to the pin. The pin is a 2.75" diameter roundbar with a tensile strength of 80KSI and yield strength of 45KSI (it's grade 1045 round bar). Since it is a catagory A lifter BTH-1 states a FS of 2 to yield (it would be 3.0 for a category B). Checking shear I found Vn=.6Fy*A=160.36K in single shear. Since it is in double shear 2*Vn=320.72K, then with the FS of 2 the allowable load is 160.36K for shearon the pin. I am just curious about analyzing the pin for bending?

It is a 2.75" diameter pin spanning 4", my gut tells me bending shouldn't be a concern but my calcs tell me otherwise. Analyzing a roundbar in bending from the AISC manual Mn=Mp=Fy*Z=<1.6My, Fy*Z=155.97K-in, 1.6My=1.6*Fy*S=147K-in.

If I treat it as a 4" span beam with a point load in the middle Mmax=P*L/4=P*4/4 so 147K-in=P*4in/4, Pmax=147K, then with the factor is safety of 2 Pmax =73.5K.

Based on analyzing it for bending it seems to not be acceptable based on the FS, but is bending really applicable in this situation? Am I over analyzing it or making too conservative of an assumption somewhere?
 
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Can you post a sketch? What are you using to grab the pin in between your two lug plates?
 
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Hi

I think you are over analyzing. In my opinion your pin cannot be be approximated as a "beam" due to the small span/height ratio. Your span isn't long enough to develop a true elastic bending stress-distribution. IMO shear lugs and shear on pins controls the design.


In Roark's 7th edition, Pg. 125 (first page of Ch. 8): Roark's 7th edition said: The formulas in this section [Beams; Flexure of Bars] are based on the following assumptions: ... (6) The beam is long in proportion to its depth, the span/depth ratio being 8 or more for metal beams of compact cross-section,



Jeff
Pipe Stress Analysis Engineer
 
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Winelandy, the lugs are spaced 4" inside to inside as I stated with the pin passing through both (the pin is 12" long with the last 2 inches on each end threaded and a nut is applied once it is inserted to keep it in position, but the threads are not in the bearing plane.) A shackle supplied with the load (that is rated for it) will then be hung from the pin.

JGard, I have Roark's 8th edition and it states the exact same thing. I felt like i was over analyzing it and that statement makes me feel even more so. So Should I only evaluate the pin for it's shear capacity? in the future should I only evaluate pins based on their shear unless the span to depth is 8 or larger?

Does anyone know by any chance how a manufacturer such as Crosby analyzes the pins for the shackles they produce?
 
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Bsimms89:
What types of equipment are you using immediately above your 2.75" dia. pin, for this lift? Why are you using 2 - 2" lug pls, 4" apart? You could use a single lug pl. on your piece of equipment, there are shackles which would work with this single pl. The single pl. might be 2.5" or 3" thk., with two doubler pls. (doughnuts) at the pin hole, welded all around, and line bored for a pin. The welding of the lug pl. to your piece of equip. is a very important detail. This must be a good clean detail, clear load paths, attention to any eccentric or out of plane loading, etc. I would make my lug pin hole a little closer in dia. to the pin I was going to use. Something you can do to help with pin bending btwn. two lug pls. is to get a piece of pipe/mech. tubing with an i.d. a 1/16 or 1/8" larger than your pin, and 3.875" long. Then, the shear is more truly double shear and the bending is shared by your pin and the pipe, and your wire rope sling (or whatever) fits over the o.d. of the pipe.
 
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Bsimms89:
RE: Crosby’s pins, they sure as hell don’t assume they will be used/treated as 4" long (4 or 5 or 6" long) bending members. Although, some damn fool, in the field, will try using them that way and then not be able to get the pin out of the shackle, without a cutting torch. Also, this really cuts into their 5:1 FoS, but the pin probably still won’t break. You will find notes someplace in their literature which strictly prohibits pin to pin bearing, this is an awful condition from the bearing stress and pin bending standpoints. Thus, you should not be using the furnished shackle just above your pin, and that leads to my single pin pl. detail, with the furnished shackle. Get the details for that furnished shackle and design around them. You are not really over thinking this problem, you should actually dig a little deeper to understand the bearing stresses in the pin and the pls. and any potential bending of the pins. They just aren’t intended to be bending members, of whatever length, and then reused, over and over. Draw a cross sectional F.B.D. through the pin and the pls., and imagine the bearing yielding shape in the pls. (a triangular stress patern) and your 4" will quickly grows to 5 or 6" of bending span length. The more so, the greater the difference btwn. pin and hole dias. The admonitions in Roark, re: bending member should be taken into account. Also, look up Hertz bearing stresses and study this, it is important, in these types of problems.
 
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I don't know that Hertzian bearing stresses do end up being important in lifting lugs, more so in wheels or other rolling apparatus. Generally, localized yielding is tolerable at the pin/lug interface, designed as per BTH-1. But like dhengr said, you should not have a gap between the two lugs, and the pin should not be spanning 4". Absolutely do not use an off shelf shackle in this situation - you could weaken the shackle and put the next user at serious risk if the strength is compromised.
 
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I agree with dhengr..I usually use one pl for the lug...I also add doughnut pl's to the lug to obtain a width that will match a correct lift shackle with say a total of 1/16" gap..this way, it forces the contractor to use the appropriate sized shackle...also usually size the pin hole 1/32" greater than the pin dia...2.75" pin in a 3 1/16" pin hole does not sound right to me..
 
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Any experience I've had with Crosby... they make their stuff... 'big and strong' and they they make it bigger and stronger. I've seen their stuff really abused and it still functions.

Dik
 
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I am not permitted to explain much more about it per the client but a single lug will not work in this situation, the ears of the lugs also cannot be moved closed together, the shackle supplied that will be connected to the piece is specified for the piece and is a 50 ton capacity. The pin of the shackle will not be bearing in our pin, the pin of the shackle will be connected to the piece so the u end of the shackle will be resting on our pin. The load is known to be 80,000lbs exactly and is a perfectly symmetrical load. everything above the ears have been analyzed and peer reviewed including their welds, I just needed to determine a capacity to rate this pin at had this question regarding pin design as to what was an appropriate way to analyze this problem.
 
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It's not unusual for something to have a high MS for one mode of failure and a low MS (or -ve) for a different failure mode.

Treating the 4" long pin as a 4" long beam is IMHO conservative ... you could use the mid-thickness of the supporting lugs.

Treating the mid-span point as being reacted at the ends of the beam is IMHO conservative ... you could say the supporting lugs have a triangular distribution, peaking towards the load side (further reducing the moment arm).

But you may need to consider the pin being restrained in the lug bore, and prevented from doing it's reflex bending (setting up a "prying" load as the bore restrains the pin from lateral deflection.

Then there are other issues like will the pin deflect about the supporting lugs (and lose support) ?

another day in paradise, or is paradise one day closer ?
 
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Bsimms89:
You say..., “The pin of the shackle will not be bearing in our pin, the pin of the shackle will be connected to the piece so the u end of the shackle will be resting on our pin.” And, you really think that is a significant/any improvement? The shackle pin is 2.75" dia., the shackle body dia. is only 2.5", and your pin dia. is 2.75". That’s hardly an improvement, you still have round bar to round bar bearing you will still have real nasty bending and bearing stress problems, with considerable yielding involved. And, per Canwesteng’s comment, we do/can tolerate some localized (well confined) yielding in these cases. You can hardly avoid it with these type details. But, round pin to round shackle body is certainly not an optimal solution.
 
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I understand that it isn't optimal and I am seeing that I'll have to look into localized bearing stress more but from the point of the shackle how it is any different then if it was hooked into a crane block hook, there the shackle is bearing on what is essentially a round piece of steel and many of them aren't much thicker than 3". I can walk out back in our shop and measure our 50 ton crane block to get an exact number but I know that the hook on that isn't thicker than 3" and a 50 ton shackle can bear on that without any damage to the shackle.

Also I know this doesn't reduce the factor of safety required but this pin will be used one time to lift the load 5", then will hold it for a couple minutes then the piece will be lowered, so deflection of the pin isn't a huge concern so long as it is if suitable capacity to meet all required specifications. The pin could Ben be cut out and destroyed if nessecary, but that will have meant it yielded which isn't acceptible to BTH-1.
 
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