Loose fitting pins in lifting lugs 3

[bc1080]

We have a lot of old lifting equipment around and are having compatibility issues with trying to design new hardware for them. In particular, many of our bridge cranes have load blocks with pins to attach lifting devices. Frustratingly, these pin sizes vary between the cranes and many of them are unusually large for the load capacity. So the challenge for trying to design something interchangeable is having to either make custom linkages for each crane or having loose fitting pins. As I expected, the in-house analyst said that the pin should be as tight of a fit as possible (which I know is the optimal situation). However, I know the guys on the floor have been using sub-sized pins for years in these cranes. Of course that doesn’t make it right, but talking with them and some vendors it sounds like this is not unheard of in the lifting industry.
Unfortunately, I am not super familiar with lifting codes/regs and I was wondering if there is an industry standard that addresses pin/lug fits or some alternative sizing method that helps determine if you can use a smaller pin in a larger hole? If we wanted to put, say, a 3 inch pin in a 6 inch hole, is there anything we could check to feel more comfortable with the strength of the assembly other than just gut feel of the lifting crew?

 

[Teguci]

http://ncsx.pppl.gov/NCSX_Engineeri...tion/Stage 3/analysis plus fea models/BTH.pdf

"Below the Hook" Design info. Check out page 25. The hole to pin size should be 1.1 max.

When I designed lifting plates, the tricky part was to select a plate thickness and hole size for the lug that would only allow the right size shackle to fit so that the "guys on the floor" were required to be grossly negligent when they used the wrong shackle size.

 

[andriver]

+1 for ASME BTH. This is what you are looking for.

If this were a onetime use situation, I wouldn't be worried about a smaller pin as long as it passes your bearing check. However, if this is going to be a typical detail, over time you will see local affects where the pin bares on the hole and could lead to a catastrophic failure.

You may be able to add cheek plates if you have a very small pin and that would extend the life of the blocks.

 

[boo1]

The thickness of the padeye at the hole should not be less than 75% of the inside width of the joining shackle.
Check Contact stress

 

[bc1080]

Thanks for the answers and the links to the standards and lifting beam design. I had actually browsed though the BTH 2014 standard, and they appear to have removed all the helpful diagrams and changed the methodology for determining lug/shear plate calculations. The documents shown here are much more straight forward. The reliability and durability over time is a definitely concern, so for any lugs/pins that are not trivially loaded, we are probably going to use the BTH standard guidelines as rules of thumb and contact vendors for specific permission if we have to violate this for COTS items.

 

[IFRs]

+1 for Teguci

"When I designed lifting plates, the tricky part was to select a plate thickness and hole size for the lug that would only allow the right size shackle to fit so that the "guys on the floor" were required to be grossly negligent when they used the wrong shackle size."

Bingo !!

I generally also make the hole far enough away from the edge so only the correct shackle would fit. Lots of screaming in the shop and field but I got what I wanted!

 

[andriver]

IFRS,

I have sized the hole for the required shackle, but never thought about moving the hole low enough so the smaller shackles can't fit! Brilliant.

 

[dhengr]

Bc1080:
There are a number of things you have to consider here, and a copy of ASME BTH-1, is a must to get started and CYA. Also, take a look at what OSHA has to say about material handling/lifting operations within a plant/a business. You really should have a handle on these issues to keep you and the company out of trouble. While I never downplay the experience and knowledge of a good mechanic or craftsman, their “gut feel of the lifting crew?” won’t get you very far in court or with OSHA, if anything ever goes wrong. You really do have to educated the workers as to what some of the limits are, their seat-of-the-pants just doesn’t cut it, nor does ‘that’s the way we’ve always done it.’ In all cases, a regular inspection program of all lifting equipment, with records of same, is a very good idea.

I assume that all of your cranes do not have the same lifting cap’y. Obviously, the lifting hardware should typically be matched to the crane cap’y., it’s often stored right under that crane. At the same time, there is nothing wrong with using a 10t spreader beam on a 30t crane, as long as you pay attention to their compatibility at their interconnection point. With the smaller pin on the beam and a lighter pin pl. you have the condition you originally asked about, and one thing you can do is to bush the larger hole or holes with a heavy piece of mech. tubing. This is bored to fit the smaller pin and then the o.d. is made to properly fit the larger hole in the crane block. This bushing can also be longer than the center pl. thickness to improve pin bending conditions due to excessive gaps btwn. the three pls. Many times a pin pl. does not have to be as thick as the throat opening in the shackle, or the like. These pls. can be thickened by welding cheek pls. to them; we called these doubler pls. Then the pin pl. and its one or two doubler pls. are bored for the pin. These holes should be neat and clean and you should break/chamfer the corners on the outer edges.

Obviously, the pin must be strong enough to lift the load below with some reasonable margin of safety. If the pin dia. (shackle pin?) and the pin pls. are designed properly, that is checking the pin for double shear, and this cap’y must be sufficiently larger than the load being lifted. From there, the analysis and design get a bit more complex. The pin bearing and in turn its bearing on the pin pls. and bearing stresses are a function of the relative difference btwn. the hole and pin dias. A loose fit leads to very high bearing stresses and plastic deformation which causes the two different parts to deteriorate quickly, or possibly fail. Thus, the 1.1 Dh/Dp ratio that Teguci mentioned. Also, the pin can flex in holes, much like a simple beam with a concentrated load from the center pl. This kind of flexing can cause excessive edge yielding on the three pls. from bearing loads. If the gap btwn. the side pls. and the center pl. gets too large, you really do have a pin bending problem. Then the pin tends to act like a cantilever supported by the center pl. and the slope or deflection can get larger and more damaging to the pin pl. edges. Also, the pin is not particularly good as a bending member with high shear and bending stresses in the same region.

 

[boo1]

"3-3.3.5 Pin-to-Hole Clearance. The static strength provisions of para. 3-3.3 apply when the diameter of the
pin hole is not greater than 110% of the diameter of the pin. If the pin hole diameter exceeds this limit, the effect
of the clearance shall be taken into account when determining the strength of the connection."

 

[bc1080]

Right, you hit on more or less my exact concerns. The gut feel of the lifting crew is regards to "standard" day to day lifts that are made without engineering consult. I.e. the shop guys just grab a crane and reposition a 500lb machine or something across the floor. When they are using equipment/rigging within its posted capacity and picking things up by designated lifting points, they generally do not contact us for assessment (this is a battle that might need to be fought as well). I am not comfortable using this methodology on heavy/unique lifts.

You're correct in assuming the lifting capacities vary. They are all well within what we are trying to lift, just with different interfaces. I think the best plan is what you have described in having unique bushings and connection hardware for each crane to follow the guidelines laid out in the BTH standard (110% hole/pin ratio), etc. I believe doing this work correctly up front to adapt all cranes to a common size pin is probably the best solution for the vast majority of what we need. I am glad to hear there is not a precedent for use of undersized pins because I was starting to think I was crazy by thinking this was a bad idea after the suggestions recommending we do this.