Multiple Shear Lug 1

[frannypants]

Hi everybody,

I have a problem with a multiple shear lug (also called "finger type" on Niu's Airframe Stress Analysis and Sizing - page 323).

I have to make a quick check on the lugs, and I cannot find how the shear load is reacted in the lugs in no-failure and failure cases (which lug gets the highest load, ...).
In literature (Niu and Bruhn) I couldn't find anything helpful and I was wondering if there's something available about this kind of joints in order to optimize the design (number of lugs in each side, thickness, ...).

Any advice would be greatly appreciated!

Cheers

 

[dhengr]

Advice...? Proved some sketches and more details, loads dimensions, materials, etc. Or copies of a few pages of that text, we don’t all have copies of that book.

 

[frannypants]

Hi, I found online a good picture:

There is a component like the one in the picture (3 lug heads) plus another one with only 2 lug heads. They are connected through a bolt.

 

[dhengr]

Are you a student? What is your engineering background and current engineering duty? Why didn’t you draw a sketch of your actual application, and what is that? This picture is only one of many which might have been drawn from the description in your OP, what gives? Again...., more details, loads, dimensions, materials, bolt size, pin hole size, etc.; these details and proportions are important to a meaningful discussion of this type of problem. Don’t be so darn lazy, you are the one who wants help. Don’t make us do all the work. Do you have a local mentor or a boss who gave you this problem, who you can discuss this with, and get more direct and immediate feedback?

How many shear planes are there on the pin? How does the load on the outer two lugs compare with the load an the middle lug in the picture you show? And, how do these loads compare with the loads on the two lug part of the joint? What is the stress in the rectangular bar behind the lug heads. How does the picture you show compare with an eye bar? What is the bearing stress on the pin at various locations?

I’m not going to participate in this thread any longer until you answer the above questions. Awaiting your answers, there is plenty which could be discussed about that kind of a joint.

 

[rb1957]

a rational assumption would be each lug reacts 1/3 of the load. (i guess Niu considered this is obvious, and focused on fail-safe designs).

if you want to go to town on it, i guess the bending stiffness of the bolt would redistribute the load between teh lugs, probably peaking in the center lug. but that'll depend on fits, etc

 

[frannypants]

Thank you rb1957, you're right, there's a peak in the central lug.
I ran some analysis and saw that with the particular design solution (3+2 lug heads) is correct to assume that the central lug gets the 60% of the entire load applied in normal operating cases (without failure).

 

[rb1957]

if the three lugs are the same width, 20:60:20 sounds quite skewed, indicating a very flexible pin.
i think if you've got this distribution, then your pin is heading for trouble in bending.

30:40:30 would be more reasonable (given a sensibly designed lug/clevis/pin ... ),
25:50:25 can be rationalised (evenly distributing the load for the two clevis lugs).

if you're thinning down the outer lugs, you'll lose your fail-safe capability (if you want that)