Lifting Shackle Stresses 1

[willwoll100]

Hello All,

I'm currently designing a lifting shackle to aid the recovery of a ROV, my problem is that I'd like to do some hand calculations to find the stresses in the shackle but I'm really not sure as to where to start.

I know how to calculate the stresses in the fixing holes but it's the point where either a hook or another shackle will be used.

I've looked all over the net and in Roark's but to no evail.

Please see the attached image.

Regards

Will

 

[BigInch]

P/A + Mc/I for a curved "beam".
Strength of materials textbooks always have one of those problems.

Moment Maximum will be P/2 * 1/2 distance between the bolts. Limit stresses to at leasat 1/4th the elastic yield point so you don't get something that looks like your sketch, and watch stress concentrations in the threaded sections.

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[BigInch]

PS Shear load across the section may control too, since bending span is very short, but effective area becomes deeper with the curve as shear load increases, so may not control.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

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http://virtualpipeline.spaces.liv

 

[willwoll100]

Cheers for this biginch, one question regarding the moment.

You say that the max bending moment will be the force applied/2*1/2 distance between the bolts. What is meant by 1/2 distance between the bolts?

Cheers Again

 

[BigInch]

Just using a simple pinned end beam model, which should be conservative, so moment at the centerline is P/2 * L/2, L being length of "beam span", or distance between supports, I considered that as distance between bolt/or, in this case "nut" centerlines. So P/2 * L/2 = M = P * L/4

That could probably be reduced a tiny bit, because you might get some end moment resistance from bolt and nut fixity, thereby reducing the centerline moment, but I'd not count on it and just use PL/4 * c/I to get the bending stress. Tension load is P/2 to each bolt end, with maybe PL/10 as a fixed end moment.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[Struct71]

Why not take a standard shackle eg from

http://www.thecrosbygroup.com

?

 

[willwoll100]

Cheers for the site Struct71 but unfortunately the pin dia needs to be a specific size along with the gap between the 'arm' points of the shackle (internal distance).

I did have a look around briefly before.

 

[TheMan008]

Willwoll100, if you don't know how to do a rigging analysis you should have your boss teach you how.

Or you could hire one of the many experienced unemployed engineers to do the calculation for you.

To do this analysis. You need to figure out how much Load is going to be lifted. The center of gravity will play an important part in deciding how the weight is divided.

Also, you need to read the American Petroleum Institute Recomended Practices on crane lifts and wire rope.

Hint: Two of the four wire ropes must be able to carry the whole load.

While the weaklest link of the whole system could be the pin, it is often to over-engineer the padeye, so be careful.

The shackles are selected based on the weight.

Weight-> Shackle -> Wire Rope -> Master Link -> Block -> Crane -> Padeye -> Shackle -> Wire Rope


It's an iterative process.


Try not to kill or injur anyone.

 

[swearingen]

Don't forget that for a curved section, the I and neutral axis are different than for a straight section. If the cross section of the shackle bow is round, the neutral axis does NOT pass through the center...


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[dooron]

You can look at Roark "Curved Beam analysis" section,which basically calculates a straight fixed end beam with point load at mid point (width of shackle) and multiplies by a factor to allow for curvature.
You can also calculate as a fixed end DEEP beam , Roark "Beams of relatively great depth", section, assuming a pinned ends.
Don't forget to check the Bolt end for tearout.
Finally you can check for contact stress using Roark section "pressure between elastic bodies", cylinder on cylinder, Allowable=2xRe.