Stainless Steel Square 1/2" U Bolt Failure 6

[mtgski]

Looking for some insight on a failure of a Stainless Steel Square 1/2" U bolt. 3 1/2" legs and 5 3?4" wide inside. Rated for 5000 pounds capacity with a 20% over factor so 7000 pounds.

Background: this is one of 4 U bolts used on the lifting bunks of a boat hoist that hold up the boat on the lifting frame. Overall capacity of the lift is 4000 pounds ( these U bolts are normally used on a 5000 pound capacity rated boat lift) Boat wet weight is 3500 pounds. The U bolts in questions were brand new last year, used for 5 months on a boat lift in fresh water. The U boats and bunks were removed from one lift to install on another lift this year. On the second day of the boat being loaded on the lift the failure occurred on one of the four U bolts about 45 minutes after the boat was put on the lift.

Do you think the tooling mark creases or cuts into the inside corners of the U bolt contributed to the failure? ( since the failure point was at or very near these tooling marks)

Would the angle of the single U bolt when mounted, with the upper and lower portions not being vertically aligned with each other, have any affect on the load contributing to the failure?

Brand new U bolt

One of the three remaining one year old U bolts

Failed U bolt, inside of bend area

Failed U bolt, outside of bend area

Angle of the lifting bunks, U bolts were offset at an angle about 3 1/2" off center due to 5" cross-member tube and 5 7/8" inside U bolt width.

 

[Compositepro]

That notch is a very intentional part of the bending process. You could not get such a sharp bend without it, without tearing occurring on the tensile side of the bend. The overall design the of the boat lift may be flawed, or less than perfect, but that is the way square bends are made.

The whole boat lift is obviously not terribly robust. It is designed to be affordable. Its purpose is to lift a boat slightly out of the water while at dock to reduce maintenance. It is not for overhead lifting. It uses a hand wheel to lift the boat with cables. Most of the lifts I have seen use floats that are filled with air from a blower on the dock. Those have cradles as well that have to be adjusted to each boat hull.

 

Compositepro,

1/2" solid bar is plenty of material to make a robust U-bolt for 5000 pounds rated capacity. That is unless it has been severely compromised by the manufacturing process. Ductility has been exhausted in the material near the sharp notch in this example, and as I said earlier, was possibly even cracked before it was installed.

Carbon steel with suitable coating protection would not have been a bad option here.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[r13]

mtgski,

Thanks for your feed back. Sadly I was wondering whether there is commercial bolt produced in the US, the percentage, and price.

 

[jimstructures]

retired13,

Yes there are many US bolt suppliers which manufacture their bolts in the USA.

Two that come to mind are Portland Bolt in Portland, OR and Unytite in Peru, IL. I do not work for either company. Both are good companies to work with.

Jim H

 

[r13]

Yeah, I forgot Portland Bolt.. Thanks.

 

[LittleInch]

This is what I would suggest. Anything else including the U bolt puts a whole lot of unknown tension and bending stress on the bolt / U bolt.

Or make your upright out of a pole and use a normal U bolt...

You may just have had a bolt which was particularly bad, but the design isn't great and the bigger diameter you can make your bolt the bigger I would say.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydtools]

If you use the u-bolts the other way around, would there be less mechanical advantage to cause bending/twisting failure at the bend? Insert them through the strut and use a tie plate or short piece of strut material under the nuts.

Ted

 

[mtgski]

Thanks for all the observations and suggestions. I am planning to go the route similar to what LittleInch diagrammed with 9/16" or 5/8" grade 8 coated regular bolts with a backing plate. But I still plan to pursue contacting a state regulatory agency to get those inferior 1/2" SS U bolts pulled from the boat lift supply stores so no one else goes through what I have!! Thanks again.

 

[mfgenggear]

Good for you mtgski

 

[LittleInch]

Still think a two pronged support is better but good luck with the bolt idea. Send us a photo when you've done it, we love seeing results even 3 months later.

The notch is a bad idea when that notch is exposed to forces it wasn't expecting. Anything other than a direct pull on both shanks could result in failure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[oldestguy]

An observation by OG. How's come China got in here anyhow when the design and use had nothing to do with where it came from. My bet if that U-bolt came from Germany no one would bring it up, yet same breakage likely.

 

[EdStainless]

OG, not really, because no one else would ship ones with sharp inside notches in them.
The structurals that these fit have rounded corners, so the inside of the U-bolt should be rounded as well.
Either these were designed for a special use and then just supplied as generic, or the tooling in FUBAR.
Leaf spring shackles and trailer axle mounts have used squarish u-bolts for ever and they do fine, without sharp inside corners and no angular loads.

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