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.

 

[MJCronin]

I agree with LittleInch ... The V notches are a defect and not a "product enhancement"

Review the information on the link that I posted above by Portland Bolt.... They take extreme measures to ensure that there is a minimum required diameter at the corners of their products.

The notches are there because some shop in China can more quickly get more bolts out the door if they bend these against a sharp corner. There are no quality standards and they are being paid by the ton ....

Again, square u-bolts are a bit of a mongrel fitting that do not really follow any ASME, ANSI or MSS dimensional, product or quality standards.

MJCronin
Sr. Process Engineer

 

[r13]

LittleInch,

In my opinion, the purpose of the notch is two fold - 1) easier to bend, 2) the stresses in the remaining area tends to be more smooth.

Stress concentrated at the inner face due to necking.

 

[mfgenggear]

we can all agree that this the wrong tooling to use.
should be an Ibolt or bracket, to prevent moment, that is the main reason it failed.

 

[desertfox]

Hi

I think the bend in the ‘u’ bolt is detrimental, with such a sharp bend on the inside it’s likely the outer diameter of the bend would have developed cracks which would reduce the bolts capacity to carry load.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[r13]

When bending round bar, a specified inner radius must be maintained, and the force shall be applied gradually. Bend bolts/bars serve many functions, important thing is to follow manufacture's recommendations in bolt selection and use, pertaining the application. Everything will fail, but what is the percentage and frequency need to be addressed before scrap an idea.

 

[desertfox]

Hi retired13

Looking on this site it recommends inside bend radi of a rectangular ‘u’ bolt should be at least twice the diameter of the bar, looking at those pictures here those radi are nothing like 2x dia.

https://www.portlandbolt.com/technical/faqs/guidelines-for-ordering-u-bolts/

However the bend radius is only one part of the problem because as already stated in other posts we have suitability for the job, loading on the bolts and the bolts have been re-used so they might of been over stressed the first time they were used which really means they should have been re-placed.



“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[r13]

desertfox,

I think OSHA only requires inspection on the lifting device by a competent person on each use. The question here is who is competent enough to notice and pick out defects. Un-trained eyes usually won't, so replace after each use maybe is the desirable solution.

 

[desertfox]

Hi

Normally lifting devices are tested with a test load, design of lifting rigs are usually to a code of practice so by the time a load test is carried out the design of the lifting rig has under gone various inspections of material certificates, NDT testing Etc, some crackS, flaws Etc cannot be detected by the naked eye that’s why it’s tested but in this case some of the defects are noticeable to the naked eye namely the atrocious bends on the ‘u’ bolts, in addition there seems to be no evidence as yet of any paper work relating to either the design or manufacture of this rig. It would be interesting to know whether the boat repair is being paid for by the insurer cause if I was the insurance agent I would be asking lots of questions, worst still if someone had been killed or badly injured

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[desertfox]

Another good source of info on u bolts here

https://www.engineersedge.com/hardware/ubolt_application_and_design_9899.htm

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

1) Was your failed U-Bolt Hardware made in China ?

I think you nailed the ultimate root cause right there.

If I wanted to be philosophical about this I wouldn't say this was a manufacturing defect. The entire idea was so flawed that it can only be described as amateur. They were trying to bend a bar over what was essentially a chisel, instead of a smooth die with an actual radius. There's a fair chance it was cracked when it left the basement, er, factory.

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

 

[kingnero]

These are bent in a press brake, I've seen the exact same notch in rebar as well. I always wondered why the rebar didn't break, so hey, it must've worked, right?

 

[mtgski]

LittleInch

Also yes but that single picture is unclear as to how the boat "lifting bunks?" are actually attached to the frame and how , if at all, these are tightened. Needs more pictures or a drawing / side on view. Are these bunks designed to slide to fit the particualr boat? Is there any tension on the bolt before the boat is lifted out of the water? The existing bolts seem to have scrape marks on them?

I diodn't quite understand this bit "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. I didn't quite understand this bit " Can you explain / draw what you mean?"

Where is the load path?

Does the bolt go through the holes visible in the lifting bunk to adjust height?

Answers Yes, the U bolt goes over the rectangle cross beam and into the holes of the adjustable height bracket. The bunks are adjusted to fit the hull of the boat and then tighten down solid. They do not ( or should not) move once mounted and tightened down to the rectangle cross beam.

By the "Angle of the lifting bunks" I meant that the top of the U bolt is at an angle or offset from the bottom of the U bolt. Since the U bolt is wider than the rectangle cross beam, the bottom of the U bolt does not contact the bottom of the rectangle cross beam unless it is slanted at an angle until the bottom portion of the U bolt contacts the bottom of the rectangle cross beam. Most boat lift bunks mounting brackets are mounted at a slight angle

I believe the load path would be on the shank of the U bolt that is contacting the top and bottom of the cross beam they are bolted to.

Let me know if these photos help or not:

 

[LittleInch]

Yes that helps hugely - would have been nice at the start....

Anyway - how tight was that U bolt? Or did it vary? I can see that the angle is important to match the boat size, but must have been tightened to stop it slipping sideways? Th scrape marks on the other bolts indicates it has been moved sideways quite a bit.

You can imagine someone really giving that nut some serious torque to clamp the U bolt and lifting frames. Do it too tight on the top angle first and you will get a world of pain at that elbow.

Then unless you get it really well centred you have a moment on the U bolt trying to twist it off the frame.

There is also the distinct possibility that the top of the U bolt is not on the top of the frame at the start, but once the weight increases as you lift the boat out of the water, the frame could easily slip down a few mm but now with shock loading or equally moves under load as the boat lifting pads move under load

At best you get all of the shear stress going through the top angle of the bolt and virtually none on the bottom leg.

Then it gets loosened, moved for the next boat, tightened up again etc etc?? This bolt I would guess wouldn't last 100 such cycles, maybe as low as 20.

Whatever load these bolts were tested to (??) it wouldn't have been in that direction. I can only assume the load is based on a sling along the top bar of the U using BOTH shafts and then lifting something upwards.

This use is totally different so your load strength is meaningless.

In short this is a pretty terrible "design" and to avoid a second or third collapse you really need to re design the connection between the bunks and the frame. IMHO.



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

 

[mtgski]

LittleInch

The U bolts & mounting brackets were loosely assembled over the rectangle cross beam, then moved into the desired position, with the top of the U bolt resting on the top of the rectangle cross beam. At that point, the top mounting nuts were very slightly tightened just to the point of a little tension so they would not move so the bottom of the U bolt could be re-positioned at the angle to have contact with the bottom of the rectangle cross beam. Then the bottom nut of the U bolt was very slightly tightened just to the point of a little tension so they would not move. Then both the top and bottom nuts were tightened in an alternating pattern, a few strokes on top then a few strokes on the bottom until the U bolts were tight by hand with the use of a 1/2" drive ratchet wrench with a 3/4" socket. However, no torque wrench was used so the amount of tension on each U bolt nut probably had some variance to the amount of tension.

The scratches you see on the U bolts are not into the metal of the U bolt but actually a "growth" of a hard water type deposit onto the metal surface that easily flakes or chips off with your finger nail with no damage to the metal of the U bolt. The "scrapes" of that growth could have happened during the removal process and/or repositioning during remounting as you can see only a little on the failed U bolt sections. But you may be correct that there is the possibility that the mounting could slip or compress even a couple of mm during the boat lifting.

The U bolt tightening cycle were only cycled two times, tightened once during original install last year and then again this year for the re-install on the different hoist.

I am planning to change the mounting hardware and use normal bolts and a backing plate with the adjustable mounting bar. Should I use grade 8 or grade 5 bolts for extra insurance that I do not have an similar issue again?

 

[desertfox]

Hi mtgski

Therein lies the problem in that you have no idea what tension is in those U bolts and even if you replace them with conventional bolts you might encounter the same problem, someone needs to go through the design of that rig, we can’t say it will be okay using this grade or that grade.
Looking at the last picture you posted I would say that the angled arms pointing downwards need to sit on top of that channel section and that way the load path utilises the full channel depth and gives a stiffer joint, the design at present relays totally on the u bolts to transfer the load in shear, if I understand your new proposal Correctly the load path will be no different.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[MJCronin]

Drill out the holes and go with NEW 5/8" SS square U-bolts...Use a locking method on the nuts

Stay away from Chinese U-bolts with sharp notches.....

MJCronin
Sr. Process Engineer

 

[r13]

Hate to ask that, but how do you verify a parts wasn't made in China? Does the seller provide certificate of origin?

 

[BUGGAR]

I did some more research. It seems the notches may be to "weaken" the bar in just the right locations to get the leg bars at the desired spacing. Similar to the notches sometimes seen at the edges of sheet metal to make sure the metal bends in the right place.
It is "assumed" that the threads define the load limit.

 

Hate to ask that, but how do you verify a parts wasn't made in China? Does the seller provide certificate of origin?

These days that is the working assumption, unless stated otherwise.

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

 

[mtgski]

retire13

The owner of the company of the brand of lift that these u bolts are made for verified that he has them made in China.