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SS Bolt Stretch

11/03/2015 1:37 PM

We tightened a bunch of 18-8 SS 1/4-20 HHCS x 1" long with one time use Top-Locknuts a week ago. Upon inspection today, it took another 1/4 turn to torque the same 8 ft*lbs. Has anyone noticed this before? How do you deal with it? Hate to have to torque twice, once for assembly and once again for delivery.

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#1

Re: SS bolt stretch.

11/03/2015 1:45 PM

You don't need to torque twice......@ 8 ft*lbs torque, it looks like the bolts were over-tightened. and now they lost their elasticity and are failing.

18-8 SS 1/4-20 bolts should only have been torqued to 75.2 in-lbs dry - 63.9 in in-lbs wet.

or 6.3 ft-lbs /5.3 ft-lbs respectively.

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#2
In reply to #1

Re: SS bolt stretch.

11/03/2015 2:02 PM

One other thing,.... were these bolts 'used'? or already have been in service.

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#4
In reply to #1

Re: SS bolt stretch.

11/03/2015 3:53 PM
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#3

Re: SS bolt stretch.

11/03/2015 2:10 PM

What is the manufacturer country of origin?

We have experienced severe issues with bolt quality that does not meet standards resulting in bolt stretch and loosening before any load is applied to them.

We are suffering a high incidence of in-service new bolt failure also.

Replacing the poor quality bolts with high quality bolts manufactured in reliable countries has solved the issues.

We have had to place several items such as bolts, valves, and lifting devices in our warehouse Global Sourcing "purchase not allowed" category.

Choosing lower price over quality never works out for the best.

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#6
In reply to #3

Re: SS bolt stretch.

11/03/2015 5:11 PM

Yes, In China, their production practices are so advanced that scrap steel melted bots can be turned in Stainless Steel with a rubber stamp on the box that says "Stainless Steel"

It is a wonderful way to save money.

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#12
In reply to #6

Re: SS bolt stretch.

11/04/2015 7:31 AM

Several years ago there was a drug dealer that got out of the drug business to make grade 8 bolts. All he did was to imprint Gr. 8 on the head.

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#5

Re: SS Bolt Stretch

11/03/2015 5:06 PM

OOPS! This could kill a lot of Atta-Boys.

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#7
In reply to #5

Re: SS Bolt Stretch

11/03/2015 5:18 PM

Yep. I don't know about anyone else but in my world 1 Oh-Sh*! cancels 200,000 "Atta-Boys"

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#8
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Re: SS Bolt Stretch

11/03/2015 11:00 PM
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#9
In reply to #7

Re: SS Bolt Stretch

11/04/2015 12:04 AM

I like my world better. It's always been 50 got wiped out.

Good Luck, Old Salt

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#17
In reply to #7

Re: SS Bolt Stretch

11/04/2015 9:18 AM

Yup, just take a look at the paperwork for your Employee Review the next time you ask for a raise:

You could have saved the company several hundred thousands of dollars, improved the company's output 300%, AND saved the CEO's life TWICE during the past year, but "This pattern of you clocking in late every Thursday for a month" (Which was your supervisor stopping you before you got to the time clock to try and get you to buy the horrid candy for his little girl's school fundraiser, incidents that were clearly caught on the company security cameras, you would have been at the time clock on time, a few minutes ahead of your shift in fact, but your supervisor's antics were causing you to clock in two minutes past the start time) means you're not eligible for a raise this year, you really need to learn to be a 'team player.'

Yeah, that's an extreme example, but it seems that no mater what miracles you pull off, your boss seems to find ways to negate them with something trivial.

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#19
In reply to #17

Re: SS Bolt Stretch

11/04/2015 9:37 AM

I was the engineering manager at a company in the 90's and the owner and president of the company always sat (stood) by the exit door after you clocked out to talk to you on the current or new projects or issues. Always at the minimum of 1/2 hour usually 45 minutes to and hour long.

It became a joke and quite comical. So I told the engineering group lets make it difficult for him and see what kind of reaction we get by all (and these were my exact words) Herd up in engineering and make a quick dash to the door like a herd of water buffalo crossing a crocodile infested river.

There was so much laughing going on as we charged to the gates especially the look on the bosses face of 'wtf,..... I wanted to talk to these guys'.

A lot of the guys just lost it. It was a National Geographic moment.

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#10

Re: SS Bolt Stretch

11/04/2015 7:09 AM

These appear high end brand new one time use bolts with top-lock nuts. The customer spec is 8-10 ft*lbs. So I had them torque to 8. Since I also believe the spec is too high. Looks like I am going to try to convince a large international company to review/revise their specs. I said 18-8 but these are 316.

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#11
In reply to #10

Re: SS Bolt Stretch

11/04/2015 7:15 AM

The customer specification is incorrect.

The problem is on their end..... did they get confused with coarse thread (1/4-24) and fine thread (1/4-20) specifications?

I believe that's where the problems lies.

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#21
In reply to #11

Re: SS Bolt Stretch

11/04/2015 1:37 PM

Looks like you mixed up your fine thread with your coarse. 1/4-24 is finer than 1/4-20.

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#23
In reply to #21

Re: SS Bolt Stretch

11/04/2015 2:04 PM

good catch... .. and let that be yet another lesson on how easy it is for an error to happen.

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#24
In reply to #10

Re: SS Bolt Stretch

11/11/2015 5:45 AM

My first thought actually...316 is no good for any application requiring a torque setting in my experience, simply too soft.

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#13

Re: SS Bolt Stretch

11/04/2015 7:52 AM

Always get a mechanical certificate on hardware purchases and if from a suspicious source have another test done on home soil. This advice comes from numerous bad experience which was solved with the certificate of compliance. My designer draftsman handled this issue as part of his contract.

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#14

Re: SS Bolt Stretch

11/04/2015 8:03 AM

Thanks all. This was customer supplied hardware and torque specs.

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#18
In reply to #14

Re: SS Bolt Stretch

11/04/2015 9:29 AM

"Here are some bolts, torque them to 8-10 foot-pounds."

"Um, the bolts you supplied aren't rated for that level of torque."

"Nonsense, take the bolts I gave you and torque them like I said."

(Some customers just don't understand that you can't get Red Roses from a White Rose Bush.)

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#15

Re: SS Bolt Stretch

11/04/2015 9:07 AM

I was taught to loosen the nut slightly before re-torquing. If you mark the nut and bolt before loosening and then re-torquing, it would be interesting to hear the result.

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#16
In reply to #15

Re: SS Bolt Stretch

11/04/2015 9:12 AM

That sounds similar to preloading.

Preload is the term used for a bolt's clamp force.

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#20

Re: SS Bolt Stretch

11/04/2015 10:01 AM

Without looking at all comments, the two issues are likely micro-galling (esp. with locknuts) and temperature rise (esp. at rapid torquing). With the former, try going to full torque, then backing off slightly and re-torquing to spec.

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#22

Re: SS Bolt Stretch

11/04/2015 1:50 PM

I would think that just turning a locknut (with it's higher amount of friction) would "use up" a portion of the torque applied.

It may take a few pounds just to move it on the threads before clamping force is even a factor.

Add this additional friction to the torque required for the clamping force (or bolt stretch) and it may explain their higher torque requirement.

If you torque charts took that into account already, then nevermind.

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