Why the Torque Wrench is Your Best Friend

[Mike.Bray]

I do R&D work to pay the bills so I'm constantly trying new things and (hopefully) learning new things. A few years ago I was having issues with fasteners on a project, specifically either breaking or coming loose. I had the opportunity to sit down with an automotive engineer one day and he passed on some extremely valuable (and potentially lifesaving) information about fasteners that I would like to share. I think it's fair to say the automotive industry has fasteners figured out.

The first thing he asked me was if I ever thought about why fasteners in an automotive engine never have "lock washers" while at the same time they never come loose. Think about connecting rod and cylinder head bolts. Not a washer of any kind in sight. Hmmmm Now that peaked my interest!



You wouldn't think of piece of solid steel like a bolt being a spring but it is. When that bolt is stretched the proper amount the "spring effect" applies enough tension or clamping force to prevent it from coming loose.



This is Hooks Law





Correct torque can be calculated.




Lubrication of the threads factors in to the torque calculation.

And here's a handy chart if you want a quick reference. Of course there's tons of information on Google, just be careful that you know the material and lubricated or not.



I recently had a project where a connection being driven by a servo motor was coming loose. I changed the design so that there was a 5/8" titanium stud about 6" long being used to apply clamping force. I did the calculations and it should torque to 185 ft/lbs and this would give it 0.026" stretch. I made the stud and a test piece with a hole through it so I could torque it and physically measure the stretch. At 185 ft/lbs it stretched exactly 0.026" So it all worked.

One last note, about those silly little "lock" washers or split washers. Sorry to be the bearer of bad news but they're not helping any. If they make you feel better or you need a small diameter flat washer they're not going to hurt anything but there isn't enough tension generated by them to keep a fastener tight. Again, when's the last time you saw one in an OEM engine?

I hope some of you find this helpful. Keep that torque wrench always by your side and happy & safe building.

[BadAsp427]

As a jet engine mechanic, torque was everything. Even the entire rotating mass through the engine was torqued using a dial caliper to measure how much we stretched the shaft. And the wrench we used was a torque multiplying wrench and the actual torque was never really known. Only the total stretch of the shaft. The shaft was about 8 feet long and roughly 4" in diameter. I do not remember how much we stretched it, but it was always fun to watch the faces of the new guys as we pulled the 3' breaker bar on the multiplier.

[Al_C]

Thank you for posting this, Mike! There is a wealth of expertise on our forum. I appreciate everyone who shares it!

[CraigS]

I have also read that when tightening by measuring stretch you can check the quality of the bolt by measuring TQ at the same time. If you get the stretch before getting to the expected torque, your bolt may be weak. Your formulas show why that works.

[Mike.Bray]

I have also read that when tightening by measuring stretch you can check the quality of the bolt by measuring TQ at the same time. If you get the stretch before getting to the expected torque, your bolt may be weak. Your formulas show why that works.

That is true. I've got more information on that subject as well as torque wrenches. Outside of the aerospace industry most people don't have their torque wrenches calibrated so that can be a problem also.

[GT53]

That is true. I've got more information on that subject as well as torque wrenches. Outside of the aerospace industry most people don't have their torque wrenches calibrated so that can be a problem also.

I have 2 questions.

First question: if a bolt gets “over torqued” i.e. let’s say torqued to 110 ft. Lbs. and the recommended torque setting was 60 ft. Lbs., will the over torquing result in damage (over stretching) to the point that said bolt needs to be replaced?

Second question: I have seen some DIY methods of calibrating a torque wrench that seem questionable to me so in lieu of an unproven DIY method, where can one get a torque wrench properly calibrated?

Greg

[Mike N]

You can over torque (yield) a steel bolt by a small amount and it will be fine. All you have done is stretched (yielded) the steel. Take the factory torque to yield fasteners as an example. Think of it this way, as you stretch (yield) the bolt the cross section of the bolt reduces. The more your stretch (yield) the bolt the thinner the bolt will get. As it gets thinner the ability to carry load also reduces. By the way the formulas above are a bit of a simplification. Look at the same diameter threads but with different thread counts the torque is different because the diameter at the root of the thread is different.

[Mike.Bray]

I have 2 questions.

First question: if a bolt gets “over torqued” i.e. let’s say torqued to 110 ft. Lbs. and the recommended torque setting was 60 ft. Lbs., will the over torquing result in damage (over stretching) to the point that said bolt needs to be replaced?

Second question: I have seen some DIY methods of calibrating a torque wrench that seem questionable to me so in lieu of an unproven DIY method, where can one get a torque wrench properly calibrated?

Greg

Excellent questions!

A simple answer to number 1 is if you stretch a bolt beyond it's elastic limit it loses the "spring" in the metal and will not return to it's original shape or length. That's why torque calcs seldom go over 75% of the bolt's Yield Strength. Once this occurs the bolt cannot maintain tension and will either relax/loosen or fail outright. So the bolt is junk.

Second question. Obviously the best method is to calibrate your torque wrench on a machine designed for the purpose.





Or here's a simplified method. I'm not sure I would trust it but it's better than nothing.



Hope this helps.

[Jacob McCrea]

"where can one get a torque wrench properly calibrated?"

Process Instruments, Inc. in Pittsburgh, PA. https://procinst.com/ A good friend of mine owns the business.

I don't recall what the price was, but it was meaningfully cheaper than buying a new wrench and you'll know for sure that the wrench is calibrated correctly.

[Derald Rice]

I was on a tour of the engine bay of a Royal Caribbean cruise, and one of the things that I remember is that
they did not use torque wrenches.....

Bolt stretch was all that mattered.

[Jeff33Ford]

Just curious question. All this talk about places not using/having torque wrench. Wouldn't that suggest they use every bolt with a nut? Don't they have/use any bolts without nuts? Like main journal bolts? I would think the same principle apply which would require a torque wrench. Or am I missing something. Or would they use the one time use bolts where you tighten a specific amount of turns after contact?

[Mike.Bray]

Just curious question. All this talk about places not using/having torque wrench. Wouldn't that suggest they use every bolt with a nut? Don't they have/use any bolts without nuts? Like main journal bolts? I would think the same principle apply which would require a torque wrench. Or am I missing something. Or would they use the one time use bolts where you tighten a specific amount of turns after contact?

I've only seen it two ways. One is actually measuring the stretch when you can access both ends of the bolt. This is how hi-performance connecting rod bolts are done. There's even a special tool that looks like an OD micrometer except with a dial indicator.

Otherwise its done by torque with a calibrated torque wrench.

There are some tables for number of turns but the accuracy is not very good IMO.

[SnakeBitten14]

Just remember surfaces must be clean and consistent so the torque actually translates to the right bolt load. Extra friction will reduce stud load even though you carefully torqued the bolt to the correct specification.

[Countrymile]

I’d agree, angle alone is not as accurate as torque.

[CraigS]

Yep lube is important. I have never seen wax listed before so that is interesting. I have seen type of oil makes a difference especially if one had moly in it. Also depends on grade of bolt, material it is made from, especially w/o a nut it matters what it is threaded into. Once you get into this, it is almost like there is another world that most people have no idea exists. Here are a bunch of charts all in one place that cover a lot of, but probably not all of, the possibilities.
https://www.fastenal.com/content/mer...ce%20Guide.pdf
There is a bunch of info here from ARP who BTW have their own lube.
https://arp-bolts.com/p/instructions.php

[AROCK]

As a former drag racer I rebuilt my engines a few times a season. I invested in a bolt stretch indicator used to check previously used bolt from engine build to engine build. It is very simple and inexpensive tool.once you know the bolts original measurement you insert the bolt between 2 points on the tool to check for any over sstretch. The cost is under $10.