Torque Limiting Extension Question

[YoshiMoshi]

TL;DR Summary

100 ft lb torque limiting extension that is 8 inches in overall length, I remove the piece in the middle and weld back together to reduce the overall length of the tool. If I were to use this new tool, would it still work as intended and limit torque to 100 ft lbs?

Torque limiting extensions act as a torsion device by flexing once a certain amount of torque is applied onto the fastener that they are tightening down. This helps prevent from over tightening the fastener when using an impact wrench. Seems like a great tool that can speed up the process of tightening down fasteners. However the standard seems to be about 8 inches in length, making their application very limited to anything other than tires in the automotive industry. If I were to cut one of these devices into three separate pieces and welded the 1/2" square male drive and 1/2" square female drive ends together, removing the part in the middle, to reduce the length of the extension, would the tool still work as intended and limit the torque to the expected amount?

100 ft lb torque limiting extension that is 8 inches in overall length, I remove the piece in the middle and weld back together to reduce the overall length of the tool. If I were to use this new tool, would it still work as intended and limit torque to 100 ft lbs?

 

[Baluncore]

You cannot shorten torque limiter bars, because they must twist along their full length when subjected to torque. The total angle of twist is made proportional to the product of torque and length. That twist angle along the bar defeats the impact driver by springing back, and so prevents further tightening.

 

[YoshiMoshi]

You cannot shorten torque limiter bars, because they must twist along their full length when subjected to torque. The total angle of twist is made proportional to the product of torque and length. That twist angle along the bar defeats the impact driver by springing back, and so prevents further tightening.

Hey thanks! How come companies don't make torque limiting extensions, except smaller in overall length? For example, brake caliper bracket bolts on my car are supposed to be 80 ft lbs, but using a torque limiting extension rated to 80 ft lbs is out of the question, not enough clearance. Torque limiting extensions that are shorter in length would be great and could be used in multiple applications.

 

[Lnewqban]

You should not use impact tools for caliper bolts, that is what torque wrenches are for.
For hand tools, you can find non-impact torque limiters, which are much shorter.

Please, see:
https://www.checkline.com/product/TL-IT/TL-IT-5

 

[jack action]

How do torque sticks work?

Googling this simple question returns basically the same answer everywhere:

Torque sticks work by flexing (like a torsion bar) when a torque limit is reached. When the stick flexes or turgid, it resists further tightening of the fastener thereby helping to regulate further application of torque thus preventing eventual damage that might occur.

Anyone who understands physics knows that is the stupidest answer you could get. EVERY extension bar you would use - no matter the size or material - will begin to twist as soon as a torque is applied - no matter how small it is - and the torque at both ends will always be the same, no matter how large is the twist, as shown in this figure:

The only way "it would resist further tightening" is if the bar breaks.

For a torque stick to work, you NEED to use it with an IMPACT gun. It will be obvious how it works once you understand how an impact gun works:

​

As you can see, the impact gun stops applying torque every 20° of rotation or so. So if an extension twists 20° under the specified torque, the "hammer" of the impact gun lifts, the torque goes to zero, the extension untwists, the torque is reapplied, and the cycle repeats itself.

This should answer your question regarding the length of the extension. If you want to make the bar length $L$ shorter, the diameter (in $J$) must also be smaller to get the same amount of twist $\theta$ for the given torque $T$. But with a smaller diameter, the stress $\tau$ will be higher and the bar may reach its allowed maximum shear stress and break.

 

[YoshiMoshi]

Thanks everyone. I now see that I can't shorten the length of a torque stick.

Why do manufactures not make torque sticks that are shorter in length? It would seem that they would have a lot of applications and would be beneficial, or perhaps even torque sockets that begin to twist once a certain amount of torque is applied. An 8 inch torque stick takes up a lot of space. By the time you add the length of the impact wrench, you need well over a foot of clearance above the head of the fastener.

I know you are supposed to use a torque wrench because it's calibrated. I'm just thinking that if say for example you have lug nuts that are supposed to be torqued to 120 ft lbs, and you use a 110 ft lb torque stick with an impact wrench, you can speed up the process of getting close to the specified torque, then go around with a torque wrench to bring it up to 120 ft lbs requiring little if any movement of the head of the fastener. This would be much faster over hand twisting the fastener, and then using a manual torque wrench to torque it down requiring several rotation of the fastener.

I know there are electronic torque gun wrenches for precision fastening. Were you put in the desired torque and pull the trigger, and the gun will tighten the fastener for you to the specified torque. This is great in all but these tools are like 3,000 dollars, and a big investment for a DIYer working on their car. Probably not worth it if you can just get a torque wrench for under 50 dollars.

 

[Lnewqban]

In my humble opinion as a wrenching enthusiast, impact guns usefulness is limited to unbolting stubborn-stuck bolts and nuts.
They can easily damage threads, not only because the impact, but because the speed with which they make anchors spin, generating a lot of heat.

I have seen many good threads trashed due to galling.
Please, see:
https://en.m.wikipedia.org/wiki/Galling

 

[jack action]

The true goal of tightening a bolt is to stretch it by a certain amount to create a tension preload. The actual torque to create that preload is actually irrelevant. When high precision is required (racing for example), the bolt is measured directly like so with a stretch gauge:

The torque - because of different factors, not just friction - can still be just an approximation:

To hammer this lesson home, we lost a 355ci small-block Chevy on the dyno when a rod bolt failed because it had not been properly stretched. The under-torqued rod bolt nut eventually backed off after several hours of dyno testing, completely destroying the engine in the process. The cause was traced to a torque wrench that fell short by 10 lb-ft combined with new rod bolts and using engine oil as a lubricant. The resulting tolerance stack caused the failure. When we disassembled the engine, the rod bolt nut was laying in the bottom of the oil pan. All of the remaining rod bolts were badly under-stretched as well. Lesson learned.

There is another specification that exists for preloading a bolt: Measuring the torque angle.

How Torque Angle Works​

To explain how torque angle works, we need to work backward. Engineers first determine the desired clamp load for the items. Once they know that, they can run the numbers based on fastener size and material to determine the required bolt stretch to achieve that clamp load. From there, it’s simply a matter of taking the thread pitch of the fastener and determining how many degrees of rotation are required for the fastener to be stretched the desired amount.

While it seems complicated, the reality is that all of the big-brain calculations are done by the engineers of the application ahead of time. All you have to do is be able to accurately set the first-stage torque (which is usually relatively low, in order to be able to better withstand any variations in conditions) and then properly measure rotation from that point forward.

[COMMENT VOIDING WARRANTY]
For the cheap DIY project, this is the method I use when tightening a bolt or screw by feel. You tighten the bolt just enough to set everything in place and then you give it about a quarter of a turn. With a power tool, you can actually see the bolt's angular velocity slow down when the preloading phase is beginning, and about 90° later you can see it almost going to a stop (Of course, it will break before it stops, so this is where experience comes into play). When done by hand, the feel of the torque applied combined with the torque angle gives you a good idea of when to stop tightening. When you are tightening old bolts - used, rusted & dirty - I would probably trust this method to be more accurate than a torque wrench reading.

Measuring the torque angle is the extra step you need to watch for in the Ugga Dugga method: evaluating the torque angle is more important than the time the power tool is working.
[/COMMENT VOIDING WARRANTY]

 

[YoshiMoshi]

Thank you everyone.

So does galling occur on the threads of a fastener when using an impact wrench to disassemble or re-assemble or both?

I have noticed that the fastener gets considerably hot when using an impact wrench.

 

[Baluncore]

I have noticed that the fastener gets considerably hot when using an impact wrench.

As a bolt is tightened, thread friction heats the nut. The process is quicker with a torque wrench than by hand, so there is less time for the heat to escape, and the nut temperature is therefore higher.

So does galling occur on the threads of a fastener when using an impact wrench to disassemble or re-assemble or both?

Both. Galling occurs due to thread lubrication failure, when metal from the two sides contact and cold-weld together. Galling occurs about as often when hand tightening as when tightening with an impact wrench.

If a thread has galled, you may remove the nut slowly by hand, or use an impact wrench to do the work of damaging the thread. You may bias your view by turning to an impact wrench for any galled thread that takes more effort.

 

[anorlunda]

Galling occurs due to thread lubrication failure, when metal from the two sides contact and cold-weld together.

Thanks. I'm learning from you. What about seizing when dissimilar metals are in contact for a long time; Is that also cold welding? I think that anti-seizing compound the same thing as anti-galling compound.

 

[YoshiMoshi]

Thanks. I'm also learning here. I've heard anti-seizing shouldn't be used on fasteners, because the recommended torque specification is for "dry" situations.

If for example, the recommended torque specification for the lug nuts on my car is 76 ft lb, this is the recommended specification without using anti-seize. If you use anti-seize the lubrication will make it easier to tighten the nut down, meaning you would need a higher torque to achieve the same clamping force?

I kind of follow the logic here on this. But is there harm on say tightening down lug nuts to the specified torque with a torque wrench (without anti-seize), then putting anti-seize on the remaining exposed threads of the stud? This would help prevent rusting and keep the threads on the stud in better shape? Some type of grease would also help. I would imagine if you were to take the lug nuts off right away, the anti-seize would accumulate a bit on the threads of the nut, making the nut "wet", and not knowing what torque to tighten them back down to. If however I wait until my next oil change, and most of the anti-seize on the exposed threads is now gone, then would this still be an issue?

^This is for situations where lug nuts aren't "capped" and you have a portion of the threaded stud sticking out after properly fastening the nut down.

 

[Baluncore]

What about seizing when dissimilar metals are in contact for a long time; Is that also cold welding?

I suspect not. The alloy formed at a contact would need to be stronger than both parent metals for the gall to occur.

I think the electrochemical reaction between dissimilar metals would result in either external corrosion, or generate a salt or an oxide that might choke the gap between dissimilar metals. You would need to be more specific about the metals, surface treatment, and the environment before predicting an outcome.

If you use anti-seize the lubrication will make it easier to tighten the nut down, meaning you would need a higher torque to achieve the same clamping force?

I think you have that backwards. Lubrication of the thread lowers friction, which reduces the torque required to stretch the bolt to the same clamping force.

I've heard anti-seizing shouldn't be used on fasteners, because the recommended torque specification is for "dry" situations.

The torque will be specified for either dry or for oiled threads. Use the specified lubricant.

The difference in friction coefficient between dry or oiled is not huge, so the bolt tension when tightened will not be that different. Only critical threads must be tensioned accurately.

For wheel nuts, assume dry, but blow any dust out of the thread before fitting. Wheel nut torque is not as critical on pressed steel rims because the rim is designed to flex and spring against the hub near the wheel nuts, and will usually be centred by the hub. Lubricated wheel nuts sometimes undo and disappear, then the a wheel falls off.

But is there harm on say tightening down lug nuts to the specified torque with a torque wrench (without anti-seize), then putting anti-seize on the remaining exposed threads of the stud?

The risk is that you will forget what you did, or someone else will change the wheel tomorrow.

Once installed, I sometimes paint nail polish that dries brittle, onto external thread contacts to help seal the thread and lock the nut. For agricultural applications, I paint the assembled fastener to keep water and corrosion out. The thick paint will flake off when the nut is removed, so the unknown history of contamination will not lubricate the thread in the future.

Use your head when tightening nuts.

 

[anorlunda]

You would need to be more specific about the metals, surface treatment, and the environment before predicting an outcome.

How about a stainless bolt in a hole drilled in an aluminum mast for a salt water environment?

 

[Baluncore]

How about a stainless bolt in a hole drilled in an aluminum mast for a salt water environment?

This has nothing to do with impact wrench torque. We are well off-topic, and you are asking for marine electrochemical corrosion troubles.

The mast should not be highly reactive pure aluminium, but a marine grade alloy. You must paint the mast. You must use grade 316 stainless nuts and bolts.
I believe you then have no choice, but to electrically insulate the dissimilar metals in a way that prevents droplets of water bridging the insulator, or entering the internal thread gap.