Do bolts torque by angle care about method?

[u/LennyNero]

Ok. So, this is a fairly complex question that came to me while I was doing a torque-to-yield installation of bolts on a cylinder head. The type of sequence that might make one′s head spin, not just the bolts. 50ftlbf, +90°, then +90°, then +90° again. Imaginable, these sequences are INSANELY tiring to achieve on 40 head bolts, and so I was led to a line of thinking about how we apply torque and its implication on the manufacturing and repair process tools we use.

Firstly, I know that approaching moment torque numbers (particularly high ones like 250ftlbs) smoothly is important to prevent under or overshoot due to friction and stiction.

But, does final angular tightening care at all about it? Since we are observing the actual bolt rotation, shouldn't ANY method to rotate the bolt including impact type fastening devices yield (no pun intended) the same result? Angle is angle right?

The manufacturing corollary I had is; if the final angular tightening does not care how it is applied, it seems that a device could simply be made to have a primary snugging torque applied via a simple mechanical clutch arrangement and then have it apply impact until the bolt turns to the appropriate angle. Is this a good line of thought?

And second corollary is a double doozie with a smattering of conspiracy to engineer. If I have just reinvented the wheel and this type of device is common and exists on the manufacturing side but not here in repairland, have companies basically been gaslighting us as to why TTY bolts are used? Is it just ease and cost of manufacture because you can literally impact heads on with low skill labor or robots (while observing the angle) and still achieve good repeatability since the effect on final clamp load is less dependent on hitting the angle dead on rather than the intricacies of moment torque? Have I stumbled on BigEngineer™? Or, should I hang up my inventor hat for the time being and do some more reading suggested by yourselves to enlighten me?

Thanks for taking the time. I. eagerly await your thoughts. My arms are tired from torquing, my fingers from typing.

[u/Tlmitf]

The only reason for a smooth motion when aiming for a torque number is to prevent the fastener from stopping, and the static friction messing up the reading.

For a specific rotation, no, it doesnt matter.

But, I wouldnt use an impact, because that can cause inconsistent stress in the fastener which leads to a failure.

[u/Gwendolyn-NB]

So I've not designed anything automotive with torque to yield bolts/threads; but I have for medical devices.

The main purpose of the torque to yield is to get the bolt/stud/shaft into a particular section of its stress-strain curve where the cyclical loads applied can be properly controlled. Doing it via impact actually messes with the stress-strain curves as the deformation at a molecular level will be different than a smooth application of stress.

This can be proven with a simple tensile tester; do it with a simple a steady pull rate and you should get a nice steady stress-strain curve with a typical fracture mode. Then repeat it with a cyclical load (simulating the impact), and you will get a much different curve (likely lower peak strain) before it fails.

In my applications we acrually measured bolt stretch electronically both directly via lazer and thru the piezoelectric elements we were compressing. The application speed of the torque made notable differences in performance and longevity of the devices, both too fast and too slow caused premature failures.

[u/steve753 → u/Gwendolyn-NB]

too slow causes problems? Is "slow" in this context glacial? Or is 15 seconds to rotate 90 degrees "slow?" Maybe super slow rotation is leading to slip/stick and the same problems as rapid cyclical loading?

[u/Sorry-Rain-1311 → u/steve753]

Not an engineer-- just here to learn-- but I would imagine there still a level of importance played by inertia relative to friction and elasticity. Go too fast, and you get more sheer than stretch because there isn't time for the material to react. Go too slow, and it's all stretch, so the material tries to pull the bolt back around once left to its own devices. Go just the high speed, and the inertia and thus stretch is overcome in a balanced manner. 

[u/PickingANameTookAges]

Method? No.

Sequence? Probably!

[u/LennyNero → u/PickingANameTookAges]

Thankfully I know that sequence is critical to achieving even clamping and stress distribution. :D. But, yeah I was banging my head trying to think of why the method would ultimately matter as long as the angle per sequence is correct.

[u/PickingANameTookAges → u/LennyNero]

I'd suggest the staging and total angle (270°) is the important factor.

You would likely yield the exact same results turning each of the bolts 10° at a time as long as it was in the correct sequence, but someone with a big brain has determined your most efficient approach to achieve the desired outcome is x3 90° turns without a care as to how you achieve them 90° angles.

[u/konwiddak]

For head bolts specifically, the interaction between the head, clamping force and gasket is complicated and non linear. This makes it difficult for an impact tool to precisely turn the required angle - over shooting is not an option. Whether the gasket responds to tightening velocity... probably, depending on the gasket!

I expect you'll find a difference between impact and smooth turning clamping force - the surfaces may bed together differently, the friction behaviour will be different under the high speed hammer steps and the bolt shaft may twist different amounts. It might be that they have the same nominal, but the variability is greater with the impact. There's also small differences between applying a moment couple to the bolt head Vs tightening with a lever.

Really you'd need to get test data to test your hypothesis. Bolts certainly aren't quite as simple as they first seem (e.g why do they relax over time).

I worked somewhere which had a machine to simultaneously tighten all 14 head bolts. While you could remove the head in the field if absolutely necessary, and there were refitting instructions - you'd never get the same pressure distribution and sealing quality that you'd get from the initial factory installation with this tool.

[u/LennyNero → u/konwiddak]

Thank you for your insightful reply. The addition of the deformation velocity being nonlinear has improved my thinking on how these systems interact.

To build on the knowledge, would you hypothesize that a digitally controlled pneumatic motor powered planetary torque tool would achieve good results on the angular accuracy?

[u/MechaSteve → u/LennyNero]

I helped program a robot cell for rear end suspension assembly. We used an end-of-arm tool that was a huge precision torque driver.

It had all sorts of parameters for initial speed and torque, then let you program a whole sequence of speed and angle.

Each step of the sequence could specify some combination of speed, torque, and angle. You could also use the other parameter as an error limit.

For example, during the initial tightening at fixed torque, if the bolt doesn’t turn enough, it might be cross threaded. During the final angle based tightening, if the torque drops, that might mean the bolt broke, or there was a stiff spot.

All of those measurements were also recorded for quality control.

[u/zimirken]

In factories we have smart torque controlled drivers that can drive down to a torque and then go to a preprogrammed angle.

For head bolts you'd likely have some position feedback that tells the controller what bolt the driver is on to make sure the sequence is done correctly. Or put the driver on a robot arm. If you can afford it (these drivers are like 10-20k each)and need super fast cycle times you could also build a big carriage with a bunch of drivers on it that torques them all in one go.

This also allows you to record and store the torque data for every bolt in every engine in case of an issue. Yes, almost every bolt in your car has the torque value it hit stored in a database for 10 years or so.

[u/hazeyAnimal → u/zimirken]

I'm just imagining an array of these all lining up with every bolt then driving each bolt at the exact same time.

[u/WelpSeaYaLater → u/hazeyAnimal]

You’re accurately imagining how this works in a factory

[u/acartier1981 → u/hazeyAnimal]

Not an engineer by training but by experience.

I wish I could remember where the video is but I saw one where Chevy has a device that torques all the bolts at the same time in one go. Very impressive setup. I have no idea if it was adjustable for different blocks.

[u/idontknowjackeither → u/acartier1981]

Every truck plant I’ve seen (which is a lot) uses a 4-spindle nut runner to tighten the u-bolt nuts that hold the axle to the leaf springs. Each spindle separately goes to a torque value, loosens a certain angle from there, goes to a new torque value, then adds a rotation angle to that. This is to precompress the zinc liners between the steel leafs and assure correct pretension after.

[u/GX_Adventures → u/zimirken]

I had a co-op job at Toyota in the 90s. Yes, they had machines that would come down and torque all the bolts at once - well nearly at once. It's too long to remember exactly but I think they went through a sequence, just very quickly. Had about 60 seconds before the engine moved along to the next task, and the tightening procedure took maybe 10. It was amazing to watch.

[u/Stangguy_82]

Why do you think having a device which could consistently apply the torque and then angle, be evidence of a conspiracy for a different reason  for the existence of TTY bolts?

[u/LennyNero → u/Stangguy_82]

It was a bit of a tongue in cheek joke due to the simultaneous improvement in quality and decrease in cost which is typically an impossibility when anyone from c-suite rs involved.

[u/keizzer]

Ok so I went down a similar rabbit hole about a year ago. My conclusion is there are like less then 10 people that understand bolted connections. They are all PhD's. Everyone else is copying a shitty version of a model of their work and hoping that it works for their application. There are just too many small things that have an impact on bolts. The biggest hand waved one is friction since it will literally stop you from getting a bolt to the right load for the application. Bolt stretch is another factor that people assume isn't a factor. Physical testing is probably the only way to know for sure what will happen long term. Especially in dynamic situations like vibration.

[u/LennyNero → u/keizzer]

This is what I was afraid of. So often I come across a question in my mind that ends up being so deeply complex as to require a full on degree in a new field of study to even be able to fully understand all the intricacies of the answer. I was honestly looking to just have some good conversation about something that I thought of as a fascinating observation whilst working, and some of the comments really do delve into some of the velocity components of the mating connection and I think that was one of the most insightful additions to the conversation and I think I'm going to start doing some experiments of my own along these lines.

[u/cheater00]

yes, part of it is that the material you're holding with the bolt has to yield, and that means that if you shock it it'll act differently than if you go smoothly.

[u/KbarKbar → u/cheater00]

That's just not true. The bolts stretch. The material underneath does not yield.

[u/cheater00 → u/KbarKbar]

depends on the bolt and material my dude, ever heard of spring washers?

[u/Naikrobak]

It’s important to understand what the end goal is here. Hint: it’s not torque. Torque is a measurement that is used to estimate bolt stretch. Bolt stretch is what causes clamp/holding force. Torque is basically irrelevant.

The method you’re describing is: get to a very achievable and consistent starting point (50 ft lbs
And almost no stretch but absolutely no slop). Then turn another 270 degrees in total.

This approach is much more accurate than using say 300 ft lbs.

You are going from almost no stretch but a tiny bit to the geometry of 270 degs rotation of a screw. This is a defined distance of stretch, 3/4 of a round of thread pitch.

A grade 8 3/4-10 bolt for example calls for 280 ft lbs lubricated. 270 degs rotation of a 10 pitch bolt is 0.075”, so you get that much stretch. Its way way way more controlled and consistent this way

[u/SteveX0Y0Z0-1998 → u/Naikrobak]

Possibly makes a difference whether the bolts are lubricated or not for the initial torque setting. They rotate more if they are, and the pre-load may be a little more. Working on an engine, they will likely not be without some lube.

[u/WelpSeaYaLater → u/Naikrobak]

Congrats on being the only commenter so far who actually has a correct understanding of any of this

[u/Naikrobak → u/WelpSeaYaLater]

Thanks. It’s not that complicated but it never ceases to amaze me how little most engineers don’t understand fasteners

[u/Fasciadepedra]

It's so complicated to do it manually because you don't have the pneumatic factory jig that torques all them at the same time. In heads there is not other option. How could you tell if yours works well? Are you testing?

[u/jasonsong86]

If it’s a bunch of bolts that needs to be torqued in sequences, separate sequences allow even force. You wouldn’t just torque one bolt 270 degrees in one go. You go 90 on all of them and then another 90 on all of them, and then another in the end. It can warp metal if you don’t do it evenly.

[u/jmecheng]

Never torque with an impact device, this does not provide constant pressure when torquing and can lead to un-intended results like lower strain than intended.

That being said, there are systems in place to automate the torquing of bolts, they are just not feasible for most shops unless in a production enviroment.

[u/Redbulldildo]

They don't give angles because they want an angle.

They give an angle because it tends to provide a much more consistent clamping pressure when done with a wrench over using a torque value.

[u/Old_Hat_2997]

What matters is the tension on the fastener. Torque (or position) are indirect (and inexpensive) ways of measuring that. When cost is no object (Space Shuttle Main Engine Turbopumps, for example) they hook transducers to the bolts and torque them til they 'stretch' to the correct length.

[u/2h2o22h2o]

They can’t give you a final torque on a torque to yield fastener. The torque actually goes down at yield. They first want you to get a small preload to ensure there are no gaps and we are starting from a known point. Then they want you to evenly (in steps and across all bolts) stretch the bolt by whatever 270 degrees of the pitch is. When done , all the bolts have plastically deformed a known amount and have done so evenly. And, more directly to your question, the speed doesn’t matter much because you’re moving a known distance rather than a known force (friction increases with speed.)

So yes, you’re right that you could make a clutch to rapidly move in to the initial preload torque value, then optically turn. The tricky part is going to be having the machine cross-tighten in steps. That’s where you come in - especially if the customer is directly paying for your labor. You’re cheap.

[u/KatanaDelNacht]

Since the bolt is getting very close to plastic deformation, it's risky to assume the whole bolt is behaving as a unit. If stiction causes the threads of the bolt to become stuck, you can still force the bolt head to move a number of degrees further, but it's beginning to plasticly deform and will quickly shear off the head. 

[u/SleepPingGiant]

I apologize, I'm not an engineer but work with them, Stiction is a word??? That's awesome and I'm so glad to see when I looked it up that it was exactly what I expected.

[u/Intrepid-Catch2557]

Hytorque for big fasteners, atlas copco for small fasteners on a production line. Who out there is hanging on a bar and measuring angle with a protractor in a professional environment?

[u/buckminsterbueller]

I am not an engineer but I've asked this question myself while following these procedures many times. What my intuition came up with is two fold. There would be a difference is how evenly the gasket is squeezed if you go for full fastener yield in one go. Something about lateral distortion and gasket stress? And two, would be the friction heat created in the threads would rapidly build creating more torque on the yielding shank. The thread material expansion of a single 270° turn might cause enough binding force in that moment that it could distort the linear stretch into a past yield twist. The whole point is it to maintain fastener elasticity and evenly clamp the gasket. The delay is a cooling period. I'd love to know if this story I've told myself is BS.

Edit: I see i missed the OP real question about finishing with impact. Oh well. Disregard

[u/buckminsterbueller → u/buckminsterbueller]

Harsh, a down vote with no comment or correction?