Quick way to go from course thread torque to fine thread?

[BlackEngineer13]

I find myself in situations in which our crews in the field realize that the studs available are fine thread rather than the standard course thread we generally use. Clearly there are issues with this happening, but that's a separate concern for me to deal with.

I know that fine thread studs have a different torque requirement than course threads to achieve the same bolt load, but I was wondering if there is a known ratio that our established torque can be multiplied by to make the change from course thread to fine thread. It would be nice to let the guys know quickly what the new spec would be rather than using b1.1 and pcc-1 to calculate it every time for custom required forces.

This is a new issue for us, but it's come up three times this year, and I just want to know if I am missing something easy. Thanks for any insight you might be able to give!

 

[SnTMan]

BlackEngineer13, fine thread studs will have a slightly larger cross sectional area than coarse thread. For quick and dirty, look then both up, ratio up the torque value accordingly. Get on with life

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[BlackEngineer13]

SnTMan, that's a good idea generally, but I'm looking for something fast and clean. if it isn't out there I don't mind doing the calcs. They are pretty straight forward. if there isn't a known established way I'll just end up making something in excel like I usually do. Just wanted to know if I was missing some bit of info on this subject.

 

[SnTMan]

BlackEngineer13, your choice

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[drawoh]

BlackEngineer13,

The torques for coarse threads will work fine on fine threads.

The standard screw torque equation T=CFD does not directly take thread pitch into account. Since the fine threads have higher tensile strength, the value for F should be a bit higher. You can easily calculate the strength of your studs. You may be able to read the strength from a handbook or catalogue.

I would assume that there is not a reliable factor for increasing torque from coarse thread to fine. There is not a precise factor for geometry to vary from coarse to fine threads.

--
JHG

 

[BlackEngineer13]

drawoh, That makes sense. I am also under the assumption that there is not a simple factor for this. Thanks for the input!

 

[hydtools]

The value of F in the torque equation is the desired clamping force generated in the joint, not the screw strength. The logic still applies.

Ted

 

[drawoh]

hydtools,

How many people try to hit a specific clamping force on a screwed or bolted joint?

As a rule, I apply as much force on the bolt as I can so that it does not come loose. If I wanted to hit a clamping force, plus/minus some value, I would not use the torque equation above. I would probably call up a spacer and a stiff spring of some sort.

--
JHG

 

[SwinnyGG]

If I wanted to hit a clamping force, plus/minus some value, I would not use the torque equation above.

Is there sarcasm here that I'm missing?

When you design things and specify bolt torques, what are you doing if not specifying a target for clamping force?

 

[3DDave]

Torque + friction variation = 50% to 200% expected clamp load. On s good day.

Which is why direct extension measurement or load cell measurement are preferred.

Pre-torque + turn = 90% to 150%, which is why it's used in critical automotive.

At least the "turn" part gives a ratio between fine and coarse threads to get the same elongation. Half the pitch - twice the turn.

 

[BlackEngineer13]

So, Just for reference, I work in a major industry where the standard is torquing to apply clamping force. Our methods range from basic equations for non critical joints to joint analysis programs for more critical joints. The amount of load application controls used is also dependent on criticality. So, new studs and nuts every make-up, torquing everything with some method of control, from clicker wrench to multi head hydraulic torque devices, if not tensioning for the most critical joints. run outs on flange surfaces, etc. Part of this process is to ensure we have clean studs and nuts (new through clean bolt holes), and to use a specific lubricant with a specific k factor. While it's true that it is still not exact due to friction, 50 to 200% is not the realistic spread we have found in practice or testing. clicker wrench might have a 50% spread, but that's more due to some yahoo who thinks more is better and hefts on it, or double clicks it, not knowing that adds additional torque. well lubricated studs and nuts on hardened washers will have a 15 to 40% spread with a single head hydraulic torque application. multi head that goes down. obviously tensioning nearly eliminates friction, aside from the modest torque applied to maintain the stretch. Definitely have my answer to the original question at this point, though. Thanks, all!

 

[hydtools]

Well, ok. Let's call F bolt force instead of clamp load.

Ted

 

[3DDave]

So much testing in exhaustively precise ways and yet no pocket conversion chart or using a phone for when the wrong thread is discovered?

Precise control of cleanliness, lubrication, precision torque wrenches and yet asking here for a by-thumb guess about how much difference the thread pitch makes?

 

[drawoh]

SwinnyGG,

I am being technical, not sarcastic. If I want to exert 300lb[±]10lb on something, I am going to specify a spring that fits around a spacer. I will clamp the bolt down as hard as I can. The spacer will control the compression of the spring, and the force.

If I am specifying the installation of a bolt or screw, I want as much tension on the fastener as possible, because that is what keeps it from coming loose. I am not looking for precision. I am looking for brute force.

--
JHG

 

[SnTMan]

I haven't seen anything yet that makes me want to change my first post

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[chicopee]

Here is my input, I worked on bridge construction for a number of years and we never used fine threads in this type of field work for obvious reasons. So BE 13 what is the nature of your field work.

 

[BlackEngineer13]

3DDave Great use of snark, but I suppose you are right. I mean, how dare I even ask if there is someone out there that might know something that I don't know that could make things easier! I guess I should remember that there's always one in every group. Everyone else, I appreciate your responses.

 

[3DDave]

Umm - you cannot both have access to the reports of extensive testing and not understand SnTMan's answer. The snark wasn't for asking - the snark was for you having all the required information but lacking understanding of what the question means (what factors about fine threads influence the torque requirements to produce identical tension to a given torque on the same nominal sized coarse threaded fastener)

Also, course is a path, such as the course of a river or a course of study.

You are so unengaged as to misspell the word that is core to your problem all that is left is to experiment to find what information you do possess. Apparently it is limited to insult.

Is it oil and gas or chemical plants? Is one of those the secret major industry you work for? Every industry runs the chance of idiomatic approaches, but withholding that information is a great way to get bad answers. Much as you withheld all the k-factor control lubricants that are used on the wrong fasteners.

I feel for your field guys, getting sent unprepared to handle a century old problem and no one in the home office to contact to look up or calculate the right answer.

Anyway the nut factor changes with thread pitch.

https://www.hextechnology.com/articles/bolt-k-factor/

recommends testing - so get that test group to repeat the tests with the fine thread studs that should not be in the field. It looks like ASME PCC-1-2019 dropped the precision formula that was in the 2013 version (you have a copy?,) likely because of inaccuracy in the torque prediction that only testing can expose. Or keep looking for shortcuts. Or use DTIs.

 

[BlackEngineer13]

3DDave Wow. You are so incredibly off the mark that I can't help but laugh. The assumptions implicit in your additional attempt to look down your nose at me for asking IF there is an easier way shows quite a bit more about your own dogmatic approach than my question does mine. As indicated in my own replies earlier, I regularly calculate the right answer. I use PCC-1 very frequently. I also not only understood SnTMan's answer, but accepted it pretty much right off the bat. As to you chalking my typo up to me being unengaged, perhaps you would do better to reflect on the hypocrisy of following up that comment with chiding me on stooping to insults.

The reality is that it is exactly people like you that turn a place like this, where people might come to enhance their knowledge with discourse among peers, into a lame forum for people to try and raise themselves up by driving others down. You're on your high horse talking about looking for short cuts when the reality is that there often is a way to do something faster with equivalent results. Why WOULDN'T a good engineer ask those questions? Aside, of course, from random douche bags who try and feed their superiority complex by attempting to make someone feel dumb for asking.

Keep doing you. You're clearly making the world a better place by how you treat people.

 

[3DDave]

"SnTMan, that's a good idea generally, but I'm looking for something fast and clean."

You didn't accept it, you specifically wrote it wasn't what you wanted. And you did not say you already understood it as part of the original question.

Wow.

"I regularly calculate the right answer," which cannot be calculated correctly because the nut-factor changes with the thread type, assuming you meant to limit that response solely to fine threads when coarse threads are specified and expected. There are extensive reports you depend on that tell you the clamp load for given torque on each fastener size so aside from calculating the necessary clamp load, you're task is done.

If there was a faster way to get equivalent results Google search would have found them for you or the ASME PCC-1 committee would include that in the documentation.

Which is it? Oil and Gas or Chemical plant?

Is this how you deal with people in person? Calling them douche bags? You should not feel dumb for asking, but perhaps you should not be dismissive of help. What did you respond with about torque-turn? You ranted that you are doing everything so perfectly that torque alone is just fine, having not mentioned any of those previous factors. That seemed to be exactly what you complain about - "look down your nose at me" for not knowing your secret company process that is so well worked out - HOW DARE YOU QUESTION MY LUBRICATION AND CLEANING AND ALL MY STUDIES! - is how what you wrote reads.

If you would switch to DTIs this would not be a problem at all. No calculations in the field. Turn til they pop. Done.