Studs vs. Bolts 4

[jdogg05]

What is the functional difference between a stud and a bolt? I am not refering to obvious assembleability concerns. I have heard that studs eliminate (or greatly reduce) the torsional load and thus provide a more accurate clamp force... I don't quite understand this mathematically.

Why are studs used for pipe flanges? Why not bolts? Is a stud actually a better fastener? COULD, studs theoretically be used in place of bolts (with arguably better results) if ease of installation and practicality of assembly were not concerns?

 

[desertfox]

Have a look at this article

http://www.thomasnet.com/articles/hardware/head-stud-bolts

 

[jdogg05]

Yah I have read that one. It still does not explain to me why a stud would reduce the torsional force. The bolt threads still slip relative to the threads in the engine block. When you tighten the nut on a stud, although the stud is not itself turning, there is still a torsional force acting on the threads (created by the coefficient of friction, normal (axial) force on the threads, and the plastic deformation of the threads). Why is this reduced with a stud relative to a bolt... both have sliding threaded interfaces.

Also, more generally, if mathematically a stud actually does reduce the torsional force, this obviously would provide a more reliable clamping force. So wouldn't this be applicable to all fastened joints? And thus, isn't a stud unequivocally a better fastener?

 

[desertfox]

Hi

To answer the question regarding less twisting, a stud is not subjected to being twisted during tightening other than the friction you have mentioned, however a bolt and nut are very often twisted at the same time during tightening particularly when you have a spanner on each end.
Now if during tightening a bolt you only twist the nut and just hold the bolt from turning, then I would probably agree that the bolt in that instance is no different from the stud.

http://www.autoserviceprofessional....dvantages-of-using-studs-vs-bolts-and-tips-on

 

[CoryPad]

That article is wrong. I have seen the same argument written elsewhere, so the error is propagated regularly. The clamping force for torqued fasteners is derived from the shank torque, which is the same for a stud or bolt (assuming same sizes and coatings, etc.).

One reason flanges use studs is because they are tightened by hydraulics, i.e. without torque. The stud end is pulled with the desired clamping force and the nut is rotated with essentially zero torque until it stops and then the hydraulic force is released.

 

[tbuelna]

CoryPad makes an excellent point. Hydraulic tensioning is a great method of pre-loading studs. A hydraulic puller is used to apply a controlled force to a stud, then the nut is simply tightened to where it contacts the mating flange, and the hydraulic force is released. This removes all frictions from the process.

 

[jdogg05]

CoryPad and tbuelna, thanks for your input. I was unaware of hydraulic tensioning. It sounds like an absolutely fantastic way to pre-tension fasteners. I mean, I have seen where wheel stud lug nuts are tightened/loosened/tightened/loosened for 2-3 repetitions (using a brand new stud and nut) and the resultant clamping force is reduced by up to 30% by the last time. This plastic deformation of the threads is pretty much unavoidable during tensioning of any fastener (through the method of torquing).

How common is this in industry? I am working on an oil and gas project right now and have not yet seen this. Most of the pipe flanges are torqued using a "star" pattern. Using this conventional torquing method I am still unsure of the reason to use studs on pipe flanges rather than bolts...

 

[Tmoose]

My limited exposure to industrial piping was with thru bolts into nuts. It's nice to be able to start with fresh new threads ( bolts and nuts) on 30 year old pipes. I think non nuclear clamping requirement is low, so torque spec for flange bolts is typically pretty low, and not requiring fancy bolts, and installation and maintenance environment requires simplicity which precludes precise tightening methods.

The first page of Google image hits shows thru bolts and nuts pretty much exclusively.

I can imagine situations where poor pipe design would limit access to the nuts on the "other" side and require one flange to be tapped for studs or bolts.

 

[jdogg05]

Hmm. I actually have yet to see any bolts (cap screws) and nuts on pipe flanges anywhere I have been. It is almost exclusively studs with a through hole and a nut on each side.

 

[desertfox]

Hi jdogg05

CoryPad is spot on, the article is incorrect, I have to confess I got sucked in by the article at first but thinking it through made me realise it was wrong.

Here is a link to the SKF site with some very good information relating to fastener tensioning.

http://www.skf.com/files/880426.pdf

 

[jdogg05]

Yes, to be honest, those fallacious articles were actually the reason I made this post!

 

[tbuelna]

COULD, studs theoretically be used in place of bolts (with arguably better results) if ease of installation and practicality of assembly were not concerns?

One way you could eliminate the effect of torsional strain due to friction when pre-tensioning a nut/stud fastening would be to add some sort of wrenching feature to the nut end of the stud. This would allow the stud to be torsionally restrained right where the friction from turning the nut is produced. With this wrenching arrangement, it would be possible to accurately pre-tension a stud using just wrenching angle and thread pitch. This technique would probably not be possible with a bolt.

As a side note, one unusual technique I recall seeing used to pre-tension or remove nuts from large, highly-stressed studs fastening together sections of a steam turbine housing, was where each stud had a hole down its center that a heating element could be put into to heat the stud. Heating the stud would cause it to grow a controlled amount, allowing the nuts to be installed/removed by hand. I thought it seemed quite clever.

 

[Hoxton]

On large engines (500kw + or so) the head bolts are stretched by a hydraulic tensioner, and the nuts spun on. In some cases, the designer specifies round nuts (i.e. Without flanges) so that over enthusiastic technicians cannot apply a wrench to provide excess torque!

This provides the required clamping force, without the errors introduced by friction in nuts, threads etc

 

[moltenmetal]

There are many practical reasons that studs rather than bolts are used on industrial piping flanges. Bolts are very much an oddity for that application.

 

[jdogg05]

moltenmetal could you kindly elaborate?

 

[Tmoose]

Hi moltenmetal,

Are the studs you mention fitted with nuts on each end, or threaded into one flange or otherwise fixed at one end (which is the first thing I think of when someone says stud) ?

regards,

Dan T

 

[Duwe6]

FYI -- flanges in Code piping systems are typically made up using 'free-floating' studs with a nut on each end. It just works better. My knuckleheads have tried bolts many times in the past - the flanges cause problems, or have to be hand-calc'd because what they used didn't meet B16.5 minimums. B-7 stud with 2ea. 2-H nuts is an industry standard for many good reasons. "Trying to reinvent the wheel is usually a waste of time & money"

 

[moltenmetal]

Oh boy... do I have time to make that list?

Yes, industrial pipe flanges use an all-threaded stud with two free-floating heavy hex nuts, rather than bolts, generally assembled without washers. The only place you'll typically see bolts is on OEM equipment or plastic flanges (where SAE bolts are often used).

You can use them in studded outlets (where one flange is female threaded) without worrying about insufficient thread engagement or worse still, bottoming out in the female part before the gasket is fully compressed. And the same fastener can also be used for through bolting. That alone requires a lot less stocking.

If they're uncoated, you can trim them to length, unlike strength bolts which are not threaded along their entire length. That means you need a LOT fewer pre-cut lengths in stock.

You can cut long ones (for a wafer-style assembly for instance) from a length of threaded rod of the right material- again, no need to stock them. Lots of field mounting arrangements rely on an extra-long stud or two.

They can be torqued or tensioned from either side- whichever is more accessible- and that may be different sides for each stud on a particular pair of flanges depending on what else is in the way. You wouldn't believe how beneficial that is for installation and maintenance, rather than relying on the designer's forethought about maintenance access.

 

[fegenbush]

"You wouldn't believe how beneficial that is for installation and maintenance, rather than relying on the designer's lack of forethought about maintenance access."

Revised for accuracy.

Additionally, we always use the red wrench to remove old flange bolting for maintenance (carbon steel only) so bolts v. studs are not a concern there.

 

[SnTMan]

For many applications the designer does not have sufficient information to realistically plan for maintenance access

All-thread studs provide uniform elongation of the fastener when tightened, as there are no changes in cross section. ASME Sec VIII, Div 1 places conditions on studs with unthreaded portions, but oddly, not on bolts.

Regards,

Mike