Studs vs. Bolts
Studs vs. Bolts
(OP)
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?
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?





RE: Studs vs. Bolts
http://www.thomasnet.com/articles/hardware/head-st...
RE: Studs vs. Bolts
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?
RE: Studs vs. Bolts
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.com/article/920...
RE: Studs vs. Bolts
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.
RE: Studs vs. Bolts
RE: Studs vs. Bolts
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...
RE: Studs vs. Bolts
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.
RE: Studs vs. Bolts
RE: Studs vs. Bolts
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
RE: Studs vs. Bolts
RE: Studs vs. Bolts
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.
RE: Studs vs. Bolts
This provides the required clamping force, without the errors introduced by friction in nuts, threads etc
RE: Studs vs. Bolts
RE: Studs vs. Bolts
RE: Studs vs. Bolts
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
RE: Studs vs. Bolts
RE: Studs vs. Bolts
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.
RE: Studs vs. Bolts
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.
RE: Studs vs. Bolts
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
RE: Studs vs. Bolts
What is a "red wrench" and what is your point about bolts vs studs in that context?
RE: Studs vs. Bolts
A "red wrench" is a vernacular term for an oxygen-fuel torch. Maintenance work often involves simply using a torch to cut off the existing fasteners therefore, from a maintenance standpoint, it is irrelevant whether studs or bolts are used. The comment was made in reference to the comment made by moltenmetal regarding ease of maintaining a stud versus a bolt.
/Apologies for using American vernacular.
RE: Studs vs. Bolts
RE: Studs vs. Bolts
When you torque a stud with two nuts, torsional movement will take place at either end of the nut-stud-nut assembly, creating a much better retention of clamping force throughout the fastener. When you tension, it really doesn't matter, you are lifting the nut, stretching the shank against the underface of the head, and then running the nut down to capture preload.
Rick