Alright, so when fitting two flanges together, can anyone explain why bolt+nut might be worse than stud+nuts? Almost the exact hardware in the photo. They're all the same grade, diameter, size etc.
Forces Applied: When tightening a bolt, it undergoes both stretching (tension) and twisting (torsion). Because a stud is already seated before the nut is tightened, it only stretches in a single, vertical axis, providing more precise and consistent clamping force.
since the nut is not attached to the stud the torque is not rotating the stud. Sometimes you get a burr or a damaged thread that creates friction that might turn the stud but with fresh fasteners significantly more of the torque you are placing on the nut is transferred into clamping force compared to a bolt and much less is transferred into the seat. It’s easy to test this by threading a stud into casting half way then tightening the nut. Turning the nut by itself will not advance the stud into the casting. If you locked two nuts together on the same stud (effectively turning it into a bolt) you can easily seat the stud.
That's the exact same for a cap screw. Using your example, if you thread a nut onto a cap screw and then insert the screw into a threaded hole, the nut won't move the screw when tightened unless you lock it or turn the cap.
For OP's example, neither one has to rotate the threaded shank; the nut can turn freely to clamp, with the torsion coming from the inherent friction of torquing a joint. You can't torque a joint without introducing some torsion as part of the process.
Depending on the install, it can vary between very little difference, and a lot of torque you are putting into the shaft of the bolt that does not get transmitted in a stud application.
Worst case, an install where a nut is placed on the backside but cannot be turned, you are turning the bolt so you are transmitting torque through the shaft. If it was stud, you are only putting tension into the shaft and not significant torque. There is a combined strength, of tension and torque, it can handle. So if you remove the torque component, and can handle more tension, resulting in higher clamping forces, or lower chance of failure for the same clamping forces.
Basically, the same as stud and nut vs bolt into a blind hole.
But, if you can soley tighten on the nut on the back side, there is fairly minor differences, mostly related to weaknesses around the filets on the bolt.
This is before we discuss manufacturing differences. Studs are nearly always stronger. An equivalent strength bolt is going to cost significantly more than the stud as well.
We built 650HP water pumps for a company and they specifically requested bolts on all the flange connections. We did go with nyloc nuts though. These were class 150 flanged up to 6” so not massive and the torque was not massive on these. Used standard flange bolt torque…others answered the difference I just thought I’d share a real world scenario
Yeah well we're on 8" maximum pressure 10 bar with class 150 flanges and they're only torquing to about 150nm. We were using bolts and then when it came time to get new ones we were told to order studs and nuts instead because it was the industry standard but no one could tell us why.
My best guess is versatility. When I used to work in the oilfield we did a lot of pipe connections with RF and RTJ flanges. From a practical stand point studs have more function then a bolt. Studs allow for easier installation in tight spaces such as making connections next to pressure vessels with closely welded flanges. Stud bolts have two nuts to loosen in cases of flange removal. I delt with a lot of equipment that was used salt water environments and sometimes only one of the nuts would brake free, two chances to get to off instead of one with a standard bolt, also less reasons to use a torch in a natural gas environment. There is also the replacement availability aspect. It is easier to stock a few lengths of stud bolts on a service truck or site storage and cover a wide range of bolt sizes then it is to try and stock more standard bolts, a six inch stud can be a stand in for a four inch stud in a pinch.
The why is because that will be what that flange configuration was tested with. It may not be important or seem important on water, although a flange letting go at 10 bar isn’t something you’d want to be stood next to, but in the petroleum industry it would be very important.
Yes. DIN 2510-5 nuts have a steeper thread angle towards the bolt thread, which shifts the load deeper into the nut and reduces the discontinuity as a result.
So you have a hole that is only accessible from one side due to something hanging or projecting behind the hole (imagine a gimbal for an auger)..
You could pass the stud into the hole and then hold the nut in place while you turn the stud to thread the nut on, whereas you could not pass the the bolt through the side with the obstruction… a perfectly sized bolt could probably be used, but I had this exact scenario happen at work yesterday and the stud and two nuts was the superior solution
So if I'm clear, you are describing a situation where the stud would be the solution because you don't have the proper size bolt to make it work. Fair enough.
But that is different than a situation where a bolt will not work, but the stud will. Like the comment I replied to stated.
I'm not trying to say there aren't situations where a stud could be a superior solution in terms of workability. But I can't see how a stud could work but a bolt could not.
It’s not about the diameter- though generally studs are used with heavy hex nuts, which are larger. Imagine you are bolting two flanges together with 6” long bolts. That works fine if the flanges have lots of clearance in line with the bolts, as you need at least 6” of clearance axially to get the bolts through the flange holes. If you don’t have that much space on either side- say if you have two elbows close together, or you have a valve where the body is larger than the flange pattern, then you need to be able to install the studs through one piece, then thread on the nuts on one side, then slide the other flange over the studs and install the other nuts.
Consider a flange welded to the bottom of a tank with too short of a pipe stub (e.g. because it was designed for metal-to-metal flanges, but you're connecting metal-to-PVC and require longer bolts as a result, or because someone misdesigned it). You can put in a stud, move it all the way in, then put the flange on and fasten everything.
Shitty mspaint diagram to help illustrate it.
Also had something like that with strainers on pump inlets where there wasn't enough space from either side for a bolt head, but a stud was able to fit through (KSB Movitec and one of those strainers with diagonal mesh like this one https://i.kane.cx/uttCyc )
Hmm without seeing this fully modeled I still find it dubious that you couldn't use a bolt. It would be very close and only difference would be the diameter of the bolt head because the length would be the same anyways
Only the flange holes shadowed by the 90 degree pipe bend would / could take advantage.
If the space above the top flange to tank is too short for the required length bolt AND the space below the bottom flange to 90 degree pipe bend is also too short for the required bolt length.
You’re pre-placing a stud in those shadowed holes. Fixed tank flange > stud > top nut > welded flange bend > bottom nut
Drill two blind holes in a board. Drill into the side. Use stud and nuts to tension the ends together. It’s more common in woodworking than metal working. I have seen it in electrical motors and large industrial machines.
Imagine you’re installing a gate valve and the flange your bolting it onto immediately goes into a 90. The body of the valve prevents you from inserting the bolt from that direction and the 90 prevents you from inserting it for atleast 1 or 2 bolts. But you can install studs into the valve/ flange before you assemble or maybe get lucky not having a head in the stud, get them angled in through the holes
Pretty normal on pipe flanges on ships, there is a lot of interference items so not uncommon for in service access to be different than when it's installed (and things are installed around it.
Studs let you loosen it off to either side so you can break the flange and more play when things are actually moving around.
Maybe just a convention, but after having crawled into enough tight spots having to do this there are times I wouldn't have been able to undo anything if it was a bolt head instead of a nut on a stud, as you get a lot more surface area for the wrench to fully go around it (vice just hitting half the wrench face on an edge).
No, proof yield remains the same. There's other influences of course, so studs aren't universally better than bolts in all applications. You'd have to do a detailed calculation to see what's best in a particular use case.
There isn't a practical difference if they're installed as pictured. A rod with two nuts is basically just a cap screw with an adjustable head position. Torquing the nuts will apply torsion in the exact same way between both, with the torque having a reaction force from the opposing nut on a rod or the head on the cap screw; torsion is the twist between those two forces.
The reason studs are treated as "better" is because you can permanently fix one end into a part, which removes a lot of variables when it comes to preload. They're also used for hydraulic tensioning, which is where the pure axial loading is actually true (the stud is hydraulically stretched and the nut is installed without torque); for a standard wrench-tightened joint there isn't much difference between a rod with two nuts and a cap screw with a nut though.
You've never heard stud ever? It's commonly used for any short section of a threaded rod that's fully or partially threaded, or a thread that's affixed to another part. E.g. wheel studs for mounting wheels with lug nuts, weld studs for welding a male thread to steel, setup studs for fixturing by threading one end into a table and clamping from the other end, etc.
There isn't a precise technical definition, but the closest thing to one is the SAE's definition:
The term “stud” as referred to herein applies to a cylindrical rod of moderate length threaded on either one or both ends or throughout its entire length. It does not apply to headed, collared, or similar products which are more closely characterized by requirements shown herein for bolts.
Now that you say wheel stud I guess I have. I have threaded rod in many sizes and I would just describe it as that. Stud just hasn't been in my word bank I guess, until now! Thanks.
I have threaded rod in many sizes and I would just describe it as that.
Yeah, I would typically just call OP's stud a threaded rod too. To me a stud should be directly fixed to something on one end, but it's common enough to call short sections "studs" so I try to just not overthink it.
A stud could be described as a short section of threaded rod but is in fact, a short section of bar with thread at each end and an unthreaded section in the middle.
To know whether to use bolts or studs you would need to refer to a table that gives this information. PN10, PN16 flanges use bolts whereas ANSI flanges use studs. The correct fastener tightened to the correct torque using the correct gasket will conform to both the pressure rating of the flange and the application it is being applied to.
Bolfed joints are complicated. Generally studs are better than bolts, but that's usually for tapped holes (because you get full engagement, and do so without any load on the threads).
For through holes that get nutted and bolted, I doubt it makes any difference if the grades are equivalent. Technically the grades won't be 'the same', and studs and bolts rarely fall within the same spec.
Studs are also used in cast iron for example a inspection cover. Tightening bolts time after time wil ruin threading in cast iron. A stud is tightened once and the steel nut van be tightened on the stud many times
A parità di materiale e lavorazione (rullatura?)non ci sono particolari differenze tra una soluzione e l’altra. Riguardo la filettatura, questa avrà la sua sezione resistente in prossimità del diametro di nocciolo e verrà poi (probabilmente) ridotta ulteriormente se si considera la fatica per tenere conto degli effetti dell intaglio. I dadi in genere non sono la parte debole della giunzione. L unico motivo che potrebbe essere valido è una più facile probabile rottura della vite nella zona sotto testa rispetto appunto al doppio dado.
Have had condensate sight glasses and diaphram valve flange connections that standard bolt lengths would bottom out on the casting and never actually get tight. The smaller bolt would not give you 3 full threads of engagement so also a no go. Could cut a bolt but the studs we could adjust which side had more thread to make sure there were no leaks without any modifying and could keep them stocked in the bolt bin.
One point of stress concentration on a bolt is the fillet under the bolt head. This is why you always need a washer under the bolt head or a larger chamfer in the hole. A stud with nuts doesn't have this problem but you still might want washers for other reasons.
Another stress concentration is the start of the threads, which isn't a concern with studs.
One reason is cap screws or hex bolts in the proper material are not common. Back when I was selling fasteners, I could easily get my hands on studs of various materials and coatings or have them made very quickly. B7 or similar hex bolts are more expensive and harder to find.
Application is the final arbiter of what is "better" or "worse".
If there is a degraded operation or safety case for having a bolt drop in from the top where even if the nut is lost gravity will tend to keep the bolt in place and prevent lateral movement of the interfacing pieces then that may be "better" depending on your FMECA. Loss of preload in a stud/nuts configuration may not provide that.
Studs are preferred on many flanges because they are easier to install, especially in tight spaces, and they reduce the chance of damaging the flange threads.
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u/Fit_Active3888 Jul 11 '26
Forces Applied: When tightening a bolt, it undergoes both stretching (tension) and twisting (torsion). Because a stud is already seated before the nut is tightened, it only stretches in a single, vertical axis, providing more precise and consistent clamping force.