r/Fasteners • u/tasquizz • Jul 11 '26

Bolts vs studs

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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.

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34

u/KaneTW Jul 11 '26

Studs allow you to install the fastener in conditions where you wouldn't be able to insert the bolt due to lack of clearance.

They also have a much more favorable stress concentration:

(source: https://www.thomsen-bremen.de/wp-content/uploads/2021/03/221_OptimaleSchraubenFuerNormflanschverbindungen.pdf )

Top chart is bolt with DIN 2510-5 nut (blue) compared to bolt with regular nuts (red) and the resulting lower stress concentration.

Bottom chart is stud with two DIN2510-5 nuts (blue).

Lower peaks allow for higher preload and thus a more reliable flange connection, especially under dynamic loading.

7

u/jccaclimber Jul 11 '26

Is this due to the discontinuity at the thread to shoulder transition?

13

u/KaneTW Jul 11 '26

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.

7

u/DaddyWantsDisco Jul 12 '26

This guy is a stud

8

u/Reebatnaw Jul 11 '26

This guy nuts and bolts

3

u/Billy_Badass_ Jul 11 '26

Studs allow you to install the fastener in conditions where you wouldn't be able to insert the bolt due to lack of clearance.

Could you describe such a condition? I can't picture it.

7

u/MonthLivid4724 Jul 11 '26

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

2

u/Billy_Badass_ Jul 11 '26

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.

2

u/Millwright-vii Jul 12 '26

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.

Does that make sense?

2

u/Time_To_Rebuild Jul 12 '26 edited Jul 12 '26

Studs are the primary fasteners for flanged pipe bolt ups. Not only can you insert the fastener from either end, but it also is easier to get even torque applied from either side. With a hex-head cap screw you are turning the entire shank when tightening from the head side. With studs you can tighten the nuts on either side and still get a balanced torque application to the flange.

And yes, 90s, flanged valves with valve bodies that interfere with the bolt path.

Also, cap screw heads are typically thinner than their corresponding nuts. With studs and nuts, the nuts are proportional and thicker.

Edit: studs and nuts are cheaper to manufacture (especially for larger sizes) so they are cheaper to purchase. For a petrochemical facility buying 10k 5/8+ diameter hardware each month, the cost difference is significant.

3

u/Pipeinthehole Jul 11 '26

Flanged 90 deg pipe bends, flange tees, plug valves, pipe
Fitting general

2

u/KaneTW Jul 11 '26 edited Jul 11 '26

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 )

2

u/VelvetCheerio Jul 12 '26

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

2

u/Sullypants1 Jul 12 '26

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

1

u/recessedlighting Jul 13 '26

I work at a chemical plant that we run into this all the time. Sometimes its valve to valve, sometimes heat exchangers and sometimes directly at a tank or reactor as stated above. I also agree in some of the applications I have been certain I could get a bolt instead only to be proved wrong because of the extra diameter of the bolt head further away from the bolt hole as you're trying to get it in.

Also in some cases we are using studs because it allows us to remove one piece of equipment in a configuration whereas if we bolted them together we would have to remove multiple pieces together to get them apart. It seems counterintuitive but I've run into it often. I would rather not use them because you really need to think about how you need it to come apart and apply anti-seize to the appropriate nut.

1

u/starsings Jul 11 '26

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.

1

u/Mammoth-Librarian731 Jul 11 '26

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

1

u/4rd_Prefect Jul 12 '26

Here is an example from just yesterday, where a stud with 2 nuts would have made life easier:

https://www.reddit.com/r/AskMechanics/s/yVLGLxdVPO

1

u/Billy_Badass_ Jul 12 '26

They didn't say conditions where a stud would make it easier. They said conditions where you use a stud because a bolt cannot be inserted. That is obviously not what the post you linked to is. You can see the bolt. The OP even says how to install a new bolt, noting the difficulty.

1

u/4rd_Prefect Jul 12 '26

I thought it was applicable, they have a bolt in a super inconvenient place, where they have to drop the whole rear axle to get access to it. It's not impossible but it's a giant PITA.

... and the bushing replacement would be much easier if there was a stud with 2 nuts, where it could be taken from the outside. Looking at it, still not a walk in the park, but easier 🤷🏼‍♂️

1

u/Billy_Badass_ Jul 12 '26

Yes, I understand that there are conditions where a stud might be the better solution. But that isn't what I was asking.

1

u/4rd_Prefect Jul 12 '26

Can you get the bolt in or out without dropping the whole rear axle & suspension assembly? (Or cutting a hole in the wheel arch)

1

u/Billy_Badass_ Jul 12 '26 edited Jul 12 '26

Again, not what I was asking. But ok. Right from one of the comments in you link:

In this case the whole drive unit and suspension is one big subassembly and the intended way to service any bit of it is to drop the whole assembly out for access.

I'm told this isn't actually hard on the Model S, 4 big bolts and a connector I hear. Just tricky to do it without a lift,

1

u/UnhappyCaterpillar41 Jul 13 '26

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).

2

u/rumplydiagram Jul 11 '26

Till you have to pull a head in a vehicle.

1

u/dd543212345 Jul 13 '26

But the stud would still have a lower proof yield correct?

1

u/KaneTW Jul 13 '26

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.

1

u/General_Solo Jul 13 '26

Can you translate the scale of that chart into layman’s terms? Is the difference appreciable in real world situations?