Backing stud exposed to high-vibration loading - gr12.9 & lock method?

[CrazyJ]

Hi All,

I'm a mechanical engineer working at a hydropower plant. The turbine guide bearing takes quite a beating, and they've asked me to look into a larger diameter stud and/or different material.

The current arrangement is a threaded insert welded in place in a thick walled tub/cylinder that the stud screws through. The stud pushes directly onto the back of the guide bearing. Outside the "tub" are a lock nut, and a jam nut behind that.

The current sizing is M30, with what looks to be a 1mm pitch (very very fine).

My first thought is to upgrade to an M33 made from 12.9 with rolled threads. I'm hearing alot of negative things about 12.9 and I was wondering if you would recommend it in an application like this that sees a lot of hammering on the stud (as opposed to the normal application where it is called on to clamp a joint).

With a hard bolting material, would you expect the lock and jam nut to sufficiently hold it. Getting the lock nut and jam nut to "bite" in to the stud may not be easy, or last.

Any bolt materials ideal in this situation?

 

[MintJulep]

What is the problem that you need to solve?

You shouldn't start working on the solution before you understand the problem.

 

[CrazyJ]

Sorry, I should have stated that.

The high vibration from the shaft through to the bearings is causing the studs to back out and the clearances to open up on the guide bearings. The plant believes it has to do with poor threadform and materials used (Chinese turbine)

 

[Metalguy]

Red "Stud Lock".

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[Tmoose]

From the description I think when the "lock nut" is tightened the stud is preloaded/stretched between the nut and the threaded insert. If the lock nut is tightened sufficiently against flat surfaces there should be little tendency for it to loosen. If the bearing shell wiggles in service then the stud must withstand that, or embedment and wear will relax precious preload. In that case the lock nut (and jam nut) will not have to move at all for preload to be lost. The shortness of the stretched portion (grip length?) of the stud may be working against you. If there is room to put a nicely made spacer between the threaded insert and lock nut then the grip length will increase and the joint will be more resistant to preload loss and loosening.

 

[MintJulep]

I was thinking exactly along the same lines as Tmoose.

 

[CoryPad]

The best way to prevent joint loosening due to applied forces (including vibration) is to use adequate preload. This is one of those better, faster, cheaper concepts. Extra features (thread adhesive, deformed threads, underhead serrations, stepped washers, ad infinitum) are really just ways to hide the problem rather than fix it.

Your variables are fastener diameter, fastener material strength, and pretension level and method (hydraulic tensioners, torque, etc.). Property class 12.9 provides very high strength, but is susceptible to environment assisted cracking. What is the environment? Temperatures, fluids, other? Property class 12.9 fasteners usually have limited applications due to this issue.

What is the current material strength? What is the current preload level?

 

[CrazyJ]

The environment is a warm 80ish celcius oil bath. I can't comment on the preload/torque as this is discovery work and I'm not able to find bolting records for this. Material, I belive is 4140. The threads don't appear deformed. I agree with Tmoose and others in that, I believe the short stretch length (and probably a reluctance to overtorque the adjustment stud) is likely our root cause.

And you confirmed my suspicions about 12.9. I'll have to look into that further.

Anyone have recommendations on bolting materials?

 

[flash3780]

Welding threaded fasteners always gives me the heebie jeebies. What's that do to your preload? Material properties? I think thread-locker or a locking insert is a better solution for a stud. You can look into Keenserts or Rosan studs, which are the most reliable options (although somewhat expensive).
You're dealing with pretty low temperatures, so the sky's the limit when it comes to locking features. Loctite would even work fine.
As far as the material goes, I've always been under the impression that 12.9 was a good material. Perhaps you could investigate why people have a negative impression of it.

 

[flash3780]

^^
Just noticed the post about 12.9. I'd wager that stress-corrosion cracking wouldn't be an issue when you're bathed in oil.
If you really want to put it to bed, you could always use Inconel 718 fasteners; they don't corrode, but they get pricey fast.

 

[CoryPad]

Property class 12.9 can be a great material for fasteners. The "negative impression" is fact-based concern for the fracture resistance of these parts. You can read more here: thread725-95071