Fine thread bolts - do they back off 12

[RCMRon]

These are 5/8 " fine hread studs holding 70 lb fan blade liners on a 500HP fan rotating at 712 RPM. the liners are held in place with 20 1 1/2 inch studs. When properly torqued, will the nuts back off and or shouyld they be tack welded or double nutted?

Does anyone know of others that use stud mounted fan blade liners?

 

[IRstuff]

Not by themselves. But, if you have enough incentive...

TTFN

FAQ731-376

 

[RCMRon]

With the rotation and vibration be enough incentive.

Should they be tak welded or double nutted?

 

[PeteDB]

Investigate using Nordlock® washers to prevent the nuts from loosening.

 

[unclesyd]

Are you talking about the 5/8" or the 1 1/2" fasteners?

 

[IRstuff]

If you're contemplating tack welds, then they aren't intended to be disassembled? If so, why not something seriously permanent, like rivets?

TTFN

FAQ731-376

 

[vpl]

Are the studs 5/8" in diameter and 1.5" in length or are you talking about two different sets?

How well are the fans balanced; the rotation isn't that excessive but you didn't say anything about vibration?

What are the consequences if the studs do back off?

Patricia Lougheed

******

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[hydtools]

If the fasteners are tight enough, they will not back out. Coarse or fine thread.

If they are now backing out, they are not tight enough.

Double-nutting is so not a choice anymore.

Ted

 

[drawoh]

RCMRon

Bolt Science has an article on loosening screws that you ought to read. It gets recommended fairly regularly here.

JHG

 

[flash3780]

Any fastener in a vibratory environment is subject to loosening and must incorporate secondary retention. It sounds like there's quite a lot of energy in these parts; a failure would certainly be a big deal. You must incorporate some sort of secondary retention.

Lock welding is fine, but needs to be ground off for removal. Safety wire will keep the bolts from falling out (which might be important) but cannot ensure that you don't lose clamp. Loctite is pretty good stuff, but the bolt torque cannot be rechecked after the loctite sets up. Threaded locking inserts (keenserts, rosan fittings, etc) are quite good, but may wear out after several uses and can be expensive.

Hope that helps. Good luck.

 

[MintJulep]

A properly selected and TIGHTENED fastener will not loosen due to vibration.

But you need to actually know the loads and a bunch of other stuff to properly select a fastener. You can't do it by guessing and using handbook values.

If you really need to preload a fastener to a known value, then torque control is definitely NOT the way to tighten it.

If you can trust your assembly technicians to actually clean the nut and stud and follow the directions, use Loctite.

Otherwise use a prevailing torque nut.

Or, as others have suggested, use a rivet or Huck Bolt.

 

[rmw]

I took it that the studs were 5/8" dia X 1-1/2" long. The presence of wear plates and the HP and speed of the fan indicate the strong possibility of a hot fan. Because of the heat, the possibility exists that there is differential heating or differential expansion of the (hot) fan parts which could loosen the bolts and cause loosening.

I'd tack weld the studs. The presence of wear plates tells me that they will be junk anyway (if they outlast the wear plates) so dis-assembly isn't a concern.

In fact, the more I think about it, I don't even like the idea of one end of a relatively soft (with respect to the wear plate) stud and nut sitting out there in any flow stream where it takes a wear plate to protect the fan blade.

I'd recommend looking for a countersunk head type of fastener that you can sink below the surface of the wear plate and weld over with a wear resistant material weld rod.

I've seen that done, but sorry, it was way too long ago to remember a lot of details about with respect to what type of flat-headed screw was used.

You aren't too worried about the nuts and exposed threads on the back side of the fan blade.

rmw

 

[Tunalover]

I see the simplest solution is to use prevailing torque style locknuts.


Tunalover

 

[RCMRon]

All of the answers given have been very helpful

Hydtools mentonned that that double nutting was not a choice any more. Why is that, is it because there are better solutions

Tunalover just suggested a prevailing torque. In oiur applications, the operating temperature of the fans, minimum 80" in diameter, would these still work? which kind of prevailing touque nut would you reccommend.

what about tack welding after the liners are torqued? (When we remove the liners we arc off the nut anyway)

I have attached a picture of the damaged blade liner showing the failed stud welds as it might provide more perspective.

Again, I appreciate all of the advice

Ron

 

[MintJulep]

For high temperatures, use an all-metal type of prevailing torque nut.

"Tack welds" are for fixturing purposes only. Stop misusing the term when you want to discuss a final weld.

I don't like welding nuts and bolts. You change the metallurgy and build in a nice crack to propagate. Both introduce new undesirable failure modes.

Perhaps you've noticed that all of the studs in your picture failed at or near the toe of the welding?

 

[unclesyd]

Can you give us a cross section of the stud in the service condition that will should all the metal contact areas. Also the nut landing landing area.

Essentially are all contact areas nice and parallel and perpendicular to the stud axis.

 

[hydtools]

See disc lock washers:

http://www.disc-lock.com/standard-washer?item=110

Ted

 

[byrdj]

instead of welding the bolt, I have used pieces of angle iron to capture a flat and tack angle to body. (or keystock, flat bar, etc)

 

[Tmoose]

Is this design proven in similar installations?

I think the pictures shows broken studs, mostly all with curiously brittle looking fracture surfaces. What is the stud material, and what is the atmosphere in the fan?
How many of these liners have failed?
How has stud pattern shift due to thermal expansion been considered? ( extra clearance for stud body, controlled stud placement)
What is the current assembly procedure in regards tightening the nuts?

Some of those stud/bolt shanks are worn severely, like the liner was moving around or buffeting for a while ( hard to imagine at 50 radial g at 700 rpm) . With the uneven and poorly matched surfaces it may not be possible to make this a proper structural joint, that can be torqued to provide reliable clamping via fastener stretch. If so, the studs are reduced to serving as mere pins, with very few being in contact with the liner at one time

The broken studs' ~1/2inch of exposed threads beyond the nut are in nice shape, like they are in clean airflow. (or things failed real quick) Could you use that 1/2 inch space for a metal lock nut sitting on a short stack of stout belleville washers to provide >some< comliance?

What does your weld stud supplier have to say regarding how and therefore why the studs failed?
I'd be looking into alternative stud designs, with an unthreaded shank or shoulder to move the threads away from the weld, up closer to the nut. Maybe even invert the design, using a tapped stud or pad.

http://studweldingfasteners.com/studs/tapped_arc.php

http://www.nelsonstud.com/Portal/Portals/0/pdf/Catalog/StudWeld/Threaded/Internal/PBL TBL.pdf

http://www.midwestfasteners.com/products/swproducts/arcfasteners/shoulder_threaded.php

 

[rmw]

What damaged the liner?

I think there is much more to this story than what has been presented.

rmw