Fatigue with Stainless Steel Bolt 2

[ChipFuller]

We have a 5/16 SST hex head bolt that keeps failing because of fatigue at our plant. The bolt is a ASTM A593C. I can't remember how much increasing from a 5/16 to a 3/8 will increase the fatigue strength and I don't what hex head stainless steel bolts are available in higher strength.

Any thoughts?

Thanks

 

[Goahead]

It seemas as something essential is missing.

SST does not identify the stainless steel involved and the properties required.
ASTM A 593 does not designate a bolt material
ASTM A 593 -
SPECIFICATION FOR CHARPY V-NOTCH TESTING REQUIREMENTS FOR STEEL PLATES FOR PRESSURE VESSEL

http://www.welding-advisers.com/

 

[ChipFuller]

I'm sorry my typo. The bolt is designated ASTM F593C.

 

[metengr]

Can you provide more specific information regarding your bolt application including temperature and environment? The cause of failure could be several reasons, one of which is inadequate (lack of) preload that would subject the bolt to damaging cyclic loads resulting in fatigue crack initiation and propagation in service.

The only benefit to be gained by going to a larger bolt diameter is the increase in load carrying capacity. However, an adequate preload is still required to avoid subjecting the bolt to fatigue.

 

[ChipFuller]

Thanks for the reply. The temperature is ambient (70 F) and the environment is somewhat corrosive. The head of the bolt is exposed to an oxidizer. It's located in a hopper. The bolt is used to hold a 1/4" nylon pad and a 1/4" piece of neoprene on the outside of the hopper. A bin vibrator made by a company called Vibco strikes the nlyon on the outside of the hopper. We have not had this problem in our other buildings with the same application.

The nut used is a cadmium plated nut.

You bring up a good point. What would a good preload/torque be for bolt like this? I'm little hesitant about using a torque wrench with a stainless bolt because it's seems like they can gall and then give false readings with a torque wrench.

 

[diamondjim]

Are the bolts in both building from the
same manufacturer? You might have inferior
bolts.

 

[ChipFuller]

You know it's hard to say. We bougth them Fastenol. We've these bolts in for years and never a problem. The ones in the other buildings are 3/8".

 

[Carburize]

I go with metengr I think they are probably loose.

 

[ChipFuller]

Thanks for the reply. What would a good preload/torque be for bolt like this?

 

[NickE]

also you can get better consistant tension/torque by using a bit of anti-sieze on the bolt. Although then the torque to produce a specific tension generally goes down.

Nick
I love materials science!

 

[CoryPad]

If a polyamide (nylon) and an elastomer are your clamped materials, then preload loss is a huge concern.

Regarding your question of a good preload, something around 75% to 100% of the proof load is a good start. Look up the ASTM standard to learn what is the proof load for your fastener.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[diamondjim]

Are you using a large diameter washer under
the heads of the bolts on the neoprene side as
well as on the nylon side? I think I would
use 85 percent of the yield of the bolts as a
minimum if the neoprene and nylon can take the
bearing forces under the washers or head of
the bolts. How hard is the neoprene? It does
seem strange that by going to a larger bolt,
you are starting to have failures. Same number
of bolts?

 

[ChipFuller]

I'm sorry if I mislead you. We are not having failures because of a larger bolt. We are having failures with the 5/16 bolt but not in the other buildings with 3/8" bolts.

The cross section for the set up goes cadmimu plated nut, nylon, neoprene, 1/8" stainless sheet, then bolt head.

Like I said earlier I'm a little concerned with getting false readings with a torque wrench if the nut catches on the thread of the bolt. Is there any other methods for torquing?

Thanks

 

[diamondjim]

So your length of engagement is only
5/8 or 1.67 times the 3/8 diameter.
If you would use a thick washer on
each side it would help to improve the
clamping length and may give you
better torque control and reduce the
strain in the bolts.

 

[MikeHalloran]

Always, always, always, apply anti-seize, or _something_, to the threads of a stainless bolt. Even ear wax is better than nothing. Also better than nothing is Loctite Pipe Sealant, a good goop to carry if you only want to carry one goop.

I'm guessing that what's happening now is that when the bin shaker whacks the nylon, it's compressing the rubber enough to remove all or most of whatever preload has been applied, and the head is pushed away from the inner bin wall, where it's exposed to the corrodent on its underside. Corrosive environments do not improve the fatigue life of fasteners.

Torquing the nut to anywhere near bolt yield will just cause the rubber, and to a lesser extent the nylon, to ooze out from under the retaining washer/nut.

A larger bolt might actually help a little.

As might putting a stainless bushing around the bolt shank, through the nylon and rubber layers, so the bolt is compressing a stack comprising the bin wall, the bushing, and the retaining washer under the nut, and the compression of the nylon and rubber is limited by the bushing, which should be a hair shorter than the combined thickness of the rubber and nylon.

Are you sure there wasn't a bushing in the stackup when it left the factory?




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[aviat]

You can check if you are thread bound by torquing in the normal way, and then trying to turn the bolt with a wrench. If you can turn the bolt without much difficulty, you may have to remove nut and install another washer, making sure that bolt is still in safety.

 

[ChipFuller]

Thanks for all of the replies. Is ASTM F593C the most common type bolt that has ASTM markings? Or is there another ASTM F593 designated bolt that has higher strength and is fairly common?

 

[Paskenell]

Chip

ASTM F593 is the most widely available. ASTM A193 is another spec you might look at as its availability is fair.

And Mike, I have been selling stainless bolts for 15 years now but I will never be able to look at them the same way after that ear wax remark

 

[ccw]

Hi ChipFuller,
I am not sure your original req. about strength increase 5/16 to 3/8 was answered. The stress area for 5/16 = 0.0524 sq.in., 3/8 = 0.0775 sq.in from Mach.Handbook. The ratio is 1.48 or 48% increase. That is significant and may explain the difference between this application and the ones in the other building.