Is there a general rule in using washers or lock washers with nylock type nuts?

In a structural, vibration enviroment?

Mating material is steel, fastener grade 8.

Split - spring type lock washers are useless, with or without nylock nut.

Use nylock with flat washer.

I've found Nylock to be a good solution, particularly in tension type attachments.
Alternatives are:
1: A325 or A490 bolt/nut properly tightened
2: Double nutting to lock
3: Tack welding nut to bolt body

Another alternative:
4. Wreck threads AOBE

I don't think wrecking threads is a good idea with bolts in tension! It is not preventing the nut from losing its pretension, which eventually may cause the bolt tension force to be redistributed among the other bolts not designed for it.
TL

Sounds like a fatigue situation or cyclical loading. Therefore the idea would be to keep the nut on.
If it's pretensioned and normally in tension, then there should be little if any increase in axial stress. Mind you I'm speaking of high strength bolts (A325 & A490). Nylock nuts wouldn't apply.
Another alternative:
5. Load indicating washers

If I have an application that involves a bolt in tension and the nuts need to be locked in place, is it best to use one nut and a lock washer or 2 nuts, or even two nuts and a lock washer?  Is there a rule of thumb for this situation?

Thanks in advance for any help.

Tony Billeaud
Mechanical Engineer
Franks Casing Crew

How much tension?  Why is the tension not sufficient to prevent nut rotation?  Can you increase the tension?  By any measure (cost, ease, safety, etc.), sufficent joint preload is best.  If you must you something extra, thread adhesives are very good; nuts with polymer inserts or deformed threads are less good.  Two nuts can work, but are tricky to use in practice.  I have never seen anything I would call a lock washer - I would avoid using them.

Regards,

Cory

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

Cory,

It two 7/8" coarse thread bolts and they are torqued to 30 ft-lbs and 100 ft-lbs.  That is the max torque that can be applied to the bolts.

Thanks,

Tony Billeaud
Mechanical Engineer
Franks Casing Crew

Cory,

I would also like to add to the above - is there a rule of thumb for the minimum torque required to be applied to a nut in order to keep the nut from rotating on a bolt?

Thanks,

Tony Billeaud
Mechanical Engineer
Franks Casing Crew

tnteng,

Your torque appears to be about half what it should be.  The rule of thumb is based on force, not torque.  A fastener torqued so that the preload is about 50% to 100% of the yield stress should be adequate for most joints.

Regards,

Cory

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

Minimum tensile strength on your fastener is 69,300 lbs.  As a general rule of thumb, you want to obtain a tension of 75 - 85% of UTS to get the most out of the fasteners in a cyclic, tensile loaded environment.  Using the old T=KDW formula, K=.20, W=69,300 X .80 = 55,440, D = .875,  you end up with T = 9,702 inch pounds.  That works out to 808 ft. lbs.
To get the joint to stay together you need to apply a whole lot more torque to it.
The only other thing that I would suggest is trying high strength Locktite on the threads.
With the torque that you currently applying to the bolts you are only getting somewhere in the area of 7,000 pounds of clamp force.  With that low force level you can get by with using unheat treated, grade 2 bolts.  

Thanks Screwman,

To have a good SF we have chosen Gr 5 bolts for this application.  The connection will be on a piece of equipment that will be transported on truck and by barge.  I am limited to the 30 ft-lbs and 100 ft-lbs listed above and the nuts need to be removed after the transport is complete.  Will a lock washer and 2 nuts work for this problem?  If so, is locktite still required and will the nuts be removeable if locktite is used?

Thanks in advance for any and all help.

Tony Billeaud
Mechanical Engineer
Franks Casing Crew

Thread adhesive (such as Loctite) is not really recommended for use-reuse applications.  If you need to use a washer, use a hardened flat washer.  To prevent nut rotation, you can use two nuts, but the technique necessary to prevent problems is complicated.  Good luck.

Regards,

Cory

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