Is a Lock Washer an Effective Torque Retention Device?

Mechanical Engineering general discussion ME FAQ's
#1

By @btrueblood and @MintJulep

Courtesy of MintJulep
When a lock washer is fully compressed, as it is in a joint that is holding its design preload, the lock washer behaves exactly like a flat washer with a crack in it. Or, if you prefer, a spring that has gone solid. It no longer acts like a spring in further compression and its stiffness, as it relates to the joint performance is determined by the modulus of elasticity of the material.

If the joint loosens sufficiently so that the lock washer acts like a spring again then the joint has lost its design preload and the only tension left in the bolt is whatever small amount that is provided by the lock washer. It doesn’t matter. The joint has already failed. The lock washer may postpone the inevitable departure of the nut from the end of the bolt.

But if you don’t want to believe me, perhaps you would believe NASA.

NASA Fastener Design Manual RP-1228
“The lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking.”

US Naval Ships’ Technical Manual, Chapter 75
"Although lockwashers may be encountered, using the flat washers with selflocking nuts, self-locking fasteners, self-locking inserts, or thread sealants such as MIL-S-22473 anaerobic compounds is preferable.

If loosening has been a problem, however, replace the lockwashers with self-locking fasteners.

The helical spring lockwasher (Figure 075-5-11) is flattened when the bolt is torqued down. Once compressed, it acts as a flat washer, contributing normal friction between the nut or bolt and the bearing surface during tightening."

British Ministry of Defense
“Vibration Loosening of Threaded Fasteners (Light):
If the plain fastener is taken as the datum any washer reduces locking effectiveness”

ASME B18.21.1
“The word lock appearing in the names of products in this standard is a generic term historically associated with their identification and is not intended to imply an indefinite permanency of fixity in attachments where the fasteners are used.”

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#2

I have one more case to add:
The FAA specifies the use of split lock washers in 1 (one) single, specific instance. That is the case where one requires a fastener to be electrically bonded to the structure (some people use terminology such as “earthed” or “grounded” but the term is “electrically bonded” in aviation.) The policy that specifies this bonding fastener emphasizes that it is NOT a structural attachment.

It is very important to be aware that the purpose of this split washer is NOT to secure the fastener but to scrape some of the finish off of the substrate material so that the electrical circuit can be guaranteed with minimal electric resistance between the washer and the substrate. Also, there are washers that are more effective at this bonding function (star-lock) therefore it is never necessary to use the split washers anywhere in an aircraft.

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#3

Not so good… Belleville OK, never use them for anything serious… Loctite Red, lots of times… check the link:

https://www.boltscience.com/pages/helicalspringwashers.htm

Dik

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#4

I believe Nordlock has some good data that their washers actually stop bolts from coming loose. We use them on machines with a lot of high frequency vibration. I have never seen one come loose. I think that they actually work.

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#5

Nordlocks (like split washers) will do a small amount of damage to the base parts every time they are removed. If your structure/machine is designed to be “never” taken apart, then maybe you can consider that. If you have a machine that must be disassembled for service or inspection, that’s not an option. They’re also a problem for scoring through a protective coating on the surface (starting corrosion) and putting radial grooves on the edge of a hole which is basically another way of saying “have initiated fatigue cracks”. I do not expect to see anything like a Nordlock in my entire career in aviation.

The aviation industry uses a combination of structural nuts with self-locking features (deformed threads, nylon inserts, etc.), castellations for cotter-pins or wire-ties, installation torque requirements, and thread-locking adhesives. Washers with funny features are not on that list. We insist on 2 locking mechanisms on any joint subject to rotation or heavy vibration. That still doesn’t earn points for split/locking washers to get them on the list.

If the bolted joints you are looking at were installed and torqued properly, then ask yourself why you think the Nordlocks helped?

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#6

@Dik,

The boltscience guy has some ideas about bolted joints that don’t sit well with me. While accounting for friction in the installation torque calculation is necessary, that doesn’t mean that friction should be used in the structural joint design.

He uses friction in the joint shear strength calculation (see BoltCalc.exe program) . Bad idea. This may be an industry-specific bias. In aerospace, we design joints with no consideration of friction between the fastened members at all. I don’t know if other industries do this, but it’s forbidden to me.

#7

Not all split washers are created equal. I have encountered split washers that have the ends slightly upset.
These are the split washers that tend to remove base metal when the nut is removed.
While this design will put the grooves that Spar mentions in relation to induced cracking, I see these mostly on thicker members where this type of cracking is not common.
With a little experience in electrical stuff, I would never depend on a plain split washer to provide dependable grounding.
I would use a star washer to ground a bolt.
Most electrical lock nuts for conduit or fittings are designed to penetrate paint in one way or another.