Value of helical split and toothed lockwashers 3

[Tunalover]

Does anyone out there have any pubished material on the vibration performance of helical split and toothed lockwashers?

I have heard from several "experts" that they can actually be detrimental rather than helpful.

 

[gdtdave]

Hmm, experts?

The main problem with locking washers is that using them is no good guarantee that the fastener won't loosen. A look at

http://www.boltscience.com

will give quite a few good reasons for fasteners to come loose.

It's that false sense of security that's a real problem. Is there a specific problem or just general interest in the subject?

 

[Tunalover]

gdtdave-
Thanks for your input and the good link. I don't have a particular case in mind, but when fastener consultants openly state that lockwashers of this kind are generally worthless, I have a yearning to see scientific proof rather than opinions.
Bruce

 

[Lcubed]

If the helical washer is flattened when the nut is torqued down, so that no sharp edge of the washer is able to dig into the nut, and if the tension in the bolt is increased only infinitesimally by the flattening of the washer, it seems clear that the washer cannot provide any significant locking action. These conditions would be present in most joints.

Toothed washers arguably offer some locking effect, but they must deform or incise the nut and the clamped surface to do so. That surface damage can lead to premature cracking and corrosion, which may be why some people think they do more harm than good. Toothed washers probably should be evaluated case-by-case, whereas helical spring washers do not seem likely to help in any application.

 

[Tunalover]

Thanks guys!
People seem to have differing opinions on this. I'm having trouble getting data from unbiased sources. Opinions are welcomed but not conclusive. If you run into anything please let me know.

Tunalover

 

[Lcubed]

tunalover,

I don't blame you for wanting something more concrete than opinion, but I don’t think I have any test data to present. I dug around in my library and came up with this:

"A wide variety of vibration-resistant washers are also available. Toothed washers can be used successfully only with relatively soft joint members. Wave washers, like Belleville springs, are intended to maintain preload even after embedment of nut and bolt threads. Their stiffness, however, is so much less than that of most fasteners that they are of questionable value."

This is from: Bickford, John H., An Introduction to the Design and Behavior of Bolted Joints, Second Edition, Revised and Expanded, Marcel Dekker, New York, 1990

Bickford shows an illustration of a helical lock washer, but makes no mention of it in the text. I think it would have the same problems as the wave washer.

I am always interested in finding out if what I "know" is true, so I’m going to see what else I can find among my references in the near future, and I’ll pass along whatever turns up.

Happy New Year!

 

[EngineerBSME]

It has been a while, but I did some research to prove/disprove how to best prevent nuts from becoming loose.
Bottom line is this:
After a period of time (1 yr +), all materials change and fastener integrity must be checked.
Metals subjected to continuous heat will loose their "spring" no matter what type configuration (wafer, split, electrical "toothed" washers, deformed threads)
Metals continuously subjected to vibration gradually loosen due to fatigue. Rivets loosen. Material dissimularity should always use a more noble material for the fastener and/or in conjunction with sacraficial materials like magnesium. For example, corrosion experts theorize the Titanic sunk because the salt water corroded the brass rivets which originally held the carbon steel shell together. The ice berg just happened along as the rivets were about to pop from corrosion.

Non metalic materials often used to lock threads in place will "creep" or "cold flow" and eventually become ineffective.

Thread sealants such as Lock tite is effective but certain types make future nut removal difficult to destructive.
Fasteners used for electrical equipment like copper grounding bars will form a non-conductive coating if not cleaned annually or earlier in corrosive or moist atmospheres. For example, approx 12 months after installation a SS bolt with SS lock washer is more electrically conductive than a brass bolt because of the corrosive coating on the copper bus bar.
To prevent loss of integrity and strength, the air line industry and military use the inspection method. Periodical visual inspection of fasteners or vibration analysis of equipment is necessary to ensure adequate integrity of critical fasteners.

 

[stuff]

I can't offer you any unbiased data on the vibration peformance of those washers, but I have seen a great demonstration of their failure by the guys who sell Nord-Lock washers. I am not affiliated with Nord-Lock, by the way.

They used the Junker test (?) - a load cell is under compression in a bolted joint that is laterally vibrated at 40Hz. The test shows the pre-load dropping rapidly (within 5 sec) for locking solutions such as Nylocs, split rings and star washers. Their washer solution, of course, retained 90-95% of the initial pre-load. See the links for the test diagram and results:

http://www.nord-lock.com/default.asp?id=6&language=4

http://www.nord-lock.com/default.asp?id=7&language=4

I have since used those washers (flight simulators on motion platforms) and found them to be reliable and extremely useful, although I have not formally verified them through testing. I have not seen this Junker test elsewhere and suspect it to be a proprietary Nord-lock test - please correct me if I'm wrong, anyone. The lateral load in the test seems favourable to the function of the washer, I'd be interested to see in-line excitation or some random vibration applied.

These washers also bite into both the fastener and the joint, which introduces both corrosion and stress concentrations concerns, albeit slight. My concern is that the right amplitude and frequency of excitation could cause the biting face of the washer to allow the bolt to rotate relative to the washer.


Sorry this is not quite an authoratitive source, but I found it interesting nevertheless.


Stuff

 

[les52]

The amount of bolt or capscrew that can stretch and act as a spring is important for keeping the fastener tight. I think sone engineers at Caterpillar have determinrd that 19 mm is the min. free length ( dist. from conact surface of head to the start of the nut or tapped hole threads )for grade 8 fasteners. The major function of a washer then is to increase the stretch length and a plain hard washer is as good or better than a lock washer.