How many of you use helical split lock washers in bolted joints? 8

[Tunalover]

Anyone want to weigh in on their pros and cons?


Tunalover

 

[CoryPad]

Common topic, these parts usually are considered to be useless. I do not use them. See:

faq404-1257

thread108-114051
thread404-94197
thread725-85323
thread725-63343
thread725-57042
thread725-145968
thread404-287279
thread404-230741

 

[desertfox]

hi
this link says it all for me:-

http://www.boltscience.com/pages/junkertesthelicalspringwasher.htm

 

[chicopee]

The link from desertfox shows a very coarse thread, so I am wondering if you would have the same effect with medium and fine threads; in addition that thread in the video did not appear to be of a "V" shape.

 

[Screwman1]

Helical lock washers have such a low spring rate that before they start to return you will have lost about 90% of the clamp load in the joint and if that happens, you have a failure going on. If they are fully flattened, then all you have is a hardened flat washer- so use one of those instead.
I would never recommend one at work and I take them off things around the house.
A waste of time and money and one more component that can be left out of the assembly.

 

[Sparweb]

They are "standard practice" in my company's procedures manuals.
Just one of many surprises when I arrived a few years ago. Change is hard. May make some progress on such issues. Had some luck on bigger problems so I have some hope.

STF

 

[Tunalover]

Ditto when I worked for GE Transportation. When I told people the washers didn't work and that they actually tended to lower the natural frequency of the bolted joint they looked at me like I was crazy. After all, they've always used them. I cited the NASA Fastener Design Manual RP-1228 but it didn't make a difference. You can lead a horse to water but you can't make it drink!


Tunalover

 

[StoneCold]

After looking at a lot of the threads I believe that if you are going to do anything other than bolt preload, serrated face washers are the way to go. Nord Lock being the original name brand.

Regards
StoneCold

 

[Tmoose]

Nordlock has some recommended torques for their differentially serrated washers.

http://www.nord-lock.com/nord-lock/wedge-locking/washers/torque-guidelines/

Looks like they are 25% or so higher than generic lubed bolt torque values.

 

[Tunalover]

What is needed are industry standard or mil standard part numbers for the serrated washer. People in Purchasing are pretty sensitive about buying from a sole source. Does Nord-Lock still have a patent on the design? Can you buy them from multiple sources besides Nord-Lock?



Tunalover

 

[tbuelna]

They are "standard practice" in my company's procedures manuals. Just one of many surprises when I arrived a few years ago. Change is hard. May make some progress on such issues. Had some luck on bigger problems so I have some hope.

The OP asked for comments on the suitability of split lock washers in bolted joints, and the general consensus seems to be that they are ineffective. But SparWeb's comment above points out the more general problem with changing long established practices used in some companies or industries that may be suspect. Sadly, the decision to change an existing practice is usually made by non-technical management. And they are usually reluctant to implement a change that could give the impression that the existing products they have been producing and selling might be deficient.

I work in aerospace and the issue of securing threaded fasteners is taken very seriously. I have seen the opposite situation that SparWeb describes, where it is difficult to train a new-hire engineer coming from another industry to follow the very explicit and tightly controlled design practices with threaded fasteners used in aerospace.

 

[Tunalover]

Well it looks like the overwhelming opinion is that helical split lock washers are useless and only serve to drive up part counts, hence complexity and costs. I contend that they also tend to drive down the vibration natural frequency of the joint. Anyone concur with this?


Tunalover

 

[Sparweb]

I went to the NASA RP-1228 for a refresher. Short and sweet, I like it.

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.

It's as if WKTaylor wrote it, isn't it?



STF

 

[Sparweb]

Tunalover,
Sorry, forgot to address your last question.
I can't see how it would affect the natural frequency of the joint, provided that the bolt has been torqued the same as it would be with a regular flat washer. Unless, perhaps, something is hammering on the bolt itself... Nah I really can't picture it. Find any research on the matter?


STF

 

[desertfox]

Hi tbuelna

My gut feeling about lowering the natural frequency is the same as yours however, I'm not sure how much of an affect it as in real terms or even if its significant, I found this paper but it refers to ordinary washers.

http://dspace.nitrkl.ac.in/dspace/bitstream/2080/160/1/bkn1.pdf

 

[Tunalover]

Can't point to anything published I'm afraid. BTW, does Nord-Lock still have a patent on the serrated surface lockwasher design? Can you buy them from multiple sources besides Nord-Lock?


Tunalover

 

[moltenmetal]

What if the bolt is being used essentially as a pin, such that it can drop to near zero clamping force and still be effective? In that circumstance, the helical split washer would seem to serve at least to some extent to keep the fastener from rattling completely out of the joint.

Although I think I understand the argument against them, and know that post-assembly wicking grade threadlocker or Nylock nuts are much more effective, we do continue to use split helical washers at present on the theory that they are somewhat better than nothing at keeping fasteners in the plant modules we ship rather than laying in the belly tarp of the truck used to ship them. That isn't proven by statistics but is not on argument in the minds of the guys who build and then re-assemble our plants at destination. That could just be superstition of course- wouldn't be the first time that something widely held as folk wisdom was proven to be untrue.

 

[tbuelna]

moltenmetal- your last comment brings up a good point about how locking of threaded fasteners is approached from a reliability standpoint in industries like aerospace.

As I understand it, the helical split lock washer works by creating an axial preload on the bolt/nut when it it compressed below its free height. And if this axial preload force produces sufficient friction at the thread contacts to prevent any relative movement/loosening, then it does provide some thread locking function. But unfortunately, the amount of axial preload force produced by the typical helical split lock washer, even when fully compressed, is often not sufficient to prevent loosening of a bolt/nut, especially if the joint is subject to any vibration or dynamic loading.

In aerospace, threaded fastener installations where the bolt/nut is loaded in tension often require a single fault tolerance level of reliability for the fastener locking function, which basically means each fastener must employ two separate methods of securing the threads. One common approach used to meet the single fault tolerance requirement is through a combination of using controlled installation torque to create fastener preload, and using a locking feature like a deformed section of the internal threads on the nut/insert that will continue to maintain a locking function after the loss of preload. Since a split helical washer will lose its locking function once it is no longer compressed, it is not considered a suitable form of thread locking device for aerospace applications.

Interesting topic.

Terry

 

[MikeHalloran]

I was thinking about the issue the other day, and realized that the only place I have ever seen evidence of helical spring lockwashers actually working was on carburetors, where the torque necessary to make a paper or cork gasket seal is not sufficient to actually flatten the washer, and it's common to find witness marks of embedment of the washer's sharp ends in the zinc die castings used for carburetor parts, and under the heads of the soft-ish Fillister head screws typically used for assembly.



Mike Halloran
Pembroke Pines, FL, USA

 

[Guest102023]

Throw them away, I've seen one come 6 inches from killing a workman.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"