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

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

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

(OP)
Anyone want to weigh in on their pros and cons?

Tunalover

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

2

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

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.

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

2
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.

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

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

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

(OP)
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

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

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

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

Nordlock has some recommended torques for their differentially serrated washers.

http://www.nord-lock.com/nord-lock/wedge-locking/w...

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

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

(OP)
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

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

Quote (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.

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.


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

(OP)
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

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

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

Quote:


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

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

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

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

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/2080/1...

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

(OP)
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

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

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.

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

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

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

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

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

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?"

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

Quote (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.
...

If if dropped to near zero clamping force, why should it not continue dropping?

--
JHG

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

OK, I get that the washer behaves as a flat washer when completely compressed, and that it has a huge spring rate when fully compressed. But it does act at least to some degree as a spring once the joint has loosened a bit, generating at least some useful force to keep the fastener from falling completely out of the joint. A flat washer ceases to be any kind of spring once the bolt head has left contact with the face.

I was commenting to screwman1's post. My point was that not all joints where bolts are used will fail once the joint merely becomes loose. Some joints rely primarily on the presence of the fastener in the assembly rather than needing the fastener to be fully tightened.

Why wouldn't it continue to loosen? It certainly would! The washer in our case merely buys us time to find the loose fastener and re-tighten it rather than finding out what hole it rattled out of to end up in the belly tarp of the truck. Is that a meaningful amount of time? Dunno. That could be tradition rather than fact- but it's pretty strong tradition around here to the point of being not on argument with the folks who install our plants.

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

moltenmetal,

Bolt Science has a page on Vibration Loosening of Bolts and Threaded Fasteners.

Quote (Bolt Science)


It is widely believed that vibration causes bolt loosening. By far the most frequent cause of loosening is side sliding of the nut or bolt head relative to the joint, resulting in relative motion occurring in the threads. If this does not occur, then the bolts will not loosen, even if the joint is subjected to severe vibration. By a detailed analysis of the joint it is possible to determine the clamp force required to be provided by the bolts to prevent joint slip.

The force required to completely compress a spring washer is way below the tensile strength of your bolt. If the bolt has been loosened from its proper snugged down condition, the helical spring washer will slow the failure, at best.

Incidentally, I used to work with a guy who claimed that you tightened down a screw by carefully watching the helical spring washer. When the washer is compressed flat, the screw is tight. Articles that claim helical spring washers are a cause of failure, never explain the mechanism. Could this be it?

--
JHG

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

(OP)
I've worked on projects where this type of lock washer was prohibited not because they simply don't work, but because they can break, separate from the joint, and cause metal fragments to short out electrical connections. The same argument was made for the Nordlock washer because the wiping action of the teeth against the flat washer might cause metal flaking. I never believed the split-lock washer would ever break but I wasn't in the position to make policy. Regardless, the decision was made not to use them and that was a good decision IMO.


Tunalover

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

banghead

Despite overwhelming quantifiable evidence, despite sound engineering explanations of their worthlessness....

Many organizations have someone of sufficient influence who remembers seeing a failed joint at just the right moment before the nut fell off and concludes "Look Look! The lock washer prevented this nut from falling off!"

And apparently there is never anyone around at that moment who knows enough to point out:

"But it's a 'lock washer' and obviously it didn't 'lock' this tight"

"What good is this bolt doing now? It's rattling around. It's not holding anything"

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

Oh poor MintJulep- I feel your pain! Sorry for adding to it...but the revolution that would occur if I went into our shop and started dumping all the bins of "lock washers" into the scrap bin is just not worth the effort! LPSs to both you and drawoh for your help in educating me on the topic.

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