Bolted fastener vibration loosening and spring washers

[Doove117]

Hi
we supply skid mounted process plant for marine and oil and gas sector.
All 2" lines or less.
Ambient temp, <10barg.
Seawater with some chenical.
One line has bubble entrained gas flowing to a degas tank (atmospheric).
There is a small fan and a small pump on the skid.

Our designers put helical split spring washers everywhere and I am told the reason is vibration which seems wrong to me for the following reasons:
1. NASA fastener design manual describes helical split springs as useless as locking washers as they flatten out to effectively become flat washers.
2. There is no significant vibration; if there is then the fan or pumps are seriously unbalanced or there is something wrong in the pipe system (like chattering valve, or air)
3. The oil and gas companies' pipe classes do not specify any type of locking washer, so I have them on small bracket joints and labels and cable tray but not on the pressure envelope!
4. I have seen Junker tests on joints with and without helical split springs that show no significant variation in time taken to loosed or amplitude or frequency of cyclic load.

The company has been putting them in for 40 years and I notice I see a lot of them particularly on stuff from electrical suppliers; but noone can explain this to me.
I thought maybe it was to allow for different coeffs of thermal expansion but really that is me grabbing for ideas.

I do not like doing something by rote without knowing what the reason for it is; eg if we do have vibration issues then we should be using a more robust solution.

Can anyone advise please?

cheers

Doove

 

[CoryPad]

Everything you wrote is correct. There is no good reason to use them for your application. Using those washers is one of those design ideas that have been propagated over time when they provide no value.

 

[desertfox]

Hi Doove117

I would agree there is no need to use helical spring washers and if you do then they actually serve to help loosen the joint.

see this link and maybe you should show it to your colleagues:-

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

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[MintJulep]

I explained why people still use lock washers at the end of this thread thread725-370111

 

[Doove117]

Thanks for the responses chaps. As you might have guessed i am after evidence against design by convention which is what i suspect this to be.
I think the elephant in the room here is the assumption that there is vibration of the right frequncy and amplitude to cause jost effect loosenning.
Can anyone direct me to a standard, cop or text that provides direction or a range or matrix to assess this type of risk?

 

[tbuelna]

You mentioned in the OP that the NASA fastener handbook recommends against using split lock washers. The reason for this is that most threaded fastener installations require operational single fault tolerance capability. This means the fastener installation must have two independent locking methods. The most common approach is installed preload in the fastener body and a locking feature using interference between the mating threads. Since the split lock washer relies on installed preload, it cannot perform its locking function independently.

As for your particular application, is there a requirement for functional fault tolerance capability? What would result if one of the threaded fasteners lost preload, or even if the nut/bolt/washer got loose and bounced around inside the skid? Would a leaking pipe connection create a hazard? Would a loose fastener pose a jamming hazard to moving machinery?

If you determine your threaded fastener installations require self-locking features, why not use a deformed thread metal lock nut? They are cheap and readily available.