You need to look at the total dynamics of what happens during vibration,
shock, and thermal cycling to a connecter held in place with split
lockwashers, or bellville washers, or nylock nuts.
During vibration, if the PWB is thermally cycled, the corresponding
Z-axis expansion of the PWB increases the compression forces placed on
the washers. Upon cooling, the z-axis expansion is reduced. This
sometimes alleviates the pressure and the nuts are actually loose (don't
go there, Dewey!).
The same phenomena happens for Nylock nuts, but unlike the standard nuts
that now have reduced pressure and can spin off during later vibration,
the Nylock prevents any further spin, and they at least remain snug.
When working in a torpedo factory in the late 70s we actually recorded
this and made a short movie of it happening. It was weird to see the
nuts begin to spin off slowly (at fast forward) during the vibration
after the thermal cycle occured.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Tuesday, June 26, 2007 7:10 AM
To: [log in to unmask]
Subject: Re: [TN] To lock or not to lock?
I have found at least small (<#4) split ring spring washers to be
unreliable for high vibration environments. I agree that they certainly
do not deserve the label "lock washer", and if you consider the physics
in this situation you will probably agree: If the vibration environment
is such that the spring washers become unloaded or nearly unloaded, then
on the opposite side of the vibration waveform they will attempt to yank
the fastening out. If the screw rotates during that re-loading even the
tiniest amount, then you are doomed. I only use nylon lock nuts or
thread locking compound.
BUT, qualification is qualification, and this seems a large enough
change to have the qualification test repeated. Sometimes we find
surprises, and that is the reason there are tests. I would certainly be
ready to place big money on the nylon lock nuts passing the quals with
flying colors if installed properly, but the tests must be re-done.
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Tuesday, June 26, 2007 2:35 AM
To: [log in to unmask]
Subject: Re: [TN] To lock or not to lock?
We have used the selflocking plastic insert nuts for twenty years with
no problems, and we shock and vibration test a lot according to MIL
specs. The disadvantage with split-ring washers is that they can corrode
in harsh environment and lose some of the good locking properties. E.g.
imagine that you do service maintenance in field, and have to reuse the
lock washers. In such a situation, plastic insert nuts are better. As
most locking is based on friction and material constants, one has to
select bolts, nuts, screws and washers after the actual construction. If
you want very reliable lockings, and there is no quick-service demand,
use Flange nuts with plastic inserts. These nuts have a larger friction
area, and also grooves that make a tremendous grip. Sometimes, they are
too good: you nearly can't unlock. And, as I said, you need do
selections from the materials you use. Locking bolts and nuts is an
art!
Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of - bogert
Sent: den 26 juni 2007 00:07
To: [log in to unmask]
Subject: [TN] To lock or not to lock?
June 25, 2007
Folks, we have an OEM who designed and qualified an assembly for
military (NAVY shipboard) shock and vibration requirements via
performing shock and vibration testing. His qualified design used split
ring lock washers. To save $ (assembly time primarly), the OEM proposes
to replace the split ring lock washers with Nylok type locking nuts.
Question is, would the OEM need to repeat the shock and vibration
qualification testing. My understanding is that although split-ring
washers are sometimes called lock-washers, in fact, they do not "lock"
the fasteners in place, as is specifically stated in the applicable MS
standard/mil spec for the washers. Apparently the Nylok locking nuts
that have the plastic
(nylon??) insert are true locking devices and as such perform better as
a locking feature than do split-ring lock washers. Therefore, there
should be no need to repeat shock and vibration qualification testing if
the OEM changed from split-ring lock washers to use Nylok type locking
nuts. The only possible concern I am aware of is that the locking nuts
can only be used (removed and re-installed) a specified number of times
(e.g., 10 to 15 times?).
I would like any Technet folks information on the above. Thanks.
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