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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