In a message dated 10/25/2001 10:22:04 AM Eastern Daylight Time,
mgagne@unc.edu writes:
> I finally got my drive shaft removed last night and found that the PO had
> installed bolts with both a nyloc and a lock wash. Seems strange to my
> untrained mind. Which is best for this application and what is the best way
> to think about using a nyloc versus a nut and lock wash.
>
Quoting from the Bowman "Fastener Facts" tech manual, "Spring type lock
washers have been in existance for so long that many mechanics will use them
on everything. They are OBSOLETE AND DANGEROUS. The SAE J489 states, 'It
should be noted that the word 'lock' appearing in the names of the products
in this standard is a generic term historically associated with their
identification and is not intended to imply an indefinite permanency of
fixity in attachments where they are used."
"Lock washers are made from carbon steel heat treated to a hardness of
Rockwell C45-51. If electroplated, they are very susceptable to hydrogen
embrittlement; i.e., they may crack easily during assembly or shortly after
installation. Their devices do not improve a fastener's lockability or
prevent loss of preload. When a lock washer is assembled, the open ends are
compressed forming a hard flat washer."
"The only time it is effective is when preload is lost either initially, in
service from part movement or eembedment, or if the bolt becomes longer due
to yielding. Thus, when sprung open, the sharp ends can dig into the work
and the nut's bearing surface or the bolt head's washer face to prevent
counter-rotation. If under the head of the bolt, contact with the fillet can
cause fatigue damage to the bolt itself."
A slightly sprung lock washer may go undetected because it is only opened by
a few thoudands of an inch. The opening may also be obscured to visual
inspection by position or grease. When the cyclic service loads continue to
impact upon the sprung lock washer, it compresses and relaxes with each
additional load. Because of this cyclic motion, fatigue stress develops.
The lock washer ultimately cracks and falls fromt he connection. The nut,
which is usually a standard free-dpinning nut, is then free to vibrate off
the bolt."
"Due to the potential dangers of helical spring lock washers, the SAE
recommended in 1969 that they not be used in any critical applications on
vehicles, other than sheet metal or inspection places. Instead,
through-hardened heat treated flat washers used with all-metal
prevailing-torque-type locking nuts are now the standard of the industry."
As the guy who gave me the book said about lock washers: "You keep using
them, we will keep making them."
--David C.
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