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Mechanical engineering general discussion FAQ

Mechanisms

Is a lock washer an effective torque retention device? by btrueblood
Posted: 3 Aug 06 (Edited 11 Aug 06)

Courtesy of MintJulep:

When a lock washer is fully compressed, as it is in a joint that is holding its design preload, the lock washer behaves exactly like a flat washer with a crack in it.  Or, if you prefer, a spring that has gone solid.  It no longer acts like a spring in further compression and its stiffness, as it relates to the joint performance is determined by the modulus of elasticity of the material.

If the joint loosens sufficiently so that the lock washer acts like a spring again then the joint has lost its design preload and the only tension left in the bolt is whatever small amount that is provided by the lock washer.  It doesn't matter.  The joint has already failed.  The lock washer may postpone the inevitable departure of the nut from the end of the bolt.

But if you don't want to believe me, perhaps you would believe NASA.

Quote:
NASA Fastener Design Manual RP-1228:

"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."

Or the US Navy.

Quote:
Naval Ships' Technical Manual, Chapter 75:
"Although lockwashers may be encountered, using the flat washers with selflocking nuts, self-locking fasteners, self-locking inserts, or thread sealants such as MIL-S-22473 anaerobic compounds is preferable.

If loosening has been a problem, however, replace the lockwashers with self-locking fasteners.

The helical spring lockwasher (Figure 075-5-11) is flattened when the bolt is torqued down. Once compressed, it acts as a flat washer, contributing normal friction between the nut or bolt and the bearing surface during tightening."

Or the British Ministry of Defense.

Quote:
"Vibration Loosening of Threaded Fasteners (Light):
If the plain fastener is taken as the datum any washer reduces locking effectiveness"

The ASME also has a standard for lock washers.  In that standard it states:

Quote (ASME B18.21.1):
"The word lock appearing in the names of products in this standard is a generic term historically assoicated with their identification and is not intended to imply an indefinite permanency of fixity in attachments where the fastners are used."

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