Feb 082010

Recently, at my place of work, I’ve been reviewing lots of drawings and mechanical designs.  Along the way, I’ve questioned a lot of things.  In particular, we have specified split ring lock washers in some places.  In other places in the assembly we don’t have lock washers, but have specified thread locker, typically Loctite.   In other cases we’ve got neither lock washers nor a thread-locker.

Now putting a thread-locker on a screw makes sense (if you don’t want it to come undone by mistake), except in places where you might worry that the stuff might not cure soon enough and might contaminate things.

So what about the split ring lock washer?  I decided to try to find out whether they actually lock anything, and whether they would still work reliably if used with a flat washer.  The answer should be easy, right?  After all, they’re called lock washers.

It turns out that the answer isn’t quite so easy, although some people think it is.   There are numerous articles and fora (that’s just two) on the subject.  There are at least a few simple articles that indicate that split ring lock washers help keep screws, nut or bolts from coming undone.  It seems people are confused, perhaps as confused as I am.

There’s a NASA Fastener Design Manual that asserts:

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.

Well, that’s pretty damning, and widely quoted on the internet.  It seems odd to me, though.  How does compressing a spring down 100% suddenly cause it to stop being a spring?  That doesn’t seem quite right.  Unfortunately, the author doesn’t indicate which of the 22 references in the bibliography give him the confidence to write this brazen statement.

So one author thinks it’s the spring component that’s supposed to do the trick, other people are convinced that you need the diagonal cut in the lock washer and hardened spring steel so that the washer bites into the opposing surfaces.  If that’s the case then maybe our NASA guy has it right:  once it’s completely compressed, there’s no way it could bite until things loosen off a bit.   And then it’s loose and you didn’t want that.  Bolt Science has some nifty videos showing how poorly a split-ring lock washer performs under transverse vibration.  The nut under test just unscrews.  Apparently sometimes even more quickly than just a nut, with no washer at all!

Ok.  So what’s going on?  Some people swear by them and some swear at them.  In many cases people seem to simply be expressing opinion, often quite vehemently, with only a bit of evidence.  Now, they’ve been used for years, and there’s enough experience apparently out there that they make a difference.

Perhaps one of the most convincing arguments for them is in the Handbook of Bolts and Bolted Joints (page 242).  Here they’re also referred to as helical spring lock washers. This argument is well thought-out, even referencing tests.  The author indicates that the washer shouldn’t really help much more than a flat washer because not only is it flat when compressed, but the spring force it supplies is significantly less than that supplied by the bolt itself.  A properly designed joint involves putting enough tension on the bolt or screw that it deforms under the load.  It stretches.  In doing so, it is supposed to keep the joint tight under a variety of conditions.  Even a flat washer can help in such a joint because it increases the effective length of the bolt, thereby giving a little more stretch.  In the case of the helical spring lock washer, apparently there is something much more devious afoot.  The washer, in cross section isn’t rectangular, it’s trapezoidal.  This apparently causes the washer to deform in a way that’s beyond the simple flat compression.  The trapezoid rolls somewhat and the ring opens.  The force required to do this, and the amount of movement involved is apparently significant.   The example given is that of a 7/16″ diameter fastener.  The spring component of the lock washer adds about .72″ to the effective length of the bolt, not including the thickness of the washer itself!  This length makes the joint a little more springy, which may seem like a bad thing, but 100% rigid joints are also risky, simply due to stress concentration.

This sentiment is also mentioned in An introduction to the design and behavior of bolted joints.  These books are long, so I’m rather thankful that Google has digitized them – they would be impossible to search in reasonable time otherwise!  On page 561, we get some more details on the research and testing of helical spring lock washers, referencing the same research as the previous book.   However by the top of the next page, there’s a table that gives a different picture.  The relative effectiveness of such a lock washer is about 5 times more than that of a plain nut, with or without a tooth-type lock washer.  But that’s not all that bad, especially if you consider that a Castellated nut with a lock wire (not to be confused with a cotter key or a spring pin)  is only 8 times better, at least under vibration.  Apparently, however this relative effectiveness is provided by a manufacturer of such locking schemes.  The author wisely indicates that you should do your own testing.

And testing is probably where it’s at.  This applies all over the place, whether I’m sourcing sealants or plastic tubing, the manufacturers’ spec sheets all end with a statement that indicates that you should do your own testing to make sure the product is suitable for your purpose.

 Posted by at 9:23 pm

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