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View Full Version : Star lock vs split lock washers



sbmathias
07-22-2009, 05:55 PM
Does anyone have any comments on when star locks should be used vs when split locks should be used? I know that some don't like any of them.

It seems like star locks, both internal and external, are usually used on smaller bolt sizes, while split locks are more available in the larger sizes.

Are there indeed any accepted standard practices?

aboard_epsilon
07-22-2009, 06:39 PM
loctite and stretch bolts have taken over .

wired head bolts look good on certain things though ..makes it look R&D.

best if you must ..is some nuts have integral washers with ridges on the underside


http://www.aaronscapscrews.com/images/SerratedFlangeHexNut_50x50_Orange.jpg

http://www.aaronscapscrews.com/images/SerratedHexWasher1.gif

then yopu got nylocks and those squashed head nuts

all the best.markj

Jim Shaper
07-22-2009, 06:42 PM
They lock in different ways, so the application should designate the one used.

Star washers provide a deformed metal disk with edges to "bite" into the fastener and the base material, but only to the extent of the disk's elastic memory.

Split washers are springier steel which actively resists the compression they're under and forces the edges into the fastener and base. They're also completely useless on left handed fasteners unless they're designed with the twist opposite of standard right hand units.

You can get split washers down to the number sizes for screws just as easily as you can get them for bolts. I haven't seen star washers in very large sizes, probably due to the fact that the tension needed to properly stretch the bolt would likely crush the edges and render it useless.

GadgetBuilder
07-22-2009, 08:13 PM
Some info on split washers here:
http://www.boltscience.com/pages/helicalspringwashers.htm

Interesting links on bolting here:
http://www.boltscience.com/pages/info.htm

John

Fasttrack
07-22-2009, 08:47 PM
Some info on split washers here:
http://www.boltscience.com/pages/helicalspringwashers.htm

Interesting links on bolting here:
http://www.boltscience.com/pages/info.htm

John


That verifies what I was going to say. I may have read that 1969 article - I just remember reading an old, yellowed article tapped next to my BIL's bolt bin that said those split lock washers are useless. :)

Jim Shaper
07-22-2009, 08:59 PM
That shaker test is faulty in it's logic and results.

If the fastener was sufficient to the joint design, there would be no movement between the plates and the lock washer wouldn't allow the nut to back off.

The mechanism providing the actual holding force in a bolted connection is the friction between the base material and the item being affixed to it. The bolt merely holds the two together to facilitate that friction.

Fasttrack
07-22-2009, 09:16 PM
That shaker test is faulty in it's logic and results.

If the fastener was sufficient to the joint design, there would be no movement between the plates and the lock washer wouldn't allow the nut to back off.

The mechanism providing the actual holding force in a bolted connection is the friction between the base material and the item being affixed to it. The bolt merely holds the two together to facilitate that friction.

If the fastener was sufficient to the joint design, you shouldn't need a split lock washer... yes?

Jim Shaper
07-22-2009, 09:27 PM
Under tension loads, you would eventually lose the nut if there wasn't something to prevent it from spinning. As has been mentioned, thread locking compounds and self locking nuts are another means to deal with the problem, but that doesn't mean the split washer is obsolete.

The vibration test clearly fails the application, not the fastener. Repeat that test with a pinned connection or even a tightly fit bolt to the hole and see which unit comes apart first. My hypothesis is that the results would be reversed. If there was tension applied to the fasteners in that test, the split washer would've prevented rotation of the nut rather than accelerating it. That you believe that demonstration means you don't understand the relationship between the components in the design of the application. The friction between the components being fastened had already been compromised to allow either method to fail.

Jim Shaper
07-22-2009, 09:32 PM
For example, anything mission critical experiencing far less vibration than that test on a helicopter would be safety wired or be assembled with self-retaining bolts. The application is clearly wrong for the stated goal of the demonstration.

Fasttrack
07-22-2009, 09:38 PM
And how does a split ring lock washer prevent a nut from spinning off if the bolt has been tensioned to the proper preload?

I did not see the demonstration until you mentioned it. I agree that the test does not mimic the behavior of a properly executed bolted joint. However, the point is that with a properly executed joint, the split ring lock washer is not needed. In cases where the preload on the bolt is not sufficient to keep the two surfaces in contact (i.e. the friction between them not high enough), then the lock washer actually worsens the situation (or at least has no effect)

There is no way that the force of a split ring lock washer can produce the proper preload in a bolt. Thus, once the joint is properly produced, the split ring acts as an ordinary washer for all intensive purposes. If the preload were to be reduced for any reason, the preload provided from the spring action of the washer is still not enough to keep the normal force between the two surfaces sufficently high. Thus the situation is now similiar to that in the Junker test.

What they have done is tightened the bolt with the lock washer just enough so that the only preload is that provided by the lock washer. And it was not sufficent to prevent the nut from backing off.

Jim Shaper
07-22-2009, 10:07 PM
The ends of a properly manufactured split washer are actually sharp edges that force themselves up into the fastener (which must be softer than the lock washer to actually function) and it's that interference between the two sides of the connection that prevents the rotation of the nut or bolt.

I fully agree that poorly made, or improperly installed lock washers are worse than no lock washer at all.

Fasttrack
07-22-2009, 10:54 PM
Ahh - see all of the lock washers I have seen certainly do not fulfill those requirements. It would seem that they would not be of any use on, say, grade 8 or higher bolts.

Jim Shaper
07-22-2009, 11:07 PM
They make grade 8 split lock washers. I buy them all the time locally.

Paul Alciatore
07-22-2009, 11:48 PM
One real difference in the application is that the star style washers are often used in electrical/electronic connections. This is probably because the multiple points will better assure a good electrical conduction path than a single point of the split style. They are often used on high current terminals and on standoffs for PC boards where the ground plane on the board is being grounded to the case. (Before you flame me, I said "often used" not "always used".)

Another consideration in some assemblies would be balance. A split washer will add stress on one side only while the star style will evenly distribute the stress all around the screw/nut.

tattoomike68
07-23-2009, 12:32 AM
They make grade 8 split lock washers. I buy them all the time locally.

Every bolt on a cat tractor is grade 8 and many bolts have lock washers. A tractor is a perfect example of why we need lock washers.

Aircraft have wired bolts and tend to be insane hard, you have to drill the bolt heads for wires. the poor bit squeeks and squaks and tries to die all the way. I had to charge them a lot and they keep giving me those rock hard %#$@ they did not care about the bill at all. I was wishing lock washers would work for them.