Studies on the use of lock washers?
Studies on the use of lock washers?
(OP)
Does any one know of any engineering studies performed to determine what if any affect the use of lock washers has on the integerity of s structural joint?
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Studies on the use of lock washers?
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Studies on the use of lock washers?Studies on the use of lock washers?(OP)
Does any one know of any engineering studies performed to determine what if any affect the use of lock washers has on the integerity of s structural joint?
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RE: Studies on the use of lock washers?
TTFN
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RE: Studies on the use of lock washers?
RE: Studies on the use of lock washers?
RE: Studies on the use of lock washers?
http://gltrs.grc.nasa.gov/reports/1990/RP-1228.pdf
Once a split lock washer is made flat by fastener tension, it's basically just a plain flat washer at that point.
I think www.boltscience.com has a video of the split lock washer coming loose under vibration, and they compare it to other locking methods on video. You might contact them for references to other studies, I think they have a section on their web site that lists studies and references.
I think the lock washers from www.nord-lock.com/ are supposed to actually work.
Loc-tite works well.
John
RE: Studies on the use of lock washers?
But yea, I read that NASA article and it's changed my view. Then again my industry (food), any exposed split lock washers are just another food trap so it didn't take much to sway me.
James Spisich
Design Engineer, CSWP
RE: Studies on the use of lock washers?
RE: Studies on the use of lock washers?
http://www.nord-lock.com/default.asp?url=51.16.37
RE: Studies on the use of lock washers?
http://itwmc.com/content/default.aspx?cID=78
RE: Studies on the use of lock washers?
RE: Studies on the use of lock washers?
RE: Studies on the use of lock washers?
A split lock washer probably does bite or gouge into the mounting and/or bolt/nut surfaces a little, but I'm not sure how much it would help with locking and gouging of the surface does not seem a dependable locking method.
If the split lock washer had knurled surfaces or a surface that could "bite into" the mounting surface and fastener faces, I would think that might help a little with preventing loosening. Maybe rough them up a little with a file or something ?
From Unclesyd's link...
"When tension in the assembly is reduced and loosening occurs, it provides resistance to the back-off rotation of the bolt".
I think the statement above is true, but the problem I see is that the vibration, movement, or force that initially overcomes the friction of a properly tightened fastener causing it to loosen, would probably very easily overcome the friction resulting from the axial tension of the split lock washer, which only decreases as the joint gets looser. I guess it depends on the application and what forces are at play but it seems to me that as soon as loosening occurs, the game is basically over that point.
"Technology University have shown that the typical helical spring lock washer exhibits a spring rate after flattening which is approximately 70% more effective than a flat washer of the same thickness".
70% is significant but I would be interested to see how the test was conducted. There can be more friction variation in identical fasteners than 70%, so I'm not sure how they accounted for that in their tests. Did they measure actual fastener tension or stretch in their comparison or did they just torque the fastener down ? I would think they should have tested many fasteners from different production lots and manufacturers.
The shakproof metal components site mentions that the split lock washer is twisted in addition to having the split ends at different elevations. However, whether the washer is twisted or simply has ends with different elevations, flat is flat, and at the point at which the washer is flat (ends at same elevation and twist removed) it would seem to have to act and respond like a flat washer. At the point at which the washer is flat, any difference in spring rate would seem to have more to do with differences in washer material, i.e., elastic modulus of the washer material, etc. as opposed to washer design.
NASA is an authoritative source, but like everyone else their not infallible. I would be interested in seeing actual tests from both sides before forming more rigid opinions.
I'm guilty of using split lock washers for certain things even after reading the NASA report, I guess old habits die hard.
John
RE: Studies on the use of lock washers?
So how effective is a "helical spring lock washer" that is 70% more effective then zero???
RE: Studies on the use of lock washers?
RE: Studies on the use of lock washers?
Good thread guys and some good links in there.
-Dustin
Professional Engineer
Certified SolidWorks Professional
Certified COSMOSWorks Designer Specialist
Certified SolidWorks Advanced Sheet Metal Specialist
RE: Studies on the use of lock washers?
It may be true that the split lockwasher may prevent the screw from turning completely out ONCE THE JOINT IS LOOSE but the idea is to prevent the joint from coming loose in the first place. The best way to do this is with one or more Belleville washer in addition to prevailing torque style fasteners (aka self-locking threads via a plastic locking element). Of course if one can figure out the necessary installation torque to produce 75% of yield and can figure out a way to inspect that torque then no locking features would be needed at all. The problem is figuring out what torque to apply. The T=kPD formula is fine if you can only determine what the empirical constand k is for a particular joint.
Tunalover
RE: Studies on the use of lock washers?
TTFN
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RE: Studies on the use of lock washers?
To prevent loosening, the best option is high preload, the next best is adhesives. Everything else is much worse in terms of performance, cost, space, etc.
Regards,
Cory
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RE: Studies on the use of lock washers?
Not a problem at all on either point:
- One really can't practically calculate the required torque necessary to accurately achieve a desired bolt stress (one can never know what the K factor will actually be). An educated stab-in-the dark is necessary. However, the answer to the second wish made by tunalover solves this dilemma...
- Calculate the fastener's elongation at the desired preload. Then, measure the stretch after tightening. Tweak the "torque" either up or down until the correct elongation has been achieved. Simple.
I've posted it before but, here once again is a small Excel file that'll help to calculate both...Ciao,
HevïGuy
www.heviitech.com
RE: Studies on the use of lock washers?
"To prevent loosening, the best option is high preload"
Technically, I would disagree. The mechanism that keeps the thread locked in most bolted joints is friction. High preload will produce high thread friction as long as the preload is maintained. But in a bolted joint with low elasticity subjected to vibration or large CTE mismatch, the preload can increase or decrease substantially. Sometimes enough to relieve the preload and allow the fastener to back out. So I would argue that the best option to prevent loosening is a joint with adequate elasticity to prevent loss of installed preload. And not necessarily a joint with high preload.
RE: Studies on the use of lock washers?
You're not going to get "elasticity" without preload! In most cases, the more, the better (within the limits of the fastener's strength).
Two things must be considered:
- the stiffness of the bolt
- the stiffness of the components being clamped
The component's stiffness must be greater than that of the bolt's. A bolt is, essentially, a spring. This spring-effect is what keeps things clampled together. One's not going to get this without adequate preload (usually quite high) "stretching" the bolt!On the other hand, if the component constituent parts were to be "elastic", joint separation would soon occur upon external load being applied.
Ciao,
HevïGuy
www.heviitech.com
RE: Studies on the use of lock washers?
Hence the NASA, MIL spec, etc recommendation/requirement for fastener length equalling at least 6 diameters.