Proper flat washer orientation 1

[GumbyDammit]

I found some information in thread404-198093 but it doesn't completely cover my application and am wondering what others think about this question...

I build lasers for everything from medical to military applications and we always use split lock washers and flat washers on all our screws. Lately (ever since I came back to work here) we've been getting lower quality (ie. cheaper) stamped flat washers that have a radiused top and a sharp bottom edge where they've been stamped from a sheet of stainless. Production personnel consistently place the sharp outer edge down causing it to bite into the material being fastened, be it anodized aluminum, stainless, or printed circuit boards. I say this is incorrect, because of the damage, albeit cosmetic, that is being done to the underlying material and the washers should be turned "upside down" so the radiused side is in contact with the material being fastened. The lock washer OD is smaller than the diameter of this stamped edge so it does not interfere with the lock washer.

Am I just being anal about something that has little significance, or do you feel there is a method to my madness? My only arguments for my approach are that the washers do damage the surface under them if positioned edge down, and when disassembled, they will stick to what was underneath them.

Thanks for your comments!
-Scott

 

[racookpe1978]

IF there is real damage to the substrate material affecting form, fit, or function (scarring the surface that causes erosion or corrosion, scratching a surface to cause unacceptable visible damage, scratching through a surface to hurt insulation, or what ever) then - yes, you need to include instructions to change the installation "thought process" of the workers when they put on stamped washers.

But don't expect much success. Lockwashers are pretty much cosmetic anyway, they work very, very poorly in the real world, though most designs include them "because everybody does" ... Best retention is the double-sided washer pairs that prevent the nut from backing off. (Nordlock is the best, but they are expensive.) Beware of cheap Nordlock imitations!

Almost everybody thinks that the "gouging" of the substrate is a "good thing" because it "catches" the washer and (seems to) prevent movement of the bolt and nut. Also, it "looks prettier" and "more professional" even though putting the washer on that way may harm the substrate in your case.

 

[btrueblood]

Buy a parts tumbler, and run all those cheap washers through a deburr process to knock off the sharp corners. But I'd still have your techs flip them over if the substrate is delicate - nuts are cheap, circuit boards not so much.

 

[ctopher]

Seems like you are buying your washers through a China supplied vendor.
Call out a MS or NAS part from another supplier.

Chris, CSWA
SolidWorks 13
ctopher's home
SolidWorks Legion

 

[Compositepro]

There was another thread on this subject a coupe years ago. I do as your technicians are doing. The purpose of a washer is to srpead the nut load to a larger contact area and to take the wear of of the nut turning against it. Contact between the substrate and the outer washer diameter provide the torque to keep the washer from rotating with the nut.

 

[JohnRBaker]

I've always assembled flat-washers with the 'sharp' side down, for several reasons. First, it just looks better. Second, it would seem that they would 'seat' better and perhaps, depending on how 'high' these sharp edges are, it might also afford some degree of pre-load, albeit minimal, like what you'd get if you replaced a flat-washer with a 'Belleville' washer. And last but not least, if the edges of a washer are really that sharp, I'd rather have them pointing AWAY from where someone's fingers, or anything else easily damaged, could come in contact with them.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[kingnero]

I agree with Compositepro and JohnRBaker, as I do it myself the exact same way in hobby-bob tinkering, but as suggested by ctopher, I'd get higher quality washers, especially for your application.

I doubt btrueblood's suggestion is cost effective (you also need media, square footage in your plant and electricity), but it's certainly worth mentioning.

 

[3DDave]

Washers don't spread the load very much. They are often there because they are 'always' there. One function is to keep the sharp corners of the screw or nut or wrench from damaging the underlying surface. The other is to cover the sloppy big hole, because no one makes fender-screws or fender-nuts ...

I don't think I've seen any sharp enough to hurt someone, but the stress concentration is enough to damage the paint, starting a corrosion path directly where it can't be easily seen.

Instructions can be tried, but they won't work long. There's eventually someone who figures it's just wrong and will 'fix' it for you. The best is to use hardware that can be reversed without causing any problems so no special instructions are required.

 

[JohnRBaker]

Let's not forget that flat-washers are almost mandatory when fasteners are used to assemble something that uses a 'slotted-hole'.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[GumbyDammit]

Guys, I really appreciate the quick replies to this. I figured I was being really anal about this, but needed a reality check. In the past, we used external tooth star washers to ensure good electrical continuity between the anodized aluminum covers and the anodized aluminum laser housings (screw holes masked for electrical continuity). That practice has gone away. We've been shifting over to metric hardware since a lot of sales and service work is done in Europe and Asia and interestingly, this washer problem is only seen on the metric hardware. We go through these things by the thousands, so cost is a huge consideration so cheaper will always win out unless product failures can be shown ant that has not been the case.

Thanks once again for all the quick responses!

 

[KENAT]

If I care about the substrate then I put the rounded edge down, to avoid damaging it.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[MikeHalloran]

Cheap, flimsy, burred washers will destroy whatever reputation you have managed to accumulate. Bean-counters don't have a way to count that, until sales decline precipitously, at which point the damage is done and no action you could take will save your company.

Specifically, pieces of the burred edges will fall off and get in your optics or your precise mechanical bits.
If installed 'up', someone will get scratched, and curse you for being a lazy bastard.
If installed 'down', the burrs will scratch or scar the expensive finish you applied, and someone will find the scars or scratches, deduce the source, and call you a cheap, sloppy bastard.


If you can't get the bean counters to buy better quality washers in the interest of keeping their evil jobs, other tactics exist:


If you specify somewhat thicker washers, they will have sturdier burrs on them that may do less damage, or be easier to strain out of the vibratory finisher should you elect to deburr the washers in house.

If you double or triple the washer thickness, the burrs may actually break off in the stamping die, becoming the washer supplier's problem, as they should be.



Mike Halloran
Pembroke Pines, FL, USA

 

[boo1]

What about matching the bolts radius between the shank and the head, and the washer radius?

 

[GumbyDammit]

All good ideas and suggestions. Unfortunately nobody else thinks it's a big deal. Even one of our mechanical engineers who's usually good about picking nits says it's not worth the effort. Thanks again everyone!

 

[MikeHalloran]

If you have to fight that hard to put a little quality in your product, you are working for a schlock outfit, and it's friggin' doomed.

Plan your career accordingly.


Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

an airplane crashed some years back because of exactly this thing ... the sharp edge of the washer (it might have been a machining burr) bit into the substrate and initated a crack which doomed the flight

Quando Omni Flunkus Moritati

 

[tbuelna]

As others noted, in this situation the purpose of the flat washer is to prevent damage to the substrate surface from the bolt/nut face sliding against it during installation. Stamped flat washers of decent quality can be installed either way, since the edges should be free from burrs. Here is a link to an (older) aerospace spec for a commonly used stamped flat washer. General note 1 of the spec clearly specifies removal of all burrs and sharp edges.

Like ctopher suggested above, the wise thing to do would be spending an extra 1 cent per washer to get a burr-free product. The AN960 washers shown in the linked spec are commercially available in stainless steel for about 3-4 cents each.

Good luck to you.
Terry

 

[Tmoose]

Thick enough, hard enough, well fitting and well finished flat washers almost always do significant, even vitally necessary good things for the performance of bolted joints.

To point to the bad things sometimes done by shabby thin washers and then write manufacturing standards banning washers based on ST washers deficiencies is most unfortunate.

 

[ornerynorsk]

Personally, I'd rather use elephant dung as a thread locker before resorting to a split lock washer. In any situation that's worth using a locking method, I can think of at least a dozen better alternatives.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.