Why do some washers have a countersink option? 1

[Baratheons]

Comparing the NAS1149 series and NAS1587 series washers, the NAS1587 has a countersunk washer 'for use under bolt head' option. Why is that? Isn't plain good enough?

 

[3DDave]

They usually have a closer fit, one such that a non-countersunk washer would cause a stress riser in the fillet under a high strength bolt head. Ordinary flat washers have an ID large enough to not interfere with the fillet.

 

[WKTaylor]

Baratheons... on-a critical 'side-topic'... A word of CAUTION RE NAS1587-** and NAS1149C** WASHERS...

NAS1527-** and NAS1149C** come in 3xx ANNEALED CRES alloys, thus... are intended for applications in low stress installs of CRES/titanium. Alloys 321 and 347 are [primarily] intended for low stress in high heat applications... so stick with NAS1527 if KNOWN high heat.

NOTE/CAUTION.

301, 302, 304, 316, 321 or 347 annealed CRES [NAS1149C and NAS1527] has such a low bearing strength, that in-service crushing can actually lead to pre-load-tension-loosening of the fastener install and eventual hole/bolt wear-out-elongation and/or hole or bolt or washer fracture. I have witnessed this many times.

APPLICATION NOTES, RE various washer alloys and finishes...

IF Your application is paired with a low/medium strength bolt/nut [steel, CRES, titanium, aluminum], then use...
Aluminum alloys HT -T3 or -T81 [clad or CCC or anodized]
Carbon steel, Cd plating [except do NOT pair cadmium-plated-anything with titanium]
A286 with Cd plated or aluminum coated
titanium with anodize or aluminum coated

IF Your application is paired with a high strength bolt/nut [steel or steel-aluminum-CRES] then use...
A286 Cd plated or aluminum coated

IF Your application is paired with a high strength bolt/nut [CRES, HRA, Titanium], then use...
A286 bare or bare-passivated

IF Your application is paired with a high strength bolt/nut [Titanium or titanium-CRES], then use...
A286 bare
Ti-6Al-4V [bare, anodized]

There are lots of reason for these recommended materials/finishes selections/combos... but I have to get back-to-work.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[tbuelna]

Good posts from 3DDave and WKTaylor.

With single side countersunk flat washers it is easy to install them upside down if the tech is not paying attention or inexperienced. Make sure your engineering documents and work instructions clearly describe proper orientation of these washers when installed under a filleted bolt head.

There are also flat washers countersunk on both sides (MS9768), which simplifies things.

 

[WKTaylor]

tbuelna... hahahaha...

I was called to a jet because a MLG bearing-cap bolt [MS20016-X] couldn't be installed since it was 'too short'. The bolt threaded into a barrel-nut, so this was a very serious problem.

When I started feeling-around where the bolt-head should mount, I felt an odd 'step-up'... Huhhhhh? I had 'found' a sealed-over MS20002C16 washer, that was almost invisible. OH YEAH... and it had been installed backwards, IE: chamfered side down [away from the head].

Mystery solved! When mechanics tried to reinstall the bolt with another MS20002C16 washer... the stack-up was 'way-off' with no thread protrusion thru the BBL-nut.

NOTE.
The last time the bearing cap was installed the [0.078-thick] chamfered washer was mounted backwards [with a dab-sealant to hold it in-place] and the MS20016 bolt was installed/torqued-down. The MS20016 bolt shank-to-head fillet radius [~R0.07] crushed/rolled the thinned-inner diameter edge over [~0.015 at the edge of the ID] and polished-it. Because it was only [1] washer and the edge crushed-over, the installation was 'OK'.

NOTE.
I still have this MS20002C16 washer in my collection... along with some 'crushed' NAS1587 washers. I'll find them and post images later.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[tbuelna]

WKTaylor-

Thanks for the story. In your example the mechanics performed an inspection after installation to verify bolt thread projection beyond the barrel nut's locking feature and caught the problem. The purpose of this inspection is to catch problems with tolerance stack-up in the fastener installation. Your mechanics did their job and the system worked. Imagine that!

The reason I even brought the issue up was because I have also seen it happen. Can't wait to see those pictures.

 

[WKTaylor]

OK... examples of 'damaged washers'... part 1.

Common flat washer crushed in-service... both sides of [1] washer

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

OK... examples of 'damaged washers'... part 2.

NAS1587C washers crushed in-service... both sides of [3] damaged washers... with both sides of [2] new washers 'side-by-side'

Countersunk/chamfered sides...

Flat-sides...

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

OK... examples of 'damaged washers'... part 3.

MS20002C12 Washer installed 'backwards', IE: CSK/Chamfered side against structure, flat side under-head sharp-bearing into the bolt fillet radius and being 'rolled-over'.

Photos are of [1] washer, only...

CSK/chamfered side...

Flat side... rolled-edge visible...

Undamaged washer next to the 'backwards washer'...

CSK/Chamfered side...

Flat side...

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[tbuelna]

WKTaylor-

Thanks for the autopsy photos. Well trained techs that scrupulously follow procedure are worth their weight in gold, and I wish there were more of them.

In case anyone is interested, here are some excerpts from NASA-STD-5020 regarding the subject:

 

[3DDave]

At least the chamfer weakened the edge a bit to allow the fillet some room.

The washer should have had a matching chamfer on the outer edge so that a visual verification could be made and to emphasize to the installer the correct way. No doubt some supplier would get that out of sync and cause big problems.