Flat washer under bolt head

[Steve P]

Hi Guys,
I came across a design proposal to use thin .016 or .032 NAS1149 (Al or CRES) flat washers under NAS6706A (CRES A286) bolt head in 7050 structure, loose running fit.
The fasteners are grouped in patterns, from 6 up to 12 fasteners per group and is shear loading. The bolts need to be torqued from the head side.

In my view this is not a correct design, a CSK washer shall be used. A tolerance analysis shows also that in 80% of the cases will be some sort of interference between the washer and the bolt radius (shank to head radius).
In my scenario, depending on the interference level, some washers will bend at the inner edge (sort of dimpling). If interference is high the bending could be above the yield point, also small cracks could develop at the inner edge. In time the cracks will extend and the washers could break.
Probably the washer will not fall out, but the bolt might loose clamping when the washer is broken or missing a part.

This type of joint is not a pure shear type like a tierod bushing for instance, but could be compared I guess with a friction type shear where the load is transferring thru friction. Loosing clamping could further bring total joint failure even when using a pattern of fasteners.

Do you guys have any experience with such design?

 

[rb1957]

are the holes CSK ? (you don't say) if there are CSK holes then yes, they should be filled. If you're trying to "bridge" over the CSK then that is IMHO asking for trouble.

retro-fit or new design ?

if tolerance study shows 80% interference and you don't want interference, then change washer hole to suit (no?? easier than turning the bolt shank down).

how much preload defined ?

Why thin washers as opposed to standard 1/16" ?

another day in paradise, or is paradise one day closer ?

 

[SwinnyGG]

I assumed that OP proposed CSK washers to avoid crashing between the washer ID and the radius between the bolt shank and head.

 

[rb1957]

I don't understand using CSK washers without a CSK (or dimpled) hole.

another day in paradise, or is paradise one day closer ?

 

[SwinnyGG]

The countersink on the washer ID could prevent the bolt shank root radius from contacting the washer

I'm not advocating that usage but it sounds like what he's describing to me.

 

[Steve P]

Yes is about contact between washer and bolt shank root radius. The bolt is not CSK.
Some washers standards, MS20002 have even two versions, flat and CSK. This standard specifies "CSK for use under head".
The question is: Shall we use flat washers under bolt head? What could be the stress and fatigue implication of such usage knowing that in 80% of cases there is some interference between washer and bolt root radius?

 

[SwinnyGG]

Where in your analysis are you getting that much root radius interference with the washer?

Are you saying that in 80% of washer-bolt pairs, the ID of the washer is smaller than the OD of the root radius?

If that's the case, why are you not using a washer with a larger ID that eliminates this problem?

The answer to this problem seems to me to be very easy. Either use a CSK washer or use a washer with a larger ID.

 

[rb1957]

use a washer designed for the role. if you want the hole diameter of the washer to clear the fillet radius of the shank then "make it so" I'd've thought that if this was such a design issue that there'd be guidance on selecting washers for these bolts. I note that many bolts use this fillet radius to improve the fatigue quality of the hole.

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

SteveP... run Your dimensions/analysis again using these numbers... I'll post my conclusions after Your reply.

NAS6706 Bolt
Plated/coated Dia = 0.3735-to-0.3745 D
Head-shank fillet = R0.015-to-0.025
A286-PHT160-TKSI
Corrosion protective coating?

NAS1149#06**## Washer
ID = 0.390+/-0.010 D
NOTE (1) REMOVE ALL BURRS AND SHARP EDGES.
#'Aluminum' ?? = 2024-T3? or 2024-T81?
#'CRES' ?? = 3xx SHT? or A286-PHT160-to-190-TKSI?
##Corrosion protective coatings?

NOTE. Typically, tension-rated bolts are torque-turned by the head. I rarely recommend torque-turning 'standard shear-tension 160-TKSI hex-head bolts', by-the-head, for installation.... and ONLY [then] to relatively low torque-values and/or under very strict torque-controls. It is very hard to manage actual torque-applied to the fastener by head-turning due to unknowns of friction forces... turning nuts is far-easier to enssure proper torque-tension.

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]

 

[SwinnyGG]

use a washer designed for the role

Are you implying that CSK washers are not designed for this? They are called out for use with socket-drive bolts all over the place, for this exact reason- to provide clearance for the shank root radius.

 

[rb1957]

I can see this, then a washer with a CSK is "designed for the role", as would a washer with an oversized hole. I was also noting that sometimes interference between the bolt head fillet and the bore of the hole is desirable and intended; I'm not saying that this is such a time. I also asking "what does the standard installation require/suggest/recommend ?"

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

CSK washers [ala MS20002C#] are designed to fit tightly under the head of tension rated bolt that has a relatively large head-to-shank transition fillet. The 90-Deg CSK [ID edge chamfer] provides relief to the Bolt fillet radius for a tight/flat fit.

In the case of the NAS6706 shear-tension rated bolt the transition fillet = R0.015-to-0.025
In the case of an MS20006 tension-fatigue rated bolt the transition fillet = R0.047-to-0.057

NOTE.
I have actually seen [have an example of] an MS20002C16 washer [~160-TKSI] that was installed under the head of a I718 [220-KSI] tension-fatigue rated bolt BACKWARDS [CSK ID away from the fillet]... the resulting ID knife edge was curled/rolled to match the Bolt fillet radius that was slightly scored. Installation torque was ~9,000-in#... washer sharp-edge is nicely rounded. I think the fact that the washer's ID edge had a fairly sharp/thin lip of metal made this possible. I'll try to find the washer and submit a photo.

StevenP spoiler...

NAS1149#06**## Washer under the head of the NAS6706 shear-tension bolt [with it's tiny head-shank fillet R] is a perfectly acceptable 'fit' combination.

NAS1149#06**## Washer under the head of the MS20006 [and similar] tension-fatigue bolt [with it's huge head-shank fillet R] is a REALLY BAD 'fit' combination [NO-GO].



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]