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In my aerospace company we have repetitive discussions regarding the stacking of low tolerance fasteners with non-countersunk washers. For example a fastner NAS6703-X (size 10) has a max. base radius of .02". A non-countersunk washer NAS1149 size 10 would clash with the base radius if modelled.

Having done a fit check, there is no clash but to avoid issues we have used bigger sized washers on some occasions. I find it odd to use different sized bolt/washer combinations.

Are you aware of any guidance material regarding this, respectively has anyone else met this topic?

Thank you!

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  • $\begingroup$ Don't you have to follow the material build specs /. documentation to continue to meet certification? $\endgroup$ – Solar Mike Oct 20 '20 at 7:32
  • $\begingroup$ We are the type design holder so the documentation/design is from us. I was hoping there is some guideline regarding standard parts to streamline the design. I was just wondering why two fastener parts can have clashes when the tolerances acc. to spec are being used. $\endgroup$ – Vitus Oct 20 '20 at 9:14
  • $\begingroup$ In your design guidelines, how much deformation is allowed for the washer? There is some assumption in the usage of washers that your washer will deform anyway, at least in non-aerospace applications. $\endgroup$ – jko Oct 20 '20 at 11:48
  • $\begingroup$ We have no such guideline internally. The concern from stress dept. is that a washer clashing with the head radius will weaken the bolt and shall therefore be avoided. $\endgroup$ – Vitus Oct 20 '20 at 14:01
  • $\begingroup$ I would go back to the NAS documents for guidance. Certainly you don't need software to determine if a washer fits the head without interference. I would start with what the math says about the OD right below the head. If there is interference from the basic math I would question manufacturers. Do they know this and as a result build to the "right side" of tolerances? My last thought is wondering if the standard just assumes the washer will be used on the a nut only and not the bolt head. $\endgroup$ – Tiger Guy Oct 21 '20 at 2:15

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