Spacer in Solid Rivet Stackup (NAS42)

[YoungTurk]

Does anyone have experience with this spacer sleeve (or any spacer sleeve) in a solid (BJ) rivet stackup? NAS42 spec calls this a "Spacer, Sleeve - Rivet" but I have not previously seen spacer sleeves used in rivet stackups.

ID is appropriate for a solid rivet installation, and aside from the obvious detrimental effect of having the overlaying part unsupported in the area of the spacer, I cannot find a solid technical objection. Parts are lightly loaded and lay flush over the majority of the surface, with a corner hanging off the adjacent bracket where the rivet lands. Spacer is limited to 4/64" height (0.063) and presents advantage to custom cut shim for fabrication & accounting...

 

[rb1957]

i guess you'd use these if bucking the rivet would compress the structure (ie if it's composite) ... much like you use spacers in composite panels (as attmt fasteners). what is the structure material ?

 

[YoungTurk]

Structure material is this case standard Al sheet. Spacer is proposed in lieu of standard shim stock. I really do not know what the standard use of the NAS42 part is.

Apparently shims are going out of style.

Actually, the argument for use of the spacers in lieu of shims is that the rivets requiring the shims/spacers are located such that they would require the creation of numerous (due to quantity of installations) small and oddly shaped shims and subsequent match drill of same shims for the single rivet.

 

[rb1957]

IMHO ... that's insane ! when did shimming "go out of style" ?

if not solid shim, then why not liquid shim. or a large (Al?) washer (so that the rivet head has good seating).

just as you observed, it sounds has though the spacer won't support the head/tail of the rivet properly.

is this an isolated rivet, or are there several involved ?

 

[YoungTurk]

Rivet is 1 of 2 on tie down brackets (low load), in numerous locations throughout fuselage (frame webs). Brackets are properly seated over 80% of surface so "rocking" under the rivets isn't really a concern. Rivet (existing location) falls on unseated portion of bracket.

Liquid shims are basically a no-go in our facility due to wait time and schedule.

Spacer OD is 0.25 for a BJ4 rivet; theoretically "enough" to seat head and/or tail. A larger washer (a) wouldn't fit in all locations and (b) wouldn't have the correct ID for a rivet (unless customized, which brings us back away from the goal). "Look, the specification says rivet right in the title!"

Agree that it seems out of the realm of sane practice, but I'm not seeing a specific reason to shoot it down. Was hoping someone else had seen/tried similar and could share outcome.

Regarding shims going out of style, it would be more accurate to say that a confluence of many non-technical (aka nonsensical) factors make the "opportunity" of using a standard spacer in lieu of custom shims attractive to parties involved. (Think end-item engineering meets lean manufacturing meets innovative re-engineering meets behind schedule and over budget.)

 

[rb1957]

the spacer is filling a gap under the bucked tail, yes ?

why ? why can't you buck onto the surface ? but then why would you shim ubder the tail ??

i guess then that the shim is going inbetween two flanges. but one of two rivets seats well, and the other gaps ?

how can you cut the NAS43 to the length (maybe something like 0.03" ?) ? ie filling a small gap between two sheets.

isn't it just as much worj to make the spacer fit as it is making a shim (and peelable shim is pretty easy to work with) ?

IMHO, this isn't standard practice.

are we talking sheet metal, with formed angles or extrusions ? how heavy a gauge ? how similar are the two pieces ??

 

[YoungTurk]

Picture would be worth 1000 words.

Spacer is NAS42, not NAS43 (yes, everybody assumes I'm talking about NAS43, nobody else had heard of NAS42 except, apparently, the person who chose to use it). Can order the NAS42 (theoretically) in necessary size of 4/64", so no fabrication necessary.

Stackup is tie down bracket, other bracket, frame web. Tie down bracket does not sit fully on other bracket, creating slight overhang at location of existing rivet hole in tie down bracket and frame web, which design intends to pick up. Standard gauge parts, 0.050 to 0.063.

Obvious solution would be to enlarge other bracket so that tie down bracket is fully seated, which is unfortunately not an option (or at least not a popular one).

 

[rb1957]

another obviuos solution (and equally impractical in the context) would be to make a separate angle (instead of a formed flange).

4/64 = 1/16 = standard washer (no?).

can't see the objection to shimming the flange.

why NAS42 ? why not Al bar stock ?

maybe at the end of the day there's no real reason not to ('cept the rest of the world will think you were nuts, and then remember when they had to make similar silly choices and feel sorry for you).