Blind rivets in dimples 1

[verymadmac]

HI

I have a repeat bug barer on a part 23 aircraft, in that there is numerous places that its uses cherry maxs in dimpled skins. Every once and a while I end up with a repair on my desk that needs strength data for these.

Does any one know of strength data for cherry maxs in dimples, I have asked the OEM & cherry and haven't gotten anywhere.

Duncan

 

[WKTaylor]

Were the BRs installed by the OEM,or by a mechanic?

IF Installed by the OEM, or per OEM repair data, then there is allowables data available, somehwere.

IF installed by a mechanic NOT using approved data, then beware.

Years ago I was in same boat. Problem is that solid rivets swell and fill the irregular surfaces [steps] in the hole that are created by the dimpling process, locking everything together. BRs will NOT fill these irregular steps in the hole and are essentially loose.

The only way I was able to over-come this issue with an existing hole/dimple [sometimes] was to drill-thru the the stepped hole for a next larger dia [or next learger OS], which straightens-out the hole walls... thes installing a flush shear head [FSH - NAS1097 style] BR**.

There are ways to "work" this problem in new production: IE: drilling [significantly undersize], dimpling [FTH - MS20246 style head] for a smaller rivet; then driling thru the stepped-hole for, and installing, the next larger/next-larger-OS Dia FSH BR.

NOTE.
** I always recommend using high pull-up, bulb-forming aluminum-aluminum BRS where possible, since these are the most ductile for hole filling/shank-swelling.
As-far-as real allowables go, I always just cleared these substitutes with the stress guys who usually hand-waived it becaused dimple-dimple construction was rarely used for primary structure.

NOTE. In-lieu-of dimples, I always preferred using Cherry-Max unisink ["flanged dome head"]: these are highly useful for very thin skins and have avery low external profile. Unisinks were used in many locations on the first non-homebuilt I worked-on: [Ted Smith] Piper Aerostar [ailerons, elevators, flaps, fairings, doors, etc].


Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[verymadmac]

Thanks Wil

The fasteners are original &/or SRM repairs. There is also an unapproved data issue but that is replacing cherries with solids (which would nice to be able to clear up as is).
Is there much data available for Cherry-Max unisinks (can't see them in MMPDS-04).

 

[WKTaylor]

verymadmac...

The folks at Cherry have the MMPDS allowables for their CRxxxx Unisinks BRs.... for typical alloys and most typical [lower] thicknesses. Ask tech support folks nicely. Most of this data was developed for clients [OEMs] for specific cases [rivet type, specific sheet alloys/thicknesses, etc] so the full gambit of rivet alloys, sheet thicknesses and alloy types has not been run... but available data is too not bad for starters.

NOTE. I try to emphasize hole dimensions/quality and chamfering/deburring or countersinking dimensions for BRs & BBs, so there is NO second-guessing by the Techs [what drill bit do I use?, etc].

Note. I always recommend designing to blind fastener yield allowables. There are many reasons for this philosophy, mostly related to my field experience and the fact that once the installation yielded BRs get sloppy and ineffective. Keep the stress guys happier [most hate BRs/BBs].



Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[rb1957]

particularly into dimples !

Quando Omni Flunkus Moritati