Cadmium Plated AN4 Bolts vs CRES AN4C Bolts 1

[Messydeer]

Hey

Are standard cadmium plated ANx-x bolts as strong as stainless steel ANxC-x bolts? According to page 6 of Mil-B-6812E

http://www.everyspec.com/MIL-SPECS/MIL+SPECS+(MIL-B)/MIL-B-6812E_11135/

they both have a 125 kpsi minimum uts. Recently an aircraft manufacturer issued a service bulletin advising the replacement of two AN4C bolts with two AN4 bolts, saying the cadmium plated alloy steel is stronger than the stainless steel AN4C bolts. It seems only way this could be true is if it is known that all cadmium plated AN bolts were much stronger than the minimum 125 kpsi while the SS bolts were known to be weaker.

A related question: If strength is the only consideration, is it allowable to substitute CRES AN bolts for cadmium plated AN bolts? For example, if an AN4-7 is called out, could an AN4C-7 be substituted for this?

Thanks,
Dan

 

[sigmatau]

If you look at the spec, all steel bolts are the same strengh. Yes on the subs.

Jim Brown, DER Structures

http://www.faader.us

 

[WKTaylor]

Warning Will Robinson Warning...

The issue may not be a static strength issue as much as a corrosion issue [materials dissimilarity].

NASM6812 [was MIL-B-6812] specifies that LA Steel bolts be cadmium plated; and that CRES bolts be passivated.

Passivated CRES plays well with CRES, Titanium, Inconel, copper alloys, etc; however it is highly dissimilar to aluminum, magnesium, LA and carbon steels.

Steel Bolts, cadmium plated with post chromate treatment (properly installed) will provide a significant improvement in corrosion protection within most al, mg & stl joints.



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.

 

[Messydeer]

Thanks for the feedback. It's a strength issue, according to the service bulletin,

http://www.sonexaircraft.com/support/servicebulletins.html

Even though the Mil-Spec cites a minimum and both types of bolts meet this, I was told by Sonex the alloy steel bolts were stronger than the SS ones, which is why they made the change. The only way for that to be true, is for the alloy steel bolts to be way over the minimum.

Thanks,
Dan

 

[WKTaylor]

Messydeer...

Be careful... there is NO direct statement in the (linked) SONEX SB saying the AN4 is higher strength than the AN4C.

*********************
Description:
Sonex Aircraft has received 4 reports of the stainless steel bolts that secure the gear legs in standard gear motor mounts breaking inside the titanium gear leg. All affected aircraft have logged more than 20 hours and/or more than 25 landings on turf runways. To date there have been no reports of bolt breakage on aircraft operated on paved runways. Three of the four reports have been on aircraft operating on rough runways.

Additionally, over-tightening of the nuts may have contributed to these failures.
*********************

There IS a significant change in the hardware installation: from an AN4C + self-locking nut (torqued to ???in#s)... to an AN4 + castellated nut & cotter-pin ("torqued snug, then backed-off").

CAUTION: I can see why CRES bolts were originally choosen.
Generally speaking titanium and cadmium are a poor mix: Above ~300F titanium can become cadmium embrittled. Friction can sometimes cause this heating effect.

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.