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FASTENER SUBSTITUTION

FASTENER SUBSTITUTION

(OP)
Hello, everyone!
It seems that Boeing's SRM 51 FASTENER SUBSTITUTION TABLES do not allow *bolts* to replace *rivets* -- and vice-versa. (1) Why is that?
Now, looking at a drawer with several "look-alike" fasteners, it makes me wonder: (2) what features or characteristics allow MS21140/21141 hardware, for example, to be called "bolts" and NAS1919/1921, "rivets"?
And, finally, please notice that there are FAA-aproved repairs within specific ATA-chapters of that same SRM which call for hi-lok bolt/collars to be installed where solid rivets were originally installed (usually for thick double-doublers). So, (3) where do we find substantiation for said substitution when desired for an eventual beyond-SRM repair approval?

RE: FASTENER SUBSTITUTION

(1) bolts shouldn't be used to replace rivets as bolts are much stiffer than typical rivets (ok, monel not so much) and so load distribution is different.
(2) rivets tend to be AL and smaller than bolts, bolts tend to be steel. Ok, there's a case for a 1/4" monel rivet vs a 1/4" bolt.
(3) an approved repair is "approved". I suggest that this means the OEM has investigated the change and the effects are understood ... if that's what you meant. Other than that, you normally won't have access to the OEM's substantiation of the SRM; if you want to make a substitution that's "beyond SRM" then you've got to make you own substantiation. Replacing a 3/16" Al rivet with a 3/16" steel bolt is not QED.

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

RE: FASTENER SUBSTITUTION

J Souza... Adding to what RB1957 said...

Generic 'rules' applied within SRMs... such as generic fastener substitution protocols... are narrowly developed to ensure that the generic guidance is NOT improperly used/applied in situations that could-be completely inappropriate/dangerous for many reasons [not always obvious or intuitive].

However, when engineers understand the specific circumstances involved then a refined/expansive approach is possible... and 'narrow rules' can be broken.

This is where I give You related quotes to consider for engineering [and in life,]...

Know the rules well, so you can break them effectively." --Dalai Lama XIV

Learn the rules like a pro, so you can break them like an artist." -- Pablo Picasso, Spanish artist

Know the rules [before You] [to] break the rules." – Quick/Dirty version of these quotes.

The laws and rules made by God [physics, chemistry, biology, etc] can only be violated at great risk to mind, limb and body. The laws and rules made by man are guidelines for daily living in a civilized society and may have-to-be broken occasionally for the greater good... but know the rules to break the rules!" – Me

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]

RE: FASTENER SUBSTITUTION

In regards to the specific nomenclature used by the standards noted in the OP, NAS1900 is "RIVET, BLIND, GENERAL PURPOSE, BULBED, SELF PLUGGING, MECHANICALLY-LOCKED-SPINDLE", and MS21140 is "FASTENER, BLIND, HIGH STRENGTH, PULL TYPE, POSITIVE MECHANICAL LOCK, 100 DEG FLUSH HEAD, CORROSION RESISTING STEEL, 95 KSI FSU".

Both appear to be similar types of pull-type blind rivets having a mechanically locked spindle.

RE: FASTENER SUBSTITUTION

but the MS21140 is typically large sizes and typically steel, vs the NAS1900 typically Aluminium and smaller sizes.

Sure there is some overlap.

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

RE: FASTENER SUBSTITUTION

Generally speaking...

Rivets [solids and blinds] are intended solely for shear applications with incidental tension loads [IE: to withstand peel... but be careful]. Never use rivets where tension loads are significant [important]. Even though blind rivets have a tensile strength requirement [listed], that value is primarily for fastener OEMs to ensure integrity of the parts when tested.

Bolts [smooth shank, threaded, blind and lock-bolt] may carry shear and significant tension loads. Due to bolt design, a significant cost/weight penalty is incurred when used primarily as a rivet substitute... not to mention it would usually have excessive strength and stiffness resulting in higher stress concentrations.

Screws [mostly-to-full-threaded shanks] are intended solely for tension loads with incidental shear loads. Use almost always for light/thin panels or mechanical assys where shear pins or other shear mechanisms are employed.

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]

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