Fastener substitution 2

[Nmoyer]

Good day,
I was wondering your thoughts on a subject. Is it permissible to substitute a bolt that was pre-drilled for a cotter pin, as long as you use a self locking nut? I would see no difference in the shear strength of the bolt. As long as the grip is correct, and it fits well would this be permitted? This issue has come up a few times, and people have varying opinions. The bolts in question wre installed that way from the OEM, and a few people wanted to change them to a bolt that was not drilled. Any thoughts or opinions welcome. Thanks much.

 

[tbuelna]

Interesting question. Bolts that have a hole drilled for a cotter pin are usually shear type bolts. And these shear bolts with cotter pin holes typically use a castellated nut. The castellated nut also typically has a self-locking feature.

For a shear application, I don't think the cotter pin hole in the end of the threads would present a problem. In fact, if you are using a normal shear-type locknut with this bolt, the reduced height of the shear locknut is probably sufficient to clear the edge of the cotter pin hole when installed.

 

[MintJulep]

How nicely are the holes drilled and are they chamfered? What type of self- locking nut?

I have seen the sharp edge of a cotter pin hole work as a cutter when Nylon element self-locking nuts were threaded over it.

 

[der8110]

If the application requires compliance with 25.607, then no, it would not be advisable to omit the cotter pin (or safety wire).

 

[WKTaylor]

Nmoyer...

IF the original installation called-for a "plain-jane" bolt and a self locking nut, then the military would accept the installation of an identical bolt... with the exception that the threads were drilled for a cotter pin... assembled with the exact same self-locking nut. Likewise, if there was ONLY an identical bolt available, but with lock-wire holes drilled thru the head, then that installation with the same nut would be acceptable. Per the performance specs, there is NO 'performance' difference between a "plain" bolt, VS an identical one with cotter-pin and/or lock-wire holes.

However this is rarely the case: usually the situation exists that the installation calls for a bolt that has a cotter-pin-hole or lockwire holes... and guess-what: all You have available is an identical plain bolt... but WITHOUT holes of any kind?!?! What do You do now?

For the military, we [engineers] are able to alter the basic part [within specification boundaries] by liaison/service engineering disposition to add the cotter-pin and/or lockwire holes to come into conformance with the spec requirements [assumes that spec allows cotter-pin and lockwire holes that are clearly defined].

CAUTION.
Adding these holes should is not to be done lightly and without carefull attention to provide details exactly extracted from the spec. Work MUST be done by a machinist with excellent condition tools and 'bits'. Follow-on visual/dimensional inspections and/or NDI may be required. Also, for steel-parts, I usually direct that the hole(s) and chamfer(s) [now bare-steel] be cleaned and then coated with: (a) brush cadmium plating and/or (b) epoxy primer for improved corrosion/rust resistance [generally NOT required for CRES or Ti bolts, W/WO coatings].

WARNING.
Altering any other aspect of the bolt would likely violate the basic bolt specification. This is often done for a number of important reasons, but requires significant attention to detail and true fastener engineering Knowledge. Also, the altered part is NO LONGER the same part, so it MUST have a unique drawing/identifier number applied... and complete review by design, stress, M&P, Man-Es, etc.

Regards, Wil Taylor

Trust Me! I'm an engineer!

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.

Unfortunately, in science what You 'believe' is irrelevant – "Orion"

 

[rb1957]

"The bolts in question were installed that way from the OEM, and a few people wanted to change them" ... the OEM presumably had that option too (not to wire-lock) but chose to wire-lock; so you guys think you can over-turn the OEM's presumably careful thought ... why? to save a buck ?? why do you think the OEM chose to wire-lock ? to spend an extra buck in assembly ?? what are the consequences if the nut comes loose ? how much do you want to be on the wrong side of a Mayday episode ? how would you approve this substitution ?? does the OEM's SRM give you any guidence ?

Quando Omni Flunkus Moritati

 

[WKTaylor]

rb1957... thanks for reality check.

My 1st "circles-and-arrows-with-paragraph-on-the-back" answer side-tracked my thoughts on the real question.

It sounds like this is a situation where dual locking redundancy is mandatory; or there is a 'witness' requirement [nut loosening or removal].

There are SLNs with castellations where the cotter-pin [rarely ever lockwire] can physically restrain the nut [thru castellations] from turning.

There is also a very obscure practice that installs a thin [low height, shear] SLN on a long-thread bolt. SLN is installed/torqued down below the pin hole; then a cotter-pin [rarely lockwire] is installed to prevent SLN movement past the cotter-pin.

NOTE. Thick [CRES, monel, Inconel] lockwire might take the place of cotter-pins, only if it is sufficiently thick/tough and the SLN is relatively thin.

Regards, Wil Taylor

Trust Me! I'm an engineer!

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.

Unfortunately, in science what You 'believe' is irrelevant – "Orion"

 

[tbuelna]

Based on what the OP states, it appears that this is a case where the original installation used a drilled shank bolt simply because that was what they had available. They apparently installed a conventional locknut on the bolt and the drilled shank hole was not used. For a shear application the presence of a drilled hole at the extreme end of the bolt threads would not seem to pose a stress problem.

I have experienced similar situations where the design called for a specific MS/NAS/AN bolt part number but the delivery lead time to get that exact part number (18-24 months) was way beyond what the project schedule allowed. However there were some vendors that had existing inventory of bolts with the same basic part number/material/finish/etc but with a slight difference, like a drilled shank or a grip length 1/16" longer, that would have no effect on the function of the bolt for the given application. This is actually a common situation in aerospace and most OEMs have general process specs that allow for things like substitution of a bolt with a slightly longer grip length and adding washers to compensate. In other situations where manufacturing or procurement wanted to substitute a bolt part number that was available for one that was not, they would submit a change/disposition request to engineering. If this was done in the example described, the OEM should have some paperwork regarding the change.

I did a quick search for any relevant documentation but all I could find was this AC from the FAA, which doesn't say much about the subject:

http://www.faa.gov/documentLibrary/media/Advisory_Circular/Chapter_07.pdf

 

[rb1957]

i guess it depends on how you read things ... i didn't get that the OEM had substituted undrilled for drilled or didn't use the drilled hole (no wire lock). clearly if the OEM delivered that then it is an acceptable configuration.

Quando Omni Flunkus Moritati