Fastener edge distance in chem mill skins. 2

[Deepaul]

My company uses a 2D minimum for fastener-to-edge in their manufacturing acceptance practice. Less than (2D) requires an engineering disposition (the company still operates under type certification). I am questioning if fastener-to-chem mill pockets follow similar requirements, throughout the industry, or is there a specification which outlines best practices.

 

[desertfox]

Hi Deepaul

I believe that the edge distance of 2D for fastener position is to do with bolt tear out in the joint and is usually the norm for most bolting applications and the minimum I have seen is 1.5 x bolt dia as an edge distance however this link might help

http://www.pulsaraviation.com/techdata/737 NDT Manual D6-28550 Rev104 15July2011/53-30__P2_104.pdf

 

[rb1957]

edge distance affects the bearing allowable of the sheet. 2D edge distance is a typical design minimum, 1.5 is usually allowed with engineering disposition (once someone has checked that the reduced allowable is ok is the specific location.

for me the edge of a chem-milled pocket isn't an "edge" 'cause the sheet continues beyond the "edge". it is probably good design practice to aim for 2D but i's accept a design that goes under this. i'd accept a rework that went under 1.5D. for me the important point is "does the rivet head sit nicely on the inner face ?", anything less than 1D is probably no good.

btw, composite panels are completely different !

Quando Omni Flunkus Moritati

 

[edmeister]

We should differentiate between edge distance from OEM production & Field repair. The OEM typically applies 2.6 D rather then 2.0 D anticipating installation of the next size up fastener or fitting factors. 'There is nothing worse for a field repair than being constrained with less then 2 D edge distance on a pressurized fuselage panel when a row of fasteners have to be increased to the next size because the holes in the stringer below are loose.'

Same argument for edges to milled skins. If the subject fastener is located on the thin material section of the boundary - provision must be made to ensure the bucked shop head (or collar diameter) does not notch the edge of the material & contribute to a "stress riser". Gap 0.1" from edge of the shop head / collar would be typical. Also note that stress risers can also arise from the edges of bucking bars or Sockets during installation of the subject fastener. Clearance has to be allowed.

If the subject fastener is located thru the thicker section of the boundary - the panel loading has to be known. Should this be a compressive panel (belly skin) then lowering the edge distance is not serious. Should this be a tensile panel (crown skin example)then the DTA / fatigue individuals of your company will most likely want to have a chat with you.

I seen numerous examples where we required blending on the opposite side of the thickness boundary due to lightning strikes & stress+DTA would not allow it due to the potential stress concentrations arising from thinner material overlay (10% thk reduction) on a milled edge. We ended up cutting it out & installing a doubler.

Also .. what is the quality of the milled radius - is it finely machined (or chem etched) .. or is it jagged + situated in a tensile panel + located in the corner + subject fastener installed with 2nd rate workmanship. Can we anticipate an issue here in 20K cyles from now?

 

[LiftDivergence]

Be careful. I believe you are confusing edge margin and edge distance. The thread says "edge distance" but in your post you use "2D". Edge distance is the physical measurement of the distance from the fastener to the edge of part, denoted "e". Edge margin is this value divided by the fastener diameter, ie "e/D". It seems trifling but too many people get this wrong. SEM nomenclature should be kept straight like everything else.

 

[tbuelna]

MMPDS-01 section 9.7 gives an excellent approach for analyzing the capability of shear fasteners in flanges.

 

[WKTaylor]

This question is probably more complex than it seems at first glance.

I think the first question I would ask is "what is the fastener head style on the flat side of the sheet [opp side from CM-step]: protruding, flush-shear [shallow countersink] or flush-tension [deep countersink]"?

The second question I would ask is "where is the fastener tail relative to the CM-step: adjacent to the CM-step or isolated from the step by another layer?"

The third question I would ask is: can the fastener [rivet, Hi-Lok, bolt, etc, flush or protruding, etc] develop full bearing/shear-out strength if it were installed in the [adjacent] thin section: or does it need the full section depth to develop full bearing/shear-out strength and stability?

Does the assymetry of the step from A to B [relative to a constant thickness across the area] make a significant difference in loading?

Configuration is every thing.



Regards, Wil Taylor

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