Freeze Plug Edge Margin

[Dez563]

Hi gents,

A simple question regarding Freeze Plug installation.
On reference to Boeing 747-400 SRM 51-20-07, on where instructions and guidelines are provided for this matter, no information is avalailable in regards of the minimum edge margin allwable.

In other words, how do you consider a Freeze Plug?
Is it a fastener? I mean do we have to have 2D edge margin and 4D-6D distance to another fasteners?

Is it an open hole? So 4D-6D is not needed for inter fastener spacing. And what about a 2D edge margin under such a consideration?

To much questions for a modest begining.
Thanks!

 

[jetmaker]

Dez,

The spacing of a freeze plug does not need to be 4-6D if it is to regular fasteners (assuming a reasonably sized freeze plug). If it is to another freeze plug, then one needs to check the stress concentration due to the infulence.

As far as the edge margin goes, it often was an issue of not being too close to the edge to cause a buldge during installation. Often we would go 1D on edge margin, with a low interference freeze plug. Factor all stresses by the KT for an open hole offest to one side (see Peterson). Then shot peen/roto-peen the part edge within 6D of the freeze plug to gain some of the fatigue ability back that is lost by the short edge distance.

Note that this is a rule of thumb, and varies depending on the specific location, joint configuration, loads, and materials.

jetmaker

 

[Dez563]

Thanks, jetmaker.

 

[CoffinsCorner]

Following are the Boeing guidelines on the use of freeze plugs:

Maintain 1.7D(plug) edge margin.

Maintain 2D(plug) to adjacent fasteners.

Maintain 2.5D(plug) to adjacent freeze plugs.

 

[Dez563]

CoffinsCorner, could you, please, write exactly Boeing referenced guidelines?

 

[CoffinsCorner]

In addition to the installation guide in your Boeing model SRM 51-20-XX:

Max. 5 freeze plugs in a part.
Freeze Plug D, equal to or less than 10 X plate thickness.
Freeze plug shall not overlap different materials.
Min. .08" material between fastener csk. and plug csk. (if plug is csk.).

All the above is for Al plugs.
Hope this will do it.

 

[Dez563]

CoffinsCorner,
I have open the SRM 51-40-07 (by the way, I already referred to the SRM in my first post) and have not find edge margin definition for freeze plug. Therefore I ask you in the forum about.
Moreover, 0.05 inch min wall thickness of a freeze plug and nothing about 0.08" material between fastener csk. and plug csk.

 

[CoffinsCorner]

The 0.05" is a general rule.

This increases to min. 0.08" material between the csk of the fastener and the csk of the plug.

If you are in doubt always consult the OEM (Boeing), they can give you all the instructions which apply in your specific case.