Freeze Plug Concentricity

[alexeu]

For all liaison stress engineers and DERs out there, here is a detail which I never got any answer to,

1) Does a freeze plug install in a parent material has to be concentric to the blueprint rivet hole (i.e. equal wall thickness on all sides), if not, then is there a reference that tells or specifies the deviation of concentricity.

2)Also what is the minimum thickness (theoretically) for freeze plug. I know that some companies specifies freeze plug to be installed in material minimum t = 0.063 inch. Some other companies says that minimum t = 0.040 inch before the freeze plug will pop out and the freeze plug loses bearingc apability below t = 0.040 inch.

 

[SuperStress]

Alexeu,

1) Freeze plugs do not have to be concentric. The decision of how to design your repair is dictated by the function of the part. If you have interchangeable/replaceable parts where you have to match a fastener pattern, then make your plug as big as it needs to be to maintain a .050" minimum wall thickness (for aluminum). This usually results in an eccentric installation, unless you make the bushing REALLY big. (I'm thinking of a repair for a double-drilled hole.)

If net section is your biggest concern, then pull the center of the hole as required to keep the plug as small as possible. This usually results in a concentric installation.

A concentric condition is most desirable since there are no eccentric loads which will tend to want to rotate the plug.

All repairs should be evaluated for their effect on the original analysis. For example, if pulling a hole changes the CG of the fastener pattern, an analysis should be performed to assess the impact. Having said that, these sorts of changes are nearly always negligible.

2) Don't know what the "theoretical" minimum thickness would be, but I imagine that the practical limits are set so that the parent material doesn't buckle when the plug warms/expands.

.063" is the limit I have seen documented for freeze plugs in aluminum structure.

SuperStress

 

[jetmaker]

Alex,

When I worked as a liaison engineer, eccentric fastener holes were permitted in low load transfer joints as long as there was a minimum of 0.050" wall remaining in the freeze plug. Additionally, adjacent fasteners on either side were beefed up to help remove load transfer from that eccentric fastener. However, if this was a high load transfer location, every attempt was made to make it concentric.

Where I worked, minimum thickness was based on maintaining a d/t ratio of 5 or better. This was very difficult in thin skin regions.

Regards,

jetmaker