Old man riveting tip ?

[mikeone]

Hi everyone,

I am working in the aircraft heavy maintenance support for deHavilland Canada Dash-8 100/300 airplanes. On the exhaust shroud ("fingernail" in jargon) they have stainless 347 parts (different thickness) riveted to 2024 aluminum parts (different thickness) with monel rivets (MS 20615M--).
Most of the technicians are using a steel washer under the rivet bucked (formed) head "for strenght" (after bucking the rivet, the washer rests under the rivet formed head).
Is this a standard practice or a bad practice (no separator, like sealant is used between the two different materials) ? Can anyone help me with some information ?
Thank you in advance.

 

[mrbliaison]

The impact force required to buck a monel rivet is higher than the force required for an aluminum rivet – the washer may be used for backup to prevent damaging or dimpling the 2024 aluminum sub-structure on impact.

 

[mikeone]

Thank you mrbliaison for your answer. If this practice is OK, do you think long term secondary effects might occured (corrosion, etc.) because of the two dissimilar materials ?

 

[mrbliaison]

There shouldn’t be a problem between the stainless steel shroud and the aluminum sub-structure since the aluminum is probably anodized and primed. The rivet, however, should still be installed wet with a high-temperature sealant since the aluminum and monel rivet are quite dissimilar.

 

[mikeone]

Thank you mrbliaison for your time and your help.

 

[edmeister]

mikeone ..
Being well acquinted with the component you describe ..
The Backup washer is standard practice because on re-assembly the original hole is usually oversized and the washer not only provides "tighter hole" but also assists the technician from "dumping" the rivet. The washer provides additional "clamp-up". For those of you who wonder why the next size fasteners is not used - edge distance is usually a problem (and the fastener usually swells up sufficiently to take up the gap)- and since these components get rebuilt on a regular basis - fastener upsize is not recommended unless absolutely necessary.

Sealant is not critical because 1/ airtight seal is not required here. 2/ Dissimilar metal corrosion not a major factor in this area. 3/ High Temp sealant is VERY viscous and prevents satisfactory riveting.

Cracking & Heat distortion of the "barrel" are the major concerns. Sealing would not provide any additional benefit in this application (only handicap the technician in accomplishing his task)

 

[mikeone]

Thank you edmeister for clarifications.