Bolting Finish Painted Parts

[roberthathaway]

What is the industry rule of thumb for assembly of painted parts. Is prime to prime acceptable, e-coat to e-coat, what about prime and topcoat to prime and topcoat. And does anyone one have an opinion on Chemical Agent Resistant Coatings used in military applications. Is it acceptable to bolt a prime and CARC topcoated part to a prime and CARC topcoated vehicle frame?

 

[CoryPad]

What are your criteria for acceptance? Mechanical? Electrical? Chemical? It seems that any of your proposed joints could work in most instances.

 

[roberthathaway]

Retention of clamp load for a structural application under low cycle high loading fatigue.

 

[CoryPad]

The goal is to limit total coating thickness, ideally < 300 [&micro;]m or so. I think you should be OK with your examples, unless you have excessively thick topcoating.

 

[roberthathaway]

What is the mechanism that limits the coating thickness to 0.012". Interested in understanding so we maintain clamp load with grade 8 fasteners.

 

[CoryPad]

The mechanism is creep of polymeric coatings. You can think of it simply as strain on a block of material - the taller/thicker the block, the larger the displacement for a given amount of elastic/plastic strain. The lost displacement comes out of the total joint displacement which was created by the elastic (usually) compliance of the joint components.

 

[Tmoose]

I believe a structural joint ( like yours )should be masked to provide metal to metal clamped surfaces, then the edges and faying surfaces finished appropriately. Protecting the inner regions requires a creeping product, like LPS-3.

"Low cycle high loading fatigue" can involve localized yielding each cycle. Are the fasteners of the material in the clamped zone stressed to yielding each cycle? Yikes. Is the coating in the high stress/strain areas for storage protection, or expected to survive "the events?"

 

[MintJulep]

CoryPad is correct that coatings can "creep" (although cold-flow may be a more accurate term) after assembly. This mechanism is one of several that are collectively referred to as embeddment.

The potential for such cold-flow to affect fastener preload depends in part on the grip length of the fastener. For any given preload, a long bolt will stretch more than a short bolt, and thus will be more resistant to loss of preload due to embeddment than a short bolt.

Any text will provide you with a formula necessary to calculate the stretch of a bolt under preload. Finding accurate information on the amount of embeddment to expect from various coatings is more difficult.