Green chromate vs. Gold chromate 1

[HiGroove]

I am experiencing a problem with a Green chomated brake nut and its mating characteristics with an aluminum body. I am getting false torques during the assembly of the two components. I am not getting false torques with a similar Gold chomated brake nut and aluminum body.

Green nut/aluminum body - the aluminum body is Gravity die cast and goes through a T-5 Heat treat process and is Anodized.

Gold nut/aluminum body - the body is extruded, no idea if heat treated or if coated

The nut is made on the same equipment, but plated differently.

HELP !?!?!?!?!?!

 

[TVP]

Are we to understand that the nuts in question are some type of steel alloy, that are coated in some way with zinc (electrodeposited, hot-dip, mechanical) and then subsequently chromated (green or gold)? And this nut is assembled to some type of aluminum component-- one is a gravity die cast part that undergoes T5 heat treatment, and one is an extrusion that may or may not undergo subsequent heat treatment?

Without additional details, this will just be speculation. The first thing I would do is confirm alloy and heat treatment for the extrusion. Next, what are the differences in the chromates? By green, do you mean the thick, dark olive drag coating? Or is it a faint green that is basically a clear chromate with a dye added to it? The OD green is a much thicker chromate, and may be affecting the tightening process. More likely, some type of lubricant is integral with the gold chromate, which reduces friction and therefore improves resistance to galling. You should confirm this with your vendor.

Have you verified the thread characteristics of the aluminum components (dimensions, integrity, etc.)? Any damage? Other than this, my first guess is the friction aspect, as many chromates for fasteners have friction modifiers added for better tightening performance. Post more info if possible.

 

[CoryPad]

This is speculation, but my initial guess is that the "green" part is binding in the threads. Zinc electroplating plus chromate conversion coating becomes olive drab when it is thick, so perhaps you have a condition where the threads have insufficient allowance for assembly. I agree with TVP that friction (coatings with or without friction modification) may play a role. You need to perform thread dimensional checks for all parts, and then look at friction modifiers for the screws.

 

[HiGroove]

TVP, I am trying to confirm the alloy and heat treatment of the extruded aluminum body. The flare nut is the generic term that is used but in reality the green/gold zinc part is actually more of a bolt. It is being threaded into the aluminum body. It is actually a nut with ISO threads and the aluminum body has ISO threads and seating surface. The assembly is leaking due to the false torque.
The green chomate is the dark olive green coating.
I have confirmed the threads of the nut and both aluminum bodies. The AL bodies are okay.
The green nut will not thread completely on M10 x 1.0 Class II Ring Gage. But it measures okay with an optical comparator.

Currently we are using a Marathon brand 'flushing oil'

CoryPad,

What type of 'friction modifiers' are you talking about ? Can you give me some examples ?

 

[CoryPad]

Ahh, these are tube nuts. You have fine threads and a high friction coating. You should look at some alternatives. Friction modifiers include PTFE integrated into the coating. You should contact The Magni Group - they have specialized coatings for automotive tube nuts (Magni 550 is used for GM automobiles).

http://www.themagnigroup.com

 

[drawoh]

HiGroove,

I do not know much about thick zinc electroplating plus chromate coating, but I have painful experience with chromate conversion coatings on aluminium.

Aluminium has a very high coefficient of friction, and the untreated stuff binds easily. The chromate conversion has no effect on this. We had to have a telescope taken apart by a machine shop a few years ago, and we learned our lesson. If something has to fit accurately, you do not use chromate conversion. Anodize works much better. If the interface has to be conductive, you cannot rely on the coating.

I think this reinforces the comments above about the zinc electroplating. When finishers prepare to anodize or coat something, they clean it by etching. This means that anodized or alodined parts actually lose material in the coating process, and clearances increase.

JHG