A question on Solid Film Lubrication

[Neubaten]

Hi guys,

I have a question for you that may seem trivial, but which I cannot seem to answer by myself.

You have a cilindrical CRES bushing fit in a hole, and a low alloy steel cad plated pin that gets into the ID of that hole, and for whatever reasons too long to explain, you have to choose to apply a MoS2-epoxy Solid Film Lubricant layer in only one of them, either the internal diameter of the bushing or the external of the pin.

Which one would you choose?

They belong to a mechanism that locks a folding element.

The pin engages/disengages the bushing with a very low frecuency, say 20 times a year maximum, and the normal condition is being locked, where the pin/bushing system is likely to suffer a low shear stress.

The film is also intended as a protection for galvanic corrosion between the pin and the bushing.




Btw, I cannot avoid to make a malicious/gross observation that unintentionally came to my mind while reasoning about this problem: Facing a similar problem, the Supreme Engineer (name it God or Mother Nature or whatever fits you) decided to put the lubrication in the bushing part of the mechanism. If you know what I mean...
;-)
Should I do the same as Him?

 

[israelkk]

First, the cadmium coating is the one that protects against galvanic corrosion.

It is cheaper and more accurate to coat the pin with the MoS2.

However, for the MoS2 adhesion to the pin the cadmium has to be Type III (phosphate treament) which is an excellent base treatment for paints and apoxy based dry lubrication such as Mos2 per MIL-PRF-46010. The MoS2 adhesion to cadmium Type II (chromate treatment) is inferior to Type III and may not meet the MoS2 spec adhesion tests.

 

[Neubaten]

Yep, you are right about the cadmium function, but notice it's acting as a sacrifice anode, because it is in contact with a CRES bushing.

Also, the part is going to be exposed to marine environment.

I did not persoanlly specify the cadmium plate for that part, but to my understanding a "least worse" solution here for galvanic corrosion.

The lube film is added as an extra protection, as the "Steel - cad plate - CRES" system weak in terms of galvanic corrosion resistance.

Very good the info about the adhesion, I didn't notice that point.

But, what if I etch the CRES internal diameter for adhesion improvement? Is CRES(etched) adhesion still much lower than CAD PLATE(phosphated) or are now comparable in terms of adhesion?

Also, I read somewhere that when doubting, you have to apply the solid film to the softer part in the contact, if you look for durability.

What is the validity of such an affirmation?

 

[unclesyd]

I would contact the E/M Coating Div of the Metal Improvement Co about there baked on MoS2 or other type coatings. I have had very good results with this type coating in similar service.

http://www.metalimprovement.com/em_coating_services.php