To grease or not to grease (metal, plastic, type of grease)

[phgachoud]

Hi Folks,
Was wondering what of those considerations apply to scuba equipments.
To grease!

• protects surfaces from corrosion
• protects surfaces from stuff to stick on it, depots, etc
• makes assembly and disassembly way easier
• makes able to screw better/stronger with appropriate torque as it doesnt jam in the the screw mesh with the other.
• Was told by a bike mecanic specialist (which makes sense for me) there is a misconception about stuff that are greased to unscrew easier, but as you can tighten easier too it doesnt apply.

Not to grease!

• In scuba equipments we dont want any grease to go into our organism
• silicon grease seems to be the non harming (or few) one for organism, but is not that recommended for metal to metal greasing as much as other types of greases.

So my question is about maintenance, where is there a reason not to grease metal to metal screws for ex.

 

[sea_ledford]

I only use O2 compatible lube on o-rings. I'm not sure I do any metal to metal lubing, with the exception of a valve into a tank neck. That is to prevent dissimilar metal corrosion. Lube is primarily important on dynamic o-rings (ie ones the seal against a moving part, like the SPG swivel) vs a static o-ring like on a port plug. I'll lube both, but the dynamic gets a lot more lube.

If you slather on gobs of silicone to parts, just because, it just attracts more gunk to stick to it.

 

[rsingler]

So my question is about maintenance, where is there a reason not to grease metal to metal screws for ex.

The primary reason I am aware of for NOT greasing has to do with torque and axial load.

Torque settings are engineered with an eye towards axial load limits for the (soft brass) metal involved.
Greasing a thread where the torque was specified for dry threads, and then applying the specified torque greatly increases the axial load and may strip the threads.
Greasing the thread to get easier disassembly, and decreasing the torque so you get the designed axial load, makes it more likely that your (yoke) bolt will come loose when you grab your pressurized tank by the first stage.

It's a conundrum. Having some lube on the threads tends to forestall the formation of verdigris corrosion as sea water forces itself into threads under pressure and then dries. But having lube there changes the engineered axial load, and may make the parts more likely to accumulate grit (especially plastic second stage parts).

My recommendation is to follow the manual regarding specified lube points. Poseidon does an excellent job here, unlike many manufacturers.
Where it's not specified, I'll lube the cap on piston regs, because the ambient chamber is wet on both sides. I'll lube HP seat caps.
I'll generally avoid lubing a yoke or DIN bolt unless it's specified. If I'm going for an extended service interval with an environmentally sealed reg, I'll at least remove and replace the yoke or DIN bolt annually, even if I wait 3-5 years on the rest.

 

[scrane]

Silicone grease is not suitable for metal to metal lubrication.