Material Choices for Underwater Kinematic Linkages

[NuclearNerd]

Hi all

I have an application which requires a bellcrank arrangement to operate underwater without lubrication. I am about to decide what materials to use. For reference:

- the pins joining the linkages together are about 1/4" diameter.
- the mechanism operates at low speeds (on the order of 1 rpm for less than one revolution)
- the linkages carry moderate loads (say, 200lbf)

I don't have room to incorporate bearings into the linkages, so I have to rely on good material choices in the linkage and pins to give a long service life. My shortlist is as follows:

1) Pin: 17-4PH SS hardened to RC38
Linkage: 304 SS unhardened
Concern: Potential for galling

2) Pin: Hard Chrome plated 17-4PH SS
Linkage: 304SS unhardened
Concern: The extra plating and grinding step to manufacture the pins is costly

3) Pin: Waukesha-88 (anti-galling nickel alloy)
Linkage: 304SS unhardened
Concern: We've used Waukesha-88 many times with success, but never as the pin/journal, only as a bearing. I'm not sure what the consequence of having the pin be the softer item will be.

I'd love to hear your opinions or suggestions for better options.

 

[ornerynorsk]

Seawater or fresh? Any other incidental exposure to chemicals? Freeze/thaw cycle, or always liquid state?

 

[TVP]

Underwater? Pacific Ocean, Lake Michigan, bucket of tap water in your lab, something else? Chemical species in the water? Expected lifetime and number of load cycles? If the water isn't corrosive, then 304 for the linkage may be ok. I would look into some type of dry film lubricant on the pin if this assembly has to be maintenance-free for a long time/lots of load cycles. FWIW, Cr plating against 304 SS (or anything for that matter) is not very good unless there is an effective lubricant.

 

[NuclearNerd]

Fresh (distilled) water, always liquid. This is a tank with tight chemistry controls. The device operates in a high radiation field however, so plastics are off limits.

 

[rb1957]

irradiated water ... temp ??

"tight chemistry controls" ... = pure or what ?
also implies you don't want the lub going into the solution.
teflon (= dry film ?)

how long is "maintenance free" ? not forever, 10 years ?

 

[ornerynorsk]

Ampco metal comes to mind. (aluminum bronze). I do not have any experience whatsoever with radioactive environments and their effect on materials, so that would be something to verify.

 

[NuclearNerd]

Answers:
- The water can be considered "pure" for this purpose.
- We're designing for a lifetime on the order of 10,000 cycles.
- Teflon's out, as are most polymers - too small a radiation tolerance.
- The temperature is no more than 50C.
- You're right that we don't want any lub going into the solution - the device must operate with only water as its (terrible) lubricant.

As I mentioned, we've used 17-4ph and chrome plated shafts operating in waukesha-88 bushings in the past with great success, but I'm looking for ways to reduce the number of parts / manufacturing steps.

 

[NuclearNerd]

ornerynorsk: We've trialed aluminum bronze before - the corrosion life even in pure water wasn't up to snuff.

I'm most comfortable with the material options I listed. I was mostly interested in your thoughts about option 3 - using a "soft" pin.

 

[CoryPad]

Nearly everything in the universe is irradiated. I suppose you are talking about gamma rays? Other?

I think your three options are not very good ones. Can you make a simple shell "bearing" with your Waukesha alloy and use the hardened 17-4 stainless pin? That would be a better option.

 

[NuclearNerd]

CoryPad: Gamma, yes. Quite a lot of it in fact.

I could make a shell bearing, but I'd have to make all the linkages thicker to make room, and that would be difficult given my constraints. As far as I have researched, the only reason to make the shaft harder than the bushing is to ensure that the bushing wears out first, as it's usually easier to replace. In my case the pin would be much easier to replace, so I'd just as soon it wear out first. That's why I'm leaning towards #3.

I just checked the forces and contact stresses. The peak contact stress on these pins is about 8 ksi - more than the ~2ksi gall-resistance limit for 17-4ph, but not so high that I'm worried about shearing the pins. Are there other failure modes I should be concerned about?

 

[hydtools]

Graphite rod for the pin?

Ted

 

[btrueblood]

Nitronic 60 alloy.