Liquid lubricant for 3D printed pneumatic ram 2

[vagulus]

My pneumatic ram actuator, built for a mechanical model I am making, is made out of what most folks would call plastic. Head, tail and piston are 3D printed in PLA, cylinder and spear are made from PVC water pipe and the O-ring is nitrile. Actually created as an air pump it is functional in that pushing and pulling the spear in and out generates air flow in and out of the orifices. Lubrication is the current issue.

Until now I have been using Gulf Western Air Tool Oil (P/N 30192) but the O-ring seal does not last long and I suspect that the oil hardens the Nitrile. Changing to using simple petroleum jelly (Vaseline) give encouraging results but leaves me with the problem of maintaining lubrication in the long term. That is to ask, "How long will this thing continue to work on one greasing?"

Has anyone any advice on a suitable liquid lubricant to use in this situation?

Thanks

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Artificial Intelligence is no match for Natural Stupidity

 

[Tmoose]

How oil-free must the delivered air be?

What is the speed and travel of the actuator ram?
Maybe add a second o-ring offset axially a little bit to serve as a lubricant reservoir ?

Has the o-ring supplier declared the finish of the ID of the PVC suitable as a "running" surface?
//Some// applications work better with controlled 10-20 µin roughness with a 45° cross hatch.

 

[3DDave]

The American Felt Association** would like to remind everyone that felt is a great material to act as a lubricant reservoir.

see

https://www.felters.com/applications.php?p=45

for an example pitch.

**(not paid or affiliated with any Felt Association or maker. Also There isn't an American Felt Association)

 

[vagulus]

Thanks Tmoose. Great suggestions.

FTR: No problem with oil in delivered air (I am not driving a respirator ). Ram stroke is 80mm, rate about 30cps.

Interesting thought about the running surface. I's pursue that.

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Artificial Intelligence is no match for Natural Stupidity

 

[Jboggs]

You are running an O-ring against the inside of a PVC pipe with 80mm stroke at a rate of 1800 rpm? I'm surprised it lasts a full minute! A PVC pipe might be "cylindrical", but it definitely doesn't have the surface finish required for a long lasting seal. Yes, lubrication will help, but there's no way around the heat you're going to build up from the friction, and the quick demise of the o-ring. In my opinion.

 

[vagulus]

Ooops!
I meant cycles per minute not cycles per second!
Having a check on my thinking here.
Make that 90cpm.

There may be a problem with the dissipation of friction heat but I haven't noticed any heat buildup in the periods (a minute or so at a time) when I have had the model running. All that happens is that the ram stops pumping air.

Perhaps I should mention that this whole system runs at very low pressure - about 0.1Bar (just over a metre of water head). All it does is push air down to the bottom of a small water tank.

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Artificial Intelligence is no match for Natural Stupidity

 

[MintJulep]

Cheaper than dirt air pumps:

https://res.cloudinary.com/engineering-com/image/upload/v1704765101/tips/1pc-12x13cm-air-pump-for-Inflatable-Toy-and-balloons-foot-balloon-pump-compressor-gas-pump-for_oxyipc.webp[/img]

What's the motivation / business case for building your own expensive and complicated pump?

 

[vagulus]

Fair question, MintJulep.
Answer, it's part of a machine. It has attachments so it can be driven by the machine. Like this - no Cylinder or Spear Yoke shown:

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Artificial Intelligence is no match for Natural Stupidity

 

[TugboatEng]

You need to use silicone or PTFE based oils with your nitrile o-rings. Pneumatic valve manufacturers seem to universally be using Dow Corning 111 grease lately. If you want an oil vacuum pump oils are available in PTFE and silicone based formulation. If you want silicone oil on the cheap you can get DOT 5 brake fluid from your auto parts store. Do not confuse with DOT 5.1. The brake fluid is usually dyed purple and may stain your plastics if that matters.

 

[vagulus]

The trick here is finding someone who will own up to being a manufacturer of nitrile o-ring cord. They seem to maintain a visibility so low that Uncle Google hasn't heard of them!

Considering the low working pressure involved would a simple grease loaded felt seal be sufficient? (I doubt there is a textbook answer to that one - its the sort of thing that only experience could advise.)

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Artificial Intelligence is no match for Natural Stupidity

 

[MintJulep]

Back in the day bicycle pumps used leather.

 

[TugboatEng]

One thing is to look on to cup seals. Cup seals are pressure energized so they can compensate for wear some.

Another option is to use x-rings. The extra cavities will hold some lubricant.

However, Petroleum oils do age nitrile rubber. They cause it to swell some which accelerates wear, and then they cause it to shrink and I've seen it harden or soften.

 

[vagulus]

Aaaah. Anno Domine there MintJulep. I, too, remember bike pumps. That's what brought it to mind.

I'll take that as a Probably !

If anyone knows where to obtain ¼"sq felt cord please let me know.

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Artificial Intelligence is no match for Natural Stupidity

 

[Jboggs]

Why couldn't you use the same machine to drive one of those cheap air pumps?

 

[btrueblood]

Nitrile can soak in oil, it literally does that in hydraulic systems, so there is no way that oil on its own is somehow damaging your Oring. Listen to Jboggs, you are overheating and damaging the oring by running it on PVC at too high a speed. If you want to dink around making cylinders, use standard drawn copper tubing/pipe from the hardware store, it has a lovely surface finish, and good conductivity to pull the heat away from the oring.