TOOK (Customer) asked a question.
What are the pros/cons to a solenoid spool valve that uses rubber (NBR) seals versus metal seals?
Application is using regular shop air to actuate air cylinders at room temp at 60 cpm for millions of cycles. The SMC VFS series has metal seals. I have been using Norgren V61 series and trying to improve longevity.
I'm guessing when you write metal seals, you mean lapped spool and sleeve? A con with lapped spool and sleeve is your air MUST BE dry. A second con is - depending on where you plan to mount them - it it best that they're positioned so the valve spools move horizontally. This is due to the spools tending to stick in the down position when not in use over a time period (my customers called this "Monday morning blues" after non-use on a weekend). A third con is they tend to have a lower CV rating.
So, you can probably gain I prefer rubber seals. I have used quite a few of the Nitra AVS valves (since 2015) in non-lubed applications and - so far - have not had to replace a one. Rubber seals are more forgiving to the things you find in compressed air: water and oil. And as long as you don't introduce Marvel Mystery Oil into your pneumatic system, should last a long time.
Good luck!
NBR seals can be a good choice for pneumatic spool valves because they provide flexibility and good sealing even with minor surface imperfections. For an application running millions of cycles, though, I think seal wear, lubrication, air cleanliness and spool surface finish become just as important as the seal material itself. Metal seals may have better wear resistance, but they also depend heavily on tight tolerances and surface quality.
In addition to above another consideration would be speed of operation. Extremely high frequency cycling would be better served with lapped spool/sleeve.
If used in a vacuum system, then rubber sealed spool would be recommended due to less leakage across the lands. lapped spool and sleeves have what is referred to as an "air bearing" that provides extemely low friction between the spool and sleeve but does allow higher amounts of leakage and is why the rubber sealed spool yields a higher Cv flow factor for a given valve.
As for lubrication - in either case it depends on the circuit and what you're trying to accomplish in the environment you're operating. Most components are prelubricated and can be ran dry. If however, you apply a lubricator you will need to contiue lubricating as that will wash the original lube away. And an important note on lubricators, Don't over lubricate and make your pneumatic system an hydraulic system. My preferred setting for a lubricator is to see ~1 drop fall per minute of flow.
For extremely high speed operation I recommend the SMC VQ series. You can achievet 100+ million cycles if your air is clean.