I am interested in doing an oring failure analysis on MacDev spools in particular. The more I research the more I think that a great deal of improvement can be made to our guns by optimizing the oring profile/ material/ durometer.
The analysis will be done based on Appendix 10.1 in this book: (great read for nerds)
Based on my early research I am thinking that the following materials/ durometer are ideal: (static orings don't make a ton of difference and due to it's low cost/ high availability buna will still be used)
Bolt Keg Inner (017) - 75 Durometer Viton Xring - This is currently in test, but viton and buna have similar wear characteristics and since the higher durometer viton has a lower friction coefficient it is well suited. It seems urethane xrings ~80 durometer would be best, but I haven't even tried sourcing them yet.
Bolt Keg Outer (020) - Static
Bolt Tip (014) - 70 Durometer - Unknown - Early research suggests urethane is the best material since the probable failure mode is abrasion (breaking contact with the breach) , however, I doubt cost makes this worthwhile since it is not a high-failure item in my experience. MacDev could add a slight chamfer to the breach and make this a non-issue completely in the future.
Bolt Bumper (15x1) - Viton - Unknown durometer - Based on the probable failure mode (compression set) viton appears to be the most well suited. I will be looking into this further seeing as how this is a VERY important oring.
Bolt Switch (016) - Unknown - This is one oring that intimidates me a little. Abrasion is likely going to be the most common failure mode, but it will be imperative to retain an even seal at all times. Using an xring in this location is not ideal because it runs over the small holes in the bolt keg and a slightly rougher surface on the bolt keg inner. I am thinking that urethane, buna, or viton will be used, however, in the past HNBR has been used successfully in similar locations. Based on the material selection guide urethane looks ideal.
Spacer Inner (017) - 75 Durometer viton xring - In test - Again the slick surface of the gold bolt, constant sealing pressure, slightly harder durometer of the viton has been working well so far.
Spacer Outer (017) - Static
Valve Front (17x1) - I am using 75 durometer viton, however, these really need to be urethane. According to the material selection guide viton is still better than buna in dynamic applications, so this will work for now.
Valve Middle (17x1) - Same as above
Valve Rear (15x1) - I am currently running the stock 70 durometer buna orings and have never had an issue. Again, as you probably guessed, viton or urethane are most desirable considering it is a dynamic oring. Likely I will be using viton because the low failure/ higher cost of urethane won't be worth it.
Valve Keg Outer (020) - Static
Powertube Tip (011) - This is another oring that intimidates me... Much like the bolt switch a low friction coefficient is desirable and viton or urethane seem to be the best choices. I am likely going to use viton here as well because it is riding on the slick surface of the gold bolt making a higher durometer (lower friction coefficient) a solid choice.
Powertube Mid (10x1) - I have never had one of these fail. Viton/ urethane would be ideal, but for cost reasons I'll probably just keep it buna.
Powertube Cap Inside (020) - Static
Powertube Cap Outside (020) - Static
Also worth mentioning silicone seems to be ideal for static orings, but it's higher cost is not even worth exploring for me.
Also used for material selection: http://www.marcorubber.com/material_chart.htm