ProSumer Compressor

I've been running an 80 gallon prosumer grade 3.5hp compressor in the shop for a while. Its up to the job, but bigger would be better. I'm
not going to tell you the brand because when I went all over town they were the same. Just different paint. Even the same size/power Quincy I saw seemed to only differ by price. That was kind of disappointing.
I've had to work on it a bit, but usually its a same day fix. Usually a same hour fix. The most recurring problem is the check valve. The first time it failed I dropped a buna rubber o-ring in between the plunger and the valve seat. That lasted for six months. I did the same thing six months later. Then I tried a viton o-ring. It lasted for a year. I tried a buna-n o-ring, and it only lasted a couple months. Pretty terrible.
This last time I took it apart I decided to see if I could do something better. I had originally thought some sort of seal/gasket was missing (The o-rings did fix it), but upon closer inspection I could see the dirty plunger was made out of either HDPE or PTFE. Based on the way it cuts I would guess PTFE. The plunger was the seal. It just quit sealing. I don't know why. The plunger looks ok, and the seat looks perfect. Regardless it wasn't working.
I cut a new plunger out of black acetal and it seemed to work, but I could hear some air hissing past and out the pressure relief tube. I'd gone for a simpler design. A recess in one side for the spring and a shallow cone on the other. As indicated. No joy. When I took it apart I looked at it and decided my finish was mediocre, but what got my attention was a shiny ring about 7/8 of the way around. I guess if I left it in there it might have eventually beat it self into seating. I might also have been able to do it by gluing a mandrel onto it and spinning it in place to sort of lap a seal. That all sounded like to much work for a "maybe" solution.
Instead I cut a new plunger out of black acetal, but this time I put a 45 degree cut on it. (90 degree cone) I also used a fresh sharp uncoated TNMG insert instead of the hand ground HSS tool I had used before. The whole surface of the cone looked bright and shiny. It was cut with a single near perfect tool edge. Of course the other side had a recess for the end of the spring like the first. I dropped the new plunger into the check valve and put everything back together. After letting the compressor come up to full pressure I thought I might have heard a hint of an air leak, but I had work to do. I fired up some machines, ran some jobs and didn't worry about it. I close the tank valve and shut off the compressor every night anyway.
This morning I checked the tank pressure and it appears I didn't loose any pressure at all over night. I don't know how long the acetal plunger will last, but it seems to be doing the job for now. I imagine it can't be any worse than a buna o-ring. I suppose I should stock some other sizes of acetal rod. It seemed like a shame to turn down a 2 inch piece of stock for a 7/16(apx) inch plunger.
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On 02/11/2020 22:35, Bob La Londe wrote:















IIRC from my engineering degree a standard check for PE or PP is to burn a bit and if it is one of those it should smell like a paraffin candle burning, after that IIRC one floats and the other doesn't but it depends on the details of the polymer as both can be very near the density of water or slightly less. If you think it really is PTFE don't burn it, PTFE is about twice the density of water so should sink fast.
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On 11/2/2020 6:00 PM, David Billington wrote:















I am not sure I even know where that old plunger is. It might still be on my welding table, but it will get tossed in the trash now that I have a new design one that works. The newest acetal one is working fine so far. They use acetal plungers in knock valves in airguns up to 3000 PSI so it should be fine at 150-175 PSI. I think they switch to PEEK for airguns that run 3600-45000PSI. The first time I fixed the check valve I was concerned about heat, but if buna held up for six months and viton held up for a year this acetal cone should hold up indefinitely... as I hear air powered hissing and banging in the back shop... LOL
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