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:
I've been running an 80 gallon prosumer grade 3.5hp compressor in the
shop for a while.Â* 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.Â* Just different paint.Â* Even the same size/power Quincy
I saw seemed to only differ by price.Â* 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.Â* The most recurring problem is the check valve.Â* The
first time it failed I dropped a buna rubber o-ring in between the
plunger and the valve seat.Â* That lasted for six months.Â* I did the
same thing six months later.Â* Then I tried a viton o-ring.Â* It lasted
for a year.Â* 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.Â* 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.Â*
Based on the way it cuts I would guess PTFE.Â* The plunger was the
seal.Â* It just quit sealing.Â* I don't know why. The plunger looks ok,
and the seat looks perfect.Â* 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.Â*
I'd gone for a simpler design.Â* A recess in one side for the spring
and a shallow cone on the other.Â* As indicated.Â* No joy.Â* 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.Â* I
guess if I left it in there it might have eventually beat it self into
seating.Â* 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.Â* 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.Â* (90 degree cone)Â* I also used a fresh sharp
uncoated TNMG insert instead of the hand ground HSS tool I had used
before.Â* The whole surface of the cone looked bright and shiny.Â* It
was cut with a single near perfect tool edge.Â* Of course the other
side had a recess for the end of the spring like the first.Â* I dropped
the new plunger into the check valve and put everything back
together.Â* 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.Â* I fired up some machines, ran some jobs and didn't worry about
it.Â* 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.Â* I don't know how long the acetal
plunger will last, but it seems to be doing the job for now.Â* I
imagine it can't be any worse than a buna o-ring.Â* I suppose I should
stock some other sizes of acetal rod.Â* It seemed like a shame to turn
down a 2 inch piece of stock for a 7/16(apx) inch plunger.


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.

On 11/2/2020 6:00 PM, David Billington wrote:
On 02/11/2020 22:35, Bob La Londe wrote:
I've been running an 80 gallon prosumer grade 3.5hp compressor in the
shop for a while.Â* 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.Â* Just different paint.Â* Even the same size/power Quincy
I saw seemed to only differ by price.Â* 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.Â* The most recurring problem is the check valve.Â* The
first time it failed I dropped a buna rubber o-ring in between the
plunger and the valve seat.Â* That lasted for six months.Â* I did the
same thing six months later.Â* Then I tried a viton o-ring.Â* It lasted
for a year.Â* 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.Â* 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.Â* The plunger was the seal.Â* It
just quit sealing.Â* I don't know why. The plunger looks ok, and the
seat looks perfect.Â* 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.Â* A recess in one side for the spring and a
shallow cone on the other.Â* As indicated.Â* No joy.Â* 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.Â* I guess if
I left it in there it might have eventually beat it self into
seating.Â* 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.Â* 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.Â* (90 degree cone)Â* I also used a fresh sharp
uncoated TNMG insert instead of the hand ground HSS tool I had used
before.Â* The whole surface of the cone looked bright and shiny.Â* It
was cut with a single near perfect tool edge.Â* Of course the other
side had a recess for the end of the spring like the first.Â* I dropped
the new plunger into the check valve and put everything back
together.Â* 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.Â* I fired up some machines, ran some jobs and didn't worry about
it.Â* 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.Â* I don't know how long the acetal
plunger will last, but it seems to be doing the job for now.Â* I
imagine it can't be any worse than a buna o-ring.Â* I suppose I should
stock some other sizes of acetal rod.Â* It seemed like a shame to turn
down a 2 inch piece of stock for a 7/16(apx) inch plunger.


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.


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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