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PostPosted: Wed Oct 31, 2018 7:58 am 
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I picked a Crosman 1400 recently, not holding air, missing iron sights and with a cracked butt pad but otherwise in good shape.

Interesting "Valve train" on this one:

Image

At first I thought the trigger/hammer mechanism was broken but it turns out that unlike most similar airguns that use a hammer to strike a valve open, the trigger on this one holds a "blow off" valve shut and it's the pressure that opens it.

1) and 4) is the chamber body, with the check valve 2) being held shut by spring 3). Air from the pump comes from the left of the image.

5) is a cup shaped piston that slips over the nipple on the right of chamber 4) keeping it sealed as long as the trigger keeps part 9) in place, locking it through rod 7).

When the trigger is pulled, 5) is free to move to the left of the image through cylinder 6), allowing the chamber to dump through the transfer port into the barrel.

There's room for improvement, a slightly unbalanced spool that would have much more flow, allowing the transfer ports to be opened up too. The o-ring on the pump end of the spool would be floating to act as a check valve for the pump.

Image


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PostPosted: Wed Oct 31, 2018 9:18 am 
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looking at the Valve train diagram above part #5 has a quad seal inside the cup end
that's what the problem is and why its not holding air

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PostPosted: Wed Oct 31, 2018 9:50 am 
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Ace wrote:
looking at the Valve train diagram above part #5 has a quad seal inside the cup end
that's what the problem is and why its not holding air


I didn't draw it in the diagram but the seal is there on my example and it appears to hold air so I don't think that was the problem. It appears that it's more likely either the the seal between the chamber halves or the pump head seal.


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PostPosted: Wed Oct 31, 2018 10:35 am 
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Looking at your diagram for the original, in addition to the missing quad seal, I'm not so sure that the O-ring on the quad seal carrier travels across the exhaust port of the valve, as it would have a tendency to get cut doing so.... It's been a long time since I had a 140/1400 apart, but I don't recall it being as you drew it....

The same could apply to your revised design, I'm not so sure that your O-ring will stay in place and remain undamaged as it uncovers the exhaust port.... Just a thought, as I have never built a spool valve....

Bob

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Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
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PostPosted: Wed Oct 31, 2018 3:38 pm 
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rsterne wrote:
Looking at your diagram for the original, in addition to the missing quad seal, I'm not so sure that the O-ring on the quad seal carrier travels across the exhaust port of the valve, as it would have a tendency to get cut doing so...


Here is the relevant part with the piston in the fully closed (top) and fully open (bottom) position.

You can see the edge of the o-ring groove at the left of the exhaust port in the closed position, meaning it moves across the exhaust port on every shot.

Image

Quote:
The same could apply to your revised design, I'm not so sure that your O-ring will stay in place and remain undamaged as it uncovers the exhaust port.... Just a thought, as I have never built a spool valve...


Having the o-rings blow off is a genuine concern especially with high pressures. The solution I found is to use as thick an o-ring as possible, and use a slightly undersized ring that needs to be stretched to fit. Not the most elegant approach, but it's worked well in the past.

While you're here, great job with this one! I've never done any rebluing, is it a challenge?


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PostPosted: Wed Oct 31, 2018 5:14 pm 
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Quote:
You can see the edge of the o-ring groove at the left of the exhaust port in the closed position, meaning it moves across the exhaust port on every shot.

Are you sure that is not the inboard (left) edge of the shuttle (part #5), and the O-ring groove is to the right of the exhaust port in that photo (hence not visible)?.... According to your photo, when assembled the shoulder of the rear half of the valve body (part #4) is in contact with the steel exhaust port section (part #6).... That means the mid-diameter part of that rear valve half is nearly to the front of the exhaust port....so the shuttle cannot move far enough forward to expose the O-ring groove in the exhaust port....

Rebluing isn't that hard.... clean the surfaces down to bare metal, whatever the finish you have is what you will get once blued.... I used Van's Cold Blue....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Thu Nov 01, 2018 7:52 am 
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rsterne wrote:
Are you sure that is not the inboard (left) edge of the shuttle (part #5), and the O-ring groove is to the right of the exhaust port in that photo (hence not visible)?.... According to your photo, when assembled the shoulder of the rear half of the valve body (part #4) is in contact with the steel exhaust port section (part #6).... That means the mid-diameter part of that rear valve half is nearly to the front of the exhaust port....so the shuttle cannot move far enough forward to expose the O-ring groove in the exhaust port....


You're absolutely right, here is is out of the cylinder:

Image

Updated the diagram to reflect the standard setup more accurately:

Image

Quote:
Rebluing isn't that hard.... clean the surfaces down to bare metal, whatever the finish you have is what you will get once blued.... I used Van's Cold Blue....


Found this video, definitely much more straightforward than I imagined. Thanks for the tip!


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PostPosted: Thu Nov 01, 2018 9:20 am 
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Interesting concept
Curious where the inlet check valve is located on yours?

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PostPosted: Thu Nov 01, 2018 9:44 am 
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Voltar1 wrote:
Curious where the inlet check valve is located on yours?


I would make the pump side o-ring on the spool floating (as one would on a pump head) so air can flow past it from the pump side but not back out of the chamber.


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PostPosted: Thu Nov 01, 2018 9:47 am 
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Very cool!
Now to build one eh!

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PostPosted: Thu Nov 01, 2018 9:52 am 
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Voltar1 wrote:
Now to build one eh!


There are other things keeping me busy right now but it's on the 2019 build list :) Just wanted to put my thoughts down for the time being.


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PostPosted: Thu Nov 01, 2018 10:42 am 
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You probably noticed that on the one I rebuilt I drilled all the passages out to 5/32" (maybe even 0.166" ? ), and the barrel port to match.... I always start with the barrel port as 75-80% of the caliber and work back from there, trying to make everything the same area.... If you go too large on the barrel port, the pellet can catch and be damaged on loading.... It is possible to get full bore area on all the porting, but the barrel port ends up oblong (0.8 cal. wide x 1.2 cal long), and to take full advantage of that you need a retracting bolt or bolt probe, to get full area in the chamber.... Often that isn't worth the trouble, unless you are shooting heavy bullets.... The 75-80% works just fine for pellets, and keeps all the port round so that you can drill them.... IIRC, the limiting factor on the original 140/1400 valve was the diameter of the port in the stub that seals inside the quad seal.... If you drill it too large, the wall gets too thin.... Your design eliminates that, but you do lose a bit of internal valve volume because of the stem on the spool.... You will want to keep that as small as structural considerations will allow....

In a pumper, more valve volume means greater peak power but that take more pumps to achieve.... while smaller valve volume builds the pressure more quickly, reducing pump strokes for low to medium power applications.... You may need a groove(s) inboard of the front (check valve) O-ring to allow flow into the valve past it.... I have made check valves by using an O-ring sitting on the stem of a flat head (conical taper) screw, the idea coming from Steve in NC on the Yellow Forum.... but the flow in those is from the inside (along the threads) outwards (past the O-ring)….

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Fri Nov 02, 2018 8:37 am 
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rsterne wrote:
You probably noticed that on the one I rebuilt I drilled all the passages out to 5/32" (maybe even 0.166" ? ), and the barrel port to match.... I always start with the barrel port as 75-80% of the caliber and work back from there, trying to make everything the same area....


Sticking with the standard design, more pressure means harder trigger pull, but unlike conventional hammer valves boring out the port does not affect that as the area is governed by the external diameter of the "nipple" on the chamber.

The advantage of the spool design is that the pressure on the trigger is decided by the diameter differential between both ends of the spoon. Naturally the trigger side needs to be a larger diameter in order for the pressure to open it once the trigger releases it, but it need not be a large difference. If we're talking say 1000 psi of chamber pressure say you have 0.25" at the pump end and 0.26" on the trigger end, that's 4 pounds of force that are trying to shift the spool.

Quote:
If you go too large on the barrel port, the pellet can catch and be damaged on loading.... It is possible to get full bore area on all the porting, but the barrel port ends up oblong (0.8 cal. wide x 1.2 cal long), and to take full advantage of that you need a retracting bolt or bolt probe, to get full area in the chamber.... Often that isn't worth the trouble, unless you are shooting heavy bullets.... The 75-80% works just fine for pellets, and keeps all the port round so that you can drill them.... IIRC, the limiting factor on the original 140/1400 valve was the diameter of the port in the stub that seals inside the quad seal.... If you drill it too large, the wall gets too thin....


Good points.

Quote:
you do lose a bit of internal valve volume because of the stem on the spool.... You will want to keep that as small as structural considerations will allow...


There's also a weight consideration for the spool, because it's going to need to be arrested by the structure and you don't want it too heavy in terms of impact. What is interesting about the rod and collar (7 and 8 in the "valve train" image) is that there is effectively an area of more or less trapped air that would act as a brake, which is probably why a rubber shock absorber was not included in the design. The "air brake" is denoted in red below:

Image

In terms of lost volume, let's say you have a 0.2" diameter spool in a 0.4" internal diameter chamber. Boring out the chamber by a mere 0.05" negates the volume taken up by the spool.

The way it's configured, I think I can make the chamber one piece, negating the need for threads and allowing for a larger chamber inner diameter.

Quote:
In a pumper, more valve volume means greater peak power but that take more pumps to achieve.... while smaller valve volume builds the pressure more quickly, reducing pump strokes for low to medium power applications...


I would be inclined towards a smaller volume for better performance for a given number of pumps.

Quote:
You may need a groove(s) inboard of the front (check valve) O-ring to allow flow into the valve past it.... I have made check valves by using an O-ring sitting on the stem of a flat head (conical taper) screw, the idea coming from Steve in NC on the Yellow Forum.... but the flow in those is from the inside (along the threads) outwards (past the O-ring)….


Basically the same as a pump head:

Image

Milling a couple of flats on the side of the o-ring groove that you want air to flow past will allow it to flex and act like a check valve.


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PostPosted: Fri Nov 02, 2018 10:57 am 
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Quick thought
Spool is equalized across intake oring as the pump cup is at the same or hogher pressire than valve internal

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PostPosted: Fri Nov 02, 2018 12:17 pm 
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Voltar1 wrote:
Spool is equalized across intake oring as the pump cup is at the same or higher pressure than valve internal


You're right, I completely failed to take the pump cylinder pressure into account! As drawn it isn't balanced at all.

I should know better as I ran into the same issue making a balanced valve for the Apache Fireball, that ended up being a little more complex:

Image

Fun to machine though:

Image

Image

Back to the 1400, it would need to look like this to function as intended:

Image

Some more thought is needed, unless I go with the original design and incorporate a bleed screw for the pump cylinder... Even then though, the stress on the trigger until the cylinder is bled would be considerable.


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