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 Post subject: two-stage piston?
PostPosted: Sun Mar 20, 2016 8:53 pm 
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Joined: Fri Feb 26, 2016 2:42 pm
Posts: 50
not sure if it would be worth the effort, but has anyone tried to build a telescoping, two-stage piston for a MPP like 1322,etc?
basically a smaller piston inside the normal FTP piston rather than wasting the space.
if so, was there any sizeable improvement in pumping efficiency?
(there should be a drawing if i managed to attach it correctly...)
cheers!


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2-stage piston.png
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 Post subject: Re: two-stage piston?
PostPosted: Sun Mar 20, 2016 9:32 pm 
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Posts: 1079
Location: Calgary, Alberta
I believe the FX Indy uses a two stage pump. Maybe you could find how they set it up. If someone made one for the 13XX that'd be uber cool, get full power with only 2 or 3 pumps....oh yeah!

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The flaw with experience is, it causes us to forget what we were like when we didn't have any.


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 Post subject: Re: two-stage piston?
PostPosted: Sun Mar 20, 2016 9:44 pm 
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Location: Canada
13xx does not have a bleed hole...air enters at the end of the pump slot.

Given that the "inner" piston must have the same length as a "conventional" flat top piston, there would be no improvement in swept volume with your design...you would actually lose the volume occupied by the outer piston.

Given "stock" tubes and linkages...a full bore piston offers the best swept volume available.

Al


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 Post subject: Re: two-stage piston?
PostPosted: Mon Mar 21, 2016 12:58 pm 
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Posts: 50
Hi guys & thanks for your input,
That's the first time i've ever used any drawing app, so you'll have to bear with me...
The hole marked "air inlet" is actually meant to be the "pump slot". (*see exciting, new & improved diagram!)
Also, the diagram is not to scale & fails to show that only the fully-extended piston would equal the length of a stock piston. When fully compressed, the piston would be shorter than the stock piston (40%-ish?!?). Which leads to another challenge: the pump arm linkage would have to be extended to make up for the loss in piston length (which means a longer pump tube, eek!), or it's pivot point re-positioned on the pump arm (which means less leverage, which means more pumping effort, yikes). The spring alone (size v. strength) brings its own problems. The adjustability of the piston may be affected by the reduced threaded area as well.
Eeek, I smell a can of worms.
So, back to my original question: has anyone tried to construct such a piston and if there were any advantages are they worth all the trouble?
cheers

(ps-who doesn't love a can of worms?)


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 Post subject: Re: two-stage piston?
PostPosted: Mon Mar 21, 2016 1:44 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8986
Location: Coalmont BC
You would need quite a heavy spring to prevent the small piston from moving into the large one once the air pressure started to increase.... If you wanted to use the small piston to boost pressure, you would need to do that, otherwise it would only be boosting volume (subject to other factors).... If the spring was strong enough to prevent movement of the small piston inside the first until it bottomed (as drawn), you would hardly get any benefit in reduced pumping effort.... unless the small piston was much smaller in area, and then it wouldn't add much volume....

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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 Post subject: Re: two-stage piston?
PostPosted: Mon Mar 21, 2016 3:22 pm 
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The sole purpose of the spring is to prevent the inner piston from moving inside the outer piston at the beginning of the compression stroke, the goal being to temporarily "lock" the two pistons together such that the outer piston passes the air inlet (pump arm slot) before the inner piston begins to move forward. (That's a lot of pistons for one sentence. Did any of that make sense?) Otherwise, the inner piston would just be pumping air back out the pump arm slot.
The spring is never intended to bottom out. If it's not clear in the diagram, the inner piston threads into the knurled adjustment nut and the pump arm link just like a normal FTP piston.
Once again, for all the effort, this may simply be case of a solution looking for a problem which doesn't exist!
Cheers.

(PS-Mr. rsterne, allow me a moment of gushiness: I've been on this forum for less than a month and immediately recognized your projects as the work of a epic, mad genius! Most impressive, keep them coming!)


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 Post subject: Re: two-stage piston?
PostPosted: Mon Mar 21, 2016 3:54 pm 
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OK, so the inner piston is just a way to get more stroke without changing the position of the end of the slot in the tube?.... That would work, if you had a longer pump linkage that would permit that.... but that would need a longer tube to mount it in as well.... At that point, just move the slot forward as well.... The tube from a 2100/2200 has 50% more swept volume, and the one from a 140/1400 is double that of a 13XX....

Bob

_________________
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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 Post subject: Re: two-stage piston?
PostPosted: Mon Mar 21, 2016 9:27 pm 
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Location: United States
Another could be a large/normal dia piston pushes air into a smaller dia tube on the extension stroke, then the normal direction pump stroke drives the smaller piston in the smaller tube.
Have also thought about a CO2 boost system where it's regular CO2 pressure normally feeding gas into the valve chamber in front of the pistons face, and makes normal CO2 power, but optionally you could extend the piston which the gas would fill up under CO2 pressure, then you pump it once to force all that additional gas into the valve. I suppose you could make it so just one pump gives you what you need, or two easier pumps, or however you want but a one pump would require less valving I spoze. Picture a 13xx but CO2 fed and a small 1/4" dia piston. Dunno where the tank would go pistol but that was just an example.
So plink and what not on CO2psi, then add a pump for hunting power. It may be easier to just use a normal pcp gun but at least for me CO2 is much easier to deal with. The problem is the pressure is too low so that's my thought process...


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 Post subject: Re: two-stage piston?
PostPosted: Mon Mar 21, 2016 10:05 pm 
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Compressing CO2 would just liquefy it and send the pressure off the charts.... Follow the 70*F (or any temperature line) line on this chart from left to right and you will see what I mean....

Image

Bob

_________________
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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 Post subject: Re: two-stage piston?
PostPosted: Tue Mar 22, 2016 11:43 pm 
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Posts: 1274
Location: United States
I thinking it'll stay a gas. I think it needs to be much colder to drop back to liquid, like a negative F# of some digit I forgot. Even if does try a room temp then maybe the pump will add enough heat to prevent it? You're the gas guy so I'll take your input over any other, but my current belief is you could pump gaseous co2 to 2k, maybe 3kpsi and it will remain gas. Possible? Or are you positive it cannot be done. Like how tanks of co2 argon mix stay gaseous.
If I had a pcp pump I'd try it myself, but alas no pump... Yet....
If say that's a total failure then how about use the co2 to charge the valve, then the small tubed pump only adds air?
These are just random ideals for the OP, I can guarantee I'll never make said gun.


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 Post subject: Re: two-stage piston?
PostPosted: Tue Mar 22, 2016 11:54 pm 
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Location: Coalmont BC
Add air (or Argon) and you could have the CO2 stay a gas.... Pure CO2 will follow that chart, pick a temperature, move from left to right, and you will see the phase change as you decrease the space it is filling (ie compress it).... Once it is a liquid (blue area), a small decrease in volume (compressing it) causes the pressure to skyrocket....

Bob

_________________
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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 Post subject: Re: two-stage piston?
PostPosted: Wed Mar 23, 2016 12:15 pm 
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Joined: Fri Feb 26, 2016 2:42 pm
Posts: 50
rsterne wrote:
OK, so the inner piston is just a way to get more stroke without changing the position of the end of the slot in the tube?.... That would work, if you had a longer pump linkage that would permit that.... but that would need a longer tube to mount it in as well.... At that point, just move the slot forward as well.... The tube from a 2100/2200 has 50% more swept volume, and the one from a 140/1400 is double that of a 13XX....

Bob


Correct, that was the idea! Basically trying to wring every last drop out of the existing 1322 format (or 2289 in my case) without it mutating into a whole new gun. My initial thought was to "simply" move the pivot point on the pump arm backward (toward the piston) to make up for the shorter piston, or to extend the linkage itself... well, as I'm sure you're aware there's nothing "simple" about either approach. I'll have to do some hands-on experimentation when I have time.

In related news: I looked up the ugly-as-sin FX Indy (referenced by HandyMan) and the much nicer-looking FX Independence and was quite impressed. They seem like the holy grail of PCP's: field "top-upable"™ via on-board pump. Bob, did you not design & build several of these years ago?!?


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 Post subject: Re: two-stage piston?
PostPosted: Sat Mar 26, 2016 10:56 am 
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Joined: Thu May 07, 2015 8:30 pm
Posts: 1190
Location: Eastern Townships
I've been thinking about that for a while, Arden. I'm currently working on an SSP 1377 pistol, and because of the grip frame screw hole in the 1377 tube, I found that a two-stage piston could be an interesting way of building additional stroke. Still a lot of work to do, but I'll post my results eventually. And thanks for sharing your drawing :) .


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 Post subject: Re: two-stage piston?
PostPosted: Sat Mar 26, 2016 2:49 pm 
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Location: Coalmont BC
I did the "Millennium Pumper", and it's prototype the .25 cal Disco Pumper Carbine.... both threads should be available here on Search....

Bob

_________________
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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 Post subject: Re: two-stage piston?
PostPosted: Thu Apr 14, 2016 11:17 pm 
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Joined: Thu May 07, 2015 8:30 pm
Posts: 1190
Location: Eastern Townships
The project is started. Made an ''outer'' piston out of brass and an ''inner'' piston out of stainless steel. Also found a suitable spring (0.451'' OD, 0.048'' wire). Still need to cut the O-ring grooves, find the correct lengths and make the base for the pivot pin. I will also have to design a new pump linkage. I haven't made any calculations yet, but I think I'll get around 4 to 5 cc of additional volume.


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