Spring piston HDD?

Spring piston HDD?

Joined: March 1st, 2002, 12:22 am

June 14th, 2012, 5:08 am #1


I know that spring guns and PCP/pumpers deliver air to the pellet in different ways. Seems to me though, they both have large chunks of metal that move, and secondary movement too.

So the pcp/msp crowd is working toward arresting the hammer bounce. Why couldnt you apply some of that HDD tech to reduce piston bounce in springers?

OR not.... much greater mechanical energies internally than my 397 or 2250 has..... 


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the Lunatic Fringe of American Airgunning
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Joined: November 24th, 2007, 12:21 pm

June 14th, 2012, 11:11 am #2

there will be no "bounce". The piston will come to rest gently against the end of the compression tube, slowed by the compression of air ahead of it. The pellet provides just enough resistance to allow that to happen. For example, we know we should never dry fire a springer because of the possibility of damage to the internals -ie. no resistance, and the piston will slam against the front of the compression tube.

An HDD is a device that helps to preserve air or gas in a reservoir, by preventing the hammer from bouncing back from the valve stem on its initial strike, recompressing the hammer spring, and then opening the valve a second time. Even though the duration of the second (or third) strike of the valve stem is very short compared to the initial opening, some gas is still wasted. There is no need for that technology in a springer. A springer makes its own gas. As do we all.....

Maybe someone else can do a better job of explaining this.

Regards,
Michael Chavka

Success is not an entitlement.
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Joined: October 11th, 2011, 6:25 am

June 14th, 2012, 2:03 pm #3

how does a lighter or heavier pellet affect the piston then.
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Joined: September 25th, 2006, 2:19 pm

June 14th, 2012, 2:57 pm #4

there will be no "bounce". The piston will come to rest gently against the end of the compression tube, slowed by the compression of air ahead of it. The pellet provides just enough resistance to allow that to happen. For example, we know we should never dry fire a springer because of the possibility of damage to the internals -ie. no resistance, and the piston will slam against the front of the compression tube.

An HDD is a device that helps to preserve air or gas in a reservoir, by preventing the hammer from bouncing back from the valve stem on its initial strike, recompressing the hammer spring, and then opening the valve a second time. Even though the duration of the second (or third) strike of the valve stem is very short compared to the initial opening, some gas is still wasted. There is no need for that technology in a springer. A springer makes its own gas. As do we all.....

Maybe someone else can do a better job of explaining this.

Regards,
Michael Chavka

Success is not an entitlement.






Last edited by oo7fuzz on June 14th, 2012, 2:59 pm, edited 1 time in total.
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Joined: September 25th, 2006, 2:19 pm

June 14th, 2012, 2:58 pm #5

there will be no "bounce". The piston will come to rest gently against the end of the compression tube, slowed by the compression of air ahead of it. The pellet provides just enough resistance to allow that to happen. For example, we know we should never dry fire a springer because of the possibility of damage to the internals -ie. no resistance, and the piston will slam against the front of the compression tube.

An HDD is a device that helps to preserve air or gas in a reservoir, by preventing the hammer from bouncing back from the valve stem on its initial strike, recompressing the hammer spring, and then opening the valve a second time. Even though the duration of the second (or third) strike of the valve stem is very short compared to the initial opening, some gas is still wasted. There is no need for that technology in a springer. A springer makes its own gas. As do we all.....

Maybe someone else can do a better job of explaining this.

Regards,
Michael Chavka

Success is not an entitlement.
Here is what I think.

Considering the design of the springer where the piston moves forward at great speed, the sizing of the transfer port will never or can never pass the instantly compressed volume so as not to create resistance.

There is great heat produced in dryfiring. So that says that the only way said heat can be produced is through the compression process.

I would be willing to wager that, in any springer, in good mechanical condition, it would be impossible for the piston to slam because of zero resistance.

Could it be that we percieve the piston slamming because the dry fire cycle is, IMHO, devoid of piston rebound? Or very little at best.
Here the opperator experiences the shot cycle quite differently than when the piston rebounds durring the cycle which launched a pellet.

So do we think the piston slams? Or do we know?

The heat generated durring the dryfire says no.

But we could and should discuss what damage comes to the piston because of dry fire.

If indeed there is little or no rebound durring dryfire, could the lack of rebound be the culprit?

Because in any rebounding instance, air has to expand to drive the piston rearward. And the volume involved is a good chunk of the charge. Then might the expanding air also cool the piston face?

Another point which might be discussed is wheather the entire firing cycle of the springer adiabatic? Or is only the compression portion of the cycle adiabatic?

My view is that only the compression of the air is of adiabatic nature. Where the moment the piston stops forward progress, the shot charge begins to loose heat because of expansion and heat transfer to gunframe.





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Joined: February 17th, 2010, 3:30 am

June 14th, 2012, 3:19 pm #6

how does a lighter or heavier pellet affect the piston then.
Lighter pellets slow the piston less because they start to move sooner.... Less heat and pressure are produced, lessening any chance of combustion.... The piston has more chance of hitting the end of the compression chamber.... If the pellet is light enough the condition is similar to dry firing.... which can result in damage to the seal....

Heavier pellets tend to stay in place (inertia) requiring more pressure before starting to move.... This additional pressure also increases the temperature, increasing the chance of combustion of any lube film present on the chamber wall (per Cardew).... The piston will have a larger air cushion in front of it, and may, in fact, reverse travel and bounce before reversing again and coming to rest against the end of the chamber.... If the pellet is heavy enough for the amount of lube present, the chance of "dieseling" (severe combustion) is increased, which may be the reason for reports of spring damage from firing heavy pellets....

The size of the transfer port also enters into this equation, with larger ports favouring heavier pellets.... The weight of the piston is also a very important factor, with heavier pistons, again, favouring heavier pellets.... Increasing the spring force for a given swept volume will, generally, favour heavier pellets as well.... Even seating depth of the pellet has an effect, and generally when pellets are seated deeper, they act as if they are lighter (in terms of piston braking).... There is more volume in the transfer port / breech, so the pressure peak is less, and the velocity tends to drop....

Bob
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Joined: March 1st, 2002, 12:22 am

June 14th, 2012, 3:24 pm #7

there will be no "bounce". The piston will come to rest gently against the end of the compression tube, slowed by the compression of air ahead of it. The pellet provides just enough resistance to allow that to happen. For example, we know we should never dry fire a springer because of the possibility of damage to the internals -ie. no resistance, and the piston will slam against the front of the compression tube.

An HDD is a device that helps to preserve air or gas in a reservoir, by preventing the hammer from bouncing back from the valve stem on its initial strike, recompressing the hammer spring, and then opening the valve a second time. Even though the duration of the second (or third) strike of the valve stem is very short compared to the initial opening, some gas is still wasted. There is no need for that technology in a springer. A springer makes its own gas. As do we all.....

Maybe someone else can do a better job of explaining this.

Regards,
Michael Chavka

Success is not an entitlement.
Guess what I was thinking was that the 39x HDD uses "frictional interference" to stop a second strike, but why could you put a similar gizmo on the piston to stop the bounce.....

And I think the operative phrase there in Michael's explaination is "tuned correctly".  But for the uncorrectly tuend guns, maybe such a device would help?

Dunno, thinking out loud on a non-linear tangent from thinking thru the origins of the heavy tar......

 


dr_subsonic's pneumatic research lab

the Lunatic Fringe of American Airgunning
Southwest Montana's headquarters for Airgunning Supremacy
Proud Sponsor of team_subsonic
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Joined: November 28th, 2002, 6:26 pm

June 14th, 2012, 3:40 pm #8

I know that spring guns and PCP/pumpers deliver air to the pellet in different ways. Seems to me though, they both have large chunks of metal that move, and secondary movement too.

So the pcp/msp crowd is working toward arresting the hammer bounce. Why couldnt you apply some of that HDD tech to reduce piston bounce in springers?

OR not.... much greater mechanical energies internally than my 397 or 2250 has..... 


dr_subsonic's pneumatic research lab

the Lunatic Fringe of American Airgunning
Southwest Montana's headquarters for Airgunning Supremacy
Proud Sponsor of team_subsonic
http://www.network54.com/Forum/79537/th ... usly%29---

Steve
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Joined: March 1st, 2002, 12:22 am

June 14th, 2012, 4:16 pm #9


hehehe

ok, so piston arrestment is probably a bad thing. So we are back to Michael's well tuned springer.

thanx guys!

back to the lab.......


dr_subsonic's pneumatic research lab

the Lunatic Fringe of American Airgunning
Southwest Montana's headquarters for Airgunning Supremacy
Proud Sponsor of team_subsonic
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Joined: November 28th, 2002, 6:26 pm

June 14th, 2012, 4:45 pm #10

there will be no "bounce". The piston will come to rest gently against the end of the compression tube, slowed by the compression of air ahead of it. The pellet provides just enough resistance to allow that to happen. For example, we know we should never dry fire a springer because of the possibility of damage to the internals -ie. no resistance, and the piston will slam against the front of the compression tube.

An HDD is a device that helps to preserve air or gas in a reservoir, by preventing the hammer from bouncing back from the valve stem on its initial strike, recompressing the hammer spring, and then opening the valve a second time. Even though the duration of the second (or third) strike of the valve stem is very short compared to the initial opening, some gas is still wasted. There is no need for that technology in a springer. A springer makes its own gas. As do we all.....

Maybe someone else can do a better job of explaining this.

Regards,
Michael Chavka

Success is not an entitlement.
To understand why, just a few facts are needed.

1. The pressure required to get the pellet to begin to move has been measured at several 100 psi.
2. The compression ratio required to generate 100 psi is 5 or 6:1, meaning the piston must complete at least 80% of its stroke while the pellet remains stationary.
3. Taking typical mainspring energies, stroke lengths, and piston masses into account, this means that at the moment of first pellet motion, the piston is less than 1/2" from the end of the tube and travelling ~1000 inches per second, meaning that the piston must stop in ~500us.
4. It typically takes the pellet several times that long to traverse the barrel, and in 500us can only traverse a couple of inches.

Consequently, it's impossible for the bore to accept the volume of the compressed charge in the time available. The only place the air can go is back into the compression tube - by literally forcing the piston to bounce to create a temporary reservoir.

Steve
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