Modeling PCP Valve Lift and Dwell

[rsterne]

Try plotting a few known points on the graph and see where they show up....

DAQ .308 - 190 grams, 180 FPE - near the "heavy - slow" line
Hayabusa .257 - 115 grams, 130 FPE - right on the "typical" line
2560 PCP - 70 grams, 50 FPE - right on the "typical" line
Disco 9mm - 70 grams, 120 FPE - just above the "light - fast" line (This gun required conversion to a rear cocker and a very stiff spring to reach that power and was still hard to cock)
1750 CO2 detuned - 30 grams, 5 FPE - near the "heavy - slow" line (I couldn't get the hammer any lighter)
MRod .25 - 78 grams - on the "typical" line at 60 FPE
Disco .22 - 58 grams - on the "typical" line at 33 FPE

The latter two fall on the heavy side of "typical" at stock power.... IMO if you try and push the power over those numbers, you should try and fit a heavier hammer.... I'm guessing Motorhead's light MRod hammer is right on the "light-fast" curve....

Bob

[PakProtector]

Also plotted Scott's light hammer in my 20 cal. Power for me is not at 950, but a bit lower. Agrees well with Scott's numbers in 22 cal.
cheers,
Douglas

[rsterne]

Using FPE instead of pellet weight should work for all velocities IMO....

Bob

[Motorhead]

The latter two fall on the heavy side of "typical" at stock power.... IMO if you try and push the power over those numbers, you should try and fit a heavier hammer.... I'm guessing Motorhead's light MRod hammer is right on the "light-fast" curve....

Bob

Indeed the .22 cal right on the LIGHT / FAST line.
At 30ft lb energy & 27 gram hammer weight right on the line.

* Made another LW hammer last night for the .177 20 ft lb gun and lightest I could get it was 22 grams which by luck again rests right on the LIGHT / FAST line for that power level 

Will get it installed do some testing and share findings.

[PakProtector]

I can think of one way to deliver a variable hammer strike that is pretty much automatic. It requires a solenoid assist, and power to the solenoid is varied with input from a reservoir pressure sensor. Set the main hammer spring for mid-pressure peak velocity, and give no assist there. Deliver assist at high and low pressure ends of the string. The calibration ought to be variable, as it is run by a wee bit of carefully drawn, doped silicon... A near reg-perfect shot string ought to be possible.
cheers,
Douglas

Lloyd did something like this, though perhaps not exactly that way and I beleive Crosman applied it to the Rogue.

[rsterne]

During work on the .30 ca Disco Double last night I took some lift measurements.... 0.07" at the beginning and 0.13" at the end of 4% ES strings.... This is greater than I have measured before (but I've never done larger than .25 cal).... I think that lends support to the argument that the area under the lift/dwell curve is proportional to the FPE level of the gun.... and hence to the most recent Hammer Wt. vs FPE graphs above....

Bob

[Bill G]

This is part of a post that I made, from another thread.  Here I'm using 25.4 JSB's.  It isn't very efficient at this point, but it does give a good place to start.  The hammer in this instant is a stock Marauder hammer.  I think that the published weight of the hammer is 78 grams.  My hammer is actually 4 grams lighter than that, as I measured it.  The transfer tube and barrel port are .156.  I think that the stoke values can be an interesting comparison to your .30cal disco double.  I think that when we try to achieve an energy output that approaches that which is maximum for the valve design, we will see that the stroke ends up being excessive at the beginning of the string.  This is where the we can be killing the efficiency.  I see this as a clue that a larger port is needed.  The education that Bob has passed to me about the port being max at 75% of bore is extremely valuable.  When I model this same string but employ a .187 port, Efficiency goes up a bunch.  It indicates an efficiency that exceeds 1.0 in comparison to the .74 as it is now.     



vel                 stroke
947.00          .0772
954.00          .0792
960.00          .0814
963.00          .0838
971.00          .0864
969.00          .0893
963.00          .0924
960.00          .0959
954.00          .0996
943.00          .1038
932.00          .1084

Shot 11 is there to illustrate the degree of drop.  This is when the volume used per shot is equal to what is possible to flow through the valve at the existing psi.  Valve dwell ranges from 1.16ms to 1.173ms.  For me, at 950 fps, the pellet is going to be engaged with the bore for ~1.71ms.  So, as you can see, 1.73 ms is longer than the duration of the avg velocity, yet the velocity is below the 950 fpe I want.  The volume of air used is also equal to what is possible thus a very inefficient shot. the valve is open for as long or longer than the pellet is engaged with the bore.  the idea of the cam would have to be suited for a given tune.  If I wanted to shoot at 850 fps the stroke would be smaller and have a smaller graduation from shot to shot.  If you are so inclined, you could plot the stroke intervals and see that they are not quite linear.  The further along you go, the more steeply it climbs almost exponentially.

[rsterne]

I assume your stroke values are calculated, not measured.... However, I would agree, lift vs. pressure is not inversely proportional in a linear fashion.... My calculations show that at three pressures (100%, 75% and 50%) the lift values vary more like 50%, 66%, 100% in relative terms....

Bob

[Bill G]

This is part of a post that I made, from another thread.  Here I'm using 25.4 JSB's.  It isn't very efficient at this point, but it does give a good place to start.  The hammer in this instant is a stock Marauder hammer.  I think that the published weight of the hammer is 78 grams.  My hammer is actually 4 grams lighter than that, as I measured it.  The transfer tube and barrel port are .156.  I think that the stoke values can be an interesting comparison to your .30cal disco double.  I think that when we try to achieve an energy output that approaches that which is maximum for the valve design, we will see that the stroke ends up being excessive at the beginning of the string.  This is where the we can be killing the efficiency.  I see this as a clue that a larger port is needed.  The education that Bob has passed to me about the port being max at 75% of bore is extremely valuable.  When I model this same string but employ a .187 port, Efficiency goes up a bunch.  It indicates an efficiency that exceeds 1.0 in comparison to the .74 as it is now.  The below string info is based on .156 port.     



vel                 stroke
947.00          .0772
954.00          .0792
960.00          .0814
963.00          .0838
971.00          .0864
969.00          .0893
963.00          .0924
960.00          .0959
954.00          .0996
943.00          .1038
932.00          .1084

Shot 11 is there to illustrate the degree of drop.  This is when the volume used per shot is equal to what is possible to flow through the valve at the existing psi.  Valve dwell ranges from 1.16ms to 1.173ms.  For me, at 950 fps, the pellet is going to be engaged with the bore for ~1.71ms.  So, as you can see, 1.73 ms is longer than the duration of the avg velocity, yet the velocity is below the 950 fpe I want.  The volume of air used is also equal to what is possible thus a very inefficient shot. the valve is open for as long or longer than the pellet is engaged with the bore.  the idea of the cam would have to be suited for a given tune.  If I wanted to shoot at 850 fps the stroke would be smaller and have a smaller graduation from shot to shot.  If you are so inclined, you could plot the stroke intervals and see that they are not quite linear.  The further along you go, the more steeply it climbs almost exponentially.

[Bill G]

I don't know how that happened.

[Bill G]

Bob,  Your assumptions are correct.  And I agree with your comparison

[PakProtector]

I don't know how that happened.

don't worry to much Bill... Just keep in mind that the OE throat on the Marauder valves is about 3/16" which when stem area is taken away is right about the exhaust port size in the 25 cal at .140 diameter. I think .234" throat is enough for most pellet work, with slight preferecne to .250 if you want to shoot the domed E-J in the high 60 FPE range. I hope to report shortly that the upper limit regardless of other porting on a .250 throat is in the low 80 FPE range where efficiency takes a whopping big crash past about 78 FPE with 53 grain boolits...
cheers,
Douglas

[PakProtector]

Alright, slightly below the 'typical' line on the 'light/fast' side, the Marauder is now working comfortably at 86 FPE. Hammer mass is slightly reduced thanks to a set-screw striker to grant me some more travel.
cheers,
Douglas

[Motorhead]

Got the 22 gram hammer installed last night into the .177 call 20# M-rod.
Velocity as gun was set using a 27 gram hammer was @ 915 fps .... Changing nothing but the weight of hammer the 22 gram once installed was at @780 fps ( This with 10.3 AA pellets )
@ 1 2/3 more spring preload and velocity was back @ 910 fps.
** Shot cycle is noticeably faster, tho it already was much more so than the factory 78 gram weight hammer.

20 quick shots over the chrony

907 909 907 907 905 906 909 905 905 907
906 906 909 907 905 903 908 908 904 909

Nothing wrong with that !! being just out of the tin AA's

At power level rifles is at currently pretty crazy how quiet shot cycle is    No hammer bounce happening is pretty obvious with gun just sounding like "TicK" and it's done .... Nice !


So it can and does work ... a LW hammer low on the scale of "Light / Fast" that does support just how much excessive weight is in that factory M-rod hammer that while OK for a .25 gun, is overkill in .177 & .22 caliber.  ** And again THIS A REGUULATED rifle  operating at @ 1850#

Factory hammer 78 grams ... this one 22 grams ... WOW 28% the originals weight, OR 72% lighter 

[rsterne]

Nice to see additional supporting data, and congrats on the excellent results....

Bob

[willbird]

I can think of one way to deliver a variable hammer strike that is pretty much automatic. It requires a solenoid assist, and power to the solenoid is varied with input from a reservoir pressure sensor. Set the main hammer spring for mid-pressure peak velocity, and give no assist there. Deliver assist at high and low pressure ends of the string. The calibration ought to be variable, as it is run by a wee bit of carefully drawn, doped silicon... A near reg-perfect shot string ought to be possible.
cheers,
Douglas

Lloyd did something like this, though perhaps not exactly that way and I beleive Crosman applied it to the Rogue.

Hmm what about changing hammer weight on the fly ?? The same silicon bits could be coaxed to lift a golden BB or two either up into a pocket drilled into the air tube, OR drop them into a hole in the hammer...one google source said they weigh 5.1 grains each. Maybe a fairly mild magnet in the hammer to hold the BB or ball bearing into the pocket in the side of the hammer, and a far stronger one to pull it out when desired. This could be done manually, or even depending on the geometry with the rotating portions of some repeater pellet magazines. Drop the ball bearing(s) in early in the shot string, adding hammer mass and countering valve lock....lift them out mid string, return them later.

I have a "strong magnet" example here that will grab a crossman BB and lift it from 1/2" away.

If I did my maths right each BB would add roughly 7% to the 70 grain disco hammer ?

Bill

[Motorhead]

I can think of one way to deliver a variable hammer strike that is pretty much automatic. It requires a solenoid assist, and power to the solenoid is varied with input from a reservoir pressure sensor. Set the main hammer spring for mid-pressure peak velocity, and give no assist there. Deliver assist at high and low pressure ends of the string. The calibration ought to be variable, as it is run by a wee bit of carefully drawn, doped silicon... A near reg-perfect shot string ought to be possible.
cheers,
Douglas

Lloyd did something like this, though perhaps not exactly that way and I beleive Crosman applied it to the Rogue.

Hmm what about changing hammer weight on the fly ?? The same silicon bits could be coaxed to lift a golden BB or two either up into a pocket drilled into the air tube, OR drop them into a hole in the hammer...one google source said they weigh 5.1 grains each. Maybe a fairly mild magnet in the hammer to hold the BB or ball bearing into the pocket in the side of the hammer, and a far stronger one to pull it out when desired. This could be done manually, or even depending on the geometry with the rotating portions of some repeater pellet magazines. Drop the ball bearing(s) in early in the shot string, adding hammer mass and countering valve lock....lift them out mid string, return them later.

I have a "strong magnet" example here that will grab a crossman BB and lift it from 1/2" away.

Bill

Sadly having a hammer with floating weight inside is the concept used in "Dead Blow Hammers"
When hammer is sent into motion the SHOT inside is sent rearward under the acceleration of the hammer. When hammer bottoms out all the loose shot comes crashing forward making a secondary impact that dampens the rebound or bounce.

In an air rifles hammer / valve mechanics NOT what we want to have happening 

Rebound bounce of a hammer striking a solid surface and Rebound caused by opposing springs are different animals  So don't get confused because of reading into "BOUNCE" as a singular meaning we use in these talks 

[willbird]

You might have mis understood my intent :-).

The hole for the ball bearing would go in at 90 degrees to the motion of the hammer. Straight into the side of the Outer diameter...the linear motion would not be encouraging it to move or slop around :-).

Sometimes a picture can be worth 1000 words



I had pictured a load/unload hole for the ball bearings when the gun is cocked. A magnet in the hammer could hold the ball in the hole....a stronger magnet slid into position over the hole (or maybe an electromagnet) could pull the ball from it's hole and hold it out of the hammer.

A .177 BB in a .180 hole is not going to be doing much "dead blow" hammering :-).

Bill

[willbird]

But I may have an elemental mis understanding going on here, looked at the charts again.

If I added weight to the hammer as I am thinking

The situation would be Mass A at Velocity B if the shot (say 2 of them for example) were "unloaded"

With them loaded into the hammer we would have Mass A + 10.2 grain at Velocity B - some amount.

The impact energy of the hammer would not change ? But would the added weight change the dwell ?

Hmmmm

:-).

[rsterne]

Yes, the added weight would change the dwell by the square root of the change in weight.... a 10% change in weight would cause about a 5% change in dwell.... The lift would NOT change....

Bob