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Thread: Valve Stem dwell

  1. #1
    Senior Member remtom1200's Avatar
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    Valve Stem dwell

    I do a fair amount of quick reading posts on the air gun forums, and after seeing a new leveled shot string emerging from higher flowing modded valves and I need to ask. Bob is your newly tested (and confirmed) type of adjustable hammer benefiting from increased speed and lower dwell using more momentum, and later impact to the valve stem? Is the valve block the stopper, and the adjustable stem striker the initiator excited or retarded by adjustment? I'm pretty sure this is what its turning out to be and could be opening up a new can of worms in pcp efficiency/ fpe
    For every mile of road, there's two miles of ditch

  2. #2
    Moderator rsterne's Avatar
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    I wish I understood your post better.... So far, I'm not convinced that the "bumper" on the back of the valve in my .25 cal Disco is doing any good.... The adjustable hammer travel / valve lift is likewise questionable.... From what I can gather so far, having the most hammer travel you can get allows the least hammer spring preload, and hence easier cocking.... It also allows you to vent more pressure because of the increased hammer energy/momentum....

    The added hammer travel is a good thing if you need more hammer strike.... Once the hammer strikes the bumper on the back of the valve, however, it prevents any further self-regulation, and the velocity of the shot string immediately plummets.... This does NOT appear to be what is happening with the bstaley O-ring mod.... so I'm left to assume that the resilience of the O-rings is working to "round off" the top of the shot string.... However, I've never tried it, so that's just a guess....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  3. #3
    Senior Member remtom1200's Avatar
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    Momentum being the subject of my thoughts.In perspective if I increase the preload of my Qb hammer it sends the hammer face at a higher rate of speed and the time it takes to hit the stem is reduced from the previous setting but allows a longer duration of the valve opening because of higher momentum. If the impact point of the hammer was later than the actual face(inverted by adjustment) the momentum would be the same speed, but at a later timing of the momentum nearing the return point of the hammer thus lowering dwell time, and reducing hammer bounce because the extra energy is used at the end of the travel to depress the valve stem. I Believe I see the pattern here and it makes sense. So the magic number is the adjustment timing of stem strike combined with the energy, and duration of the momentum of the hammer. The variables are hammer weight, travel distance, impact point, and fill pressure. Looks like worms to me and thanks to Eric at scopes and ammo my new Chrony is gonna be delivered Monday so I can further research this theory.
    For every mile of road, there's two miles of ditch

  4. #4
    Senior Member SeanMP's Avatar
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    The subject of Force, Energy and Momentum was chewed on pretty hard over at the GTA not so long ago, by some pretty heavy hitters. I'm not sure that anything was resolved in the discussion. I'm seeing a couple of points here.
    Momentum has a time component. The longer an object accelerates the more momentum it has. Thus if the striker is screwed in, delaying the strike, the hammer should have more momentum at impact.

    Increasing the preload on the spring seems to switch the collision from an elastic to an inelastic collision(bug bounce off windshield or bug splatter travels with windshield). Also the force of the spring is directly acting on the valve stem so now your not dealing with a simple collision.

    I think there is something here but in order to explore it there would have to be no spring preload at the time impact. Yes momentum should be able to directly correlate to dwell timing because momentum is the ability to push through. Then you could see a perfect (nearly) elastic collision and Conservation of Momentum transferred to the valve stem.
    Last edited by SeanMP; Dec 30 2012 at 03:54 AM.
    Sean

  5. #5
    Moderator rsterne's Avatar
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    Both Momentum and Energy increase if you increase the Average Spring Force or Hammer Travel.... However, changing the Hammer Weight changes the Momentum but NOT the Energy.... In terms of increasing valve dwell, all three changes increase dwell.... Increasing the dwell also increases the lift (unless it is artifically limited by the hammer hitting a bumper or buffer).... However, once the lift is greater than 1/4 the valve throat diameter, the flow RATE no longer increases, only the dwell.... Longer dwell releases more air at any given pressure....

    I happen to believe that Momentum is more important than Energy in opening the valve because Energy can change forms (to heat and sound) but Momentum can't.... I have graphed both Hammer Energy and Momentum against the valve closing force they can overcome, and the Momentum shows a more linear (and consistent) relationship.... Other than a very small amount of "buzz" (rapid and inconsequential vibration) in the valve stem on initial contact with the hammer, once the hammer hits the valve they move as a combined pair through the entire valve travel until the valve returns to the seat.... At that point, the hammer (it's momentum reversed from the valve closing force) leaves contact with the stem and bounces against (or off of) the spring.... It may or may not reopen the valve.... We don't want it to, because any air released from a second valve opening is totally wasted.... the pellet has long since left the building....

    I am of the opinion that without the interaction of the hammer with a third element (a bumper or buffer), a given momentum will open the valve a given amount (lift and dwell) against a given pressure.... I think it matters little if that momentum is achieved through the mass of the hammer, it's travel, or the spring force.... It is the effect of those three elements on the Momentum that governs what the valve does.... Having said that, IF the valve opens faster but less (eg. a light hammer moving fast) it may provide a slightly greater push in the first part of the pulse and yet a smaller total pulse.... That should increase the efficiency.... I think that's where the idea of the bumper/buffer comes in to limit the total lift.... but as yet haven't found the key....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  6. #6
    Senior Member SeanMP's Avatar
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    I'm still pondering the idea of throwing the hammer at the valve stem. Meaning the spring is at the end of it's travel just as the hammer contacts the stem....no idea how this is going to work yet but I think there are advantages.
    In a pure elastic collision the hammer should stop, transferring it's momentum to the valve stem. The valve should then close crisply owing to the absence of extra mass from the hammer and spring force. As well the tiny gap should dampen double strikes

    I know this will require a stout little spring. But I was thinking of preloading the bolt with a spring acting in the opposite direction to reduce the perceived cocking force.
    Sean

  7. #7
    Moderator rsterne's Avatar
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    I don't follow how you think the collision between the hammer and the valve stem is (or could possibly be) elastic?.... The valve stem itself is very light, but the force holding it closed is very high.... Even when the valve is open, the closing force is much greater than just the spring, the air pressure is still acting on the stem diameter.... If you have a seat diameter of 1/4" and a stem of 1/8", and 10 lbs. static spring force, the values at 3000 psi are:

    valve closed (0.25x0.25x3.14)/4 x 3000 + 10 = 157 lbs.
    valve open (0.125x0.125x3.14)/4 x 3000 + 10 = 47 lbs. (plus aero drag past the head of the poppet)

    The way I figure it (and Steve in NC agrees), when the hammer contacts the valve stem, it now has those deceleration forces at play.... If they were only acting on the mass of the valve stem, the valve might open, but it would shut instantly.... While there may be a TINY amount of relative movement initially between the valve stem and the hammer, basically the hammer momentum becomes a combined momentum (of the hammer and stem weight combined) acted on by those forces (minus any residual hammer spring preload).... The hammer/valve combination is braked to a halt in a distance of less than 0.150" (maybe a bit more at low pressure, but much less at high pressure), the direction of the pair reverses, and the valve closes in about (typically) 1-2 millisec of total dwell time.... The poppet stops, but the hammer now has significant momentum in the opposite direction, which will be braked to a stop and reversed by the hammer spring.... The period of that resonance is much larger, on the order of 15 millisec (depending on mass and spring rate)....

    Unless you are planning on anchoring the hammer spring at the back end, it's own momentum (1/3rd of it's mass is considered adding mass to the hammer) will keep it in contact with the hammer, even with zero preload or no clearance.... True, if will probably start oscillating wildly within it's own length, at the primary frequency and several harmonics.... and the fact it isn't in solid contact with the hammer may well damp out or even prevent hammer bounce to some extent.... Some of my designs have zero preload now, where you pull the bolt back a small amount before the hammer contacts the spring.... I do believe that may help prevent the harmful effects of hammer bounce, but have no concrete proof as yet....

    The energy absorbing bumper on the back of the valve I'm using in my .25 cal Disco certainly is capable of stopping the hammer.... Once the hammer hits it, the valve dwell (and the velocity) drops like a stone.... If the collision was elastic, I would not expect to see that effect until the valve lift became nearly zero, and then (and only then) would the velocity drop.... It doesn't, the decline in velocity as the lift is limited is nearly linear.... dropping to zero only when the recessed face of the hammer is no longer capable of hitting the valve stem.... I think that is pretty good evidence that the valve stem and hammer are moving together in a (nearly perfectly) inelastic collision....

    If you COULD achieve an elastic collision, and installed a bumper to stop the hammer (after it hit the valve stem), you could still change the valve lift and dwell through how hard the hammer hit the stem (ie changing its momentum) before it hit the bumper.... Actually, I guess it would never hit the bumper, would it?.... I don't think you will be able to achieve that....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  8. #8
    Senior Member SeanMP's Avatar
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    I believe you are correct that an elastic collision would be difficult. I'm thinking a solid hammer stop. And I agree it's going to have to be hit really really hard to have the stem carry on under it's own momentum given the forces acting against it.
    Sean

  9. #9
    Moderator rsterne's Avatar
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    My hammer in the .25 Disco, when the face is recessed far enough, has a "solid hammer stop" consisting of an energy absorbing elastomer on the back of the valve.... The valve lift at 3000 psi is about 0.095", and as soon as you recess the hammer face far enough that the stem is only opened physically that far (assuming constant contact with the hammer), the velocity starts to drop.... Take a look at this graph....



    I measured the lift (dotted lines, right hand scale) by using an O-ring on the spring guide, which stopped sliding on the guide when the hammer stopped moving forward.... After the shot I measured the gap between the O-ring and the RVA adjusting screw, and that was the amount the valve opened (assuming the valve didn't keep going after the hammer stopped).... The red lines are with the hammer travel set so that it isn't hitting the bumper, but varying the preload.... The more preload, the more the valve opens, and the greater the velocity.... although it is starting to approach a plateau at about 1050 fps.... The blue lines are with the preload set at 1 turn out from coil bind, and the hammer travel before stem contact is varied by recessing the center of the hammer face.... As the face is recessed more, the travel increases, but the maximum valve lift possible before the hammer hits the energy absorbing bumper on the back of the valve decreases.... The hammer starts to contact the bumper at just over 2 turns.... Up to that point, the lift is 0.095" (governed solely by the dynamics of the hammer and valve), and the velocity is basically constant at 1000 fps.... As soon as the amount the valve can lift becomes limited by the hammer hitting the bumper (at about 0.090"?) the velocity starts to drop quickly.... At 5 turns, the valve can't open, and as you can see, the velocity is headed to zero at about that point (or before)....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  10. #10
    Senior Member remtom1200's Avatar
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    I sensing opening dwell and closing dwell factors now
    For every mile of road, there's two miles of ditch

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