Misconceptions About Valve Lift

[rsterne]

Almost every week I get a PM asking about increasing (or decreasing) valve lift to change the performance of a PCP.... People seem to have the idea that how far the striker/hammer can physically open the valve before hitting the back of the valve is the lift, and 90% of the time that is NOT the case.... Only with the Korean guns where the power wheel adjusts a buffer on the back of the hammer, or a gun running a bstaley O-ring buffer, does the hammer ever hit anything except just the end of the valve stem, unless the air pressure is very low (compared to the hammer strike).... Some CO2 guns, such at the QB7X or XS-60c, have only about 1/8" of valve stem sticking out behind the back of the valve (or rear block), and when running CO2, and the low pressures involved on a cold day or when nearly empty, then the hammer does hit that stop, to prevent wasting even more CO2.... but in a PROPERLY tuned PCP, the valve simply doesn't need to open that far.... Here is a device I made to measure the valve lift in a PCP....



It is a piece of 1/8" aluminum rod, with a nut on the front end captured between the valve spring and the hammer, that travels with the hammer.... On the back end is an O-ring that is snug on the rod, and travels with it.... When installed in a Disco running a Challenger style RVA, it projects through the center hole in the adjusting screw like this....



To determine the lift, you simply push the O-ring up against the back of the adjusting screw, cock the gun, and fire it.... The distance between the O-ring and the end of the screw is how far the valve opened.... In the photo, you can see it is less than 1/16".... This is typical of a PCP that is operating efficiently.... Incidently, if you measure the distance between the O-ring and the screw when the gun is cocked, you will also know the hammer stroke/travel....

Over the last couple of years, I have made a lot of lift measurements on many different PCPs, and the lift that is required is related to the diameter of the valve throat.... This makes sense, because a poppet valve has maximum flow when the valve is clear of the seat by 1/4 of the throat diameter.... Beyond that, the flow RATE cannot increase, because the area of the throat is smaller than the "curtain area" between the valve poppet and the seat, measured around the perimeter of the valve.... Here is a typical graph, showing the actual valve lift in my recent .30 cal Grizzly regulated PCP....



You can plot a velocity curve like this for any pneumatic airgun (PCP, CO2, or pumper), at any pressure, and provided you can hit the valve hard enough with the hammer, you will get similar results for the velocity.... You will note that adding hammer spring preload past a certain point does NOT increase the velocity further, all it does is waste air.... This is because eventually, the valve is open after the pellet has left the barrel, and that is occurring on the flat part of the plateau on the left side of the graph.... Note that where the plateau starts is slightly different with different pellets.... as the weight increases, you need a bit more preload.... When the valve closes when the pellet is about half way down the barrel, you have in fact only lost about 3% of the velocity in most PCPs, so that point is where the velocity just starts to decrease.... I call this the "knee" of the velocity curve, and in the example above, it is at about 2 turns in (from zero preload) with the 45-50 gr. pellets, and 3 turns in with the 70 gr. bullets....

Now, look at the measured lift at those points.... It is between 0.080"-0.095".... The valve in the Grizzly has a throat diameter of 0.266", so that means the lift is about 1/3 of the throat diameter.... Opening it further than that only wastes air.... This is very consistent over all the PCPs I have checked, regardless of pressure, and the maximum lift required is always between 1/4-1/2 of the throat diameter for any usable tune.... In a detuned gun, it can be even less.... In an unregulated PCP, tuned for decent efficiency, the lift is typically about 1/4 the throat diameter at the beginning of the shot string (high pressure) and about 1/2 the diameter at the end (low pressure).... Therefore, you really will never use lift greater than about 1/2 the throat diameter, regardless of the pressure you tune for, if you have a reasonably efficient tune.... The only time the valve will open more than 1/2 the throat diameter is when you have the gun set up to waste air.... Therefore, allowing for more travel than that is basically useless.... I like to make sure that the hammer doesn't impact the valve, just so that I can be 100% sure that the lift is behaving in the self-regulating fashion we want from our PCPs, so in an unregulated PCP I allow about 75% of the throat diameter to insure such contact never occurs.... However, in every case, if I measure the lift, it turns out I allowed for way more lift than was actually needed....

Increasing the distance the valve can open can actually waste air.... When I got my FD-PCP, I noticed that the valve could only open about 0.10", which I thought was marginal (that is about 1/2 the throat diameter).... so I shaved 0.050" off the backing block, just to make sure the hammer didn't hit it.... The result was that I lost 1 shot and ended up with a shot string that dove more quickly after the peak velocity.... This could only be because the hammer was hitting the block once the pressure dropped below about 1100 psi, and preventing extra lift below that point.... When I machined off the back of the block, and the pressure was below 1100 psi, the valve opened more than 0.100", and the gun used more air, even though the velocity of those shots didn't increase.... The result was that I lost a shot....

I hope that this post will give you a better understanding of how much maximum lift you need to allow for in a PCP.... and how much lift is actually occurring when the gun is tuned efficiently....

Bob

[Frank_Zuccarini]

Thank you for taking the time to share your wealth of knowledge.

Frank

[hutnicks]

You bring to mind something tangential that occasionally crosses my mind.

With regards to hammers hitting the valve body and hammer bounce. Has anyone ever attempted a floating pin arrangement in an airgun? Would the extra mass required to actuate an internal pin make the action unusable?

[rsterne]

Not sure what you mean by a floating pin, there are a couple of anti-bounce hammers that use an internal weight.... no idea how well they work.... I would never design the hammer to hit the valve body on purpose, without a buffer in between to prevent damage.... Besides, as soon as you provide a rigid stop, the velocity will start declining from that point as the pressure drops, the concept of self-regulation goes out the window.... That is one of the drawbacks to the Korean hammer stop, when you engage it to reduce the power, you get a declining shot string.... Those setups could be used in a regulated PCP, however, where the pressure is a constant....

Bob

[hutnicks]

Not sure what you mean by a floating pin, there are a couple of anti-bounce hammers that use an internal weight.... no idea how well they work.... I would never design the hammer to hit the valve body on purpose, without a buffer in between to prevent damage.... Besides, as soon as you provide a rigid stop, the velocity will start declining from that point as the pressure drops, the concept of self-regulation goes out the window.... That is one of the drawbacks to the Korean hammer stop, when you engage it to reduce the power, you get a declining shot string.... Those setups could be used in a regulated PCP, however, where the pressure is a constant....

Bob

Just the hammer striking a spring loaded pin in a separate enclosure which does not contact the valve stem until the hammer strike hits it with enough inertia to drive it forward into contact. The pin then returns on the spring to a no contact state. It is how a lot of PB pistols manage a safe carry state with hammer down and a round chambered.

[rsterne]

Ahhhh, got it.... If the spring is heavy enough to prevent the hammer from opening the valve a second time, it would take a lot out of the first hammer strike.... making a heavier hammer and spring necessary.... While it might work to prevent the valve reopening, the added length and complexity may not be worth it.... In addition, the gun would end up being harder to cock, because the hammer now has extra work to do.... The Challenger has a similar setup, but nothing to do with safety, it has to do with looking after the extra room between the valve and trigger compared to the Disco.... I don't think the "transfer pin" is spring loaded, or if so, it is a very light spring....

Bob

[hutnicks]

Ahhhh, got it.... If the spring is heavy enough to prevent the hammer from opening the valve a second time, it would take a lot out of the first hammer strike.... making a heavier hammer and spring necessary.... While it might work to prevent the valve reopening, the added length and complexity may not be worth it.... In addition, the gun would end up being harder to cock, because the hammer now has extra work to do.... The Challenger has a similar setup, but nothing to do with safety, it has to do with looking after the extra room between the valve and trigger compared to the Disco.... I don't think the "transfer pin" is spring loaded, or if so, it is a very light spring....

Bob

Almost. When the hammer is sitting on the pin the pin still does not contact the valve stem there is a gap. Only the inertia imparted by a hammer strike gives the pin enough momentum to strike the valve stem. So the spring in the block housing the inertial pin need have very little tension at all. A ballpoint pen spring is more than enough as all it has to do is retract to the rear.
The physics here is simply how much hammer power do you need to give the pin enough inertia to hit the valve pin with enough force.
The charm of that design is there is no way you could get hammer bounce.

[rsterne]

The charm of that design is there is no way you could get hammer bounce.

You would have to prove that to me.... The valve still closes with a lot of force, pushing on the pin, which pushes back the hammer, which then compresses it's spring and then the hammer comes forward again.... A light pin with a light spring isn't going to prevent it from reopening the valve, the way I see it....

I've never had a PB pistol, but my guess is that the hammer in it's uncocked position sits against a second spring pushing backwards to prevent it from hitting the pin when being shaken or dropped.... or that it has NO preload on the hammer spring when uncocked.... Hammer bounce doesn't matter when the shell is being fired by a primer, it only fires once....

Bob

[oldpro]

 rsterne is dead on with his analysis of valve dwell and opening but thats no surprise Ive read all his posts. I have used his data to help develop several products as well as other data I dig up here and elsewhere. It sure is great to have people with solid data not guesses to aid us in our endeavors to further air rifle advancement. I have much appreciation. If you can adjust both dwell and lift and eliminate bounce there is no doubt your set up will be capable of very good strings.

[hutnicks]

The charm of that design is there is no way you could get hammer bounce.

You would have to prove that to me.... The valve still closes with a lot of force, pushing on the pin, which pushes back the hammer, which then compresses it's spring and then the hammer comes forward again.... A light pin with a light spring isn't going to prevent it from reopening the valve, the way I see it....

I've never had a PB pistol, but my guess is that the hammer in it's uncocked position sits against a second spring pushing backwards to prevent it from hitting the pin when being shaken or dropped.... or that it has NO preload on the hammer spring when uncocked.... Hammer bounce doesn't matter when the shell is being fired by a primer, it only fires once....

Bob

As there is no direct contact between the striker pin and the valve stem, once fired, the striker pin retracts and the valve stem rebounds with nothing to contact.

[PigButtons]

rsterne, thanks for the post. Very well explained and very intuitive explanation of the physics.

Question, In the throat diameter calculation, is it purely the throat diameter? Or is it throat diameter minus valve stem diameter?

As far as hammer bounce: The valve is going to bounce when closing no matter what you do. In race car engines they use multiple valve springs to try and cancel out some of the inherent bounce, but alas, there is always some. I don't have a clue whether this is more or less significant than the hammer bounce.
 
Bounce 2: The Theoben hammer has a mini hammer inside that is spring loaded. If I remember correctly the documentation states that it is to increase dwell time by adding a slight "second strike" an instant after the primary hammer stops moving forward. I don't know if it really works or not, but it is an attempt to increase dwell without needing more lift. And then the question becomes, Is more dwell wasting air or adding power?

[rsterne]

hutnicks.... one question.... where does the energy from the valve closing go?.... I say it launches the hammer back to compress the hammer spring....

and then what happens to that energy?.... If the hammer can overcome the gap between the pin and valve once, why can't it do it again?....

This topic is completely off course, unfortunately....

Technically it is the throat area minus the stem area, but practical applications seem to ignore the stem area.... and so do I when I'm talking valve lift.... I just use the throat diameter for my comparisions.... The flow rate probably maxes out at a fraction less lift than 1/4 the throat diameter....

Bob

[QVTom]

The charm of that design is there is no way you could get hammer bounce.

You would have to prove that to me.... The valve still closes with a lot of force, pushing on the pin, which pushes back the hammer, which then compresses it's spring and then the hammer comes forward again.... A light pin with a light spring isn't going to prevent it from reopening the valve, the way I see it....

I've never had a PB pistol, but my guess is that the hammer in it's uncocked position sits against a second spring pushing backwards to prevent it from hitting the pin when being shaken or dropped.... or that it has NO preload on the hammer spring when uncocked.... Hammer bounce doesn't matter when the shell is being fired by a primer, it only fires once....

Bob

Unless I'm missing something and I may, isn't the pin becoming the hammer (excuse me, the striker) in this scenario? I'd think the pin would have to be of sufficient mass to impart enough energy into the valve stem to open it.  Also, a primer doesn't care how many times it has been hit after the initial strike.

Tom

[ezman604]

Bob, thanks for taking the time to give us yet another gem of wisdom!!!

It certainly helps take more of the mystery out of the inner workings of our airguns. I have edited out some posts that were not relevant in order to keep this thread open and informative.
If you have good info to add, please do.
Thanks again Bob!!!!

[rsterne]

No idea what happened while I was out Grocery shopping, don't care.... it's all good now!....

Bob

[Bill G]

Oldpro at Wicked Air rifles might have the best no bounce solution that I have ever seen.  Definately eligant with the simplicity and not adding a bunch of extra mass to the mix.  Perhaps he can sound off with direction or advice.