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PostPosted: Thu Jun 20, 2013 1:12 pm 
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I have been thinking about regulated guns using a hammer stop for a while, and the idea intrigues me.... I'm not sure if it would be better to use a rigid stop (Delrin?) or O-ring(s), but I'm leaning towards the O-ring(s).... or possibly an energy absorbing polymer that could prevent hammer bounce.... The concept is this: In a regulated gun, if you have too much hammer strike, you get too much lift, and you waste air, without adding any velocity.... You can actually push the dwell to the point where "Elvis has left the building" and the valve is still open.... If you plot velocity vs hammer spring preload you will get a graph like this....

Image

You will note that from coil bound to 3 turns out, the velocity doesn't change, but the amount of air used drops by 50%.... Obviously, operating in this range makes NO sense at all.... As you continue to reduce the hammer spring preload, the velocity starts to drop a bit (maybe 30 fps by 4 turns out), but the gun continues to use less air, in this case 25% less air for only 3% less velocity.... That is where I would usually run this gun, and not just because of the excellent balance between power and efficiency.... At that point (on the "knee" of the curve) you are actually operating right near what would be the peak of the "sweet spot" in an unregulated PCP.... That means you can actually shoot somewhat below the regulator setpoint before the velocity starts to drop.... In this case, you could probably shoot down to 1300-1400 psi (the setpoint is 1600) and still only be down about 1% in velocity.... At preload settings up on the plateau, once you hit the setpoint, the velocity drops like a stone by comparison....

An interesting thing happens when we reduce the preload further.... The velocity continues to drop, and the efficiency continues to increase, both good things.... However, once you reach the setpoint on the regulator, the velocity will begin to RISE before falling off.... This is because the gun is operating like a conventional PCP but with the top of the velocity curve flattened by the regulator.... Here is an example (with compressed "x" axis while on reg.) showing what can happen when you operate in that mode....

Image

On the left side, above 1400 psi, the gun is operating in regulated mode, and the velocity for the first 70 shots is pretty constant.... Once the pressure drops below the setpoint, however, over the next 30 shots the velocity INCREASES nearly 150 fps, peaking at about 1000 psi, before it starts to drop again.... That is because below 1400 psi the gun is acting as a conventional, unregulated PCP.... It is in THIS mode of operation where I see the hammer stop as being the most valuable.... If, instead of using hammer spring preload to set the lift (and hence the velocity), we used a positive hammer stop.... we should be able to eliminate that "bump" in the velocity curve below the setpoint, because the lift could no longer increase as the pressure dropped.... Instead of the velocity increasing, with the lift limited, the velocity should decrease as the pressure declined.... This could be particularly useful where there is a "legal limit" on the velocity or energy, either by law, or by rules such as in Field Target.... You could set the gun up to be "legal" above the setpoint, and not have to worry about the velocity climbing below that.... If we used a rigid stop (or an energy absorbing one), the velocity should decline immediately after it came into contact as the pressure dropped further.... That could still be set for maybe 100 psi below the setpoint.... If we used an O-ring buffer, it may be possible to create a shallow curve (ie much as what occurs when you operate on the "knee" of the preload curve) that extends the shot string 200 psi below the setpoint....

You may ask "why not just lower the setpoint" to operate at reduced velocity, and then move back to the "knee" of the (now lower velocity) curve.... That is certainly a very good way to tune the gun, always keeping the setpoint pressure and the hammer spring preload "in balance".... However, the efficiency (but not necessarily the shot count) will tend to be higher if you run a higher regulated pressure and then reduce the preload so that you are operating on the "downslope" on the right hand side of the curve.... The reason for that is that you are using a smaller "sip" of higher pressure air the whole time you are above the setpoint.... Using a hammer stop to prevent the rise in velocity below the setpoint in conjunction with that tune could well prove the best choice.... The reason I said this MAY not result in the largest shot count is that lowering the setpoint increases the amount of air you can use from the tank.... and then operating on the "knee" of the curve increases that even further.... Therefore "knee" tuning may results in the most shots, but with lower efficiency.... I have to admit I don't have the data either way, and it is likely each gun may be different anyway....

Now, let's look at how we might use this hammer stop idea INSTEAD of preload adjustment to set the velocity.... Let's say we set the preload at 2 turns out, so that we were well up on the plateau of the curve.... and let's assume we install a bumper made of a polymer that is designed to have energy absorbing properties.... The first advantage I can imagine is that the possibility of hammer bounce should be reduced as any extra hammer energy/momentum won't go into additional valve lift, and hence stored energy available to throw the hammer backwards.... Instead of reducing the preload to lower the velocity and increase the efficiency, we back off the striker in the face of the hammer to physically reduce the valve lift.... Imagine that you are recessing the face of the hammer, so that when the hammer hits the bumper, the valve stem isn't pushed as far.... Less lift means less velocity, and less air used.... We move to the right along the velocity curve, and I would suggest that the curve would be virtually the same shape, having a "knee" and then a declining slope.... I would further suggest, that other than a reduction in hammer bounce (which could increase the efficiency), that the efficiency curve would also be similar.... Ths difference I perceive is that when you are well down on the "downslope" on the right of the curve, there would no longer be an upwards "bump" in the velocity curve when the pressure dropped below the regulator setpoint.... as the lift could no longer increase because the hammer is already hitting the bumper.... The only downside I can see is that "knee" tuning which can extend the shot string below the setpoint 200-300 psi would likely not occur, as the lift can't increase as the pressure falls below the setpoint.... However, if hammer bounce was reduced, that could (maybe) make up for the missing shots....

So, who's going to be the first to try it?.... I wish I had the time right now, as I'm pretty excited by this concept....

Bob

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PostPosted: Thu Jun 20, 2013 6:48 pm 
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I think what you describe is exactly what happens with CO2, both when the temperature is lower than I normally run the pistol, and as the pressure falls at the end of the cylinder. Both of those can add 75 to 100 fps to the current setup, forcing me to tune for slower than ideal 'normal' speeds. I'm hoping that a stack of o-rings as a buffer can tame the worst of these high lift scenarios and allow for a faster normal tune. I have 4 o-rings in there at the moment, and a very unscientific quick test suggested it may actually be helping, but I don't have good comparison numbers to back that up. It's sure an interesting idea, for which I offer my thanks.

Jim


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PostPosted: Thu Jun 20, 2013 7:27 pm 
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Assuming you're using a 22XX platform, get the hammer from a Challanger, it has an adjustable striker.... You will need a hole through the end plug to access it with a 1/8" allen key.... If you need "1/2" an O-ring to add to the stack to get the height right, use a "backer ring", they are 0.055" thick instead of the 0.103" for a standard #113 O-ring....

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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PostPosted: Fri Jun 21, 2013 12:08 pm 
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Location: England.
Its been done to my knowledge around 30 years ago by the late Mick Dawes.
I can remember him telling me what he had done around 20 yrs ago. Valve protrudes and he just machined hammer to suit the max opening travel.
My logic here is the energy is still present when hammers hit the stop and subsequent excess pressure will still be present. Much like when little pressure is in cylinder and hammer hitting dumping the air, no valve stop will cure that.

I took a different approach and only works for regulated air rifles.
I use monstrous valve return spring you will struggle to compress between fingers 1mm! I also use it to act a dead stop against serious spring tension. I can set a valve up off the rifle by feel brought about by trial and error.
What this means is hammer stroke can be as little as 1.4mm actual not the normal age old 1/2" or old hat 6mm from around 8 years ago.
While I use a monstrous return valve spring I get more power than weaker variants!
Lost the chart but use die springs with ratings per mm of compression in four weight categories.

Valves are funny things and will drive you round the bend.
We learn something new every day sometimes unexplainable.


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PostPosted: Fri Jun 21, 2013 3:55 pm 
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I have found that die spring and square wire springs have a more controlled deflection rate as per the typical round springs. Which is what I believe your trying to say here Jon. Hence this is why alot of Euro high end guns have a short cocking stroke.

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PostPosted: Fri Jun 21, 2013 7:56 pm 
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The reason I used die springs around 92 was a known deflection rate per compression.
I had a full chart with diameter and free lengths, might still have it.
The other reason was they were readily available made from decent spring steel chrome vanadium.

Heres the strongest 10mm dia die spring, must finish the thing off one day been on hold 123 months.
http://pic20.picturetrail.com/VOL15/728 ... 294937.jpg

The strength of any spring whether round, flat or square is upon the bending direction. 3mm dia round will have same compression as a square 3mm.

Only the Feinwerkbau have a short hammer stroke at 3.5mm P70 and P800. Anchutz, Tesro, Steyr, Walther have at least 5.5mm and rely on a light weight hammer non more so than a P800.


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PostPosted: Fri Jun 21, 2013 8:26 pm 
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Jon wrote:
The reason I used die springs around 92 was a known deflection rate per compression.
I had a full chart with diameter and free lengths, might still have it.
The other reason was they were readily available made from decent spring steel chrome vanadium.

Heres the strongest 10mm dia die spring, must finish the thing off one day been on hold 123 months.
http://pic20.picturetrail.com/VOL15/728 ... 294937.jpg

The strength of any spring whether round, flat or square is upon the bending direction. 3mm dia round will have same compression as a square 3mm.You will please have to explain this one. For if that was the case there would be no call for round to square to rectangular of their force being the same as your saying, of effects other than the rate of deflection that is put into them during manufacturing. So why not have all springs being round in place vs square or rectangular then.

Only the Feinwerkbau have a short hammer stroke at 3.5mm P70 and P800. Anchutz, Tesro, Steyr, Walther have at least 5.5mm and rely on a light weight hammer non more so than a P800.
Ok then why do the AA have 8.5mm to 12mm in the same case.(depending model believe Diana follows suit here) Or why does Crosman choose a 12.5mm-14mm and Air Force choose a 22mm. Please explain the theory upon this.
Thx

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PostPosted: Sat Jun 22, 2013 2:26 pm 
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Location: England.
From round wire springs are quicker and easier to produce with minimal equipment.
With square wire it needs to be straightened and kept square as its wound to make the spring though can be produced as quick.

The strength is on the thickest points not the width which has no bearing.
The main points of a compression spring are the dia of coil, pitch and free length.

Presume you mean diameter, that again has no resemblance to compression. Though a larger diameter with same pitch would be weaker than a smaller same pitch due to the extra length between each coil. ie longer easier to bend than a shorter length plus menial difference in angle of pitch.
It just means the other companies haven't moved with the times using what was used way back in the 80's and 90's.

Off subject to prove a point heres a V spring 225 thou high, before filing, turning, hardening and pulling up thickest point (turn point) is 110 thou. http://pic20.picturetrail.com/VOL15/728 ... 395274.jpg
If these deformed or flexed away ie laterally, they would break plus they are meant to lie flat on the plate no bows or gaps and look right.
Think these were 300 thou wide and around 90 thou at strongest point. Strength is determined by the 'set' and thickness. http://pic20.picturetrail.com/VOL15/728 ... 943086.jpg
By keeping it wide it has strength in not flexing laterally which Is not required.

Back on subject I had some RN10 springs made 15 yrs ago but spring maker wrongly did a shallower pitch but same diameter and coil thickness. Hardening and steel used aside and will have a bearing, these springs were not man enough for more than 760 ft/sec.


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PostPosted: Sat Jun 22, 2013 3:45 pm 
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Image

That's an interesting (and pretty!) looking trigger group.


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PostPosted: Sat Jun 22, 2013 4:47 pm 
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Yes, beautiful, but what has it to do with the topic?....

Bob

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Airsonal;
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Proud Member of the 2000+fps Club!


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PostPosted: Sat Jun 22, 2013 5:32 pm 
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Sorry Bob, I let my enthusiasm run away with me. Jon did say that section was off-topic, but I thought the image worthy of a comment.


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PostPosted: Fri Jun 28, 2013 7:58 pm 
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Jon your going to have to show us this chart. Because to my experience my opinion differs as round to square for energy level displaced by the same thickness, deflection and material composition.

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PostPosted: Fri Jul 05, 2013 4:24 pm 
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Location: England.
A spring will bend where its the thinnest, ok so far.

Since a round section say 3mm cross section will have almosty the same strength as a 3mm square section at thickest point.
Perhaps the easiest way is grab same steels of differing widths but same thickness and try and bend. There will be negligible difference gradually increasing with the wider ones.
On picky above type actions they are 'pulled up' in jigs before fitting no bows or collapsing. Too relieve areas we used to file and its only the flintlock period where the edges were rolled off or rounded to reduce breakages, this did not weaken the springs.

What you might find with a square section is less tendency for a compression spring to buckle and in turn would be stronger for that reason.

Not forgetting materials used to make the springs differ so does the hardening and tempering along with pitch and coil thickness.

Last piccy above was one of the last I made for Dave McKay Brown in Glasgow approx 2008, think I done 28 of them in all at last place of work. Based on the Dickson round action. Theres no part of any quality gun I cant make the traditional way by hand for the best in the business.


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PostPosted: Fri Jul 05, 2013 4:43 pm 
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Wire cross sectional area is the deal.
A 3mm round has the same cross section as 2.659mm square.

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PostPosted: Fri Jul 05, 2013 5:05 pm 
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3mm square its the thickest point.

Your not bending laterally or diagonally so don't matter.
So if your right a Purdey main spring at 285 thou wide will be stronger than a 220 thou wide spring. The strength is from the turn approx. 125 thou and the flat arms much thinner as they taper down.


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