Pressure buildup dump valve for springers?

[subscriber]

Thanks Jim

[WhatUPSbox?]

My understanding of the term dynamic pressure is that it is based on the velocity (kinetic energy) of the gas at the time. Not sure how much is present at the time the pellet releases.

[subscriber]

Stan,

I think the dynamic pressure release is more like the difference between pushing a nail into wood with a hammer, VS having the hammer move at speed before making contact.  Except that in the case of the hammer the nail moves easier when well out of the static friction regime.   

The claim is that slowly pushing on the pellet with hydraulic pressure takes less force to break it free from the breech cone, than slamming it with air pressure the way a springer does.  This is because the skirt acts as a rivet, that is upset by the blow just before it is driven through the leade.

[Jim-in-UK]

Thanks Jim

Sorry for the delay, it was a busy (airgun related) weekend.

Further to my last post, my figures were derived from calculation only of peak cylinder pressure according to the amount of lost volume, everything else, including volumes and whether the pellet moved, was measurement.

The 120psi start pressure pellet would not be very useful in >12 ft. lb. springers. It is excellent in terms of fps in 6 ft. lb. rifles, but progressively less so the closer the muzzle energy to 12 ft. lb. In my 85mm stroke TX200, when the pellet starts to move, adiabatic compression has added under 4 ft. lb. of internal kinetic energy to the air, enough to slowly accelerate the pellet, but it is too far up the barrel to gain from the much higher air energy later in the stroke. The 7.9gn JSB has about 4.3 ft. lb. of air energy when it starts, the 8.4gn JSB has well over 5 ft. lb., and both are nearer the breech as the air energy rises, and one or other usually gives the highest muzzle energy in <12 ft. lb. springers.

A 600psi start pressure pellet such as the RWS Hobby has over 8.5 ft. lb. of initial internal energy to drive it, but its high kinetic friction (~0.9) and light (6.9gn) mass means that it gives lower muzzle energy than the 7.9gn and 8.4gn. JSBs.

On peak cylinder pressures, the lower the start pressure, the higher the peak cylinder pressure, the 7.3gn JSB peaking at 1,355psi in my TX200, the 8.4gn under 1,100psi. (both measured).

All my tests are with sub 12 ft. lb. springers (usually circa 11.4 ft. lb. to give a safety margin), and peak pressure in my TX200 is a fraction that of sub 12 ft. lb. rifles with reduced air volume (I have measured one at ~3,000psi), let alone >12 ft. lb. springers.

I hope that I have addressed all your points, and thank you for the opportunity to discuss this fascinating (to me, at least) subject.

[subscriber]

Thanks for the detail, Jim

I am surprised that lower start pressure correlates with higher peak pressure.  I would assume that when a pellet starts moving early, it opens up a larger space for the same amount of air to be compressed into.  On the face of it, that should reduce the peak pressure.

As I said above, the inertia of the pellet is also a factor.  If one took some of the most efficient pellets and swaged their skirts perhaps 0.005" larger in diameter, that should boost the starting pressure.  It would be interesting to see what effect that has on peak pressure and efficiency.  Ditto for swaging the skirts down to groove diameter.

[Jim-in-UK]

Thanks for the detail, Jim

I am surprised that lower start pressure correlates with higher peak pressure.  I would assume that when a pellet starts moving early, it opens up a larger space for the same amount of air to be compressed into.  On the face of it, that should reduce the peak pressure.

As I said above, the inertia of the pellet is also a factor.  If one took some of the most efficient pellets and swaged their skirts perhaps 0.005" larger in diameter, that should boost the starting pressure.  It would be interesting to see what effect that has on peak pressure and efficiency.  Ditto for swaging the skirts down to groove diameter.

The low start pressure/high peak pressure is one of the very few aspects of the springer shot cycle that is easily explained. The cylinder air pressure (and with it the force acting against the piston) is lower for any point in the piston travel past pellet start, and the momentum of the piston compresses a smaller mass of air to a higher pressure.

Pellet inertia is certainly a factor, though I think it has less effect than swaging, and I don't know how much effect static friction, with molecular interlocking exacerbated by air pressure pushing the skirt against the lead-in, has but, along with lead alloy hardness, they must all play a part. I'll try spreading the skirts of 7.3gn and see whether it makes any difference.

[subscriber]

I'll try spreading the skirts of 7.3gn and see whether it makes any difference.

I look forward to seeing what that does. 

I suspect that there is an optimal skirt diameter for every type of pellet, and that exceeding it by too much, will increase piston bounce and cost efficiency.  This optimal skirt diameter for each pellet probably varies along with barrel and breech dimensions and finish; with the piston configuration superimposed on that.

[WhatUPSbox?]

On peak cylinder pressures, the lower the start pressure, the higher the peak cylinder pressure, the 7.3gn JSB peaking at 1,355psi in my TX200, the 8.4gn under 1,100psi. (both measured).

Jim, can you comment on how you are measuring peak pressure directly?

[subscriber]

Jim, can you comment on how you are measuring peak pressure directly?

Yes.  Is it pressure at the inlet to the transfer port relative to atmosphere; or pressure drop across the transfer port?

[Jim-in-UK]

On peak cylinder pressures, the lower the start pressure, the higher the peak cylinder pressure, the 7.3gn JSB peaking at 1,355psi in my TX200, the 8.4gn under 1,100psi. (both measured).

Jim, can you comment on how you are measuring peak pressure directly?

I'm glad you asked that, because I should not have written it.

I have a 400G accelerometer, so I measure peak recoil deceleration, rifle weight and piston csa, from which the peak cylinder pressure can be determined.

[subscriber]

Jim,

By that measure, if you dry fired the air rifle, the piston would see the max deceleration.  This, by virtue of clouting the endwall of the cylinder, rather than compressing air.

So, are pellets that start easily really producing higher pressure, or just operating closer to dry firing?

The answer is probably to be found by looking at the percentage energy from the spring that ends up in the pellet.  The energy the pellet has, the the better the balance between the piston bouncing, and clouting the endwall due to a lack of pressure buildup.

[Jim-in-UK]

Jim,

By that measure, if you dry fired the air rifle, the piston would see the max deceleration.  This, by virtue of clouting the endwall of the cylinder, rather than compressing air.

So, are pellets that start easily really producing higher pressure, or just operating closer to dry firing?

The answer is probably to be found by looking at the percentage energy from the spring that ends up in the pellet.  The energy the pellet has, the the better the balance between the piston bouncing, and clouting the endwall due to a lack of pressure buildup.

Good question, and I can answer only for sub 12 ft. lb. springers, from my recoil velocity recordings.

The piston always bounces, regardless of whether there is a pellet in the breech or not.

The only time I managed to get the piston and cylinder end to collide at the end of the compression stroke was when I dry fired an Anschutz335 with the piston seal removed, which I deliberately did to get the recoil plot signature of a piston/cylinder end wall collision.

[subscriber]

If the piston always bounces, does dry firing produce a more abrupt stop, nearer to the cylinder endwall, with higher deceleration?   High enough to cause damage to some component or other.   If not, dry firing would not be something we need to avoid.

[Jim-in-UK]

If the piston always bounces, does dry firing produce a more abrupt stop, nearer to the cylinder endwall, with higher deceleration?   High enough to cause damage to some component or other.   If not, dry firing would not be something we need to avoid.

I have not risked my current accelerometer on a dry fire, as I don't think I could replace it. From recoil velocity data from my magnet and coil, the piston does indeed come to a very abrupt halt.

An earlier (and now deceased as a result of this and similar experiments) accelerometer gave peak recoil of 310G with an HW95 dry fired - can't remember the rifle weight but that would equate to peak pressure in the region of 3,000psi, and the piston bounced, so it must have been air pressure rather than a collision.

The dry fire tests I have done would not be terminal for the rifle, but I would not recommend dry firing any springer, nor make a practice of it other than for experiments, nor try it with a scope on the rifle.

[subscriber]

Thanks Jim

[WhatUPSbox?]

Jim, thank you for clarifying your test method.

As far as dry firing. When I was testing with my .22 Benjamin Titan, once (or twice  ) I forgot to put a pellet in. The resulting accelerometer trace and the accompanying sound convinced me not to do it again. Unfortunately I don't think I saved the trace.

[rsterne]

A question about how you are calculating the pressure from the 310G of deceleration?.... Are you just multiplying the "G" by the piston weight to get the maximum force?.... and then using that force divided by the piston area to get the pressure?....

Why do you say that since the piston bounced it must have been from air pressure, and not an elastic collision with the end of the chamber?.... Hit a block of steel with a hammer, and it certainly bounces....

Bob

[Jim-in-UK]

Jim, thank you for clarifying your test method.

As far as dry firing. When I was testing with my .22 Benjamin Titan, once (or twice  ) I forgot to put a pellet in. The resulting accelerometer trace and the accompanying sound convinced me not to do it again. Unfortunately I don't think I saved the trace.

Excellent - someone else using an accelerometer! I cannot test >12 ft. lb. springers without jumping through a mass of legal hoops here, so any data from 'FAC' springers would be welcome.

[Jim-in-UK]

A question about how you are calculating the pressure from the 310G of deceleration?.... Are you just multiplying the "G" by the piston weight to get the maximum force?.... and then using that force divided by the piston area to get the pressure?....

Why do you say that since the piston bounced it must have been from air pressure, and not an elastic collision with the end of the chamber?.... Hit a block of steel with a hammer, and it certainly bounces....

Bob

Hi Bob.

The rifle weight, not the piston, but that principle.

I should have clarified that when I engineered the piston/end wall collision by cleverly removing the 28mm piston seal (and replacing it with a ~22mm acetal buffer), there was no identifiable piston bounce, because there was no compression and on air pressure.

[Bayman]

I don't know about all the scientific stuff but when I've accidentally dry fired my Weihrauchs it sounds like a gunshot. I assume that's detonation from a very rapid increase of pressure. My guess is the detonation reverses the piston direction violently before it reaches the end of the compression tube. I also believe it reverses much harder and further than normal piston bounce.

I've had factory Weihrauch seals ruptured and useless by a single dry fire. I've also noticed premature spring breakage in guns that have been dry fired.

Interestingly enough it's supposedly ok to dry fire a Hw45. I have one and have done it several times. It too detonates but it has a tougher Teflon piston seal. Not sure why the Hw45 spring handles the dry fire better than their rifles.

Dry firing has been a topic many times and people have varying opinions. I can say I've absolutely damaged seals and springs in Weihrauch rifles. Ymmv with other guns.