Tomba - All in One barrel sealing

[popgun pete]

I read that Garpun page. Angling the sear tooth away from 90 degrees can work on a "cam lock" trigger as the sear lever is restrained by another lever component. A pneumatic gun sear lever of the "see-saw" type has nothing holding it, so angling the sear tooth makes it less secure as instead of a restoring torque keeping the sear lever always hooked on the piston the torque is then minimized by passing the normal component to the sear tooth face of the piston pull through the sear lever pin (which eliminates the corresponding torque as the distance offset is zero). That is how the Prodanovich band gun "balanced" sear trigger mechanism works (it also has a high sear pivot pin, having once been a single-piece trigger), but you would not want to minimize the force that hooks the pneumatic gun's piston as the gun may shoot without warning. One of the reasons pneumatic guns have oil in the mechanism is to prevent the sear tooth face wearing and becoming unreliable for retaining the piston until you tip it over by pulling the external trigger which pushes on the rear of the sear lever with the short trigger transmission pin.

The only way to make the trigger itself truly independent of gun pressure would be to pass the trigger transmission pin right through the gun so both of the pin's now outer ends were at ambient pressure, which is not really practical. I have never found pulling the trigger to be a problem even on my old "Sten" which had a larger trigger transmission pin than modern guns such as the "Cyrano Apnea" model with a very slim pin. Smaller the pin, the less it is affected by internal pressure due to the very small cross-sectional area of the pin, thus producing a lower force on the pin which is trying to push it out of the gun.

The balanced sear lever condition was found to vary, probably due to changing friction, as the band load increased. A gun tuned for two bands would not work as well when subjected to four or five bands and vice versa. In a pneumatic gun this would be the same as changing the air pressure, so the tuned sear lever needs fine adjusting for the right amount of torque to turn it at the various air pressures. If you got it wrong in the band gun then at least the gun did not shoot, or you had a higher than expected trigger pull, but get it wrong in the pneumatic gun and it will shoot without pulling the trigger. Biasing a sear lever that is not mechanically trapped so as to have it on the point of rotating or not rotating is asking for trouble.

 

[tromic]

Few details were missing on a drawing, if somebody want to make calculation. For whole compensation of trigger force due to pressure in a gun also a friction(s) and spring tension should be taken in calculation. Garpun.spb.ru :: - .



Unfortunately if friction would be taken in calculation there would be dependance on pressure. For example:
atm--- kgf
15--- 3,67
20--- 1,96
25--- 0,25
26--- (-0,1)
kgf is necessary force to the trigger to shoot.

I accidentally omitted mm after the kgf. Now results seems to be OK. It is possible to adjust trigger sensitivity in wide range of pressure in a gun, adjusting (choosing) spring tension above the sear.

atm---kgf * mm-----gr
15----3,67-----166 gr
20----1,96-----89 gr
25----0,25-----11 gr
26.... (-0,1)----(-4 gr)

 

[tromic]

Interesting diagram that shows the sensitivity of trigger to angle and pressure. Very good!

 

[enross]

wow, a very long thread, can anyone tell me if i can buy one of these? im sure it might say in one of the 41 pages but if someone could give me a heads up it would be much appreciated!

 

[foxfish]

There are several versions available & all hand made by Tromics very talented son Zmajmr!
There are two basic ways of sealing out the water, one uses a slider on the spear & the other uses a direct seal on the barrel to the spear = free shaft.
Both are very effective but the free shaft has one less component as the line is fixed to the spear near the spears pointy end!

 

[tromic]

Correction to diagram.

 

[popgun pete]

Correction to diagram.

In practice this system is not going to work, although it is an interesting exercise on paper. The spring adjustment is not that easy to get right and the small catch step on the sear lever leaves little material to shape precisely. By being conservative it can be made to work for not a lot of gain in performance and the pressure would need to be set very carefully. Relying on a spring to secure the mechanism is very risky, plus the safety on a pneumatic gun does not lock out the sear lever. On single-piece trigger band guns the trigger pull used to be adjusted (by gun owners) by filing the sear tooth, but there the safety could be engaged to block the trigger and sear lever which are the same item. As pneumatic guns are muzzle loaded you only need the piston to momentarily catch and release for the spear to skewer your hand before you get it out of the way. This idea is not new, it has been around for decades, but pneumatic spearguns have opted for safety by having 90 degree sear teeth on this type of lever. The other approach is to lower the height of the sear lever pivot pin, that too lowers the torque closing the sear tooth on the piston tail. The coil spring at the back only biases the sear lever, it plays no part in the holding action of the sear tooth on the piston tail for the "see-saw" type sear levers.

 

[tromic]

Pete you are right! It is not advisable to change the angle and to rely only on spring. That might work good in some narrow range of pressure (20 - 24 bar) but would be dangerous and would require good maintenance and frequent inspection. It is maybe enough to have polished the contact surfaces of piston and sear lever. Eventually to change trigger pin from 3 mm to 1,5 mm like Cyrano has.

Here is the trigger of Mares Mirage without any modification. It seems to be secure.
If the angle that is 1,5 on picture would be 5,68 force of friction would be equal to force that try to release the sear lever from piston (for friction coefficient f = 0,1). That means that even without a spring tension, piston would be cocked while the angle is lower than 5,68 deg. Of course this would be true if the piston would not tilt, if it had some support, or the angle must be lower for the angle of piston tilt (1 - 1,5 deg).



Here is a different, home made solution, similar to trigger of Airbalete:
http://www.pescasubacquea.net/public/forum3/viewtopic.php?p=263954#p263954

 

[zmajmr]

There are several versions available & all hand made by Tromics very talented son Zmajmr!
There are two basic ways of sealing out the water, one uses a slider on the spear & the other uses a direct seal on the barrel to the spear = free shaft.
Both are very effective but the free shaft has one less component as the line is fixed to the spear near the spears pointy end!

Thank you on nice words

 

[popgun pete]

Here is a different, home made solution, similar to trigger of Airbalete:
http://www.pescasubacquea.net/public/forum3/viewtopic.php?p=263954#p263954

I see you received the same answer on the pescasubacquea forum (which I don't usually look at); the angled sear tooth on a "see-saw" type sear lever is potentially dangerous. They are really flattened out single-piece triggers and their stability depends on the 90 degree sear tooth, change it and the stability is compromised. On the "Airbalete" the trigger mechanism is completely different, it uses a yoke type (it is really a jaw type with the pivot pin behind rather than below the sear tooth) sear lever similar to the arrangement in "cam lock" band gun trigger mechanisms, only it locks with a sliding pin engaging the sear lever tail rather than with another rotating lever, which in a band gun is usually the trigger (on a mid-handle band gun it is the "locking lever" which is a shortened version of the trigger, the lower arm being cut off and the remaining stub pushed by a remote trigger via a long rod). Yoke type sear levers have a sear tooth and a backing projection, the former holds the piston or spear tail and the latter resets the mechanism when directly pushed by the piston or spear tail. We don't usually see yoke type sear levers in pneumatic spearguns as the "see-saw" type sear lever is a simpler and cost effective construction due to the smaller number of necessary parts.

The "Airbalete" trigger (which will almost certainly be in the "One Air" as well) is locked because the sliding pin operated by the trigger passes inside the travel arc that the sear lever tail has to sweep through for the sear lever to release. The push rod operated by the trigger moves this pin forwards and then the sear lever can turn with the pin out of the way. The system is rather ingenious as the sear lever tail moves rearwards and down on an arc while the pin, which has a flat on it, goes forwards. The sear tooth has an angle on it to destabilize the sear lever as the sear lever pivot pin is almost where it would be on a single-piece trigger mechanism, but here we need the sear lever to turn, which is completely the opposite to the "see-saw" sear lever situation. Time will tell if wear at the sliding surface between pin and sear lever tail will ever be able to push the pin forwards without you pulling the trigger, but the trigger mechanism geometry appears to make that an unlikely scenario.

One thing that is being forgotten in this discussion is the leverage built into the external plastic trigger on guns like the Mares and similar rear handle models. You pull on a long lever while a very short lever pushes on the lower end of the trigger transmission pin, that leverage plus the long tail on the sear lever gives the operator a considerable mechanical advantage when pulling the trigger to fire the gun.

 

[popgun pete]

The "cam lock" trigger built inside the inner barrel is interesting (shown in two photos on that pescasubacquea site), but removes some of the working course of the piston as there are now two pivot pins inside the inner barrel, the new sear lever being in front of where the sear lever used to be which has itself become a trigger with the lower "finger pull" arm folded back behind it. A sort of distorted "Undersee" trigger mechanism with a high sear pivot pin and angled sear tooth to make the new sear lever unstable. It uses a sear box roof (the black plastic piece) to insert inside the inner barrel tube to stop the piston tail riding up on the angled sear tooth and escaping, although that may not be necessary as it depends on how much the piston can tilt in the inner barrel.

It just occurred to me that the new sear lever lacks a spring to bias the sear lever tail up at the rear after the shot, so the black item may be rubber and not plastic and it is a "spring element" for the sear lever, going underneath it and not on top of it. The black item has side holes which appear to be locating holes for the pivot pin to pass through and trap the item in the inner barrel.

 

[tromic]

The "cam lock" trigger built inside the inner barrel is interesting (shown in two photos on that pescasubacquea site), but removes some of the working course of the piston as there are now two pivot pins inside the inner barrel, the new sear lever being in front of where the sear lever used to be which has itself become a trigger with the lower "finger pull" arm folded back behind it. A sort of distorted "Undersee" trigger mechanism with a high sear pivot pin and angled sear tooth to make the new sear lever unstable. It uses a sear box roof (the black plastic piece) to insert inside the inner barrel tube to stop the piston tail riding up on the angled sear tooth and escaping, although that may not be necessary as it depends on how much the piston can tilt in the inner barrel.

It just occurred to me that the new sear lever lacks a spring to bias the sear lever tail up at the rear after the shot, so the black item may be rubber and not plastic and it is a "spring element" for the sear lever, going underneath it and not on top of it. The black item has side holes which appear to be locating holes for the pivot pin to pass through and trap the item in the inner barrel.

Just to update the link: forum.pescasubacquea.net • Leggi argomento - Smontaggio fucile Oleopneumatico

 

[tromic]

I accidentally omitted mm after the kgf. Now results seems to be OK. It is possible to adjust trigger sensitivity in wide range of pressure in a gun, adjusting (choosing) spring tension above the sear.

atm---kgf * mm-----gr
15----3,67-----166 gr
20----1,96-----89 gr
25----0,25-----11 gr
26.... (-0,1)----(-4 gr)

I was still playing with this idea. Here is new diagram that shows that it is possible to achieve no sensitivity to pressure for a specific angle. That is, according to diagram, about 10,87 deg. Other practical problem is that it is very difficult to make and maintain such a precise angle. Force on trigger (with finger) is about 150 gr, for spring tension 200 gr. It could be adjusted with spring tension to be less or more, and same for any pressure.



Here is a calculation of my forum friend (Mexanik) who is the author of this idea:
http://garpun.spb.ru/forum/viewtopic.php?p=46369#46369

 

[popgun pete]

A problem which these calculations do not take into account is that when you push the spear into the gun you are exerting a lot of force as maximum resistance is encountered near the end of the latching operation. This causes the piston's mushroom tail to move down slightly past the sear tooth and then rebound back onto it to be caught by it. That subsequent jerk on the sear lever is capable of being held by a 90 degree sear tooth, but if the sear tooth is angled then the torque closing the sear tooth due to the force on the piston from the gun's air pressure is much lower and that jerk may then be enough to roll the sear lever to the position where it will release the piston tail. So while the static calculations may look OK, the dynamic situation is not so secure. Slowly pushing the spear to the latching point will eliminate the jerk on the sear lever, but loading by hand usually involves a much faster push and you don't have time to check the degree of shaft insertion as your main interest is in hearing the mechanism click as the sear tooth catches on the piston tail. If you do this sear tooth modification then you will need to keep pressure on the loading bar for a moment to hold the piston on the sear tooth and allow it to get a steady grip on the piston tail. On a short gun that is easier to do than on a longer gun where you are stretched out a lot more. The moment of truth will be when you release your effort on the loading bar and having been stretched out by a non-secured piston which had not latched I can tell you that it is not something you want to experience too often. I was using a cup style loader, the one that was supplied with the Mares "Titan", so the spear tip stayed inside the loader, thus protecting my hand.

 

[tromic]

Pete you are right! Although it would be possible to make this design working, there would be o problem while loading the gun, to move down the sear tooth with piston mushroom.
But... I did a calculation. The force that the piston must fight against to push down the sear tooth is about 3,5 kgf at 25 bar and 3 mm OD of trigger pin. If you load the shaft with a speed of about 1,5 m/s, in last section only, you will have sufficient force to cock the gun. Nevertheless, maybe that analysis of trigger would be useful for something other...
I heard that similar cocking problem (difficult to cock) had some users of Airbalete too.

If I had decided to make this kind of trigger my main obstacle would be how to make connection: sear lever - trigger pin, not changing the trigger design (housing, O-ring, pin) on a gun so mounting and dismounting of the gun would be same as before.

 

[popgun pete]

I heard that similar cocking problem (difficult to cock) had some users of Airbalete too.

Any difficulty in cocking the "Airbalete" is a function of it being a "jaw type" sear lever and the backing projection length being constrained by what will fit inside the inner barrel bore with the sear lever cocked. When the rearward moving piston tail hits the backing projection of the discharged sear lever its forward most tip is only slightly off the centre of the barrel bore, so the torque to push the sear lever around is limited by the short perpendicular offset from the sear pivot pin axis to the direction of the loading effort on the backing projection tip, i.e. straight down and parallel to the longitudinal axis of the inner barrel. This tiny transverse offset distance times the loading effort force equals the torque. As the sear levers swings around on its pivot pin this perpendicular offset of the backing projection's tip from the sear pivot pin axis increases, so the torque increases, making it easier to turn the sear lever. However the loading effort to compress the air as the piston is reaching its limit of rearward travel will be already high, so the extra effort to turn the sear lever should not be that great, particularly as the metal coil return spring on the sear lever is only a biasing spring, although what it takes to push past the release slide (or pin) which I assume is leaning on the sear lever's tail end I don't know. What may be happening is that on hitting the backing projection users don't realize the need to keep on pushing and stop just short of the full relatch, so the sear lever swings back the other way and lets the piston tail go. Whenever I load a pneumatic I try to keep the pushing effort on until after the mechanism clicks, sometimes that is easier to say than to do, especially with a lot of pressure in the gun and when you are close to your limit. The spear coming straight out again is very disconcerting and can be dangerous if you are not anticipating it. Hence my concerns earlier about modifying a sear tooth.

Not much can be done to improve this situation unless the sear lever's backing projection was slightly longer, hence increasing the initial tip offset distance (thus more torque) and then fitting its tip into another side slot in the inner barrel on the other side of the barrel to absorb the extra tip length with the sear lever cocked. The length increase would be very small and any improvement would be marginal, so I guess that is why Omer just made it all fit inside the available barrel bore space. Users who have trouble should lower the air pressure in their guns so they know where and when the latch completes, noting how far their hand is from the muzzle tip before increasing the pressure again. Maybe put a piece of tape around the shaft as a temporary indicator of where the shaft stops loading into the muzzle, allowing for a slight rebound and the length of the line slide.

 

[tromic]

Back to pressure balances trigger mechanism. Here is a comparative of old and "new" trigger behaviour, just to see if it is worth of trying it.



The angle I talk about is "alpha" on drawing.
a = 12.5 mm; b = 23.8 mm

http://imageshack.us/photo/my-images/815/proracunokidanja1.jpg/

 

[tromic]

A few details:

 

[tromic]

There is an another advantage of this design. If we connect the lower part of trigger pin to trigger (maybe similar way to upper part), we can make safety switch reliable. Locked safety switch would lock the sear lever too!
Without the spring, beyond the sear lever, operation would be also possible, with only 50 gr of force necessary to pull the trigger.
If instead of 3 mm pin, 1.5 mm pin would be used the angle "alpha" should be only 7.15 deg ( for pressure independence) what would be even better because the sliding force of lever tooth would be closer to balance with friction force and "miracle" angle of 5.77 deg.

 

[popgun pete]

There is an another advantage of this design. If we connect the lower part of trigger pin to trigger (maybe similar way to upper part), we can make safety switch reliable. Locked safety switch would lock the sear lever too!
Without the spring, beyond the sear lever, operation would be also possible, with only 50 gr of force necessary to pull the trigger.
If instead of 3 mm pin, 1.5 mm pin would be used the angle "alpha" should be only 7.15 deg ( for pressure independence) what would be even better because the sliding force of lever tooth would be closer to balance with friction force and "miracle" angle of 5.77 deg.

When I saw the photo showing the transmission pin connected into the rear of the sear lever I had the same thought about connecting the trigger end as well, but trigger and sear lever both move in arcs, whereas the trigger transmission pin is limited to up and down movement, so each end of the pin will need to use a moveable link to the respective pivoting component to prevent the pin being twisted out of alignment in its bore and breaking the air pressure seal. The existing trigger being made of plastic, any strong upward pull on the connecting pin if the sear lever turned prematurely could snap the trigger where it will be fastened onto the pin if you are then relying on the safety to stop the gun shooting. The trigger may need to be made from metal to strengthen it if it becomes load bearing rather than the unconnected pin pushing element it is now. The reliability of the converted mechanism needs to be carefully evaluated as essentially it is like a worn sear lever tooth, no longer being at 90 degrees, or very close to it.

I am reminded of the "Demka BR" guns, they used a "see-saw" sear lever, just like a pneumatic gun, but it was exposed to seawater and sand and could wear. "Demka" later replaced that mechanism with a "cam lock" trigger mechanism much like that used on their conventional band guns, so I assume the earlier mechanism became unreliable in what was a muzzle loaded gun due to it being powered by an internal rubber pipe that sat inside the large diameter outer barrel. The stretched rubber pipe formed the inner barrel around the spear, the gun being totally flooded, so it operated in a manner similar to a pneumatic speargun when the pipe contracted, once released by the trigger being pulled and the rubber pipe then threw the spear from the gun. I posted a thread here on those guns some time ago.