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Old 02-23-2012, 04:03 PM   #1
popgun pete
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Trigger Mechanism Design Rules

I just rediscovered this old trigger mechanism diagram from my speargun research which I was carrying out prior to working up a "big banger" mechanism. About that time Bill Kitto had produced his "Millennium" mechanism, the one with the heavy-duty cast steel sear case and "Undersee" based levers, it being a traditional "cam lock" design, and he was then about to embark on his multi-lever "M" series. Regular readers of this forum will know of what then followed with the successors to that original Millennium mech, or M1. They are the M3 and the M5 (four levers as the remote trigger is a necessary fourth), which I like to think of as the "MBT" mechanisms (or "Main Battle Tank") and those mechanisms for less bulky weapons such as the M2 (two levers), M4, M6 and M7 (three levers). During some prior discussions we had touched on the "speargun mechanism rules" and this diagram will serve to show you what they are. Of course like any other rules they can be broken, sometimes with more success than others. Bad things can happen, if you do not know what you are doing!

The trigger mechanism shown in the diagram is an "Aquacraft", it was also used in the "Sportsways" gun and is currently used in the "Bandito" and "Patco Inc" band guns, judging by their similar "Addict" style finger grip handles with four arm line release wheel in the handle butt. It is an "Undersee" cam locker, but with a shallow sear lever inclination angle when cocked, which makes it reasonably compact in the vertical dimension.

1. The largest circle on this diagram is the travel arc of the outermost tip of the sear lever tail, the smallest circle is the path traced out by the trigger retention step when the trigger revolves. For the mechanism not to push the cocked shaft backwards against the band pull when the trigger is pulled (otherwise trigger pull goes up in a big way) the curve of the trigger retention step must match the arc cut into the sear lever tail which contacts it and slides against it. Here we can see an ideal match. That is the first rule. Jack Prodanovich knew this rule and was careful to build it into his own two-piece "balanced sear" guns.

2. The second rule is to aim the force from the sear lever tail directly at the trigger pivot pin to eliminate any torque being applied to the trigger, only giving it a squeeze from the sear lever tail contact pressure instead. This rule was also formulated by Jack Prodanovich during his many trigger mechanism experiments. That all sounds good, but the sear lever tail has to sweep past the trigger pivot pin position and clear the bulk of the top of the trigger component, so to maintain a shallow mechanism like this "Aquacraft" mechanism is the action line of this force can be moved slightly inwards to pass in front of the trigger pivot pin. This will revolve the trigger and sear lever under band load into each other providing a strong "cam lock" (shorthand for interlocking cams) and will allow the sear lever tail to sweep up past the trigger pivot pin position and strike the sear box roof.

3. The third mechanism rule is the spacing between the pivot pins sets a limit on the achievable gearing in the trigger mechanism, but this rule has been broken as we will see later. Guns that are limited by this rule are the old JBL tube guns, that is why the XHD models have carbide inserts to resist the lever contact pressures as the gearing is otherwise insufficient to lower the inter-component forces when using big band loads.

4. The fourth mechanism rule is that in order to aim the force from the sear lever tail directly at the trigger pivot pin the contact point (actually a short curve) between sear lever tail and trigger retention step should lie on a circle whose diameter is given by the distance between the pivot pins. This is the third intermediate size circle in the diagram. We can see that this "Aquacraft" mechanism's contact point lies inside that intermediate circle, so it will have a definite lock as some torque will be produced to revolve the trigger against the direction in you will have to pull it when firing the gun. That makes it very safe, provided nothing breaks, but slightly increases trigger pull. Those who remember their geometry from school will know that any right-angled triangle with one side (the hypotenuse) being the diameter of a circle will always have the apex sitting on the circumference of that circle.

5. The fifth mechanism rule is that the trigger to fire should operate in a direction opposite to that in which the cocked mechanism will try to push it, this rule can also be broken, but care needs to be taken to avoid the mechanism popping open under band load by ensuring full engagement of the sear lever tail tip on the trigger retention step. That can be achieved by rolling a safety bar, or cam, right in behind the trigger, as is used in the latest JBL timber gun trigger mechanism, to force the engagement.

Terry Maas used the mechanism in the diagram for his Tuna gun, after cutting away the lower trigger section to enable it to be fired via a push rod activated by pulling a remote trigger in a mid-handle grip, although these are actually "rear biased" mid-handles, not true mid-handles. It has good gearing which I had calculated, from the lengths of the levers, at 13 pounds trigger pull for a 300 pound band load, neglecting any friction. If you gear this mechanism up with a remote trigger with a 3:1 mechanical advantage then the trigger pull comes down to 4.3 pounds at the remote trigger. In his book "BlueWater Hunting and Freediving" Terry, whom I have never met, indicated that it was more than adequate for the job. If you look at page 66 of that book (1st edition) you can confirm that this is the same mechanism in the photos, but there it was out of the "Sportsways" version of the gun.

Of course the "golden rule" of trigger mechanisms is that you don't bet your life on the trigger mechanism, or that of anyone else; guns can always discharge if things break, are not fully locked or have foreign material blocking their action, and in some cases, the misplacement of the shooting line in the sear box with extreme rear tail tied shafts.
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Last edited by popgun pete; 02-23-2012 at 04:42 PM. Reason: typo
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Old 02-23-2012, 04:49 PM   #2
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Re: Trigger Mechanism Design Rules

Here is one trigger mechanism that breaks some of the rules, the Scubapro "Panther". It works very well, even though vector analysis would suggest that extreme band load would revolve the trigger to "fire" by applying a torque in the trigger finger pull direction.
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Last edited by popgun pete; 02-23-2012 at 07:24 PM. Reason: adding the "circle analysis" diagram
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Old 02-23-2012, 06:56 PM   #3
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Re: Trigger Mechanism Design Rules

The "In Depth Incorporated" trigger mechanism is an example where following the rules, but not necessarily understanding them, results in a trigger mechanism that is less effective than it could have been. Generally for good leverage, and hence good mechanism gearing, the force going in, i.e. the band pull, should operate about as short a (vertical) distance from the sear pivot pin as possible, while the force going out, from the sear lever tail, should be as far out from the sear pivot pin as possible, within the practical limits of the trigger mechanism pin spacing and housing dimensions. Likewise the trigger should be pulled by your finger at a long lever arm length while where the sear lever tail contacts the trigger retention step should be as close as possible to the trigger pivot pin. This "short in, long out" lever arm arrangement lowers the torques going into the trigger mechanism and gives good trigger pull for high band loads. The "IDI" trigger mechanism is not optimized in this way and has a "cam lock" condition which revolves the trigger's toe forwards, impacting on the plastic trigger finger guard frame. If it were not for the parts having already been constructed it should have gone back to the drawing board, but the guns were still serviceable as I own two of them.
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Old 02-23-2012, 11:52 PM   #4
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Re: Trigger Mechanism Design Rules

Interesting stuff

Ric
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Old 02-24-2012, 02:53 PM   #5
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Re: Trigger Mechanism Design Rules

Thanks Ric. Here is another example.

The "Ocean Rhino" is a relatively recent trigger mechanism developed by Kevin Bruington and in the attached diagram one can see the results of the "circle analysis". The sear tooth is very low down in front of the sear pivot pin giving a very short lever arm length going into the sear pivot pin. There is good gearing in both trigger and sear levers as can be seen by the relative lengths of the force "in" and force "out" arms on the levers. The sliding contact position between sear lever tail and trigger retention step lies within the "intermediate circle" (shown in green) providing a positive "cam lock" design. This is an example of a "jaw type" sear lever as the sear pivot pin is high up behind the yoke formed by the sear tooth and the backing projection located immediately behind it. This allows a very compact design with reduced trigger pull.

This mechanism also has a well engineered safety device to block trigger movement.
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Old 02-24-2012, 05:44 PM   #6
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Re: Trigger Mechanism Design Rules

Jack Prodanovich's "balanced sear" mechanism. Jack got around the pin spacing limitations in his own alloy handle by using an angled sear tooth for vector force splitting of the band pull. A brilliant solution based on his long experience with single-piece triggers and the effects of tooth wear on unstable release tendencies. One of the great spearfishing innovators, this mechanism of his own devising is a design classic, including the trigger finger actuated safety cam to stop the gun discharging, unless you pull the trigger.
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Old 02-24-2012, 06:02 PM   #7
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Re: Trigger Mechanism Design Rules

The "In Depth" plastic trigger housing would break where it met the aluminum tube when loading a 3'rd 9/16" band. It was irreparable after that. It was my 1'st gun though and took a lot of calicos with it.

Nice post!
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Old 02-24-2012, 06:09 PM   #8
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Re: Trigger Mechanism Design Rules

Excellent thread!!
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Old 02-25-2012, 09:14 AM   #9
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Re: Trigger Mechanism Design Rules

The Kitto M3 "circle analysis" demonstrates the enormous gearing of this trigger mechanism, and there is still the trigger itself (not shown) that has an additional gearing of its own where the long trigger arm pushes the push rod close in to the trigger pivot pin mounted in the remote handle position. With stainless steel upper and lower rollers controlling the shaft tail vertical positioning and a twin thick stainless steel plate sear case, this is the "Main Battle Tank" of trigger mechanisms with a large diameter sear pin taking the main axial load from the shaft. There are four gearing reductions in this mechanism; in the sear lever, the intermediate lever, the locking lever and the pivoting remote trigger. The slight forward tilt of the sear tooth from 90 degrees means that the spear tail will bite into the bottom of the tooth contact face and possibly tear it off at extreme loads not encountered in spearfishing. Band loads at this level will place enormous strain on any gun stock, so if the gun disintegrates (folds up) then you could be in very bad shape. The similar M5 has a larger bearing area on the more vertical sear tooth face and does not use "over-push" rotation of the sear lever to relatch, so it can withstand even greater loads if you feel that you need an even higher capacity gun.

It is unlikely that the band gun world will ever need a "bigger banger" mech than this one, plus it is a true "cam lock" mechanism as the "intermediate lever" and "locking lever" lock takes place within the intermediate circle (green circle). Because of their very large diameters, the other circles are not shown on this diagram. This photo is of a prototype set mounted on an aluminium plate to check the locking action and firing sequence, but is virtually identical to the production M3, the final "jaw sear" has a slightly taller angled backing projection.
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Old 02-25-2012, 06:08 PM   #10
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Re: Trigger Mechanism Design Rules

Seems to me that this is a pretty worthwhile topic, and that the relatively few responses are because of the very comprehensive way you have dealt with it. Unfortunately, the way forums work, this means will be lost too quickly.

There was one matter that you didn’t explore – the depth of bury of the sear tail into the trigger. Most mechanisms have a slot something between 1/8 and ¼ inch. Would seem to me that the key factor is how far the trigger will be pulled (maybe between ½ and ¾ inch?) and the proportional distance of the finger location and the sear bury slot to the trigger swivel pin. But perhaps I have missed something?


I also wanted to explore a related matter. Maybe I should have started a new thread, but I will include it here.
I haven’t spent all that much time pulling triggers of really powerful guns, but I have built guns using several mechs, with four and six rubbers. My experience is that the triggers have not been all that hard to pull. I also speculate that trigger pull might not be too much of an issue with spearguns – the relatively high masses of the guns, stabilizing influence of water and the relatively close targets, all mean that aim is not going to be all that much impacted by a slightly hard trigger pull.

However, there is a factor that can make a real and significant difference, and that is the friction of the shooting line retainer hook on the sear. In Undersee style mechs, the line release hook bears on the side of the sear lever. In open muzzle guns, a fair bit of pressure is placed on the line hook (to keep the shooting line taut, holding down the spear at the muzzle). My personal experience is that the effect can be great enough to cause a problem. (I normally use a substantial bungee on open muzzle guns, which provides enough pressure to hold down the spear, but not so much as to increase the pressure on the line release hook).

Does anybody agree or disagree?

Ric
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Old 02-25-2012, 06:46 PM   #11
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Re: Trigger Mechanism Design Rules

Awesome stuff Pete. It has me going back and looking again at a mech I wanted to try.

One question though. When the tip of one lever (say the sear in Kitto's M3 mech) impacts the flat surface of the intermediate lever, isn't the direction of the resultant force perpendicular to the surface of the intermediate lever? I think that would then change the effective length of the lever arm about the pivot pin?

In the pic below, wouldn't the resultant force from the sear on the intermediate lever be in the direction of the green arrow, and then the effective lever arm about the pivot pin the length of the orange line?

I don't think it changes much in this case, but I'm trying to make sure I understand . . .
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Old 02-26-2012, 01:20 AM   #12
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Re: Trigger Mechanism Design Rules

Quote:
Originally Posted by Ric.Fallu View Post
Seems to me that this is a pretty worthwhile topic, and that the relatively few responses are because of the very comprehensive way you have dealt with it. Unfortunately, the way forums work, this means will be lost too quickly.

There was one matter that you didn’t explore – the depth of bury of the sear tail into the trigger. Most mechanisms have a slot something between 1/8 and ¼ inch. Would seem to me that the key factor is how far the trigger will be pulled (maybe between ½ and ¾ inch?) and the proportional distance of the finger location and the sear bury slot to the trigger swivel pin. But perhaps I have missed something?


I also wanted to explore a related matter. Maybe I should have started a new thread, but I will include it here.
I haven’t spent all that much time pulling triggers of really powerful guns, but I have built guns using several mechs, with four and six rubbers. My experience is that the triggers have not been all that hard to pull. I also speculate that trigger pull might not be too much of an issue with spearguns – the relatively high masses of the guns, stabilizing influence of water and the relatively close targets, all mean that aim is not going to be all that much impacted by a slightly hard trigger pull.

However, there is a factor that can make a real and significant difference, and that is the friction of the shooting line retainer hook on the sear. In Undersee style mechs, the line release hook bears on the side of the sear lever. In open muzzle guns, a fair bit of pressure is placed on the line hook (to keep the shooting line taut, holding down the spear at the muzzle). My personal experience is that the effect can be great enough to cause a problem. (I normally use a substantial bungee on open muzzle guns, which provides enough pressure to hold down the spear, but not so much as to increase the pressure on the line release hook).

Does anybody agree or disagree?

Ric
In the discontinued Ultimate brand gun, when you put any considerable pressure on the line release tab, the force needed to pull the trigger goes up noticeably.
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Old 02-26-2012, 09:50 AM   #13
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Re: Trigger Mechanism Design Rules

Regarding friction of the line release lever against the sear . . .

That's one of the things that I like about the Neptonics Reef mech with the underneath line release tab. The force of the line release lever acts to rotate the sear in the same direction as the band load, and not against the side.

Lots of other line release options, of course.
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Old 02-26-2012, 01:49 PM   #14
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Re: Trigger Mechanism Design Rules

Quote:
Originally Posted by Ric.Fallu View Post
Seems to me that this is a pretty worthwhile topic, and that the relatively few responses are because of the very comprehensive way you have dealt with it. Unfortunately, the way forums work, this means will be lost too quickly.

There was one matter that you didn’t explore – the depth of bury of the sear tail into the trigger. Most mechanisms have a slot something between 1/8 and ¼ inch. Would seem to me that the key factor is how far the trigger will be pulled (maybe between ½ and ¾ inch?) and the proportional distance of the finger location and the sear bury slot to the trigger swivel pin. But perhaps I have missed something?


I also wanted to explore a related matter. Maybe I should have started a new thread, but I will include it here.
I haven’t spent all that much time pulling triggers of really powerful guns, but I have built guns using several mechs, with four and six rubbers. My experience is that the triggers have not been all that hard to pull. I also speculate that trigger pull might not be too much of an issue with spearguns – the relatively high masses of the guns, stabilizing influence of water and the relatively close targets, all mean that aim is not going to be all that much impacted by a slightly hard trigger pull.

However, there is a factor that can make a real and significant difference, and that is the friction of the shooting line retainer hook on the sear. In Undersee style mechs, the line release hook bears on the side of the sear lever. In open muzzle guns, a fair bit of pressure is placed on the line hook (to keep the shooting line taut, holding down the spear at the muzzle). My personal experience is that the effect can be great enough to cause a problem. (I normally use a substantial bungee on open muzzle guns, which provides enough pressure to hold down the spear, but not so much as to increase the pressure on the line release hook).

Does anybody agree or disagree?

Ric
The spear tail notch is usually cut to half the available depth of the spear tail and to fit in the space, with some clearance, between the sear tooth and the backing projection on top of the sear lever "yoke". On guns without a backing projection that is not so critical as the spear tail pushes the tooth down by moving it on a sliding sear pivot pin position (as in some euroguns). On single-piece trigger guns the spear tail notch size and shape has more of an effect, but otherwise not.
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Old 02-26-2012, 01:57 PM   #15
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Re: Trigger Mechanism Design Rules

Quote:
Originally Posted by Tin Man View Post
Awesome stuff Pete. It has me going back and looking again at a mech I wanted to try.

One question though. When the tip of one lever (say the sear in Kitto's M3 mech) impacts the flat surface of the intermediate lever, isn't the direction of the resultant force perpendicular to the surface of the intermediate lever? I think that would then change the effective length of the lever arm about the pivot pin?

In the pic below, wouldn't the resultant force from the sear on the intermediate lever be in the direction of the green arrow, and then the effective lever arm about the pivot pin the length of the orange line?

I don't think it changes much in this case, but I'm trying to make sure I understand . . .
The diagram as I drew it is correct, if you were an engineer you would immediately recognize it as a torque analysis diagram. A torque is the force at a right angle operating at a certain radius out from a pivot. Same torque, less force further out, but more force when closer in to the pivot. Multi-lever trigger mechanisms are torque reduction devices. This stuff is not rocket science, in fact at this level it is taught in high school or college.
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