Trigger Mechanism Design Rules

[Tin Man]

So are you saying that the angle of contact, where the sear tip contacts the intermediate lever, has no bearing on the torque imparted to the intermediate lever? This seems intuitively incorrect to me.

Originally Posted by popgun pete

. . . 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 . . .

I agree here. I'm just questioning the direction and magnitude of the forces which generate the torques. I thought I remembered that a frictionless (making that assumption) surface could only exert a force in the perpendicular direction. So, for example, a weight sitting on an inclined ramp will exert horizontal and vertical force components on the ramp. It seems that the sear, in contacting an inclined surface, is sort of like a weight sitting on a ramp. I certainly agree that ultimately we are trying to determine torque reduction.

[popgun pete]

Originally Posted by Tin Man

So are you saying that the angle of contact, where the sear tip contacts the intermediate lever, has no bearing on the torque imparted to the intermediate lever? This seems intuitively incorrect to me.

Unfortunately intuition is wrong here, the torques are as I have drawn them, it is a standard method of analysis used in such situations. As a former engineer I used to do this stuff all the time and have worked on a number of speargun mechanisms using this method.

[Tin Man]

Sounds like I'll be sketching and trying to refresh my memory tonight.

[popgun pete]

Here is a "pull-down sear lever" trigger mechanism from a spring gun. The sear lever is in a sense a locking pawl pulled down by the trigger. This same two-piece trigger mechanism is also used in early French Arbalete's.

[fish impaler]

How does this look with regards to geometry?

[popgun pete]

Originally Posted by fish impaler

How does this look with regards to geometry?

That is pretty much the "Undersee" layout which started the progression to two-piece trigger, "cam lock" mechanisms. Riffe and Alexander are copies of it, as are many others. The sear tooth should be vertical, not tipped back as shown here. Also the trigger pivot pin is usually located higher up in the housing, that way the force from the sear lever tail can be aimed virtually right at the trigger pivot pin.

[Tin Man]

Pete - If I understood your analysis correctly, are you saying that both of the fictional mechs below would see the same torque "T" on the trigger lever? They have the same lever arms "d". The only difference is that in the lower sketch, the sear tip impacts an angled face (angle "A") on the trigger, vs. a perpendicular interaction in the top sketch. I'm still thinking that the trigger lever torque will be higher on the lower sketch, due to the angled face.

[chalkeye]

[popgun pete]

Originally Posted by Tin Man

Pete - If I understood your analysis correctly, are you saying that both of the fictional mechs below would see the same torque "T" on the trigger lever? They have the same lever arms "d". The only difference is that in the lower sketch, the sear tip impacts an angled face (angle "A") on the trigger, vs. a perpendicular interaction in the top sketch. I'm still thinking that the trigger lever torque will be higher on the lower sketch, due to the angled face.

If they were able to be locked up and those dimensions were truly the same, then yes. Think of it being like the separate gear shafts in a multi-step reduction gearbox, the shape of the gear teeth (provided they all mesh) makes no difference to the relative speed and torque produced in each of the shafts. In a speargun mech we are only concerned with the pressure felt through the trigger as everything else does not move until the trigger retention step clears the lever tip that it is restraining (being curve matched for this purpose where they slide against each other). Then everything else starts moving and we have no interest in what happens next, as long as there is no hanging up effects as the levers sweep past each other. After that the spear tail escapes from the sear box. Of course the "locked up" condition has to be reliable, to withstand a certain degree of component wear without becoming dangerous and the mechanism has enough play and clearances (in the right spots) to not jam up with the tiniest of foreign particles, yet still hold.

Because spearguns operate in non-clean room environments there has to be this capacity for operating under rugged conditions and as the parts are usually mass produced at an economic cost they do not have absolutely tight tolerances. That is what made the "Undersee" mechanism with its brass levers such an effective trigger mechanism as it was moderately rough in its construction, but was good enough in the places that count. As the number of levers increase in a trigger mechanism then the tolerances have to become tighter as any errors magnify through the gear reduction system. For example I was working on a variation of the Mori/Kitto/Kalohi mechanism with four levers to comply with the lever rotation rule (which I have not mentioned), to make it a "cam lock" mechanism (purely as an academic exercise). The tolerances became impossibly tight as it would either not relatch properly, or lock, as you would sacrifice the one at the expense of the other. For four lever mechanisms you really need a two-stage relatch, which is how the Kitto M5 works, the M3 just squeaks by with very tight tolerances to be latched with a single spear push, and a good one at that. As most spearguns can get by with two lever mechanisms there is really no need for new designs as everything is pretty well catered for now, from compact narrow cross section rear handle guns to the multi-band "timber logs".

Incidentally your upper mechanism will not "cam lock", there is no abutting of adjacent surfaces, so the levers will push past each other, hence it fails as a trigger mechanism. Your lower mechanism will also not lock for the same reason, plus both these mechanisms when you pull the trigger will tip the sear tooth up and rearwards against the band pull, which is a big no-no.

[Tin Man]

They weren't meant to be trigger mechanisms exactly, just examples to illustrate the forces. I drew the spear notch that way just to position the notch engagement directly above the pin, so that everything would be at 90° angles. I'm just trying to understand your analysis of torque transmission, and why we see it differently. So if you think those two mechanisms would impart the same torque "T" at the the second lever (I disagree), then let's refine it to the special case where the lever arm "d" is reduced to zero.

The top mechanism is now just a simple latch. If the contact point is directly above the pin of the second lever, then the top arrangement should have zero torque at pin 2.

For the lower arrangement, the contact locations are the same, but again the second lever has an angled surface. Surely we agree that the second lever would be subject to torque in this case, yes?

So how is this different from the more general case I desribed first?

[popgun pete]

Originally Posted by Tin Man

They weren't meant to be trigger mechanisms exactly, just examples to illustrate the forces. I drew the spear notch that way just to position the notch engagement directly above the pin, so that everything would be at 90° angles. I'm just trying to understand your analysis of torque transmission, and why we see it differently. So if you think those two mechanisms would impart the same torque "T" at the the second lever (I disagree), then let's refine it to the special case where the lever arm "d" is reduced to zero.

The top mechanism is now just a simple latch. If the contact point is directly above the pin of the second lever, then the top arrangement should have zero torque at pin 2.

For the lower arrangement, the contact locations are the same, but again the second lever has an angled surface. Surely we agree that the second lever would be subject to torque in this case, yes?

So how is this different from the more general case I desribed first?

Think about the gearbox analogy, this is not really a very productive discussion, but you seem to have a bee in your bonnet about it. Concentrate on the lock, not the angle of the lever interactions. It does make a difference at the sear tooth as that is how Jack Prodanovich got his "balanced sear" mechanism to work even with a high mounted sear pivot pin normally associated with single-piece triggers. Otherwise you accept what I say or you don't, that is up to you entirely.

[Tin Man]

Originally Posted by popgun pete

. . . Otherwise you accept what I say or you don't, that is up to you entirely.

Never been very good at accepting rules without understanding the details, but that is for me to ponder further.

Sorry for the derail.

[popgun pete]

Originally Posted by Tin Man

Never been very good at accepting rules without understanding the details, but that is for me to ponder further.

Sorry for the derail.

Don't worry about that. I will send you a pm on it and then you will see why the issue you are worrying about is not a factor here.

[popgun pete]

Wally Potts followed Jack Prodanovich's lead in using the "balanced sear" design in his own later model spearguns. The "balanced sear" lever design, developed by Prodanovich using his genius for innovating and perfecting spearfishing equipment, allows a pivot position for the sear lever which is essentially the same one as would be used for a single-piece trigger. Such an arrangement would not usually allow the sear tooth to roll forwards under band pull action from the shaft tail as its function is to hold the shaft until the trigger is pulled by the operator. What causes it to roll here (the Wally Potts diagram is attached) is that the sear lever is destabilized by cutting an angled sear tooth with a matching angle cut into the spear tail notch. Through patient experimentation Prodanovich had varied the tooth angles and the fore-aft offsets of the tooth position from the sear pivot pin and thus discovered that the sear lever could transition from hanging up, as is required in a single-piece trigger, to just turning with the load, but at a minimum level of torque. Thus he could control trigger pull in his trigger mechanism to be low even with very high band loads, despite no significant gearing in the sear lever component and a close spacing of the pivot pins in the mechanism housing which usually limits the gearing. Thus Prodanovich had beaten two of the trigger mechanism design rules at one stroke. His mechanisms were either set up for high power use, he had tested it for 800 pounds band load and had one in his big Tuna gun (at 1,000 pounds!) which threw a heavy shaft with a tandem lapped band layout. Like a gas operation pistol the speargun functioned best with "high power ammunition", but for lower band loads the mechanism was tuned to offer more torque at those reduced force levels to roll the sear lever. An impressive feat for a self-taught engineer and one not recognized by many who know of his pioneering development work.

The Potts version of this mechanism has been designed for 400 pounds band load and uses a sear tooth angle that suits this mechanism's particular geometry which is very different to that found in the Prodanovich gun. The diagram shown here is basically an installation instruction sheet and provides the method to dismantle it for inspection, it is not an engineering drawing, but shows the general internal layout.

http://rocknfish.com/Jack%27s_Tuna_Gun.html

[Scott Diego]

I am honored to even post on this thread... Lots of great information in these page.

Please keep the mechanism-guts examples coming. I wish I had more to offer in terms of dissected mechs (but alas have to get back to work)