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Old 01-09-2022, 09:18 PM   #196
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Re: Trigger Mechanism Design Rules

I have been asked for a full diagram of the Ermes Sub Double Roller mech so here is an approximation of it. The important part of this mechanism is the shape of the top of the trigger which has a cup shape for the sear lever nose roller to press against, but the nose roller cannot move past it. However pulling the trigger revolves this cup away from the nose roller which provides a clearance and the sear lever can then fall. Either gravity during dry firing or band power pushes the sear lever down as it has no biasing spring. The trigger has a biasing spring of the torsion type as does the line release lever (not shown).

The roller tooth has a pin driven through it to keep it captive in the side slots or windows in the cassette walls, otherwise it would fall out of the mechanism. The roller tooth moves the force from the spear tail down to the cup cut-out in the sear lever arm so that it effectively is the tooth holding the spear tail, the roller just being an intermediary element. The roller tooth does not necessarily sit snugly in the cup, any clearance to the sear box roof will roll the roller forwards causing it to wedge the spear tail between the sear lever arm cut-out and the sear box roof. Hence there is some accommodation for different diameter spears and their tails.

During relatching the nose roller acts on the trigger which has a curved ramp below its cupped retention step so that the sear lever revolving upwards can push the trigger out of the way and allow the nose roller to sit once again in the latched position.
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Old 01-10-2022, 12:19 AM   #197
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Re: Trigger Mechanism Design Rules

This second diagram shows how the "mobile" roller tooth can accommodate different diameter spears by riding up inside its cradle or cut-out in the sear lever arm to jam spear tails against the sear box roof (or ceiling). The curved euro spear tail notch is what makes this work as it drags the roller tooth forwards and up if there is any ceiling gap. The curved rear of the spear tail notch creates a force component that pushes the sear lever down as was shown on an earlier post once you finally pull the trigger.
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Old 01-11-2022, 07:05 PM   #198
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Re: Trigger Mechanism Design Rules

A few years back Sea Hornet Australia were developing a eurogun style speargun which was a first for them as previously their range consisted of cocking stock guns. Some of these guns had a very short cocking stock by butting the rear loading pad tight up against the rear grip handle, but they were still the same layout of parts.

The prototype eurogun was sent out for trials but very little was heard of it and unfortunately the company went into closedown and ceased operations, so that was the end of that, well for now that is as the company's tooling is all in storage.

The only freely available info was that the gun used an upside down trigger mechanism with the sear tooth dropping into a spear tail notch from the top rather than from underneath. By way of someone stumbling across a diagram we now know what it was going to look like.

The sear lever is a big yoke with a ramp on the lower arm that acts as a backing projection when you load the spear tail into the gun. This resets the sear lever revolving it down for its square cut tooth to engage the shaft notch. With the bands applied the shaft tail drags forwards out of the back of the yoke so that it can now revolve when you pull the trigger. The retention step is on the front of the sear lever yoke's lower arm and the trigger’s catch element drops down a window cut in the front of the lower yoke arm's tip, just rearwards of the retention step when you pull the trigger.

Certainly a rather different arrangement, but the sear lever would not be so easy to make, especially with that hole in it that creates a window whose "frame" you would not want to bend, otherwise big problems!
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Last edited by popgun pete; 01-11-2022 at 08:16 PM. Reason: adding the advert page
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Old 01-11-2022, 11:25 PM   #199
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Re: Trigger Mechanism Design Rules

Note that there does not appear to be any cam locking of the levers here, the mechanism seems to rely on the force at the sear lever's lower tip, or retention step, not being great enough to roll the trigger when pressing on the "locking" tip that extends out behind the trigger when the gun is cocked ready to shoot. Any problem with the trigger biasing spring and it would appear that the gun would shoot without you pulling the trigger! This also breaks one of the trigger mechanism rules that the trigger should revolve in the opposite direction to that which the mechanism under band pressure is trying to urge it when you come to shoot the gun.
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Old 02-24-2022, 09:38 PM   #200
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Re: Trigger Mechanism Design Rules

While discussing the above mechanism with someone else it occurred to me that the force on the sear tooth with the bands cocked would close the sear lever rather than open it up as in essence we have a single-piece trigger sitting upside down if you just consider the top part of the sear lever. Now if the line of contact of the band force runs above the sear pivot pin then the torque thus created will hold the spear tail by the sear tooth pressing down on it. The only way this set-up is going to shoot is if the tooth rocks at a slight angle emulating a worn sear tooth and creates a component of band force that runs under the pivot pin which then provides a net torque which will open the sear lever, or the force runs right through the sear lever pivot pin creating zero torque and acts as a hook on which the spear can dangle! Either way it seems not a very promising design as you don't want the spear to hang up when you pull the trigger!

The horizontal spring sitting at the rear may give the sear lever a push, but using springs to make the gun shoot is also not a good idea as springs lose their power over time.
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Last edited by popgun pete; 02-24-2022 at 09:46 PM.
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