Eurogun spear tails and their effects on trigger mechanism design

[popgun pete]

In the speargun trigger mechanism design rules thread I looked at the trigger as a torque reducer where the sear lever and trigger geometry use the lever arm lengths to reduce the force needed at the trigger to fire the gun. In many trigger mechanisms the design aim is to direct the force from the sear lever directly at the trigger’s pivot pin to make the torque on the trigger close to zero.

However on many reverse trigger mechanisms they cannot do that as the sear lever will sweep through the trigger pivot pin position, hence they are geared to make the incoming torque small. One aspect that intrudes on this arrangement is the angled sear tooth as unlike a square cut tooth there are force components not parallel to the spear pull direction. On the second diagram shown below the components of spear pull force are shown in orange, the normal to the tooth contact and the parallel to the tooth surface where they touch. The latter force component tried to push the sear box roof off, but the former pushes down on the trigger. If you calculate the torques as the sear lever moves then they are the same as before, but the angled sear tooth puts a static load on the trigger retention step as the sear lever arm cannot bend. These reverse trigger mechanisms would work better with a square cut tooth, but then they would not be able to use a wide rectangular mouth sear box. The reason that they do dates back to the shooting line running back into the sear box and attaching at the extreme rear of the spear tail because the tooth angle causes the spear tail to clamp firmly up against the sear box roof. In square cut tail sear boxes the shaft is surrounded by a tunnel and is usually round, not flat except for a dent in the roof or flats on the sides to stop the shaft rotating or rolling in the sear box.

Eurogun shafts cannot roll as their flat tails are always pushing upwards on the sear box roof and thus they cannot wobble side to side especially when the band load is on, even when the sear box has no floor and no sides touching the spear.

 

[Andrew the fish]

These reverse trigger mechanisms would work better with a square cut tooth, but then they would not be able to use a wide rectangular mouth sear box. The reason that they do dates back to the shooting line running back into the ...

I am still using that far end hole to attach the line. Can't do it on the enclosed track, but otherwise there is nothing wrong with it. Round tooth is the part that assist with trigger pull loads, I see no problem here, it is just another design route and I actually like it better. There is such thing as a trigger travel, with round notch it is only limited by safety factor and size of sear.

 

[popgun pete]

Maybe you should read this http://www.spearboard.com/showthread.php?t=199608

 

[Andrew the fish]

thank you for the link. I bothered to read the whole thing, did not add much to your point.

Spearq8 posted there "...Reverse triggers in general have a tremendous amount of pressure on the sears. There is no lever divider to reduce the stresses on the sears... " which is completely wrong.

The arm length gives you about 8:1 mechanical advantage, reducing the stress on sear about 8 times. I use "8" as an example, just from a look at the diagram. The further back you put your reverse trigger, the longer will be the connecting arm, and greater mechanical advantage . Add to it the fact that actual downward force on the lever is already reduced, because it acts on the surface of rounded tooth and have to be normalized to the lever axis. Hard to explain without a diagram.

 

[popgun pete]

Well there is a lever arm advantage and if you read my trigger mechanism design rules thread on Spearboard then you will see everything in more detail. However Spearq8 has spent a lot of time in pools shooting many guns looking for long range accuracy (maybe too long in the pool) and has found that rough surfaces develop in the reverse trigger mechanisms that he has tested, so something is causing those problems. Mind you the jerk of the spear bouncing out of the sear box mouth is very small and for most shooting purposes you could ignore it, but ranges of 6 to 8 meters and small things do make a difference. Personally I don’t shoot at those distance and for me 6 meters is a long shot, however he has threaded the needle shot after shot with a gun set up perfectly by smoothing the rubbing contact surfaces in the guns operating at those longer ranges. Recently he has been promoting the Ermes Sub Double roller trigger mechanism because the rollers get rid of any jerk as the spear tail leaps out of the trigger mechanism and there are no contact surfaces to polish, well not so far as these mechanisms have only been around for about a year.

My take on it is the problem of shafts not exiting smoothly is the sear boxes only have a short roof and effectively no sides or floor, hence the spear tail is not tightly controlled inside the sear box. Contrast this with the “Sea Hornet” trigger mechanism which is one of the first reverse trigger mechanisms and it has a very long sear box tunnel. I have used many of those guns and they shot very accurately at the ranges which I usually open fire at, but they are not 6 meters plus range guns and were never designed as such.