Mares Mirage

[popgun pete]

There may be a more prosaic reason why the power regulator in the Mirage has no coil spring on its control shaft. If you look at all the guns like the Sten their control arm or cursor has a right-angled bend in the arm that has a small knob attached on the outer end. With the guns set on high power that bent arm is jammed up hard against the back ledge of the selector gate (except for the Salvimar guns which are reversed), in fact that is all that stops it moving further back if say the back of the gate broke. If the Mirage had a two position gate then it could have worked the same way with the bent control arm under load in the rearwards position. However the Mirage with its three position gate has an angled arm that is not so square set and has a big thumb press knob on the outer end. In the rearmost position with the control arm length properly adjusted this bent arm does not rest on the gate’s back ledge, but stops just short of it. The reason it does not is because the piston plug is travel bound sitting on the metal sleeve or red plastic collar and can move back no further. In my gun that hold off distance is dictated by the two sleeves stacked up on each other, plus any sealing rings in-between. Mares probably did this to stop busting those arms off as that cranked arm would not be amenable to high loadings being placed on it all the time as it is not as robust as the arms used in the Sten and others like it. That cranked arm is a consequence of making the gun have a 3 position gate. On later models they tried to beef up that fancy control knob, but I have seen a number busted off on similar guns, although not Mirages except for one in a photo.

 

[tromic]

Did you notice a quite different approach in placing the sealing O-ring in the early and later Mirage?
In my Mirage O-ring is pushed inside the grove in brass tubing.
At the drawing below O-ring is in front of the brass tubing.

 

[popgun pete]

The tube ends are completely flat in the earliest gun, the "O" rings sits on a flat surface formed by the squared off end of the sleeves which are both plated. The ends of the sleeves are identical. I took care pulling the inner sleeve out so as not to scratch it by making a small but wide in a proportional sense flat hook to bear against as much metal as possible.

 

[popgun pete]

A sunny day, so washed the dust off and took these photos of the Mirage grip. Note you can just see the front of the metal sleeve which is the innermost one in the handle, the other being removed and seen in previous photos. Note the airflow cut-out which connects the transfer port behind the bulkhead (removed here of course) to the rectangular duct that runs directly under the main barrel to the rear of the gun which is what enables the gun to breathe by channelling air to the inner barrel.

Note the metal sleeve reflects off the shiny black wall of the entrance tunnel, the end of the metal sleeve is flat.

 

[tromic]

My is different: Maybe I have a lather, improved model...

 

[popgun pete]

Yes, they have changed it. As the "O" ring sits in that cupped end then the sleeve must be sealed separately in the grip handle as it cannot be sealed simultaneously by the "O" ring in the cup. That means I was right to put a ring as a gasket down the back and they probably did something similar with your model rather than glue the sleeve in. But this revised design still lacks a means to hold the O" ring from moving forward as the sleeve in front can move, so again they rely on internal gas pressure to hold the ring back. It is an improvement and would have been an easy modification to make as they have yet to change the handle moulding. As discussed in the previous posts that regulator shaft “O” ring is trapped in position from front and back in the Stens and Cyrano guns, but unless a fixed front cap on the regulator shaft is used there is no way to do it reliably.

 

[tromic]

I am not concerned about possibility the O-ring would be pushed out from its cupped end of the sleeve.
At 30 bar the force acting at the O-ring is 8.26 kgf. That is higher then the friction force that acts in opposite direction when one push the power regulator shaft forward. My back sleeve is very firm in its position. I could not take it out. I believe it seals excellent itself. No need for any additional gasket.

 

[popgun pete]

The force on the sealing "O" ring can only be that over the cross section of the regulator shaft tunnel as the tail end of the shaft is at atmospheric pressure which is being opposed by internal gun pressure pushing the regulator shaft out of the gun.

 

[tromic]

Yes, but as soon as the O-ring would be pushed forward even little bit, the air pressure would be applied to the whole surface of the O-ring inside the cupped brass tubing. So it would not be pushed out. It could happen only if the pressure in the gun is too low and/or if the power regulator shaft have not been operated for a long time so the friction may become higher.

 

[popgun pete]

Yes, but as soon as the O-ring would be pushed forward even little bit, the air pressure would be applied to the whole surface of the O-ring inside the cupped brass tubing. So it would not be pushed out. It could happen only if the pressure in the gun is too low and/or if the power regulator shaft have not been operated for a long time so the friction may become higher.

It is the latter.

 

[tromic]

Pete as I see, what you are talking about is presented at image a)
I talk about case at image b) where the o-ring is compressed radially.
That are two possible mode of operation, but quite different.

 

[popgun pete]

The pressure across the opening is what you need to consider, anywhere else it is balanced by a reaction from the wall opposing the pressure. That is why I left the outer arrows off that you have shown. Strictly speaking you could show arrows normal to the ring surface and radially disposed to the ring cross section, but these just squeeze the ring inwards, not backwards. If the ring was too soft or the pressure was extremely high then we would have to consider that, but not here. The pressure differential occurs across the bore hole and that is what needs to be considered in the force calculation.

 

[tromic]

I just measured the friction between regulator shaft and the O-ring in my Mirage. It was about 0.5 kpf. I still do not know the friction between the O-ring and its seat in cupped end of brass sleeve. That friction should be overcome to remove the O-ring from its place. I suppose it is higher then the measured friction because moving the regulator shaft forward did not result in pushing the O-ring out. My Mirage is not under pressure now.

 

[popgun pete]

Well we know they leak, so there must be a mechanism, every Mirage in that store had leaked because the oil had crept over the grip surfaces and onto the rear tank. If anyone had tried to wipe them off then they must have given up and that is why they were selling for 50 bucks. I checked there a few times and none had sold, I even contemplated buying another one, but one was enough. Plus 50 bucks was not a trivial amount back then. Those guns all had darker bronze tanks and the bump on the top of the rear folding handle which my gun lacks. I wondered if my gun was the initial sample the distributor received as it had the brown vinyl carry bag which these guns lacked, evidently they were from a later batch. My gun was identical to the first Mirage that I ever saw at MSD, a general sports store, there being no dive shops as such, diving being just a sub-section in those stores.

 

[tromic]

Mine was leaking with O-ring 3.68 x 1.78 as I wrote in post
#76:
"I am not sure about the original O-ring dimensions for this Mirage, but it was leaking with the O-ring from Mares spare O-rings kit (3.68 x 1.78 mm). Maybe it would not leak on pressure above 18 bar. In that case OR 3,68 x 1.78 might work as a floating O-ring!? But on low pressure it leaks! Because of that I've put OR 4 x 2 mm."

After I changed O-ring for 4 x 2 mm I have not noticed leaking.

I don't know dimensions of o-ring in your Mirage, Pete, but if the regulator shaft is 4 mm OD, and the ID in handle boring for sleeves 8 mm, I would try to use O-ring 4 x 2.5 mm. That would be for static application with compression 20 %.

 

[popgun pete]

Mirage for sale, gun only. The gun is not working and leaks oil.

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This example still has its vinyl sticker which tended to lift off and chip around the edges as the vinyl gradually dried out and the adhesive failed. I peeled mine off thinking that it might go back on, but it stretched and would never lie flat again so I threw it out.