VACUUM RETENTION OF SPEARS IN PNEUMO-VACUUM GUNS

[popgun pete]

Recently I had to replace a Mares "Cyrano" piston, which I had damaged the nose of, and noticed that the white acetal plastic is rather tough stuff until it gets loaded up unevenly and then it can be crunched. Possibly sticking a drill in the piston front end might also crack it, but it seems to me that loosening the shaft tail fit in the piston is an easy way to provide a vacuum check after your gun is cocked and which has been supplied, or retrofitted with, a captive seal vacuum muzzle.

This topic came up recently on another forum when discussing the "Taimen" and "Pelengas" spearguns. The latter uses Mares pattern spears of local manufacture, all in spring stainless steel, hence when loaded the spear tail is jammed in the piston face and you have no way of knowing if the vacuum seal is still doing its job. On the other hand the "Taimen" will let you know because the spear can fall out, but not completely as for the spear to leave something has to replace it in the inner barrel. There is a weight limit that the vacuum can support, but most common gun sizes will not be a problem.

Enlarging the hole in the piston face would probably be safest using a sanding or grinding action with a support dowel that could be pushed into the piston face with abrasive paper wrapped around it while the piston was spun in a variable speed power drill. A careful incremental approach is necessary as you want to remove just enough for a loose, but not sloppy, fit of the spear tail.

 

[popgun pete]

A simple tool such as this should do the job.

 

[tromic]

Another solution might be inserting a small plastic plug into the pistons boring. I did that long time ago. There was an image of my solution on this forum but maybe it was lost. The problem of loose binding might be scratching of inner surface of the barrel if the shaft tail end butt had some sharp damage on it.

 

[popgun pete]

You need the spear tail to sit at the same depth in the piston as before, the idea is to make it not a tight fit so that the spear can be pulled forwards slightly. When you release it the vacuum will pull the spear back into the barrel, signifying that the inner barrel actually has a vacuum. What moves the spear backwards is external pressure acting over the cross-sectional area of the shaft, which being greater than the pressure behind the shaft tail, i.e. next to zero, moves the spear back into the gun. If the spear does not move back then the vacuum has been lost. Filling any gap behind the spear tail when it is inserted into the piston may also be worth doing.

 

[tromic]

Boring in Mares Cyrano piston I had was conical, not cylinder shape as some other pistons have. Instead of 11 mm depth with depth of 9 mm the effect was similar to widening the boring. For holding the shaft only due to vacuum the depth might be 8 mm instead of 11 mm, not quite loose but having minimal friction suitable for the vacuum test.

 

[popgun pete]

I have a damaged piston to play around with, useless for shooting, so I will slice it up and check the situation. In the old days metal pistons and spear tail could wear so badly that at times the spear could fall out of the cocked gun, usually the spear tail had worn the most. Fix was to use a new spear and if necessary use a punch to upset the metal on the piston face to allow it to grip, stainless steel pistons especially. Spears are more likely to be lost or damaged today before that happens as plastic pistons don't wear down the spear tails.

 

[tromic]

If vacuum sealing were good than the vacuum would hold the shaft (7 mm) attached to the piston up to the length of the shaft 130 cm, pointing down. In that case the force of the "vacuum" would be about the same as the weight of the shaft - about 0.4 kg. This would be true if we assume there were no friction between the sealing cuff/O-ring and the shaft, and if we were above the water, in air.

 

[popgun pete]

Here is the table of shaft weights for "Taimen" speargun models in grams.
Assuming that the pressure in the charged up to shoot gun's inner barrel is zero, or very close to it, and calculating the shaft cross-sectional areas for the various spear diameters used which are 6.5 mm, 7.0 mm and 8.0 mm, we obtain areas of 33.18 sq. mm, 38.48 sq. mm and 50.26 sq. mm. Thus the maximum shaft weight, including accessories, that can be supported is 343 g., 398 g. and 519 g. respectively.