doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com psi on hingh pin

See this pipe gun:

http://youtu.be/7Va87gB_4AI

Obviously there is NO hinge pin on this construction. Observe how both the barrel and the outer "breech" pipe recoil together, dragging the "breech" backwards.

I suspect that the expanding shell sticks to the "chamber" and barrel and shell casing act as one unit pushing back on to the "breech". If something similar happens in double guns then the strain on the cross pin is minimal. From this point of view the friction coefficient of the chamber would become a factor to consider.

This is a fascinating subject and really could benefit from some high speed video so we can see what really happens to the breech during firing.

In my previous posting I mentioned the Poisson effect. Thick walled cylinders expand radially under pressure and contract axially. Obviously after the pressure is reduced the cylinder returns to its original dimensions and this recovery must involve some force on the breech face and cross pin. As far as I know there has been no analysis of this phenomenon. Most authorities focus on the breech flexing, assuming that there is no dynamic processes going on in the barrels. The authorities consider that the pressure on the breech face is via the shell alone, while the pipe gun tends to indicate that barrel and shell together press the breech face. Just looking at the imprints left on the breech face of double guns seems to indicate that there is some pressure exerted by the barrels too.

To add to the above. How much strain is there on the barrel of an auto which is held via a shallow thread on the magazine tube? If the strain is as great as calculated above those shallow threads should show some serious wear. That they do not tends to indicate that there is little forward thrust via the barrel on the threads.

Note that "the strain is as great as calculated above" is NOT forward on the bbls but rearward on the breech. You do bring up some interesting points, but that the forward thrust on the barrel is not equal to the rearward thrust on the breech is really obvious, by the fact that when the two are bolted together the gun recoils "Backward".
A barrel would have some forward component of thrust on it from the friction of the load being pushed through it, but even in the case of the autoloader the breech bolt is locked to the barrel at the moment of firing thus actually exerting a larger rearward force to the barrel than its forward component.

Poisson effect? Something downright fishy there.

Piper you have a point re the frictional force in the barrel.

Overall the locking of double guns seems to be a result of empirically derived choices rather than analysis.

Over the years I have measured many guns. Some have cross pins of 8 millimeters and are solid, others have 10 millimeter cross pins and rattle. Obviously it is hard to isolate the causes of the looseness, or to prioratise them, ie does friction contribute more or less than strain from firing, etc.

The impression left after much measuring and comparing is that the fewer locking elements there are, and more generously dimensioned they are, the more robust and long lasting the lockup. Prime example is the Winchester Model 21 which as far as I can determine has a two point lockup- the cross pin and the round locking bolt. The French Manufrance Robust is another solid locker with two lock up points, so is the Beretta 626.

Shotgunlover;
There are several American guns of renown which utilize Two Point bolting. The L C Smith, Ithaca NID & Fox guns all depend upon the hinge pin & the top rotary bolt. Lefever Arms Co guns use the ball hinge & a bolted square shouldered dolls head. D M Lefever guns use the ball hinge & top Cross Bolt, very much akin to Greener's. Parkers used the pin & Single underbolt but unlike the Win 21 had a square shouldered, unbolted, Doll's Head. Baker Bativia Models as well as many Stevens/Savage models used the hinge pin & a bolted straight rib extension. For multiple bolts to all be effective they must be extremely well fitted so they all bear simultaneously, Something unfeasible on many lower priced guns one sees advertising a multitude of bolts. Even then it is not unusual to see guns advertised as being triple or even quadruple bolted with all the bolts restricting the rotational motion of the barrels & only the hinge pin containing axial forces. Such in my opinion is purely advertisement Eye Wash.

Thanks, Tom. 2-p made a good catch on the 90% of half = 45% of the circumference. His math is right --- see below.

I don't think the pin taper, if any, will make much difference.

I agree that circle jointing, doll's heads, etc. complicate the issue. I was shooting for a SWAG.

DDA

2-p, you are correct on catching the half circumference. That is what I was thinking, but forgot to divide by two in me equation. Shame, Shame on me!!! Thanks for your keen eye. I always appreciate your review.

Even the possible 16,860 psi is still well within the working limits of steel. The above number is interesting in terms of relative rotation between the hook and pin. However, I think the more relevant number is the shear stress on the pin. That is the stress required to yield the pin (in two places) across its diameter. I measured the hook of the gun considered above and got a pin of very near 0.50". So, the section area of a 1/2" rod is pi X radius squared = 1/2 divided by 2 quantity times itself times 3.14 = 1/4 X 1/4 X 3.14 = 0.0625 X 3.14 = 0.196 square inches. Shear on the pin in two places means the shear area is 0.392 square inches. So, we have a shear stress of 4276# divided by 0.392 square inches = 10,908 psi shear on the pin. This is still well within the usual working stress for low carbon steel.

For whoever above observed that most of these designs seem to be emperical, I agree.

DDA

I like white lithium grease and, especially, Mobil 28 grease. Any of the modern gun oils will do fine; I use Rem-Oil mostly due to availability and cost. The trick is to wipe down bearing surfaces and relube frequently; like after every outting.

DDA

I don't think you can do a simple contact area /force calculation to get specific loading on the circumference of the pin. The pressure won't be the same as the surface changes from 90� to the force as it is around the curvature of the pin. I think its a very complex problem to solve. But I also think the surface pressures aren't the limiting factor.