08-19-2014, 05:45 PM
Now lets discuss stresses in the upper if you are using a device that clamps on the upper, thus causing all the torque forces to be reacted by the upper.
Torque on barrel nut is reacted by the threads, index pin, and friction, which ultimately is reacted by the cylinder section of the upper. It creates a torsion and shear force through the cylinder section of the upper. The torque value of 80 ft-lb on the nut creates about 4000 psi of shear stress in the cylinder of the upper. The shear allowable for 7075 Forging is 44,000 psi. No big deal right?
Well as that torsion load travels back down the upper, it comes to some cross sections that are not continuous like the cylinder portion of the upper.
The upper transitions from a continuous cylinder into a more rectangular section that is not continuous at the bottom and even at the ejection port.
So what are the shear stress values now at these sections?
For 80 ft-lb torque a continuous rectangular tube approximately the size of the upper still generates about 4800 psi of shear stress.
For 80 ft-lb torque reacted by a channel (3 sided square tube) of the same thickness, the shear stress goes up almost 10 fold, at around 48000 psi above the allowable for the material.
So if you clamped an upper from the middle of the ejection port and back, you could really do some mangling on an upper. Because the full torque will have to be reacted right were the clamping begins.
I did this study just as a reminder of how important it is to realize how easy it could be to distort an upper if you aren't careful about thinking where this torque load is going to be reacted.
Imagine clamping locally, just the behind the barrel nut threads. That would be much better than clamping just behind the ejection port, right? Because the barrel nut thread area is continuous, whereas around the ejection port area has 2 locations that aren't continuous.
That is why I think the reaction rod is a great idea and tool and I think I'm going to get one. It keeps all torque load out of the upper, index pin, and barrel. It is only reacted at the barrel extension lugs.
The clamshells and the cutting board types (which is what I currently have) that LRRPF52 suggested, are fine if you are installing a barrel at the ranges specified by the armorer spec, which I believe is between 50-80 ft-lbs. But if you are a removing a loctited barrel, I would use the reaction rod type.
Torque on barrel nut is reacted by the threads, index pin, and friction, which ultimately is reacted by the cylinder section of the upper. It creates a torsion and shear force through the cylinder section of the upper. The torque value of 80 ft-lb on the nut creates about 4000 psi of shear stress in the cylinder of the upper. The shear allowable for 7075 Forging is 44,000 psi. No big deal right?
Well as that torsion load travels back down the upper, it comes to some cross sections that are not continuous like the cylinder portion of the upper.
The upper transitions from a continuous cylinder into a more rectangular section that is not continuous at the bottom and even at the ejection port.
So what are the shear stress values now at these sections?
For 80 ft-lb torque a continuous rectangular tube approximately the size of the upper still generates about 4800 psi of shear stress.
For 80 ft-lb torque reacted by a channel (3 sided square tube) of the same thickness, the shear stress goes up almost 10 fold, at around 48000 psi above the allowable for the material.
So if you clamped an upper from the middle of the ejection port and back, you could really do some mangling on an upper. Because the full torque will have to be reacted right were the clamping begins.
I did this study just as a reminder of how important it is to realize how easy it could be to distort an upper if you aren't careful about thinking where this torque load is going to be reacted.
Imagine clamping locally, just the behind the barrel nut threads. That would be much better than clamping just behind the ejection port, right? Because the barrel nut thread area is continuous, whereas around the ejection port area has 2 locations that aren't continuous.
That is why I think the reaction rod is a great idea and tool and I think I'm going to get one. It keeps all torque load out of the upper, index pin, and barrel. It is only reacted at the barrel extension lugs.
The clamshells and the cutting board types (which is what I currently have) that LRRPF52 suggested, are fine if you are installing a barrel at the ranges specified by the armorer spec, which I believe is between 50-80 ft-lbs. But if you are a removing a loctited barrel, I would use the reaction rod type.