A phenomenon that I find very interesting about pressure tubes, is that given any particular material, to hold a specific volume, at a specific safety factor at a specific pressure, it doesn't matter if you use one tube, or 2 or 3 tubes to meet the volume, the total weight of the tube, or tubes, will be exactly the same.For example, if you have a 1" dia x .095 wall x 10" long 2024 alum tube, it will hold 84.4ccs at 3200 psi with a 3 to 1 safety factor.That tube will weigh .26 lbs.If you make a tube to hold twice the volume, you will need a 1.416" dia x .135 wall x 10" long tube to hold 168 ccs at the same pressure and safety factor. And guess what. That tube that holds twice the volume weighs exactly twice as much, .52 lbs. So one tube or two tubes, there is no weight advantage, if you use the same material. It often boils down to what material is available in what wall thickness.
With a threaded end cap sealed inboard of the threaded portion, there are still two separate calculations required.... one to determine the tensile (yield) force required to rip the threaded portion of the tube from the part of the tube that is full wall thickness (that failure would usually occur right at the inboard end of the threads).... and secondly to determine the shear strength of the threads themselves.... For the second calculation, the shear strength used would be 60% of the tensile strength of the WEAKER component (eg. an aluminum plug in a CrMoly tube, use the aluminum).... and more threads increase the strength, so this is seldom an issue....The only time you have to worry about the hoop strength of the bottom of the threads in the tube is IF that portion can become pressurized.... That MAY be possible if the O-ring leaks (especially rupturing suddenly) and the pressure cannot escape quickly enough.... The spiral gap along the threads (zero on tapered threads) may not vent the pressure very fast, so to be very sure, I guess a small hole drilled in the area between the O-ring and the threads would be a good idea if the wall thickness of the tubing on the OD of the threads was suspect for strength.... Never thought of that before, actually, see how these discussions can increase our knowledge !!!!Bob
This is an amazing discussion. I have learned a ton. I feel MUCH more comfortable diving into building my gun now. To be honest the air cylinder has been freaking me out a little. I still need to figure some things out but I am confident I can accomplish this safely now. Thank you guys, what a great plethora of knowledge.
Sean,It can be 60% of the yield strength, or 60% of the ultimate tensile strength, depending on the application and how conservative you want to be. Even the 60% is an engineering estimate based on test data collected over decades. For some materials, that percentage is higher than 60%, for some, it is lower. All of our other calcs are based on the yield, because I think we want to know when the material starts to fail, rather than when it is too late. It's more a matter of how conservative you want to be.
On the Ti, do keep in mind it is a stretchy stuff...a Ti bolt running in a nut significantly stiffer( steel for example ) will risk domino failure as it loads up the first few threads engaged...this in a tension loading. Putting a ti nut on a ti bolt is another thing I have been warned of...cheers,Douglas