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Of course, an additional constraint on piston radius is the allowable space inside the Avionics Bay Coupler. The Team previously found that 6491K254, which had a 1in radius, was large and provided little room for Avionics to house its hardware, especially the batteries. Thus, a logical conclusion is to restrict the new piston geometry to radii below 1 in, which will provide an even larger safety factor on premature separation due to a pressure differential.
Geometry
Given the allowable bore-radius range of 0.092 in → 1.51 in (and more accurately, a 1 in upper limit), we opt to initially select a radius of 0.50 in, which is roughly in the middle.
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This is also a logical decision because 0.5 in is the minimum typical bore diameter for tie rod air cylinders.
Tolerances
To create the bore, we plan to take an existing piece of hollow aluminum tube and turn it down to the proper outer radius (the inner radius can be achieved by drill and then reamer). The chosen wall thickness of 0.0625 is achievable within the 0.5 thousandths diameter tolerance of lathes on campus.
COTS Solutions
First, it is necessary to determine an appropriate COTS solution for the piston. Due to timeline constraints associated with the difficult task of engineering base plates (most notably, all of the required seals), it is logical to take an existing piston and modify it to meet our needs (i.e. changing the throw on the piston, making mass saving cuts, etc).
Solution No. 1: 1691T104
The first solution is a 0.5 in diameter compact tie rod air cylinder with a 4" stroke. Because the coupling section is 4.5", we need a much larger throw than that to achieve an appropriate factor of safety. Thus, it is necessary to replace the bore with a longer one (and also the tie rods). This is also necessary because the 1691T104 piston has a composite bore, which adds safety complications.
Resources:
The following resources are useful materials for learning about pressure vessel and piston theory:
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