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Kartoffelbri

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I have been assigned the task to calculate the strength of threads for the hydraulic cylinders my company manufactures. I have started doing some research and have found several guide formulas and methods to find shear area, length of engagement, force resulting in failure and so forth, but every equation or source seems to specify it applying to fasteners. The cylinders produced here are sized quite a bit larger compared to the average fastener and so I am not sure if these equations still apply. We make cylinders with up to a 3 foot diameter as far as I know or even bigger, but on average they have about a 1 foot diameter. I know in the mechanical engineering world there are many instances where one specific calculation has different equations needed to solve it based on the existing parameters. Is this one of those instances or is it a one equation fits all situations type circumstance? And if these equations do not apply for larger diameters, what equations do I use, as I cannot find them?

On another note, I see several formulas to derive the force that will result in failure of the thread based on the tensile strength of the thread material, but I do not see any equation explicitly stating how to find the acting/working stress on the thread so that I can have a representation of a safety factor. In this case, since I am working with hydraulic cylinders, I should be able to find the resultant force based on the operating pressure and the inner diameter of the top stage of the cylinder. Once I have found that, I just divide that by the shear area of the thread to find the working stress on the thread correct?

Brian