I am working on a cylindrical (externally loaded) pressure vessel,with two flat end caps connected by 4 retaining threaded rods(8-32) and nuts. We generally snug tight retaining rods with nuts.I calculated preload torque for 8-32 screw with nuts to be about 14 lb-in.
1.I modeled threaded rods as "blot connectors" with 14 lb-in pre-load and ran the study by applying external pressure.The "Pin/Bolt Check" plot showing FOS for these screws as only 0.85, which is very low value than I expected. When I ran study with zero pre-load, I got FOS of 15.9. Other thing I noticed is - FOS is increased with increase of pre-load to certain value (0.15) in this case and decrease gradually. In this case thread rods are under compression. I imagine this happened because "pre-load" in solidworks simulation expected to be used only under tension. I wanted to see how it will behave under tension.
2. Then I applied same pressure internally to see how bolt joint is under is going to behave tension. Even in this case, I got FOS of 15 with zero pre-load torque and got 0.8 for 14 lb-in preload torque.
(3. I applied force on inside of flanges(triangular corners - notice in pic. attached), in same direction as bolt, to see how its going to behave in pure tension. In this case, with zero preload, FOS is 4.5 and with 14 lb-in preload FOS is 0.75.
4. I applied same load on outside of flanges (same direction as bolt) in pure compression, in this case with zero preload, FOS is 23 and with 14 lb-in preload, FOS is 0.86.)
Now I am trying to understand results. I would greatly appreciate if someone could explain me based on above studies.
1. Why FOS of bolt joint decreases when correct preload (75% proof load calculated from handbook) is applied?
2. Why, bolt joint showing same behavior in both tension and compression in case 1 & case 2.
Although case 3 and case 4 are not required for my application, I am curious to know
3. Why thread joint showing less FOS in pure tension compared to compression? I expected to see bolt joint stronger in tension in this case (as the cylinder is sandwiched between plates & with correct pre-load) the joint supposed to be stronger?
thank you so much,