Really... air squeezes out and so does the lubricant.
Male and female threads aren't air tight, if this was true we wouldn't need seals and plugs and or flared mating surfaces (ehem injector fittings) or AN fittings around fasteners that block pressurized and pass pressurized fluid through them, we would just screw them together and be done with it.
Wow. Really?? Spoken like a true engineer. Try to think outside the box for a second as obviously you don't have the hands on experience to understand the problem here.
First off tapered threads such as NPT/MPT obviously are different as full thread engagement is the sealing surface. Hence the reason for thread tape/pipe dope or other lubricants to keep the threads from galling when they get tightened. Other methods of sealing as you stated like flared, AN,JIC have straight threads and use a tapered surface while others with straight thread will use a sealing medium of some sort like an O-ring such as the ones used in ORF or ORB fittings.
But we are talking about straight threads here with a bolt. Technically yes there is no positive seal between the threads until they reach the point of positive thread engagement. But there are conditions where this can happen.
For one, it is entirely possible to thread a bolt in fast enough i.e. via the use of an impact gun, that it will not allow the fluid to get past the threads before getting full thread engagement due to trapped fluid or from the head of the capscrew tightening against the surface it is holding down. The heavier the fluid, the more likely this will happen. This can easily and most commonly happen in high TPI fasteners or long threaded bolts with minimal clearance between the bottom of the bolt and the bottom of the hole. Also if the threads have not been adequately cleaned they can also restrict the flow of the fluid trapping it both between the threads and below. Over time, they can slowly migrate up past the threads resulting in a loose bolt.
With air it doesn't matter as much as A) it can evacuate the cavity much faster than a fluid and B) air is compressible hence the reason generally this never happens with a straight thread capscrew or stud.
I personally have had this happen to myself when I was first starting to use air tools. After discovering this problem, cleaning out all blind holes and careful use of lubricants and loctite alleviated this issue for me. It came with hands on experience, education and common sense. Hopefully I explained this clearly enough for you to picture what can happen.
Maybe that's why engineers aren't called mechanics.., they can't understand unless it's written on paper, and even then couldn't physically perform the task they "engineered" :cool2:
If only I could count the times an "engineered" procedure thought up in front of a computer was not practical or even possible in the real world. And most of the time you could argue with them until you were blue in the face and they would stand by it blaming you until they actually went out into the real world and tried to do it. Granted, a different design or procedure eventually fixed the issue shortly after, but sometimes trying to get them to see things from a real world perspective was the only way.
I've also worked hand in hand with several engineers over the years putting their thoughts on paper into real world application and the good ones were always the ones that took input from those with experience actually doing it and/or stood next to you while you did it to monitor the procedure.
Sorry not to badmouth all engineers. Just like those of us actually performing the work you designed there are good and bad at both. I know there are some really smart ones on this site, I try not to offend you all......
