How well does a thread function as a labyrinth seal? 4

[vagulus]

I am working on a hand-operated hydraulic device which involves a shaft being forced into a closed cylinder by a screw thread thereby creating the hydraulic pressure. This is hardly patentable, but it created an interesting problem.

For the purposes of sealing around the shaft, the original concept involved a simple O-ring. It soon became apparent that the diameter of the housing around the shaft itself added to the diameter of the thread that holds the housing in the cylinder head conflicted with the internal diameter of the cylinder. In other words, you can't make it. There's not enough room. That leaves the option of simply driving the screwed shaft into the cylinder - winding a bolt into the cylinder cavity if you like. My question is, how badly would that leak?

Internal pressure would force the lands of the male and female threads into tight contact and a seal, but there would still be the inevitable root gap of the thread to consider. My thinking is that the fine, capillary-like passage created by the root gap would sufficiently restrict passage of the hydraulic medium, probably heavy grease, so that leakage would not be a problem - even at 100+ MPa.

I'd appreciate experienced advice on this.

Thanks

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[3DDave]

It will leak quite well. Even if there is full contact between the threads, that's on one face. On the non-contact face of the thread it's basically a helical pinhole. The size of the pinhole is the amount of pitch clearance in the threads.

 

[KENAT]

Put the thread on the outside of the cylinder & make the piston a threaded cup with a plunger coming down the center into the ID of the cylinder?

Put a 'wiper' portion on the front end of the thread made of PTFE or something with an interference thread in it? Doubt you'd get a good enough interference thread for acceptable cost to really make it work but maybe.

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[Compositepro]

There are ways to accomplish what you ask. I have personally experienced cracking of an aluminum casting because there was oil in the blind screw hole when the screw was tightened. Placing a dollop of thread sealing paste into the hole prior to starting the screw would work for at least a few cycles.
A more professional approach would be a rubber plug in a smooth bore pushed by screw. The tip of the screw would be turned down to remove the threads and match the diameter of the bore (sometimes called gib screws).

 

[KENAT]

sure, putting the O-ring in front or behind teh thread with a corresponding plain portion of the screw shank and plain portion of the cylinder works - but overal length increased significantly if you need much travel.

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[CheckerHater]

Would you consider turning your "bolt" backwards?

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[KENAT]

Doh CheckerHater, so much better than my cup idea.

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[vagulus]

My thanks to all who contributed. Given me plenty to think about.

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[MikeHalloran]

Around 1958, Popular Mechanics illustrated a great idea for removing bushings from blind holes. Just fill the cavity behind the bushing with grease, and drive a drift into the bushing bore. If you can't find a drift that's a close fit, cover the drift with paper towels or Saran Wrap. When you have bled the cavity of air, the grease will feel like solid metal to your hammer, and the bushing will come out nicely. It works super easy for clutch pilot bushings, and even for thin bushings like you find in starter non-drive ends. Nobody believes it will work until they see it.

Similarly, you could use ptfe tape, paper towels, or Saran Wrap to seal your bolt in even a loose fitting threaded hole.


Mike Halloran
Pembroke Pines, FL, USA