×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

How well does a thread function as a labyrinth seal?

How well does a thread function as a labyrinth seal?

(OP)
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

----------------------------------------
Artificial Intelligence is no match for Natural Stupidity

RE: How well does a thread function as a labyrinth seal?

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.

RE: How well does a thread function as a labyrinth seal?

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.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: How well does a thread function as a labyrinth seal?

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).

RE: How well does a thread function as a labyrinth seal?

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.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: How well does a thread function as a labyrinth seal?

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

----------------------------------------
Artificial Intelligence is no match for Natural Stupidity

RE: How well does a thread function as a labyrinth seal?

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources