×
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

NPT Threads in Plastic
4

NPT Threads in Plastic

NPT Threads in Plastic

(OP)
I am designing a plastic drain pan with a molded-in 3/4" female NPT (tapered) pipe fitting. Does anyone know if design guides exist for designing the plastic surrounding the threaded hole? Tapered threads can produce extremely high hoop stresses and I don't want the HVAC installer to crack the threads (and pan) causing a leak. I'd appreciate some place to look for more information.

RE: NPT Threads in Plastic

You are right in suspecting trouble with plastic female pipe thread because of high hoop stress leading to leaks because of relaxation or possibly cracking. A steel ring around the joint MAY help.

RE: NPT Threads in Plastic

Lilkitty,

You have a valid concern. The fitting could crack at the time of installation - or (even worse) after it's been in service for a while. (I've seen both)

Here are a three options to consider: (note - I only "linked" Spears and Banjo - there are other mfrs of similar fittings)

Use an outer metal "collar" (as insideman suggested)
http://www.spearsmfg.com/prod_brochures/SR-2-0801_...

Use a metal insert:
http://www.spearsmfg.com/prod_brochures/SR-2-0801_...

Use a bulkhead fitting:
http://www.banjocorp.com/products/prod.asp?PMPN=TF...++++++++++

Good luck.

Donf

RE: NPT Threads in Plastic

lilkitty
Why not change the npt to a socket fitting.  Similar to PVC fittings.

RE: NPT Threads in Plastic

lilkitty,
You are so very correct to be concerned.
In my view, you are asking for trouble to provide female threads for the 800 pound gorillas to screw something into.

I have seen manufacturers try everything to protect themselves from this, including thicker (does not help much), metal rings (still a problem), warning decals, etc.

If you can find some way to get away from the thread you will save yourself tons of warrantee claims, returns, phone calls and faxes, and money.

My company has outlawed all plastic threads.

We use grooved couplings, quick connects like grooved couplings.  Adapt to a metal thread somehow, anything to get away from plastic.

PUMPDESIGNER

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