1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Left Hand Thread Applications

  1. Oct 3, 2012 #1
    Sorry for this elementary question, but I'm just trying to avoid making a mistake that will lead to a bigger problem. I'm trying to change out a water valve in my garage. The little old lady that lived here before me pulled a little too far into the garage with her car... (I'm guessing). The valve and the knob used to turn on the water need replacing. Before I use an easy out to remove the screw, I'm trying to determine if the industry standard for the screw that holds the knob onto the valve might be a left-hand thread or is it typically a right-hand thread?

    As I said, I apologize for submitting a question that is 'non-thought' provoking.

    Thanks for any help.

  2. jcsd
  3. Oct 3, 2012 #2


    User Avatar
    Science Advisor
    Gold Member

    Typically left-hand thread screws are used in rotating machinery or mechanisms where its motion would tend to loosen a conventional right-hand thread.

    In the case of a water faucet's knob my suspicion is that it would be right-hand threaded so it will tend to tighten when you're closing the valve (and the valve won't open when you're attaching the knob). Still, that's not really based on any more than a guess...
  4. Oct 3, 2012 #3
    I would guess that it would be right-handed threaded as well, but you could try checking at a local hardware store when you go to buy a new one, since they should probably know.
  5. Oct 4, 2012 #4


    User Avatar
    Science Advisor
    Homework Helper

    I would hope the knob is located on the shaft with a flat on the shaft, or splines, or something similar, and the screw is just to stop the knob failling off not to transfer the torque from the knob to the valve.

    But the easiest way to answer the question is probably buy the replacement parts first. If the new valve comes with a new knob anyway, it doesn't matter what damage yoo do to the old one.
  6. Oct 21, 2012 #5


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Left hand threads are only (a rash statement I know) found where a right hand thread could be an embarrassment. The above example of rotating machinery is very common, as is the thread found on 'one half' of a bottle screw which is used for adjusting the length of a steering link or, possibly, the standing rigging on a boat, where both ends of the part cannot be allowed to rotate.
    LH threads can also be found on some gas bottle connectors to prevent the wrong pipe being fixed to the bottle.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook