Torque (getting it right)

Discussion in 'Electrical Forum discussion & Blog' started by rerickson, May 10, 2010.

  1. rerickson

    rerickson experienced amateur

    Messages:
    14
    Location:
    Puget Sound
    How do you estimate (or measure) torque on connector screws?
    Put another way: how do you teach an apprentice what the correct torque feels like?

    I discover I over-tightened some screws on the grounding bus bar of our panel; one #12 was crushed beyond flat. On the other hand, I'm going to be using some Al connectors that call for 35 and 50 in-lb. So it's time to get my torques right.

    Could buy (or perhaps rent) an automotive torque wrench, but there's probably a better way...

    Thanks much for all your advice! Reading this forum has been very helpful.

    P.S. All of these use a slotted drive. I will use emery paper and Noalox on the Aluminum.
  2. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    He has to learn how to feel the wire tightening in the lug.

    Let him do it then check his work.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,034
    Location:
    Cave Creek, Arizona
    Automotive torque wrenchs are NOT calibrated in inch pounds they use foot pounds. Practice and experience is the best teacher, and most users would have stopped twisting LONG BEFORE the ground wires were "beyond flat" whatever that means. It is like putting stuff in a box. You may be able to put a lot of stuff into it, but that does not mean you can carry it, or that the box will not fall apart when you try. Just because you have a big enough screwdriver to keep twisting it, does NOT mean that it is not tight.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,262
    Location:
    New England
    The safest thing is to buy a torque screwdriver...they are readily available.
  5. Jim Port

    Jim Port Electrical Contractor

    Messages:
    156
    Location:
    Maryland
    Even Sears has torque wrenches calibrated in inch/pounds. Or you could convert foot/pounds to inch/pounds.
  6. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
    If you go with a torque wrench, you will want a mini one. You are looking at a torque of 3-5 ft-lbs. Common automotive torque wrenches are 0-150 ft-lbs (or more), so they aren't that accurate for these low torque values. In addition, the handles tend to be long on most automotive torque wrenches, so not great in small spaces.

    So, look for a torque screwdriver or a mini torque wrench. One that is calibrated in in-lbs is more likely to better suite the range of torque values that you need.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,262
    Location:
    New England
    Grainger has over 20 different models. Sear sells one, and there are a couple on Amazon for around $65. Lots of choices. Depends on how accurate, and the range of adjustment how much they cost.
  8. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,034
    Location:
    Cave Creek, Arizona
    And after you use one a couple of times, you will know how tight to make them and never use it again. A waste of money.
  9. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Do any of the plumbers here use the code-required torque wrench to tighten banded coupling screws??????????!!!!!!!!!!!!!!!!!!!!
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,262
    Location:
    New England
    Depending on the material and the situation, my feeling is that there is no substitute for a torque wrench. All too many times I've needed a breaker bar to remove tires, and come across stripped screws or bolts that would have come apart without stress if properly installed. When you have a fastener that may be subject to heating and cooling or vibration, getting the right tension on the material is quite important to ensure it holds adequately without the likelyhood of it working loose, subjecting the threads to deformation enough to preclude dissasembly or reuse, or outright failure. This is even more important when someone doesn't make that sort of connection every day, or there's a risk of deformation that could affect a seal or the ultimate performance of the fixture. When an engineer takes the time to figure out the optimum torque value of a fastener, while 'close' may suffice, it may not perform as well as it could. That little bit of extra effort may be worth it.

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