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Thread: Gauge of safety wire on turnbuckles?

  1. #1

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    Gauge of safety wire on turnbuckles?

    Looking at AC 43.13-1B, it seems the only gauge of safety wire for turnbuckles is .040 (page 7-45), regardless of how it's done (single wrap, double wrap) or type (brass, stainless, etc.).

    I realize that in the E-AB world we can look a the word "guide" and use our own judgement, but at work we're seeing a lot of aircraft (both certified and Experimental) with .032 double wrapped on turnbuckles, and I'm wondering if we're missing something here.

    https://www.faa.gov/documentLibrary/...-1B_w-chg1.pdf
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  2. #2
    rwanttaja's Avatar
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    I guess I'd look at this from the structural point of view: What's the tension a "spinning" turnbuckle can apply, and how likely is it to break a piece of safety wire?

    Let's look at the structural design of a turnbuckle:
    Name:  turnbuckle tension.JPG
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    You've got the turnbuckle ends screwed into the barrel. Now...what happens when the turnbuckle comes into tension? First, the threads on the turnbuckle end are going to jam up hard against the internal threads on the barrel. This will impart a rotational effect. However, the threading on the turnbuckle ends is rather fine. There's going to be a HECK of a lot of friction opposing rotation here, because of the solid, under-tension contact between the threads. Remember, we don't lubricate turnbuckles.

    The question is, can a single piece of 0.032" stainless steel wire successfully oppose the amount rotational force trying to turn the turnbuckle end? I don't have he-man biceps like Frank does, but *I* certainly can't pull on a piece of 0.032" stainless enough to snap it. Looking online, the wire seems to have a tensile strength of 75,000 psi. If my math is right, it should withstand at least about 240 pounds of pull. From what I see, brass is about ~20% as strong, so a piece of 0.032" brass should handle about 50 pounds.

    I have trouble believing that a turnbuckle will generate that much rotational tension. Remember, the higher the tension, the harder the threads are jammed together, opposing rotation. A single wrap should be adequate.

    NOW: I am occasionally an idiot, who still has to mutter "leftie-loosey, rightie tightey" when I tighten nuts. It's *quite* possible I'd mess up the direction of the wrap (e.g., apply the safety wire to oppose the turnbuckle tightening, instead of loosening). A double-wrap helps protect me from myself.

    A year or two back, I tightened a couple of internal wing turnbuckles. Had to work through a standard inspection hole, which wasn't that well aligned with the turnbuckles. Felt like a proctologist trying write his girlfriend's name inside someone's colon. I did a single wrap here (and brass at that) and check it out every condition inspection.

    Externally, the Fly Baby landing and flying wires are in sets of two. On these, I single-wire wrap each turnbuckle (0.032" stainless), then wrap them together, running the second wire through the center hole of both turnbuckles.

    Ron Wanttaja

  3. #3

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    To extend this discussion further, AC43.13 would tell you that all propeller bolts must be secured with 0.040 wire. My mechanic thought that until I showed him the page in the Hartzell Propeller manual that stated that 0.032 was used.

    Best of luck,

    Wes

  4. #4
    Airmutt's Avatar
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    Per NASM33540....

    The size of of the wire shall be in accordance with the following minimum requirements:

    a) 0.032 inch minimum for general lock-wiring except that 0.020 inch wire may be used on parts having a nominal hole of less than 0.045 inch, on parts having a nominal hole between 0.045 and 0.062 with a spacing between parts of less than 2 inches, or on closely spaced screws and bolts of 1;4 inch or smaller.

    b) Copper 0.020 inch shall be used for shear and seal wire applications

    c) When employing the single wire method of locking, the largest nominal size wire for the applicable material listed in NASM20995 that the hole will accommodate shall be used.

    There is also a good How To video accessible thru the home page. Makes a good argument to double wrap a turnbuckle.
    Dave Shaw
    EAA 67180 Lifetime
    Learn to Build, Build to Fly, Fly for Fun

  5. #5

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    Quote Originally Posted by WLIU View Post
    To extend this discussion further, AC43.13 would tell you that all propeller bolts must be secured with 0.040 wire. My mechanic thought that until I showed him the page in the Hartzell Propeller manual that stated that 0.032 was used.

    Best of luck,

    Wes
    Yep, this is the manufacturer stepping in and having a say. Amazingly enough, Piper stated that .032 single wrap for turnbuckles on Cubs is what they wanted, so in that case, using .040 is actually incorrect, but acceptable.

    A year or two back, I tightened a couple of internal wing turnbuckles. Had to work through a standard inspection hole, which wasn't that well aligned with the turnbuckles. Felt like a proctologist trying write his girlfriend's name inside someone's colon. I did a single wrap here (and brass at that) and check it out every condition inspection.
    Lordy, "No Look Twisty Wrist With Bent Elbow And Lifted Shoulder" is in the advanced level of Aircraft Yoga. Anything with safety wire puts it in the Elite.

    And yes, at work I name all the odd position stuff with made up yoga moves. It helps explain why sometimes during the day I have to step away from an aircraft, stand up, and walk in little circles while saying the healing mantra of "ow, ow, ow," while rubbing a shoulder, wrist, or other part of my body. It sort of caught on, as my boss was shoulder deep into a Luscome and shouted a Yoga Mantra, proclaiming it was a "Yoga Cramp."

    "That's how you know it's working," is the almost automatic response in the shop.

    Time to dig into NASM20995, I reckon, and find out what sort of authority it has...
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  6. #6
    Airmutt's Avatar
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    Don’t bother, It’s merely the standard for the various types of lock wire used by DOD and the industry.
    Dave Shaw
    EAA 67180 Lifetime
    Learn to Build, Build to Fly, Fly for Fun

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