[NSRCA-discussion] Metal Servo Arms

[Earl Haury]

Bob

I agree - in reality servo arm / spline strength is pretty much of a mute point for pattern. I did the tests after a friend broke an 8411 shaft on a big gasser, and some heli friends claimed the Al arms stripped the spines from nylon servo shafts. Takes excessive force to do either. 

Recall changing a clevis to a ball link on a Rossi carb arm - arm broke on the first flight, a doubled arm broke on the next - back to a clevis and never broke another. Vibratory forces aren't understood by most - but experience is a great teacher.

Somewhere I've some DSA data gathered with mini-accelerometers mounted on aileron servo mounts, firewalls, etc. One look at those data with and without a soft mount will make one a believer in soft mounts!

Earl
  ----- Original Message ----- 
  From: Bob Richards 
  To: NSRCA Mailing List 
  Sent: Friday, August 31, 2007 1:51 PM
  Subject: Re: [NSRCA-discussion] Metal Servo Arms


  Earl,

  Good information. However, this is a static test. I think the real failure mode might be fatigue in a dynamic (vibration) environment. This is what usually causes threaded-rod type control horns to fail.

  Bob R.


  Earl Haury <ejhaury at comcast.net> wrote:
    The Al arms are quite resistant to twisting - but you're correct that the nylon wheel is plenty strong and the Al wheel stronger than either. 

    A few years back I tested the strength of the JR 8411 Al spline and the equivalent nylon spline coupled to stock & H9 Al arms. The test involved making an adaptor to set over a single arm and position an inch-lb torque wrench directly over the shaft. An output gear / shaft was clamped in a vise and the torque wrench used to measure the "give" point and the failure point with different combinations. All exceeded the torque rating of the servo.

    Nylon shaft & arm: "Give" @ 320 oz in and failure @ 480 oz in - the spline shaft twisted and slipped, arm spline damage (yet there was enough binding to retain some control transfer).

    Nylon shaft & Al arm: "Give @ 320 oz in and failure @ 560 oz in - shaft spline total failure.

    Al shaft & nylon arm: No "give" point. Crisp failure @ 560 oz in - arm spline total failure.

    Al shaft & Al arm: No "give" point, Crisp failure @ 1600 oz in - no spline damage, shaft broke below spine.


    Earl



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