[NSRCA-discussion] Metal Servo Arms

Fri Aug 31 07:59:42 AKDT 2007

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
  ----- Original Message ----- 
  From: twtaylor 
  To: 'NSRCA Mailing List' 
  Sent: Friday, August 31, 2007 9:09 AM
  Subject: Re: [NSRCA-discussion] Metal Servo Arms

  Last night I was playing with them, they fit the plastic output gear just fine. I was trying to get them on 8411sa's with the metal splines. I finally did but they won't go down as far as a nylon arm. Using a magnifying glass I could see where the alum arm is fully seated internally. Just looks different than with the plastic arm, as the arm doesn't cover the entire part of the splines at the bottom like the standard arm does. I'm using this on elev with the MK bearing fitting. I'm going to replace the rudder arm with a H9 as well.

  Tim

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  From: nsrca-discussion-bounces at lists.nsrca.org [mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of vicenterc at comcast.net
  Sent: Friday, August 31, 2007 9:57 AM
  To: NSRCA Mailing List; NSRCA Mailing List
  Subject: Re: [NSRCA-discussion] Metal Servo Arms

  Earl,

  I wonder if the aluminum single arm will twist more than the round nylon wheel in similar loads and conditions.  Using MK link (which are very close to the plane of the wheel) with round nylon wheels has been working very well for me.  The reason why the nylon wheel is strong is because the structure is closed.  Of course aluminum wheel would be stronger than nylon wheel.

  --
  Vicente "Vince" Bortone

    -------------- Original message -------------- 
    From: "Earl Haury" <ejhaury at comcast.net> 

    The nylon servo arms are quite adequate when used with a clevis that applies the force in line with the plane of the arm (typical pin clevis). Pushrod ends that bolt to one side of the arm move the force off the plane of the arm and thereby impart a twisting force to the arm. (DuBro type ball links are the extreme.) Add a bunch of vibration to the normal loads and the nylon arm may eventually fail - a wheel is a much better choice if one uses the nylon accessories. In addition to better withstanding the twisting forces of offset linkage, the aluminum arm is more substantial for screw mounting the ball / ball bearing end.

    Jim is correct regarding the H9 arms on JR servos - there seems to be more variation in the spline of the output than the spline of the arm. (An arm tight on one servo may fit nicely on another.) OTOH, I've found the H9 arms to fit Futaba servos consistently well.

    Earl

      ----- Original Message ----- 

      From: vicenterc at comcast.net 

      To: NSRCA Mailing List ; NSRCA Mailing List 

      Sent: Friday, August 31, 2007 7:28 AM

      Subject: Re: [NSRCA-discussion] Metal Servo Arms

      For pattern I always use the nylon servo arms that comes with the JR servos.  I am not sure why you need the metal servo arms.  Usually I use the round one for aileron and elevator.  For rudder I use the heavy duty arm that JR offers (comes in the 8611 servo).  I never have problems with the nylon servo arms in pattern application.

      Regards,

      --
      Vicente "Vince" Bortone

        -------------- Original message -------------- 
        From: Bob Richards <bob at toprudder.com> 

        I've used the H9 arms and the SWB arms. No comparison, I will continue to use the SWB arms.

        Bob R.

        Kelly Regan <kelly.regan2 at verizon.net> wrote:

          Not an answer to your question but I find the SWB arms much nicer
          since they are split. They use a a 2/56 clamp screw and nylon locking
          nut to draw the clamp tight around the spline of the servo shaft.

          twtaylor wrote:
          >
          > I bought some Hanger 9 alum servo arms and when I put them on they are 
          > VERY hard to get to pull down on to the servo. Is
          >
          -- 

          ________________________________
          Kelly Regan
          Home Page: www.reganspace.com
          ________________________________

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