[NSRCA-dist7] Servo replacement / wear - how to tell

[Budd Engineering]

Comments below:


On May 22, 2008, at 8:05 AM, Scott wrote:

> There was some of the typical RC discussion at Hemet - and one topic  
> that came up, as it usually does, is servo wear and replacement.  
> With today's digital servos, I have the following questions about  
> digital servo replacement;
>
> 1) what are the symptoms of a digital servo that is starting to wear  
> out?

In flight, the plane will lose the "locked in, tight" feeling that it  
(if initially properly set up) had when new.  You'll think that you're  
flying in light turbulence even though it may be calm out.  Other  
symptoms to look for are the plane doesn't consistently roll axially  
anymore (assuming it once did!), due to the servo endpoint having  
changed slightly due to the change in pot resistance from the wearing  
in the pot element (I check this in flight with multiple up and down  
line rolls in both directions, 3 or 4 rolls in a row).  If you  
measured your aileron deflections you would likely find that one or  
both of the aileron "throws" had changed from their initial settings  
by 0.1 or 0.2 degrees (assuming you had accurately set them with a  
precision instrument to begin with).

>     - poor centering ?

Yes, but you will only be able to see it if you put a long pointer on  
the servo, put the radio in "servo sweep/test" mode, and look for a  
slight pause as it passes center.  I use my laser deflection system to  
check for this when the servos are installed in the plane.

>         - does this equate to a louder 'buzz' at center

Sometimes, but not usually (more likely on the rudder if the cables  
are tight).

>         - is the only way to tell this to put a servo on some sort  
> of protractor and look at the centering?

See above.

>         - does thee servo draw more current?

Not significantly.

>
> 2) What tests can be performed either 'in the plane' or 'outside the  
> plane' to determine if a servo needs to be replaced?

In the plane, see above.  Outside the plane, I mounted a laser emitter  
to a servo arm, clamped the servo to a lightweight, portable vise,  
pointed the "dot" at a point on the wall, put the system into "servo  
sweep/test mode", and look for a pause or slowdown as it passes  
through center (same as the in-plane test).

>     - In plane testing would be ideal, but we must compare against a  
> 'new' servo, so maybe this is not practical.

No need to compare, new servos don't exhibit any of these  
characteristics (at lease the ones I use don't).

> Any thoughts on the above would be considered.

For the guys flying glow, you can expect the servos to show signs of  
pot wear (primarily to vibration) on the ailerons at around 35-50  
flights, closer to 80-100 on the elevators and the rudder.  And  
contrary to popular belief, it doesn't matter "whose" motor mount you  
are using, the answer is the same.  It's also good practice with glow  
setups to change out the servo gears every other time you change the  
pots.

For electric the magic number is around 180-200 flights, with the  
culprit being pot wear due to the wiper sweeping back and forth over  
the element under normal usage (i.e. simply wearing out).


As always, YMMV

Jerry

Budd Engineering
jerry at buddengineering.com
http://www.buddengineering.com
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