[NSRCA-discussion] Metal Servo Arms

[J N Hiller]

Ron sounds like you may have experienced the even / odd number splines
mismatch being used by different manufacturers or manufacturing tolerance
extremes.
Futaba began using a 25-spline (I counted them 3 times) drive about 20 years
ago allowing a 1/4 spline offset (3.6 degrees) with each approximately 90
rotation. Being a long time Futaba user I was especially impressed with this
change from the previous octagon drive. It greatly enhanced fine tuning arm
to pushrod angles for differential travel. In recent years I have purchased
other brand servos with an even number spline and found that different
servos sold under the same label have a Futaba compatible spline. Normally
the even / odd splines will not even start to assemble but extreme
manufacturing variations may allow mismatched parts to be assembled. I don’t
know if any of the manufacturers conform to a “Standard” or if there is one
and if a manufacturer reverse engineers a product to obtain a dimensional
basis for their product it is easy to deviate from the intended mean
dimension, especially when working with injection molded plastic parts
subject to shrinkage. The aluminum arms are ‘dimensionally stable’ and may
be intentionally sized to be on the tight side to accommodate shrinkage
variations associated with non-stable plastic parts. Better tight than
loose. There appears to be enough inconsistency to warrant a fair amount of
skepticism when mixing brands and needs to be checked every time.
What a pain. I am migrating back to using all Futaba servos.
Jim Hiller

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of Ron Van Putte
Sent: Thursday, August 30, 2007 8:57 AM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] Metal Servo Arms

I know you ordered JR arms for JR servos, but be careful that you don't
mistake "tight" with "too tight".  I made the mistake of trying to use a
Futaba servo arm on a Hitec 5625MG digital elevator servo.  The arm seemed
to fit fine because it was "tight" on the servo.  A couple of flights later,
I discovered dramatic elevator trim changes.  The Futaba servo arm was
slipping on the Hitec servo's output shaft.  Wasn't fun.

Ron Van Putte

On Aug 30, 2007, at 8:36 AM, k6xyz wrote:



I had the same trouble with these arms.
It seems that the spline in the arm is the right size, but it has debris or
something in there…anyway, I was able to pick out some of the loose ‘stuff’
in the spline and it went on easier but I think it is still too tight.

Regards

Dave Harmon
NSRCA 586
K6XYZ[at]sbcglobal[dot]net
Sperry, Ok.
-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org [
mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of twtaylor
Sent: Thursday, August 30, 2007 7:50 AM
To: nsrca-discussion at lists.nsrca.org
<mailto:nsrca-discussion at lists.nsrca.org>
Subject: [NSRCA-discussion] Metal Servo Arms

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 this normal? They don’t go down
as far as the plastic arms do. They are stamped with a J on the bottom so I
assume they’re made for JR servo’s.

Tim
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