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<DIV><FONT face=Arial>Bob</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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.
</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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!</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Earl</FONT></DIV>
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=bob@toprudder.com href="mailto:bob@toprudder.com">Bob Richards</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=nsrca-discussion@lists.nsrca.org
href="mailto:nsrca-discussion@lists.nsrca.org">NSRCA Mailing List</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Friday, August 31, 2007 1:51
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [NSRCA-discussion] Metal
Servo Arms</DIV>
<DIV><BR></DIV>
<DIV>Earl,</DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV>Bob R.</DIV>
<DIV><BR><BR><B><I>Earl Haury <<A
href="mailto:ejhaury@comcast.net">ejhaury@comcast.net</A>></I></B>
wrote:</DIV>
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<DIV>
<DIV><FONT face=Arial>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. </FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>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).</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Nylon shaft & Al arm: "Give @ 320 oz in and
failure @ 560 oz in - shaft spline total failure.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Al shaft & nylon arm: No "give" point. Crisp
failure @ 560 oz in - arm spline total failure.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Al shaft & Al arm: No "give" point, Crisp failure
@ 1600 oz in - no spline damage, shaft broke below spine.</FONT></DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial></FONT> </DIV>
<DIV><FONT face=Arial>Earl</FONT></DIV></DIV>
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