|
Gordy wrote:
Anyone using plastic gears in their TD
ships? Just wondering.
There
are some very good servos with plastic gears; the JR DS3301 and the Futaba
9650.
You can ease up on your servos teeth by keeping
your servo arm as short as possible. The shorter the arm the less the
leverage applied to the teeth and case.
Less load on the
gear teeth, but more load on the case - which is not all bad if the output is
supported by a ball bearing or a properly designed bushing surface in the
case.
And lets not glaze over the fact that usually servo slop is the other name for sloppy surfaces..and one also usually has nothing to do with the other. A little play in the threaded end to the clevis' on both ends.
No excuse for this as you can use
a solder clevis on one end and a small pattern brass 2-56 nut to lock the clevis
to the threads on the other end.
a little flex in a weak linkage rod,
Again, no excuse for this as you
can always use a heavier rod. I've started to eliminate the adjustable
part of the pushrod. This allows me to use 5/64 piano wire and two solder
clevis'.
some flex in the hinge line cloth and hinge line skin (yes it happens
in the best of moldies),
It sure can, but it's poor design
and craftsmanship on the builders part if this is allowed to
happen.
some clevis pin movement at the horn
First off, don't drill the hole
too large in the servo arm. A #52 drill is .063" and if pushed through a
hole in a plastic arm with a pin vise by hand and removed without twisting, the
hole will be a very good fit - snug without lots of drag.
To correct a loose control horn,
remove the clevis from the rod and place a tiny drop of thin CA on the clevis
pin while it's assembled to the control horn. After the CA has set, rotate
the clevis around the horn a few times to loosen the glue joint. Presto,
an exact fit of the clevis to the control horn. This needs to be checked
and renewed occasionally as the glue 'spacer' wears.
and arm it all stacks up, now add in run out ...a speck of movement
at the horn connection is an 1/8" or more at the trailing edge.
This is where plastic gears are
superior to metal gears as it's possible to run them with a little interference,
mostly eliminating the play in the gear train.
The oh so popular drag saving top linkages absolutely mangle servos gears
fast and cook servo motor brushes.
I disagree with this. It's
in direct contradiction to your earlier statement to keep the servo arms
short. The real difficulty with the top actuated surface is getting the
clevis hole close enough to the centerline of the servo arm, thereby
matching the travel generated by the servo to that needed by the very short
linkages typical of top hinged surfaces - without elctronically reducing
the travel of the servo. Electronically reducing the travel of the servo
amplifies the effect of gear slop by making it a larger percentage of the total
travel. With top hinged surfaces it's essential to use the complete
rotational travel of the servo. This maintains the power/leverage of the
servo, allows use of the full angular resolution of the servo and minimizes the
effect of any play internal to the servo.
Weakened motors, let gears move more around center, digi or not, in fact
gear slop, linkage slop, long arms and weak motors, can actually be accentuated
by the new Digital resolution....Digi servos try their darndest to keep the pot
'centered' (arm) so as it slops around the digi amp is busy yelling at the gears
to put the arm back and back and back and back at really high speed...specially
if you have 5 cells to juice the action.
This doesn't fly either.
Other than actual impact loads generated by loose linkages, the servo has NO
idea of the play that exists outside of the servo. The amplifier maintains
the position of the output arm without respect to surface
position.
Same for the beloved holding power of digi's...analog left the center
'soft' you had to (and could!) move the arm some before the analog amp would
yell, "THAT'S ENOUGH!" that squish acted as a cushion against teeth
wear.
Maybe, but the inertia of the
motor/gear system is probably more important where impact wear is considered
than is a 'hard' center.
So get more nuts about linkage, and worry less about specs.
Well you ended up on a good
note!
regards,
Jim Porter
Johnston Iowa USA "The airplane stays up because it doesn't
have the time to fall."
Orville Wright |
