HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Oct 30, 2006, 05:17 PM
Deported Village Idiot
TheSaint's Avatar
A roamin' in the gloamin again
Joined Jun 2006
517 Posts
Help!
Reducing friction on pushrods

Got the elevator pushrods on the GP Tiger Moth ARF hooked up at the weekend, per instructions with two pushrods - one per elevator- in individual tubes to a single servo. Pushrods are joined by the servo using two wheel collars. The security of this setup is a different debate - they're gonna get soldered, silver soldered, that is, before it maidens.

Friction on the pushrods is high, and there is no way of modifying the setup or routing without a major tear-down. As an interim measure I used a spare high torque servo to drive them, but this is slow and a high current drain.

Ideas on reducing friction, anyone?
TheSaint is offline Find More Posts by TheSaint
Reply With Quote
Sign up now
to remove ads between posts
Old Oct 30, 2006, 07:09 PM
the-plumber
Joined Feb 2003
1,094 Posts
Quote:
Originally Posted by TheSaint
Ideas on reducing friction, anyone?
Not with that gawdawful lash-up.

Presuming you followed the book, the solid wire pushrods are running at diverse angles from the common point at the servo horn connection. That forces the two rods to flex at an increasing angle as the servo horn travels toward the rear. A major no-no.

Whomever "designed" that mess needs shooting. Soon. It's a battery-eating kludge.

One way out of the morass might be to add a second elevator servo parallel to the first, and connect one push-rod to each servo. Use a dual servo extension to drive the servos, and match the servo movements as closely as possible. Ditto for matching the push rods and elevator horns (and elevator horn locations).

Another way would be to install a rigid link between the elevator halves, and drive the elevator assembly with one significantly larger rod (4-40, or larger).

Or, learn to like short battery endurance and plan on inspecting the linkage Real Often.

The "saving grace" is that the Moth will likely be controllable when one of the rods breaks, and you _might_ get it back on the runway right side up with all the bits in relatively close formation.

The individual rods might also simply break off the common section of rod attached to the servo horn, morphing the Moth into a Grub and you into a Grump.
the-plumber is offline Find More Posts by the-plumber
Reply With Quote
Old Oct 30, 2006, 09:20 PM
Registered User
DougC's Avatar
New Hampshire, USA
Joined Sep 2004
520 Posts
This might help.

http://www.rcgroups.com/forums/showp...&postcount=195

Turned out that thought #2 was this fellows dilema.
DougC is offline Find More Posts by DougC
Reply With Quote
Old Oct 31, 2006, 02:16 AM
Registered User
USA, TX, Diana
Joined Aug 2005
270 Posts
When installing pushrods I always take 3 or 4 hundred grit sandpaper and polish the rods. You would be surprized how much friction that reduces.

I also squirt graphite down the tube. That helps also. Cut a fine tip of the graphite tube , stick it in the pushrod tube and squirt it in. Then stick the pushrod in and try it.

If you will do these two things I think it will help.

I have found that metal pushrods,even tho they appear to be smooth are not. They are rough.

Jim
In God We Trust
safebet is offline Find More Posts by safebet
Reply With Quote
Old Oct 31, 2006, 09:39 AM
Deported Village Idiot
TheSaint's Avatar
A roamin' in the gloamin again
Joined Jun 2006
517 Posts
Yes I agree it's a bunch of barstewards looking for an accident to happen, and I'm not happy with it.

From what I have seen of the kit construction, it looks like there may originallly have been a U-shaped bar between the elevators and a singler servo to drive them. There was a void under the tail fin skeg where such a U-bar would have fitted, sadly now filled with bondo and microbeads. maybe they found 1 x servo was not strong enough for the relatively large levator surfaces on the Moth.

Two servos is one obvious route, but might need to jigging to get the servos isto the available space (I'd have to relocate at least the throttle servo, maybe the rudder servo also). I looked at dual servos but couldn't get them in the space without moving the other servos and / or coming up with a way of reversing one servo relative to the other.But, is the current drain from 2 x ordinary servos equal to, more or less than that from a single high-torque?

How about "stacking" two links onto the servo arm? They would share a common fulcrum, so relative movement would be the same, simplifying synchronisation.

Or using something like this:

http://www2.towerhobbies.com/cgi-bin...?&I=LXFV37&P=M
TheSaint is offline Find More Posts by TheSaint
Last edited by TheSaint; Oct 31, 2006 at 09:46 AM.
Reply With Quote
Old Oct 31, 2006, 10:21 AM
Deported Village Idiot
TheSaint's Avatar
A roamin' in the gloamin again
Joined Jun 2006
517 Posts
Another idea someone suggested to me is to glue short pieces of tube over the pushrod at regular intervals to form a set of pushings inside the guide tube - I guess the idea is to reduce "capstan effect" friction around the bends.

I still don't like the two pushrods tied to each other beyond the servo arm though...
TheSaint is offline Find More Posts by TheSaint
Reply With Quote
Old Oct 31, 2006, 11:03 AM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,499 Posts
A Moth really needs pull-pull on the elevator and rudder.
Sparky Paul is offline Find More Posts by Sparky Paul
Reply With Quote
Old Oct 31, 2006, 11:52 AM
Deported Village Idiot
TheSaint's Avatar
A roamin' in the gloamin again
Joined Jun 2006
517 Posts
Quote:
Originally Posted by Sparky Paul
A Moth really needs pull-pull on the elevator and rudder.
Like this (photo of real thing)?

lots of work....especially on an ARF that's already been coated
TheSaint is offline Find More Posts by TheSaint
Reply With Quote
Old Oct 31, 2006, 01:11 PM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,499 Posts
Compared to the problems you're having with the elevator pushrods, pull-pull is a fast easy cure.
Sparky Paul is offline Find More Posts by Sparky Paul
Reply With Quote
Old Oct 31, 2006, 03:13 PM
Deported Village Idiot
TheSaint's Avatar
A roamin' in the gloamin again
Joined Jun 2006
517 Posts
I don't think I can make a pull-pull work without major rework, which I do not want to do at this stage (the plane is basically 90% finished).
TheSaint is offline Find More Posts by TheSaint
Reply With Quote
Old Oct 31, 2006, 04:21 PM
Think Thermals!!!!
SoCalGliderFlyr's Avatar
Costa Mesa, California
Joined Mar 2004
2,964 Posts
I've used the brass wound acoustical guitar strings in place of metal or even plastic. Have to solder the ends to keep them from flexing and sometimes solder/splice a threaded push rod on the ends outside of the guide tubing. Solved a few internally crimped push rod problems where the assembly could not be removed and replaced.
SoCalGliderFlyr is offline Find More Posts by SoCalGliderFlyr
Reply With Quote
Old Oct 31, 2006, 04:26 PM
Deported Village Idiot
TheSaint's Avatar
A roamin' in the gloamin again
Joined Jun 2006
517 Posts
The picture attached shows the servo layout per the GP manual.

Srevo arrangement from top of picture to bottom is: Elevator, throttle, rudder.

You can just make out the two elevator pushrods and interconnecting wheel collars in the picture. Per the manual I've got the servo orientated with the output shaft nearest the nose, an improvement over the other way around, but still not good.

There's enough space for four servo bodies - I've checked.

My thoughts are to use 2 x servos for the elevators and to cram a total of four servos into the bay as best I can. I figure at worst this will mean re-routing the rudder pushrod tube towards the bottom side of the bottom servo as seen in the picture. The trick will be to get the two elevator servos acting in the same direction, which may mean I also have to re-route the lower of the two elevator pushrod tubes to servo slot #3 from the top (and not #2) and put the throttle servo in the middle.
TheSaint is offline Find More Posts by TheSaint
Reply With Quote
Old Oct 31, 2006, 04:32 PM
Deported Village Idiot
TheSaint's Avatar
A roamin' in the gloamin again
Joined Jun 2006
517 Posts
PS

FWIW I'll write a little builder's notes piece on this ARF when I've finished it.
TheSaint is offline Find More Posts by TheSaint
Reply With Quote
Old Oct 31, 2006, 04:43 PM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,499 Posts
I've done something similar, but with an arrow shaft pushrod that had the Y-split to the seperate elevators at the tail end of the rod, just before it exited the fuselage.
The pushrod was supported at its back end, to keep it from bowing up, down or sideways.
Sparky Paul is offline Find More Posts by Sparky Paul
Reply With Quote
Old Oct 31, 2006, 07:02 PM
Oxford Panic
AndyOne's Avatar
United Kingdom, Oxford
Joined Feb 2003
3,661 Posts
One trick I used to get around a similar problem was to attach two double ended servo arms, one on top of the other, to one servo with enough gap between them to get a clevis onto each of them. I cut off the spline socket of the top arm and used a long screw to go into the output shaft through both. I also stabilised the top arm by putting a small self-tapper through the free end of the arms.

Andy.
AndyOne is offline Find More Posts by AndyOne
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Reducing noise on motors ? philgib Blimps 5 Oct 26, 2006 06:55 AM
Question Reducing glitches on my Corona yaremchl Electric Heli Talk 9 Oct 04, 2006 11:10 AM
Reducing airgap on motor and cutting magnets for triple stator build Dongledell Power Systems 7 Jun 18, 2004 06:50 AM
Enlarging & reducing plans on the computer planeman Scale Kit/Scratch Built 1 Mar 28, 2003 12:41 PM
Reducing current on my charger LYHTSPD Power Systems 5 Oct 04, 2001 07:38 AM