Control rod to servo slop/play advice

[Marc RC pilot]

My elevator has aprox 1 to 2 mm of play which means that the pitch controls are not accurate/don't hold true every time.  It seems that the main culprits are the larger servo holes on the servo arm where the very small diameter control rod attaches (Z-bend) into which allows for some free play and also the Z-bend on the control rod to servo end (it has more of a slanted Z-bend/no right angles. 

Was thinking of either applying some superglue to the required servo hole to reduce it's diameter or place some heat-shrink tubbing around the Z-bend on control rod to thicken it out (bit concerned of wear and ending up with same issue using heat-shrink tubbing). Also will look at making the existing Z-bend shape a better fit.

 

What do you recommend? 

 

Many thanks 

 

 

[Marc RC pilot]

Or thinking I could just drill a smaller hole close-ish/below the existing one and increase servo throws a tad to compensate?

Pic of servo/arm with the larger hole (top servo in pic)

[Andy_B]

get rid of the Z bends and put a more suitable connection in ..no point in having expensive servos and then using sloppy connections .Even maybe just go for a straight L bend and a keeper 

[Marc RC pilot]

True Andy thx. A straight L with keeper sounds like a good plan.   

The control rods are so thin...

[Marc RC pilot]

Cheers Pete,  good advice. 

The problem with having a little slop at servo arm end for me is that it translates to the elevator not being able to settle exactly at its neutral point every time. Now if it where the rudder I could almost live with that...But pitch is not as forgiving/a little more high risk

It's only a few mm play but that translates into slightly different plane pitches. I don't want to have to keep re-adjusting elevator control to keep her flying true. I want my controls to be solid, true and effective no matter what model ideally. 

Thanks for heatshrink warning 

[oipigface]

I am the proud owner of a few little devices that will screw into a servo arm, and a have hole through to take the pushrod, which is secured with a grub screw. I’m guessing that they project about 5mm above the arm. I’ll send you a couple if you think you could use them.

I don’t think sloppy linkages are to be recommended in any kind of plane.

[Marc RC pilot]

Hi oipigface. That sounds like a good idea/solution but I fear 5m clearance I do not have. Will measure/check and get back to you, if not I will most prob opt for retainer system.

Thank you m8

[mikef]

Spring return linkages have 'zero slop'.

You can put an insert in the servo arm to reduce the hole diameter.  Here's a spring return servo connection with an insert

I have used these Dubro micro clevises on small push rods.  They have 0.8 mm pins and they are adjustable for length.

 

 

[Marc RC pilot]

That's clever idea Mike; thank you for the extra options 

 

[satinet]

Not using z bends.

What's the model? 

[isoaritfirst]

Firstly use much shorter servo arms. Make them around 70% max length of the surface hole length. (hinge line to hole). Shorter if possible. 

Then use a simple L bend. Make sure it is bent accurately. Put a small pad on either side of the servo such that it forms a larger flat area together with the too of the servo. 

Push the L bend through the servo horn from the bottom before pushing the arm onto the servo. Screw it down so the L bend runs on the flat surface of the servo and the pad.  

If necessary fix a thicker rod to the end of your pushrod to make a larger diameter L. 

Solder together or use heat shrink and cyano 

 

[Andy_B]

8 minutes ago, isoaritfirst said:

Firstly use much shorter servo arms. Make them around 70% max length of the surface hole length. (hinge line to hole). Shorter if possible. 

Then use a simple L bend. Make sure it is bent accurately. Put a small pad on either side of the servo such that it forms a larger flat area together with the too of the servo. 

Push the L bend through the servo horn from the bottom before pushing the arm onto the servo. Screw it down so the L bend runs on the flat surface of the servo and the pad.  

If necessary fix a thicker rod to the end of your pushrod to make a larger diameter L. 

Solder together or use heat shrink and cyano 

 

I know what you mean Mike , but your gona need a picture for this as alot wont 

[isoaritfirst]

[mikef]

42 minutes ago, isoaritfirst said:

Firstly use much shorter servo arms. Make them around 70% max length of the surface hole length. (hinge line to hole). Shorter if possible. 

I don't understand why the servo arms should be as short as possible.  For a given lack of fit, the shorter the horns, the more angular backlash you'll get.  To get the most out of servo resolution and power, the ratio of the horn lengths at each end of the linkage should be chosen to get the required surface travel at the maximum nominal servo travel.

[isoaritfirst]

Correct.

Typically a ration of 7/10 works well. 6/10 is better.

Longer arms will give less slop, but can you maintain the ratio if you fit a long servo arm?

looking at your picture I would say your effective servo arm is 10mm so is you surface arm 13 - 14 mm effective length?

[mikef]

I work out each servo horn and control surface horn length and both rakes to suit the installation geometry and travels involved.  Here's a top drive flapperon from a DLG.  Short servo horn here (5mm) is driven by the need to get the servo horn inside the wing.  Note that at mid travel, the horns are both at 90 degrees to the push rod.  Flapperon horn came out at 7mm to get desired travel.  You can see the necessary rakes in the picture.

(As another example, an internal elevator horn set-up calculation may be driven by the elevator horn length needed to center the control run in the fuselage section.  That gave a 7mm servo horn on the DLG in question with a 17mm elevator horn.)

[isoaritfirst]

Good stuff Mike.

7 mm servo horn is also about as small as you can go before you need to do something that's a little fussy. 

[mikef]

Here's an example of the way to keep the linkage in the servo horn with a short horn.  This is the wing servo mount from a DLG flapperon linkage like the one I posted above.  The pad at the top of. the picture retains the pushrod and it's faced with 0.4mm ply.  (It would be better to have it slightly taller but it was my first attempt). The pushrod is put through the horn as the servo is dropped in.

[Marc RC pilot]

Ingenious Mike! Cheers 

[Marc RC pilot]

I tweaked the Z-bends to servo and elevator control horn (they where not at 90degree angles, made a big difference) and re-tacked control rod outers to fuz in places where needed.   The elevator has a L-bend connection to horn with a little heatshrink keeper glues in place. I also put a tiny bit more bend in the L-shape to reduce any risk of it popping out. No more slop and she's ready to go up (weather looks ok for some dlg flying/practice/tweaking etc). 

Many thanks gentlemen for the great help and advice as always.; much appreciated.  

Tally Ho!