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[CJ]

Hi all,

probably a totally stupid and daft question  but just replacing my rudder servo and removed the single pushrod, was about to replace and wondered whether anyone actually uses two, one for each side ??

Is this even possible or will it cause issues or is it advantageous in terms of strength ??

Any advice or abuse for asking a silly question greatly received as always !!

CJ

[Umi_Ryuzuki]

It's not usually necessary, but yes, people often run dual push/pull rods to
actuate the rudder and other control surfaces.

 

[CJ]

Cool, thanks Umi. Yeah wasn't sure what other people ran at so thought I would throw it out there !

I might fit 2 just for my peace of mind as the insides are out and I have access right now.

Cheers

CJ

[Andyn]

If the two are not precisely the same length, you will run the risk of stripping the servo gears. On one control surface, if anyone ever runs twin control lines, it's a bowden cable pull pull system.

[CJ]

Thanks Andy,

Yeah was pondering that also as I'm sat here....although my thinking is two being safer I can see what you mean if the mounting or rods are out slightly I could have a problem, sounds safer to keep it simple and stick with one rod 

Cheers

CJ

[Andyn]

Well put it this way, I'm using a single 3mm pushrod on a Savox servo with 20 kilos of torque, in a 70mph outrigger hydro...

[CJ]

  - sounds good enough for me then !!

Thanks

[Shipmate60]

CJ,
If the model has a single rudder there is always a danger that the rudder when going astern can bend the rod slightly and throw it "over centre".
This will keep it reversed against the hull immobile.
I have to disagree entirely with young Andyn, his hydro has very limited rudder throw so unlikely to ever happen as it can't go in reverse.
I always advise 2 rudder arms in scale boats, the reason is simple.
The pushrod can bend when in compression so can give worse response one way.
With 2 pushrods the main force is applied in tension to  pushrod which wont bend it.
As to stripping servo gears, never seen that yet.
A bowden cable is a different animal completely.
Only talking with over 20 models built this way with no problems and never changed a servo for stripped gears.
I usually incorporate a screwed adjuster on 1 pushrod for ease of setting up.

Bob

Bob

[Andyn]

hydro has very limited rudder throw so unlikely to ever happen as it can't go in reverse.

'Fraid not mate... It's a 5" blade with 20 degrees of throw, and it turns right on the push stroke. If you can find me an application in a model which has more turning force acting against the pushrod laterally I'd be incredibly surprised...

Oh, and all servo arms should be exactly parallel with the tiller arm, so if your model is set up correctly there should be absolutely no reason whatsoever why there should be any possibility of the rudder turning over centre.

[Shipmate60]

So andy, how does 1 pushrod not strip the gears when you say 2 will.
All the radial force is on one side acting both ways.
On a 2 pushrod system for the same force the tension side will produce about 75% and the compression side 25%, but eacc side taking equal loading afterall.
I just dont understand how 2 rods will strip the servo gears and 1 wont?

Bob

[RAAArtyGunner]

Good discussion so lets flesh it out.

Whilst I have no experience as such, rudder connections, it makes sense that a double connection would be far safer/functional than a single.

The action would be, both push and pull, as has been mentioned, one in compression (push) and open to bending if too thin or sudden load applied.

The other in tension, being pulled, which requires much more force to snap as opposed to compression, so could be a flexible wire, nylon line, etc or such.
 
So which is it ???????

[Andyn]

I've seen planes crash because of double linked servos that were rods, not cable, so I'm not stupid enough to attempt it myself...

If you want to do it, fine...

[Shipmate60]

Andy,
You are good at quoting the types of models you know about.
But the question was not about aircraft or I.C, boats.
I just object strongly to the over excessive reasons given.
2 pushrods  " will run the risk of stripping the servo gears".
Your words, could you please expand on this. I would like to hear your reasoning as I was always taught that spreading the load reduces the stress on rotating machinery.
With 2 pushrods you have a built in failsafe if 1 pushrod moves out of position and spreads the load on the servo horn splines therefore the side loading on the gears which REDUCES the likleyhood of gear failure.
"If you want to do it, fine..." I have been doing it this way for over 25 yrs and it is my prefered method.
I have extended the servo arms to give longer throw to the rudder and still have the original standard servo in a model over 20 yrs old working fine.

Bob

[knoby]

I used double rod linkages on everything & have so for decades without any problems. Being an ex car racer, I must admit to being a little '"xxxxx"' about linkages. Many years ago i bought a 2mm left hand thread die, I buy threaded 2mm rods, cut them to length & tap a left handed thread on to the cut end. Then i use either ball & socket joints or rose joints to attach them to the arms, & spend time setting them up so as not to cause strain on one side. never had a problem with stripped gears or jamming & have used this method in everything from scale to yachts to multi racers.

Glenn

[CJ]

Thanks gents for all the comments, I do sort of understand where Andy was coming from to be fair....hence asking the question as I'm just not sure.
Would be interested to see what you use Glenn and Bob if you could drop me a link. I was looking at 2mm threaded rod and either ball joints or Clevis, also Glenn are you saying you recut one end in reverse or cut both ends - am I assuming you do both and joint both ends for full adjustment ?

I am tempted to fit two, not that I am running anything too  but yes Bob, always been taught to spread the load !

CJ

[HS93 (RIP)]

I do similar to knobby but use a normal handed m2 dia it makes for much neater set up also put a lock nut on one end ,if you use left handed dia nuts remember to get left handed ends from a model car supplier , hear s a single rod set up with the endsthreaded to get the right length
if you use twin rods just remember to have the geometry the same at both ends  I have no problems doing it this way

Peter

[knoby]

I just use the plastic ones, so the metal rod cuts its own thread into it

[RAAArtyGunner]

HS93,

Peter, now you have thrown me a curved ball.

In the diagram example, the one that wont work, it would work if it had only one "link/rod".

That being the case, am trying to work out why it won't work with the two connections.

It's obviously geometrical and can see with the equal/parallel systems they will work perfectly but the other has got me beat, am missing something????

Had another look/rethink is it because the distance from the pivot points are not equal at each end??????????

[HS93 (RIP)]

to keep the links parrelel the V arms have to be longer so when you move the servo so that one side of the rudder tiller arm is at 90deg to the link  it bind because it is now longer, I found this out the hard way spent ages trying to stop the binding on a tug till I got them all in line then no problem there is another way and that is to use a controle overide on one side , then if your rudder gets hit (club500) and the link comes of this is still there to get you in and it stops the rudder fliping to one side.

Peter

[RAAArtyGunner]

 Yes, the ole grey matter is coming to grips with it,

 one side lengthens  whilst the opposite shortens as they are not pivoting equally around each point the same amount of travel.

[boatmadman]

Peter,

I am interested in the servo clamp in your picture - never seen one like that before, did you make it yourself?

Ian

[HS93 (RIP)]

Hi Ian I do make some but the one in the picture is from http://www.modelshopleeds.co.uk/catalog/product_info.php?cPath=105_101&products_id=1425  they have a very good section of lincage materials and are fast delivery I get my 2mm push rods from them they are about the right strenth  good for tapping. you can buy just the centre part if you look down a few pages and use wheel collets with nice grub screws then springs that suite your needs I have it on my tug because the rudder on it gets knocked very easily and it has saved a few servos it also takes any slop out of linkages without making them tight.

Peter

[essex2visuvesi]

I've seen planes crash because of double linked servos that were rods, not cable, so I'm not stupid enough to attempt it myself...

If you want to do it, fine...

Most large RC helis use double linked servos on the swashplate.... I'm sure if this was an issue they would have knocked it on the head pretty sharpish a no one wants a 4 foor wide out of control skil saw.


HOWEVER. I will concede that setting up double linkages requires more care and accuracy

[ACTion]

Metal Push Rod Connectors Ref: SL063-R - about halfway down the page  http://www.slecuk.com/catalogue/Control-Fittings.html Just add a couple of collets and tyre valve springs and you've got it.
My observations are that the distance between the two pushrod fixing points on the servo disc/arm and the tiller arm must be exactly equal or one rod will be stretched and the other compressed when the whole shebang rotates > servo and rudder binding up.
It's gonna take a heck of a lot of rudder pressure to bend a 2mm rod unless you're daft enough to make it longer than four or five inches.
Ball and socket joints have nil float in them and are the preferred method of fixing to servo discs.
I've never used the push-me-pull-you arrangement and I've never had any problems. Careful setting up will avoid the over-centre effect described by Shipmate 60.
Suit yourselves.
DM

[Shipmate60]

Over centre can be avoided by fitting blocks of something like balsa to stop excess travel on the tiller head.

Bob