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2mm threaded push rod Doobery things

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I have had massive issues with sullivan snakes changing length with temperature. In my experience, i measured 1% change in length for a 40° change in temp. This can be problematic if you build/fly in such temp ranges (as i do). A previous thread on here somewhere refers.

1% might be a small number, but over a 600mm length, compared with the actual travel provided by the servo, this can result in significant changes in length, so i am very wary of them now. 

They need to be installed correctly to minimize this effect. Ignore at your peril. (fix the ends of the outer very securely, the rest of the inners somewhat free to move, and the snake must have some gradual bends in it (can't be run in a straight line from servo to surface). 

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I was under the impression that fixing firmly at both ends and incorporating a curve into the snake was the recommended method, rather than attempting to keep them straight. For fixing the ends I wrap the snake outer in a wrap of masking tape, which is then glued into the support at each end and I like the wee plastic snake end pushrod exits that do a nice job cosmetically on the exterior. IIRC there is also a soft rubber isolation mount available, which holds the snake outer firmly on passing through a former, though I tend not to use those.  The temperature range in the UK doesn't tend to be as much as 40 degrees C, at least when it is flyable.

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22 hours ago, Nigel R said:


The topic has come up on the forum a few times, I believe.


The outer and inner are made from the same stuff and expand/contract at the same rate. If you fix the very ends of the outer, and only the very ends, and leave the middle "free to expand", then any change is matched by any change of the inner rod.  Done this way, the only expansion that affects trim is the very small piece of inner that protrudes from the outer, that very last inch at each end. The expansion on 2" of nylon rod is small. in order of a few thou per 10 deg C, if my sums are right.


I appreciate this is not possible as a retrofit for an existing install.


👍I've used them for years  and fitted correctly as you say there are no problems. I support the middle loosely just to stop them vibrating and flapping about.  Control snakes are my preferred system . Securing ends can be an issue as no glue really fixes the plastic. To overcome this I wrap the ends tightly  with Masking tape then seal this wit a thin coat of cyano . when its fitted and  in position I then glue the ends to the airframe. and jobs done . Any issues I've ever come across has been due to one or both ends sliding .  

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8 hours ago, Dale Bradly said:

In my experience, i measured 1% change in length for a 40° change in temp




Expansion for nylon is around 0.1mm, per degree, on a meter of nylon.


For 40 deg change in temp (a fairly extreme case!) - that gives 4mm on 1000mm, (or 0.4%) - note for a whole metre though.


I'd make an assumption (dangerous) that the protrusion from either end of a control run is 25mm, giving 50mm total (as this is about what I believe the instructions call for, "no more than 1" sticking out")


With a correctly fixed snake, we only need to worry about the bit sticking out - that would give, for that 40 degree rise, a much smaller change in the control run - 0.2mm change.


Another assumption - most of my installs use a standard servo with a standard arm and would probably show a total movement of around 1/2" (say 12mm) either way. In my typical install, the expansion/contraction over our 40 degree change is approx 1.5% of the servo movement in each direction.


I can't say I fly in 40 degree weather very often but I appreciate countries other than the UK exist! Most of my flying occurs between (say) 10 degC and (maybe) high 20s deg C, so the change I see is much less, about half.




For me, snakes are easy and work "sufficiently well".


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Bruce, the inner expands at the same rate. And herin lies the problem. The outer is fixed between two ridgid points. The inner is secured between the servo, which to all intents and purposes is fixed, and the control surface horn, which by definition is certainly not fixed! So any expansion needs to be accomodated /allowed for, without said expansion/contraction affecting the one "free" end of the inner. Install methodology has a huge part to play in how successfully this is achieved. 

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