# Adding R/C Control



## wslogger15 (Jul 14, 2008)

Hi Everyone,

In looking through past posts on the subject of adding R/C control to live steamers, I found several where sprockets and chain drives were used for the throttle. Has anyone used a continuus rotation servo with this type of installation? Does the roughly 90 degree rotation of a standard servo open the throttle enough to provide maximum possible steam to the cylinders? I am considering adding R/C to an Accucraft K-27 using this method but I am not sure if I should buy a standard servo like the Hitec HS-82MG or getting one from servo city which has been modified to continuous rotation.

Any Advice?

Regards,
Chuck Collins


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## Dwight Ennis (Jan 2, 2008)

I've never used continuous rotation servos in this or any other application. However, I will say that often shutting the throttle completely off requires some force, sometimes more than a small servo puts out, even the ones with higher torque specs. With space at a premium, larger servos are often not the answer. The sprocket/chain arrangement has the additional benefit of providing some mechanical leverage to the servo via the "gear ratio" of the sprockets chosen.


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## R Snyder (May 12, 2009)

I have a belt/pulley arrangement on an HS82mg servo on my Accucraft Mogul that seems to work pretty well. I think it would be virtually identical to the chain and sprocket you are talking about.


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## steveciambrone (Jan 2, 2008)

I have only used a pushrod type arangement on several models and it works fine, there is a benifit if more or less throw iis needed it can easily be changed with the holes in the servo or throttle arms. I also have a computer radio which allows even easier adjustment. I have seen gear and chain mechinisms but to me it makes it more complicated and not really required. I have built many R/C planes and boats before so R/C installation and adjustment comes easy. 

Steve


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## TonyWalsham (Jan 2, 2008)

Don't forget that whilst a gear and chain mechanism will increase the torque available it will also reduce the amount of rotation by the ratio of the two gears. 
The valve gear *MUST* be free moving, with no binds at all. Adjustable end pints are very desirable. Otherwise they have to be made using only the holes in the arms of the servo etc. 
You may also need to replace the regulator needle valve with a shaft of a different taper for R/C use. 
I find that the HS65MG is fine for small locos and the HS85MG is satisfactory for larger locos. 

I cannot comment on continuous rotating servos as I have no experience with them.


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## rwjenkins (Jan 2, 2008)

I switched from a pushrod to a sprocket-and-chain setup in my K-27. I have the smaller gear on the throttle and the larger gear on the servo (I forget the ratio off the top of my head, but it's something close to 2:1). There are a couple of significant advantages to this setup. One is that I can get 180+ degrees of rotation on the throttle, compared to a maximum of about 135 with the pushrod. The other is that the most torque is needed in the first few degrees of rotation when you're cracking open the throttle, or the last few degrees when you're closing it. With a pushrod, assuming you want to make the most of those 135 degrees of rotation, that's also when you're closest to a "dead center" situation with the pushrod and throttle arm (throttle arm and pushrod are close to parallel at this point), giving you the least amount of effective torque because you're pulling against the throttle shaft. With the sprocket and chain setup, there is no "dead center", so you're the applying maximum effective torque no matter what point it's at in its rotation. I stripped the gears on 3 micro servos in my K-27 (including a metal geared one) before I made the switch to the sprocket-and-chain setup. It has been completely trouble-free since then.


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## rodblakeman (Jan 2, 2008)

I have used the same sprocket and chain set up as Richard on both K27's and K28's with great success. I use HS82MG and HS85MG servos'


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## wslogger15 (Jul 14, 2008)

Thanks for all rhe responses. I have decided to go with the sprocket and chain setup. In measuring the throttle rod shaft it is .235" diameter. The sprockets are available in 3/16" (.1875) or 1/4" (.250) bore. Would it be better to get the 3/16" bore and drill it out for a press fit or the 1/4" bore and make an adapter collar? What size (number of teeth) sprockets are best? Sounds like 180 degrees of rotation of the throttle is sufficient to provide maximum steam to the cylinders. My locomotive will be running on grades so it will need as much power as possible. 

Regards, 
Chuck


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## rwjenkins (Jan 2, 2008)

I went with the slightly smaller bore and drilled it out for a press fit. You'll also want to add a set screw, because the delrin plastic sprocket gets slippery when the throttle shaft gets hot (don't worry though, it doesn't get hot enough to melt the plastic). For maximum power, you'll want the smallest sprocket on the throttle that will fit the shaft (I forget if that's 8 or 10 teeth), and the largest sprocket on the servo that will fit the space where you're mounting the servo. Doesn't hurt to get a couple extra sprockets in different sizes to experiment with.


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## Pete Thornton (Jan 2, 2008)

Does the roughly 90 degree rotation of a standard servo open the throttle enough to provide maximum possible steam to the cylinders? I wondered that when I did my first Accucraft loco, a C-16. Done 2 more since then but never had a problem getting enough steam to the cylinders. 
I did try different size sockets giving me a 2:1 or better but the servo couldn't handle the torque! I wrote it up and it was in Steam in the Garden - "A Tale of Two Servos". Send me a PM with your email and I'll shoot you a .PDF.


drill it out for a press fit or the 1/4" bore and make an adapter collar? I often make up a special collar, as the sprockets are designed to push on a grooved shaft. A piece of brass tube with some short lengths of wire soldered to it usually does the trick. A bolt through the throttle shaft to hold it in place - or a set screw.


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## Dwight Ennis (Jan 2, 2008)

I usually also buy the sprocket with a 1/4" bore and make a collar to adapt it to the smaller shaft. I JB Weld the collar into the sprocket and drill and tap through both for the set screw. Haven't had one come loose yet.


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