# R/C using chain and sprocket



## Nick Jr (Jan 2, 2008)

This is my first attempt at a using chain and sprocket. I want to thank rwjenkins and Pete Thornton for the inspiration to try it this way.
I first had to re-route the piping and valve for my tender heater. Pics show the rest, nothing others haven't already done. I did enjoy doing it this way, and can see the advantages and disadvantages of doing it either way
Tender Heater: the pipe running the length of the tender is closed at the end and has several #80 holes drilled into it. With the valve just cracked the trickle of steam also adds mild turbulence to the keep the water mixed, and as an added benefit the steam is turned back into water. LG


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## aceinspp (Jan 2, 2008)

Looks good Nick. How much rotation do you get on the throttle? Later RJD


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## Nick Jr (Jan 2, 2008)

RJ, thank you, I used a 12 tooth sprocket on the throttle and an 18 on the servo. That gives me just under 180' rotation, much more than is necessary. Michael Glavin mentioned in an earlier post that most LS loco's reach high speed at 90' rotation of the throttle. LG


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## placitassteam (Jan 2, 2008)

Nice neat installation. Have you had a chance to run it on track?


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## Nick Jr (Jan 2, 2008)

Winn, thank you, only on rollers. Tomorrow AM will be a track test, weather permitting. LG


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## snowshoe (Jan 2, 2008)

I was thinking about doing the same to my Forney. Using the wire is not enough on grades. I found I needed just a little more throttle but the wire system did not give me enough rotation. Ill have to try it now.


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## Nick Jr (Jan 2, 2008)

Shawn, how much rotation of the throttle does the wire give you? what grades do you have? LG


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## snowshoe (Jan 2, 2008)

Nick
I get just under half. I don't need much more but just that little bit will make a difference. Im not sure what my grades are, I would say less then 1%.


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## Nick Jr (Jan 2, 2008)

Shawn, I get just under half (180') on the 18 t sprocket on the servo, if you need more you may have to jump tp to 20 or 24 t. Keep us posted. LG


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## Michael Glavin (Jan 2, 2009)

Something to ponder...

Typical servos generally operate with 120* total travel arc without limitations the transmitter can impose electronically. If you are not rotating your servo arm the aforementioned full travel arc to suit your engines requirements the cost will be resolution (precision) and or finite steps available to you for same.

In other words if your engine operates satisfactorily within a 90 degree travel arc driving it past same has its price and serves no purpose. The finite steps of travel afforded you will be more granular than precise. Figure out the need for needle valve MAX rotation and duplicate same with the servos linkage within its full travel arc.

As mentioned previously in another thread, best possible setup will be achieved mechanically with the linkage ratio and minimal if any restrictions with the TX. As a last resort use the Transmitters "end-point", "ATV" "travel limit functions" to fine tune your setup.

Back in the day I used downloaded protractor pictures and scaled them to my needs to observe my servo linkage set-ups. Pretty enlightening IMO.

In many cases the results of what appears to be a simple setup is less than desirable. If your servo arm is longer than the driven arm (throttle needle) you are already at a disadvantage. Servo torque is reduced, servo resolution is diminished and needle valve settings are more sensitive.

Another problem often realized with servo linkage is net results if not at travel arc/servo center. In MANY cases, servo throw is not equal either side of center, you may realize desired travel at the needle valve BUT how it gets there is the issue. Imagine if your TX is programmed to move 70% in one direction and 30% in the latter. This happens when the servo and throttle needle pivots points are not in a planar relationship and or are offset too. Results from this very common setup is unequitable sensitivity from center or neutral to end point. This is correctable with proper servo arm placement (usually several degrees off center in one direction).

Michael


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## aceinspp (Jan 2, 2008)

As I recall and I did buy there is a devise that you can add to extend the travel on the servo. To obtain I believe about 160 degree rotation Later RJD


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## Greg Elmassian (Jan 3, 2008)

Heck, I think you can set them up to rotate all 360 degrees... perfect for a gear drive.

https://www.adafruit.com/product/2442

Greg


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## Nick Jr (Jan 2, 2008)

RJ, can get that with just sprockets, so I guess that device will take it even further if you didn't have room for an even bigger sprocket. .  I took advantage of the Servo City sale and found real tiny servos, HS35HD that can fit in very small spaces. Tempted to remove a 'make shift' set up for something not as obvious. LG


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## aceinspp (Jan 2, 2008)

Nick Jr said:


> RJ, can get that with just sprockets, so I guess that device will take it even further if you didn't have room for an even bigger sprocket. . I took advantage of the Servo City sale and found real tiny servos, HS35HD that can fit in very small spaces. Tempted to remove a 'make shift' set up for something not as obvious. LG


Don't know how small you are talking about but I use the Hitec HS-82MG micro ones that use metal gears and not nylon. Very strong.Later RJD


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## Nick Jr (Jan 2, 2008)

RJ, the 35HD main body it is just over 1/4" shorter and and just under 1/4" higher, and the gears are Metal Karbonate. There is an info page that comes with it. 
Quote: Check the neutral position before linkage connection!. Please be advised that this servo had been designed to rotate in 360 degree when the power is off due to it's tiny structure, Unquote. LG


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## Pete Thornton (Jan 2, 2008)

Nick Jr said:


> RJ, can get that with just sprockets, so I guess that device will take it even further if you didn't have room for an even bigger sprocket. . I took advantage of the Servo City sale and found real tiny servos, HS35HD that can fit in very small spaces. Tempted to remove a 'make shift' set up for something not as obvious. LG


Nick,
I found with Accucraft throttles the really small servos can be underpowered - especially if your sprocket ratio is greater than 1:1. The throttle can stick shut when hot, and the servo can't exert enough leverage to force it open. I tend to use HS65HD servos.


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## Nick Jr (Jan 2, 2008)

Pete, you are exactly right about the throttle. What I do is set the end of the throttle servo movement to ALMOST close the throttle, but not fully till the loco is barely creeping. I bring the locomotive to a complete stop using the Fwd/Rev servo by just pulling it back towards N until the loco stops. Can also use N as an emergency stop, LG


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## East Broad Top (Dec 29, 2007)

You wouldn't want to use a continuously-rotating servo as a throttle control. They're designed to keep turning one direction or another, where their rotational speed is proportional to the position of the stick. The farther from center, the faster the servo turns. If you were to connect one to the throttle, you'd wind the throttle valve completely out of the throttle or jam it completely closed depending on which way you moved the stick from center. And if the "center" position on the transmitter isn't 100% dead stop, it's going to creep on you, which means a very slowly opening (or closing) throttle. Technically speaking, you _could _control a throttle valve that way, but you'd have to be _very _careful, have some visual feedback on the position of the throttle to make sure it's not turning when the stick is centered, and it would not be a proportional control for the speed of the loco. It'd be more akin to using pushbuttons to control the speed of a battery R/C loco. That's not an effective way to control a steamer. 

Continuously-rotating servos _are _great for running feedwater pumps, though. I've got one set up in the tender of my EBT mikado. I move the stick either way from center, the servo starts rotating, and moves the pump handle back and forth. I've got to get a stronger servo, though, as it really strains to push against full boiler pressure. 

Later,

K


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## TonyWalsham (Jan 2, 2008)

Hi Kevin.
I agree that continual rotating servos are not a good idea for regulators.

I am playing around with a number of different continual rotation servos, mainly with the idea to use them with a chain drive for powering small critter locos.
Here is a selection from Pololu.
https://core-electronics.com.au/robotics/motors-gears/servos/continuous-rotation-servos.html

I tried the little one
https://core-electronics.com.au/robotics/motors-gears/servos/continuous-rotation-servos/feetech-fs90r-micro-continuous-rotation-servo.html
It was a little bit ON-OFF when starting.

This one is the best so far. Very smooth start and stop and plenty of torque.
https://core-electronics.com.au/robotics/motors-gears/servos/continuous-rotation-servos/continuous-rotation-servo-futaba-s148.html

This one looks promising as it has BB's on the shaft. Will get a couple to try.
https://core-electronics.com.au/robotics/motors-gears/servos/continuous-rotation-servos/springrc-sm-s4303r-continuous-rotation-servo.html

BTW the prices are for Australia. No doubt they would be less expensive in the USA.


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## Pete Thornton (Jan 2, 2008)

> mainly with the idea to use them with a chain drive for powering small critter locos.


Interesting idea. I'd have thought that the micro motors, like these that Phil Dippel sells (if he has any left) would be more robust?


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## TonyWalsham (Jan 2, 2008)

They probably are more reliable Pete.
However, the constantly rotating servos offer a combined ESC and motor/gearbox at a low price.
Once a way to attach a chain sprocket to the servo is worked out, I will be testing them to destruction to see how long they last.


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## Greg Elmassian (Jan 3, 2008)

Dear Tony:

So, I'm a bit confused. A normal servo will rotate in a certain direction until you stop.

Why does a 360 degree servo necessarily run forever as Kevin stated?

Are there more types / classes of 360 degree servos?

Greg


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## TonyWalsham (Jan 2, 2008)

Hi Greg.
The amount of rotation in a servo is usually determined by a pot inside the servo.
Whilst I have no idea how to modify them they are readily available already modified.
https://core-electronics.com.au/robotics/motors-gears/servos/continuous-rotation-servos.html

Instead of the stick position moving the servo to the predetermined position, the stick position, or knob in the case of my R/C Tx handpieces, determines the speed of rotation of the output shaft in both directions.


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## Naptowneng (Jun 14, 2010)

Thanks Tony that could be very useful information for my RC endeavors

Jerry


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## Pete Thornton (Jan 2, 2008)

TonyWalsham said:


> . . .
> Once a way to attach a chain sprocket to the servo is worked out, I will be testing them to destruction to see how long they last.


Servocity sells sprockets already set up with servo splines. 

https://www.servocity.com/0-1227-pitch-nylon-servo-sprockets










However, there are different size splines on the servos - I didn't find one that fit a mini servo.
Here's my technique:










The other pics show the servo and the various components.


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## Nick Jr (Jan 2, 2008)

A slightly different method: I made centering pins so the holes in the sprockets and the attach devices that came with the mini/micro servos were centered. I then used 2 part epoxy and straight pins as rivets to keep them together. I used Micro Screws to hold the sprocket to the collar that attaches to the throttle. The servo in the pic is the HS35HD. I'm re-doing the R/C on the K4. LG


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## Greg Elmassian (Jan 3, 2008)

Thanks Tony, seems strange you could not have a 360 servo that just moved as long as you held the stick. So getting a regular servo, perhaps extending it's range a bit and getting the right gear ratio is the way if using gears?

Greg


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## TonyWalsham (Jan 2, 2008)

Hi Greg.
Maybe you can get servos as you describe..
I am a neophyte when it comes to robotics, which is where most of the servo development is taking place.

An upcoming challenge is to convert screw reversers without completely replacing them.
First up this will be specifically for the Argyle locomotive C38 built by Accucraft. The screw reverser will require the servo to move 5 turns each way and stop. If the idea works, no doubt it will be adaptable to other screw reversing locos.

Pete & Nick.
Thanks for the information. it would be nice if there were only one or two options for fitting a chain sprocket direct onto the servo.
I have a friend who is about to play around with 3d printed sprockets that will use the technique you show using an adaptor.


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## Greg Elmassian (Jan 3, 2008)

Thanks Tony, neophyte also, so appreciate all your info.

Greg


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## Nick Jr (Jan 2, 2008)

Tony, as you are well aware of, many times you have to 'imagineer' parts and fittings to do things they weren't actually designed for. LG


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## TonyWalsham (Jan 2, 2008)

That I do Nick.
I value creativity and workmanship above all else.
Which is why I am asking all these questions. Your contribution has been much appreciated.


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## Michael Glavin (Jan 2, 2009)

The “POT” or potentiometer also known as a variable resistor is incorporated in a feedback loop used to report the servo output shafts physical location. Said “POT” is connected via the gear train, accordingly it is always cognizant of the output shafts position. The servo amplifier interprets the “POT’s” resistance/voltage and drives the motor to the position commanded by the TX or pulse driver. Servos modified for continuous rotation disable the “POT” and create a voltage splitter with two *precision* resistors soldered in place of the “POT”. The resistors are used to create a fixed neutral or center position. A 10k “POT” would be replaced with a two 5k ohm resistors. The only other thing done is to remove the pin or stub stop used to limit rotation to equitable rotation in either direction. Results are a Bi-directional variable speed continuous rotation servo.

Go for Digital verse Analog amplified servos in all cases, the extra expense is well worth it. And the “PROGRAMMABLE” digitals are even better IMO. 

Hitec’s Karbonite Metal gear train is a combination of Hitec’s proprietary metal alloy and their Karbonite composite matrix of the servos gear train. Most servos have plastic gear trains, some are all plastic with a metal output shaft gear. While others maybe all metal less one gear driving the “POT”. 

I was heavily involved in RC Aircraft with my son many years ago with the factory support of many OEM’s. Hitec provided lots of gear for empirical testing we learned early after Karbonite gear-train releases that while the gear train was very robust, precise and hard it was also subject to fracturing. The best parallel I can offer is Karbonite mimics glass in its failure mode. Sudden shock and rapid pulsating often leads to failure. 

Michael


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## Nick Jr (Jan 2, 2008)

Tony, glad I had something to offer. I know I have gotten incentive to try things from contributors right here on MLS.


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## Greg Elmassian (Jan 3, 2008)

Thanks for the explanation on the change to make the servo a "360", that makes complete sense, so the unit basically is always in the "center" so the stick input just makes it rotate, and more input is just more rotational speed. Simple.

So to have a servo with a position that could extend beyond 360 degrees would take a whole different method. Hmm... I wonder if anyone ever put something like a 20 turn pot on the end of a 360 degree servo... yeah I know you would be increasing the size of the assembly, but it would work I think.

Greg


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## TonyWalsham (Jan 2, 2008)

Greg.
All of those servos I showed feature speeding up and slowing down in both directions.

Extending the 360º rotation beyond 360º for a specific amount probably means having a gear driven external pot telling the output shaft how many turns to rotate. That is how my associate is approaching the problem of interfacing with a screw reverser.
He has great engineering skills and I hope to see his handiwork being demonstrated at the GRITH steam up the weekend of April 29th & 30th in Newcastle.


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## Greg Elmassian (Jan 3, 2008)

Right, or maybe you have not seen the 5, 10 and 20 turn pots available off the shelf, which could be hooked up to the servo shaft directly.

I guess this comes down to size and where you can fit the pot... I'd think on the servo, but do servos come with double ended shafts? I have to see how small you can get a 5 or 10 turn pot ... actually you can get them really small now that I think of it... shoot, the super miniature ones, half the size of a sugar cube only have a rotational life of 200 cycles... that's a no go.. ok... here's one that will do 5 turns and almost a million cycles:

http://www.bourns.com/docs/Product-Datasheets/3548.pdf?sfvrsn=0

If I had the time, would be fun to experiment. Would think you could get really fine throttle control with 5 turns of the valve.

Greg


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