Anyone who has converted a live steam engine to radio control has bumped in to the question of how much the throttle servo has to turn the throttle shaft in order to make the engine go properly. In general, most engines will move on 10-20 degrees of shaft movement, but on an uphill line with 6 heavy coaches you may feel that 180 degrees will be required.

Some years ago I converted my Accucraft C-16 and managed to get about 135 degrees of movement (see below.) My Accu EBT #12 had only 90 degrees as I didn’t try anything special, though I recall Kevin saying he wanted 180 to be able to give it a big burst of steam when needed. Most live steamers only seem to need about 90 degrees max to function, though you hear that more might be better. With Jason selling longer tapered fine control throttle shafts, you will definitely need to think about the shaft rotation.

My C-16 originally had a regular servo and a long servo arm connected by a wire to the throttle shaft crank, which was shorter than the servo arm. Simple geometry will show that a long arm moving a shorter one will make the short one move further. I think I got about 110 degrees, but it is tough to go further as you run in to the mechanical problems of such arms.

My C-19, on the other hand, had chain drive (1/8" nylon from Servocity.) In theory, this allows you to alter the ratio of turns between the servo and the throttle shaft and thus move the shaft as much as you want. A 12 tooth sprocket on the throttle being turned by an 18 tooth on the servo will give you a 1.5 times advantage (i.e. if the servo turns 90 deg, the throttle turns 135.)

There are 2 problems - the torque needed is directly proportional to the ratio of the connection (i.e. in the example above, you need 50% more torque than a 1:1 connection,) - and there is a size limit to what can be fitted inside a (model) loco cab. (Followng pic is my EBT#12, but the C-19 was similar.)

I found a 180 degree servo, and replaced the servo in the C-19. As it turned out, my TX (Deltang TX20) was only sending a 75% move command so my 180 degree servo was only moving 135 degrees! However, it was more than adequate. I did research changing the TX to a 100% throttle throw, but it got too complicated and the engine was performing fine, so it was left in that state.

Recently I added a throttle servo to my K4 (another thread, when I get time) and pondered the same issue. I found a bunch of servos on aliexpress that promised 180, 270 or 360 degrees of rotation, for the robot guys I believe. However, when they arrived I could see no advantage - most just turned 90 degrees on my test receiver. One did almost 180 degrees - a Futaba S3003 which was supposed to do 270 (as ordered.) More research was needed.

It turns (!) out that the pulse sent to the servo (from the RX, under command of the TX) needs to be extended to make it turn more. The throw on the servos can sometimes be adjusted at the TX - the Spektrum Dx6i will allow you to request a 150% throw on any channels. At the moment I have not found a TX that can ask for 200%. Most of the robotic guys seem to be using microprocessors and generating servo pulses in software that are as long as they like.

More to come as my musings and research continue. . .

Hello Peter.

Deltang powered Tx’s such as my RCS Dual Use range with a standard Deltang Tx2 module come with Ch #1 set to 100% of throw (90º) all other channels are 150% (135º).

Channel # 1 is normally used for the regulator and Ch # 3 for the reverser.

Whilst servo throws can be changed as long as the Tx2 core module is set to profile # 2, changing them doesn’t necessarily help. The throw options are quite limited.

As you suggested in our private correspondence, what can be done relatively easily, is simply reverse the usage of the Channels.

Channel # 1 becomes the reverser with 100% throw.

Channel # 3 becomes the regulator with 150% throw.

In the case of my RCS handpieces just swap the knobs on the handpiece and swap the servos at the Rx.

BTW. When using a TX2 core module in profile # 2 only, all the servo controls except Ch # 5 can be reversed.

If anyone needs to know how, drop me a line and I will supply instructions that are understandable.

I received an RCS TX3 from Tony last week and Winn loaned me a “servo doubler” from Servocity, so it was time for some tests. Here’s the TX3 with the knobs reversed, so the throttle has 150% servo throw preset, and the reverser is 100%.

Here’s the video of the first set of tests -apologies for the commentator who kept getting his numbers wrong.

The following day I repeated a couple of tests to confirm the findings.

I have a different servo, a Hitec HS81 spec’d at 162 degrees max, on the way so there will be some more tests.

• I concluded that RCS’ TX3 with the 150% throw does make some servos move 135 degrees instead of the standard 90 degrees. This can help with a live steam throttle which is rod or chain actuated.

• The “servo doubler” works as advertised, even on servos that pay no attention to a 150% throw request. At $20 it is a bit pricey. With chain and sprockets, it can help where you need more movement of the throttle shaft.

Final set of tests with a Hitec servo instead of generic chinese SG90s.