The fire truck belongs to the oldest daughter as one of her “collectables”. I thought it might be cool to make it go and steer via radio control, but I needed to leave all the manual controls intact. This sounds easy, but there was a lot of fussy fitting needed to put the steering and drive mechanisms in place without cutting into the fire truck. In the end, I’d say it was 80% successful because the chain drive wouldn’t stay engaged. But Raggedy Andy looks like he’s enjoying himself behind the wheel. I had two 12 volt motors that I planned to use for moving and for steering. Motor on Amazon
Step 1: Steering
The motors are geared down and have plenty of torque. But the shaft is a non-standard screw and the motor comes with no nut to follow it if you want linear motion. Somehow, with a bit of trial and error, I was able to 3d print a nut that served pretty well to ride on the screw to move the steering. Lots of other 3d printed pieces located it all under the “hood” with minimal screw holes in the old fire truck.
The steering controls begin with a servo turned by the RC receiver. The servo arm pushes against a switch that gives 12 volts to the motor which moves the switch away from the arm. If the servo moves the other direction, a different switch is closed and the steering moves the other direction. It’s a crude “all or nothing” system, but for this application it was fine.
Step 2: Drive System
This was the tough part. I decided on a chain and sprocket system, and in retrospect I think I’d have done better with toothed belts. To attach the sprocket to the axle I 3d printed a split sprocket. This is how I avoided cutting into the steel axle. The motor of course had a non-split 3d printed sprocket. And with chains I needed a spring idler wheel to keep the right tension and make sure the chain is wrapped around the sprockets enough.
In addition to the problem of proper chain tensioning, I think driving the axle from only one side made it more difficult. The bends in the axle, needed for working pedals, introduced a lot of friction when driving it from only one side, and it required lots of torque. After a bunch of fussing around I got it to the point where the chain stayed on most of the time.
The drive motor is run by an electronic speed control (ESC) for brushed DC motors, 60Amp max. The ESC is controlled by the RC receiver.
Step 3: Battery, Radio Control, and Sketchup Files
I used a 3S (12 volt) Lipo battery mounted next to the motor. It seemed to provide plenty of power for both drive and steering motors for as long as the chain stayed on.
This is a simple two channel system. The receiver gets it required 5 volts power from the “battery eliminator circuit” (BEC) of the ESC. Then it’s just a matter of velcroing Andy into place and turning up the rock and roll.
Sketchup Files for 3d Printed Parts
I include here the models I drew for 3d printing if anyone is interested. The Acme nut itself could be useful to anyone who wants linear motion from a motor like I used.