Small Johnson Motor & T-Box (Robots)

As part of the development process for Anorexia, I have been searching for better motors than the R/C Hobby motors that the wonderful Team Whyachi T-Box was designed for.

As I found out at Battle On The Beach, the hobby motors really don't like being pushed past 7.2v. Extended driving at 14.4v melts their fancy plastic endbells, which trashes a $40 motor.

During this search, I stumbled upon the so-called “Small Johnson” motor [ PDF ], which looked like a close fit for the hobby motors (which are themselves fancy versions of the Mabuchi 540 motors [ PDF ]). If they worked out, it would be great, because they seem to be tougher, more powerful (able to take 14.4v), and a steal at only $3.50 each!

A couple of emails back and forth to Jim at RobotCombat.com resulted in him sending me a free motor to test out, and the initial results are promising.

Although it is a little hard to see in the first picture (because the comparison hobby motor is missing its endbell, so the armature projects out more), the small johnson faceplate and shaft are almost identical to that of the hobby motors. The motor mount holes are identical (25mm apart, not 1“ as claimed on the robotcombat.com website, btw), and use the same screws as the hobby motors. The only differences are that the nipple (or whatever it is called) on the faceplate is larger than on the hobby motors, and the shaft is knurled (the hobby motor has a flat).

The nipple is no problem, it's the same size as the standard Mabuchi 540, and fits into the gearbox. And modifying the shaft is trivial. What I did was lightly clamp the motor with a vise, attach a battery to spin it up, and file down the knurling. This only takes a few seconds, so take it easy. Then I used a small end mill to put a flat in the shaft. I took off .030” but 0.025 is probably fine (but see below). A hex key stuck into the back of the motor ensures that the shaft doesn't rotate much, and I also used a pair of needlenose pliers (braced against the mill) to prevent the shaft from rotating during milling.

I actually ended up milling a notch in the shaft, and mounting the pinion with the gear away from the motor body. The shaft is actually not quite as long as the hobby motors, and if you take a look inside the gearbox when the motor is attached, doing it this way ensures that the pinion fully engages the first stage gear (which is not as thick as the pinion).

One minor gotcha you have to worry about is that the notch you make has to be deep enough so that the set screw can be inserted flush. If it isn't, then it can rub against the first stage gear, causing a grinding sound. This is not good.

I ended up using a dremel grinding bit to make the notches a bit deeper, because some of my original ones were not deep enough. Turns out the dremel is the perfect tool for this, it easily grinds away as much of the shaft as you want, the width is just perfect, and at medium speed and pressure, it doesn't cause the shaft to rotate, so you can just clamp the motor in a vise and grind away. I now do all my notching with the dremel, no need to mill at all.

Initial test drives are quite positive. The bot gets to top speed within about 5 feet, and steers straight as an arrow. I was a bit concerned that the motors might not be exactly neutrally timed, but the only artifact I've noticed is a bit of spin-out when coming to a stop from full-power reverse.

The Small Johnson running at 14.4v seems to be a bit faster than the original hobby motors at 7.2v (but a bit slower than the hobby motors at 14.4). The bot is very peppy and seems to have a ton of torque.

Best of all, after 5 minutes of tooling around the driveway, the motors weren't even warm, and neither were the speed controllers.

A couple of tips: make sure you solder in a supression capacitor across the leads, and wrap the endbell of the motor in high-temperature electrical tape. As I found out the hard way (with the hobby motors at least), regular electrical tape can melt!

The most common failure mode for this motor/gearbox configuration is that the pinion gears come loose, despite locktite on the shaft and set screw. As a wise man once said, “Set Screws Suck!”. I've taken to permanently mounting them with JB Weld. I put a little blob of JB inside the pinion, slide it on, then dip the set screw in JB and tighten it down. I haven't had a failure since I've started doing this.

Finally, I strongly recommend that you rigidly clamp the motor to the frame. If the only thing holding the motor in place is the two tiny screws that fix it to the T-Box, you'll have problems with big impacts deforming the motor cans. A cheap pipe clamp snugging the motor against a structural member works fine.