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.
