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madoverlord:robots-smalljohnson [2016/05/14 16:15]
madoverlord
madoverlord:robots-smalljohnson [2016/10/04 14:42]
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-==== Small Johnson Motor & T-Box (Robots) ==== 
- 
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- 
-As part of the development 
-process for [[madoverlord:​robots-ANOREXIA|Anorexia]],​ I 
-have been searching for better motors than the R/C Hobby motors that 
-the wonderful [[http://​www.teamwhyachi.com/​TBox.htm|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 [ [[http://​wiki.animeigo.com/​MO/​PDF/​johnson.pdf|PDF]] ], 
-which looked like a close fit for the hobby motors (which are themselves 
-fancy versions of the Mabuchi 540 motors [ [[http://​wiki.animeigo.com/​MO/​PDF/​rs_540rhsh.pdf|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 [[http://​www.robotcombat.com/​marketplace_motorsmisc.html|only 
-$3.50 each]]! 
- 
- 
- 
- 
-A couple of emails back and forth to Jim at [[http://​www.robotcombat.com/​|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. 
- 
- 
- 
- 
- 
- 
- 
- 
-