Let's Make Robots!

Planetary vs Spur gears.


I am in the business of building a two wheel balancing robot, and I am now looking for motors/gears. 

I seem to remember to have read that the planetary gears have more slack(play) than the spur gears, but i cant find it to confirm.

Anyone with/without experience who knows?

Is there other pros/cons with either gear?


All help is much appriciated.

Best regards,


Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.


I actually considered that for an earlier project. it would actually fit this project as well since you can mount the motors on top, and get a higher center of gravity. I could actually use that in combination with a gear box, just to move some weight up. But the plan was, and still is to keep the mechanics as simple as possible, though I appriciate the suggestion. not a bad idea at all. 

Overpowered motors it is then :D

OddBot is right about the belt drive. I should add make sure you use large wheels and high RPM (over 300) so the robot can recover fast if pushed out of balance. Otherwise it will balance, but any little bump will make it fall. Read the Balancing robot for dummies thread on the Arduino forum. You will find there links to videos of a robot that uses a belt drive.

One possibilty that is often overlooked is a belt drive. A good belt drive has little or no slack. Many hobby shops sell small toothed belt drives for RC cars although O-rings are also a good choice for small robots.

As well as no slack a belt drive is also very quiet. Admittely you would probably need a two stage belt drive to acheive a suitable torque.

No matter which drive train you use I recommend using motors that are over powered rather than underpowered. This gives the robot a good reserve of power to react in an emergency but still uses little or no power when balanced.

Thanks for the quick replies.



The link you posted was really helpful. I cant see any good reason not to use a planetary gear vs a spur. 


The motor and gear are the first I'm looking into. exept for wheels. I think the easiest would be to oversize motors(torque and speed) for the wheel size chosen.


I found some really cheap motors with planetarygear on ebay, though it is risky to buy from a seller who does'nt know what he is selling and can't provide any data.

What do you think about this(speed, torque):



Its very likely that i will use the tamiya 65mm(dia) wheels to keep the construction simple and cheap(periferal wheel speed of about 40cm/sec at no load/12v) .

Thanks again for thurough and quick replies.

Best regards, 

Daniel Aarø





120RPM isn't terribly fast for such small wheels so you'll have to make sure your system dynamics are slow, i.e. the platform won't fall over too quickly by itself, giving the motors time to right the robot.

The torque is nice, and would be tons for a typical size balancing robot. If you've got some of the other parts and dimensions figured out it's worth doing a few rough calcs to estimate how the system will cope.

The backlash/play in the gears comes down to how many gear stages there are and the precision between each stage. It's hard to compare planetary vs spur with restricting the pool of options to, say, two stages only and using the same gear material.

I suggest writing yourself a list of required properties: minimum torque at stall (and torque tolerated by the gearbox), minimum no-load speed, maximum backlash, etc. Then go out and find the cheapest/smallest/lightest gearbox + motor combo that meets those needs.

As stated here:


Next, consider backlash and reduction ratio. Backlash is a measure of positional accuracy usually specified in arc-minutes. For example, a typical spur gearhead has about 10 arc-min of backlash, whereas its planetary counterpart may have about half of that. Reduction ratios for both spur and planetary gearheads range from near unity up to several hundred to one. Spur gearheads, with a single geared input shaft coupled to a geared output shaft (single stage), provide about 6:1 reduction. Planetary units, for comparison, can reach roughly 10:1 in a single stage. For higher ratios and proportionally greater output torque, multiple stages or gear sets are stacked together axially. Increasing the number of stages boosts the reduction ratio and output torque but increases overall length and lowers mechanical efficiency.