Let's Make Robots!

Why is your robots motors not strong enough? (Gearing)

Have you ever made a robot that ran fine when you held it, but when you put it on the floor it moved really slow (or not al all)?

Hmm… Did you use a motor without a gear? Well, there is your answer.

This may make sense to many, if so you should find something interesting. But if you never had the “aha” experience, here’s some help:


I often see people making their first robot and think "Hey, motors - I do not need to buy those special ones, I have some right here", and some how the thing does not work!

Do you have a driver’s license? Tried manual (some people from the US may not have tried this)? Well, if so try to start your car in 5th gear. It will not move! Ever tried starting a bicycle in the highest gear? Not very easy is it?

The RPM of the little motors you can find are high. Very high.

Even my Wall Racers are geared down, my guess is 50:1 and they are still fast (even with small wheels which are acting as down-gearing as well). If the gearing was not there, they would not be able to start: High speed = low power, low speed = high power.

You need a gearing that can both supply enough power for your robot to start up, but not so heavy (slow) that you fall asleep before it has moved an inch :)

So you need motors with a gear attached, or you need a lot of cogwheels (or a BIG one and a SMALL one, very big & very small) or a worm-gear.

I have seen one robot without gearing. It was FAST, and the wheels was SMALL (this helped, if not, it would not work), it was able to skid (motor could get the robot under control). It was light and I also think it had way too much power:


And then there is of course the solar-powered little things, where there is no wheel, just some plastic around the shaft (=VERY small wheels)

BUT as a rule: "robots require geared down motors!!" if not, they do not work.

I always recommend high gearing (200:1 or something like that) for beginner’s projects. It makes the robot much stronger, and so it is more forgiving for bad code. And, it makes it slower, which makes it easier to capture what on earth it is doing so you can debug.

I know it may sound like fun to start with a fast robot, but it is not! On the contrary, the slow ones are fun! Check my first robot, I think the gearing must be something like 400:1 (pre-assembled 200:1 gears, and then I double up with extra gears).

There is a ton of very fine articles etc on the more deep thoughts on gearing out there on the net, so I will not go into that.

But for robots:

You can get a million different geared motors. Some you assemble yourself, and some where you can change the ratio. Pre-assembled little packs are very popular, and a good idea is to buy wheels that match! It will make you happy not having to drill or glue to get the motors to fit the wheels!

Thank you so very much to jklug80 for helping making my english somewhat understandable in this text :)

Hope it was helpful, Have fun :)

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Wow! It's really helpfull for me Thanks 4 ur Idea!!!


A cone shaped gear sounds like it has a mechanical flaw to me. At the narrow end there is only room for a certain amount of 'cogs, while you'll have many more on the wide end. So at sertain intervals along the length of your cone you will have to introduse more cogs. There is no way to make them appear smoothly, so your cone will stop being a cone and become a row of gradually larger gears placed next to one another. To make a transition from one gear to the next, you'll need a clutch to disconnect the power to avoid cogs from snapping. At that point you've built a manual transmition.

Using cones (without cogs) and a band sounds like an interresting experiment. RubberCompany har some amazing green round bands called "rundrem" that you just cut to desired length and join to a loop with a cigaret lighter. Their url is rubberco.se but it's only in swedish.


/ vzz-clck-"Maneuver"

Another good thing about gearing down is that the electronic braking that most robots use will be able to brake a lot harder if the motor has been geared down since from the wheels point of view to the motor it has been geared up and now require more torque applied on the wheels in order to roll.

- Jimmy

Youp! And there should be a comment on worm-gears as well.. Anyone?

Well yes... I have used worm gears a lot when building with lego. They have some very nice properties and some not so nice properties:

  1. It has very high gear reduction = NICE!
  2. It has a perpendicular setup with the gear that meshes with it meaning that the worm gear can rotate the meshing gear but the meshing gear can't rotate the worm gear. This is very usefull if you don't want your robot to drive around when it is not being run by the motor = NICE!
  3. It loses a lot of power to friction = NOT SO NICE!

I guess that's what I know about them... I have used them to build something like a servo motor for lego mindstorms 2.0 - which is not the most compact servo motor in the world :-D

- Jimmy

If you don't mind me asking is that your gear housing or is the pic stolen? :) I was wanting to eventually build a gear housing that I could use for my robots with holes in places so axles could be moved and new gears added. I was hoping it already existed so I didn't have to build it.
It is just a random picture from somewhere :)

This topic has gotten me thinking: are there any good "transmission" gear boxes out there that will let you change the ration on the fly; like a car transmission?  Most times you would never need this, but it would be cool to have a bot that can adjust the gears to it's needs. 

I suspect it wouldn't be too hard to create this if somebody is familiar with machining (which I am not).  You could probably do it with a cone shaped gear and the pinion gear simply slides up and down on it to change the ratios.  When it's at the tip of the cone you get a 1:1 ration, as it slides down to the other side you get more torque and slower speeds.  You'd just need a servo to move it and some sort of a sleave to allow the motor shaft to stay connected to the pinion.

Just  a random thought. :)

You would likely need a servo and an actuator (or another servo that can act as an actuator similar to what I'm using to load caps http://letsmakerobots.com/node/657). First you need to decouple the connection between the 2 gears. Then you need to move an axle down that has various gears on it. I may have to work on this it sounds like a challenge. As soon as summer is over I will have more time to play with robots. For now I want to work on my cap launcher, but after that I may try to make a transmission, I would just need to buy another servo.

EDIT: Here is another issue. When you shift a car the engine slows down because you take your foot off the gas (unless you drive I like do and keep the RPM up!). You would have to do somethign with the motors. If you power the motors off the robot slows down a LOT, if you keep the motors full speed then the gears grind and maybe don't connect, you would either need to power them down or disconnect the motors from the axles so it doesn't slow the robot down.

My idea would be to lower the voltage to the motors while the robot is shifting or tiem it just right so you only need to shut them off for a milisecond. This would be tough to work out when making the transmission...

 Maybe a conect theme if someone has a prize to put up or maybe the winner gets nothing, but the pride of making it! :)

The nice thing about having a cone shaped gear with another that simple moves up and down it is that the motor shouldn't need to disingage to change the ratio.  Then again, I've never done this and really don't know. :)

I wonder where I can buy a cone shaped gear.