Improving FLL Robot Game. How to calculate gear ratio in complex systems Pro Preview

We calculate the number of rotatios when a gear system is involved. The driving wheel will have to do a number of rotations for the driven wheel to rotate to a desired number of degrees. In our specific case when the driven gear wheel is rotate to about 90 degrees the legs will lift the robot.

  • #468
  • 06 Apr 2017
  • 5:07

There are a number of ways to approach the problem


Start small. With 10, 20 or more rotations. In the video I say a Million rotations, but please do not go for a million. It is pretty much. 

Calculating it

We calculate the number of rotations of the driven wheel. Because we have a 40 teeth gear wheels driven by a 8 teeth gear wheels. Then we follow the axle and calculate the number of rotations of each of the gear wheels in the system. 

How to's on gears and their ratios

How to on Gears (part 1)

How to on Gears (Part 2)

Search for gears

Robot Lifting Attachment With a scissors mechanism

For the FIRST LEGO League 2016-2017 Animal Allies we prepared an attachment for lifting the robot. The idea of the attachment is to show how you could lift the whole robot with a system of gear wheels, levers, and scissors constructions.

Built mainly from LEGO Mindstorms EV3 parts but could probably be constructed from NXT sets.


When lifting the Box Robot we have a system of gears right here. The question is and we have one of the axles connected to the motor. The motor is right here inside of the robot. The question is: 'How many rotations should the motor do in order to rotate this wheel to about 90 degrees?' Because when you rotate this wheel to about 90 degrees we have the legs at their highest position. How many rotations should we make with the motor in order to have this gear wheel rotate to 90 degrees? And how should we approach this task? How do you approach this problem? What you could do is experiment. Experiment with rotating the motor to let's say 10 rotations. Then 20, then 50, then 100, then 1 000 000. You can do a lot of experiments and find the correct number of rotations for the motor that will make the final driven wheel this one here rotate to about 90 degrees. But the more correct way, the way that engineers do it is to do a small calculation. And the calculation is the following. You have the gear wheel 40 teeth gear wheel. It has 40 teeth. And you have a small gear wheel connected to it that has 8 teeth. So, what you want to do is to rotate this large gear wheel to about 90 degrees. If it has 40 teeth, this means 1/4 of 40 is about 10 teeth. And you must make this wheel move forward for about 10 teeth. It might be 11 depending on how far you might want to get the scissor mechanism to expand but let's fix 10 for the video. This means that if you have 8 teeth on this small gear wheel, it should do one full rotation and then 1/4 of full rotation. Which is about 360 + 90 = 450 degrees. So, this small gear wheel should rotate to 450 degrees or 1.25 rotations. Then we follow the axle. We want this wheel to rotate to 1.25 rotations. This means that we want this wheel right here the small wheel to rotate to 1.25 rotations and this wheel has 24 teeth. It is driven by one more small wheel connected to it. It's difficult to see it from here. It's right here. I hope that you can see it. Now, the question is this small 8 teeth gear wheel that's right here how many rotations should this wheel make so that we have the 24 teeth gear wheel rotate for 1.25 rotations. And this is a calculation. What you do is you find the ratio between the different gears in the system. You find the ratio between this gear and this gear, you find the ratio between 24 and 8 - the ratio is 3. You find the next ratio here 24 to 8 which is again 3. And then you find the ratio right here which is 20 to 12 and from these ratios you can come up with the correct answer. How many rotations should this wheel that's connected to the driving axle do to have the final wheel rotate to 1.25 rotations? And that's the principle. This is how you do it. You do a calculation with the different ratios between the gear wheels. Now, this sounds probably difficult but we have a couple of resources at that explain the working of the gears, the gear ratios and how you calculate them. I'll try to provide with links somewhere in the course below the video so that you can check the other resources and then continue forward after you know how to calculate the ratios between a driving wheel and a driven wheel when there is a system of gears in between. These are 2 or 3 different videos that you should look at.

Courses and lessons with this Tutorial

This Tutorial is used in the following courses and lessons

Image for FIRST LEGO League (FLL) 2016-2017 Animal Allies. Review of solutions with explanations
  • 48
  • 118:45
  • 15
Image for Calculate and configure gear ratio
  • 8
  • 0
  • 2
  • 3d_rotation 1