Hydraulic JudoBots




About: I'm a writer, maker, and educator who's on a mission to better the world through hands-on engineering projects. Check out my work: www.MadeForSTEAM.com

Two robots face off and try to throw each other off of a table! It'll make more sense after you watch the video...

Be sure to check out Made for STEAM, an amazing collection of hands-on projects for kids!


More footage from actual student-built JudoBots:

Updated version from my book Rubber Band Engineer

I call them JudoBots because of the way they seem to throw and grapple with each other during combat.

Although designed for children in grades 4-6, I think just about anyone can have fun with this. Even parents can't resist trying it out!

The bots are assembled from four components built separate from one another: the base, the stand, the arm, and the hydraulic system. The hydraulic system uses plastic syringes as pistons and water as the hydraulic fluid.

Step 1: Materials, Tools, and Design Criteria


Design Criteria

  • The base of the JudoBot must fit within a 12x12" square (okay if the arm extends beyond that)
    • This prevents students from building gigantic, sprawling bases that are impossible to flip
  • Can use a maximum of 40 craft sticks (or adjust as you see fit)
    • This prevents students from building super-heavy robots that are difficult to move

Step 2: The Base

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Step 3: The Stand

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Step 4: The Arm and the Wedge

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Step 5: Assembling the Pieces

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Step 6: Make a Hydraulic System

This part can be a bit tricky for students. Although the process is fairly simple, it isn't easy to commit to memory by watching it done once or twice. You may want to outline these steps on a whiteboard:

  1. Connect tubing to one syringe
  2. Fill completely with water
  3. Point the tip of the syringe up and push on the plunger. This expels all of the air and fills the tubing with water
  4. Refill halfway and set aside
  5. Submerge the tip of the second syringe and repeatedly pump the plunger to expel air. Fill halfway
  6. Connect the syringes and try it out. If the total amount of water in either syringe exceeds the 10ml mark then there is too much water in the system. There should be little to no air bubbles, too.
  7. Glue on a holed cube to the end of one plunger in each set

For extra fun, use food dye to color the water

Step 7: Attaching the Pistons

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Step 8: Battle Time!!

The arena consists of two 10" squares spaced apart by 2" - 4" drawn onto a tabletop (depicted in the video). Ideally there should be about 6" between the sides of the squares and the edge of the table.

Rules of engagement:

  1. JudoBots begin by squarely facing each other with the front of the base touching the edge of the square and the arm completely extended.
  2. On the count of three, fight!
  3. There are three ways to win a fight: flip your opponent, push your opponent off of the table, or if your opponent experiences a hydraulic failure.
  4. A stalemate occurs when the bots are both active but unable to reach one another.
  5. A draw occurs if both bots are either flipped or have fallen off.
  6. Students cannot touch the JudoBot with their body during battle.
  7. Students must strive to control their JudoBot with precision.

Hydraulic failure most often occurs when the plunger is yanked out of the syringe. Repairs are not as straightforward as filling the lines, so emphasize the importance of operating the JudoBots with precision. If you see a student recklessly pulling on the syringe in the heat of battle, call for a time out. It's better to spend a few seconds reminding your student to slow down than it is to spend a minute or two fixing the line.

The first time students battle it will look like two poorly programmed machines bumbling into each other. This is normal - it just takes some practice before getting the hang of the controls and basic tactics.

Step 9: Presenting This Information to Your Class

I break this project up into two parts: 1. Construction and assembly, and 2. Attaching the hydraulics and battling

  1. Prepare two working JudoBots and an extra hydraulic system before class.
  2. Demonstrate how to operate one JudoBot in front of your class. Tell them right away about the importance of operating the JudoBot with precision, and show them what happens when a piston is yanked on too hard.
  3. Let each student have a chance to operate a JudoBot, though not in combat. This allows the students to experientially grasp what hydraulics are, which is necessary because you will be referring to that concept a lot.
  4. Afterwards, have the students focus on you as you disassemble a JudoBot into it's 4 components: base, stand, arm and hydraulic system. Show students the key features of each part, such as the materials used for the pivot column and how it operates. Do not explain how to create or attach the hydraulic system yet.
  5. Once you feel confident that the students comprehend how to construct each part, allow them to access the materials and begin.
  6. At the beginning of the second part of class, show the students how to create a hydraulic system and outline each step on the board. You may want to show them a second or third time. Demonstrate how to attach each hydraulic system to the JudoBot
  7. Finally, remind your students to operate their bots with precision once again! Never yank unrestrainedly on the controls.
  8. Once the battling begins, continue to allow your students to redesign and build. Oftentimes the very first design includes some flaws that need to be worked out.

Step 10: Further Thoughts

This project is relatively new in my curriculum lineup, so there are lots of unexplored ideas. What design aspects could be improved? How could this be motorized? What new element could make the battles more interesting and decisive? Could this idea be turned into a toy that uses easily interchangeable parts which allow children to customize their bots and battle with their friends? If you try this out, post some photos and share your insight. Or if you don't, let me know what you think of it anyway :) Thanks for reading through this Instructable - now go forth and create an unforgettable experience for the kids in your life with awesome engineering projects!

Same materials, different project

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45 Discussions


4 months ago

ooh that looks great, I'll try it with my class using the piano hammers etc that I didnt know what to do with. Thanks for the great tutorial


7 years ago on Introduction

Hi there! I just wanted to say thanks for the wonderful idea! I adapted the syringe concept and made a catapult which won first prize! Thanks!

3 replies

Reply 7 years ago on Introduction

Well it wasn't really a contest we were supposed to build a mechanical toy in Physics.


Reply 7 months ago

I recommend limiting the sticks to 40 or so, otherwise it's possible to make super-heavy robots that are difficult to move!


Question 1 year ago on Step 10

I am coordinating an event that introduces kids to STEM by having them do projects like the judo bot. I was wandering how long it takes to make one judo bot?
Thank you,
Chaitanya Pawar


5 years ago on Introduction

Very cool.
Had to make some...

Will video the battle ... "There can be only one".

Thanks for the ideas.



5 years ago

Fantastic project! My students built these in my physical science class and it was their favorite!

yo you know the whole pressure thing yay that is what works the thing what ever it is you know wat evs

Idea: create a team event after the battles......a pass the parcel sort of thing.....vid the results. For a team vid