Build a Simple Walking Robot Leg




About: I am an American teaching English at Shangluo University, Shaanxi. I like making machines that do interesting but fairly useless things - I call them Quixotic Machines.

Here is probably the simplest robot leg that allows forward and backward and up and down movement. It only requires a toy geared motor and some other miscellaneous stuff to build. I didn't have to buy anything to build this project.
The problem with leg movement is that as the leg moves forward or backward it also needs to go up to prevent dragging it's foot on the floor. The wheel has all the correct motions built in and it is just a matter of attaching the leg to the wheel in such a way as to take advantage of that range of motion (using crank/slider mechanism).

Future modifications:
1. I plan to add a knee joint to the leg that will lift the foreleg or bottom of the leg up as the thigh moves forward or back thereby making it easier to step over obstacles..

2. It would be really cool to add a solenoid to lower and raise the pivot point of the leg. This would reduce and enlarge the leg stride on the fly making a way to increase and decrease the efficiency of the robot leg while it is moving.

3. It would also be interesting to put 6 of these legs together and see how a robot can move without using computer control to coordinate the legs movements.

4. Put a shoe on the leg - noticed it is slipping a bit on the carpet.

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Step 1: Mount Motor on Tether

The motorized wheel comes from a toy construction truck. I mounted the motor on an acrylic ruler by drilling two holes through the ruler and into the motor gear box. Be careful that you do not drill into the gears. Then use self tapping or wood screws to attach the ruler, which serves as the tether, to the gear box. Note I hotglued and zip tied the motor wires to the gear box so that they don't get pulled out.

After that I drilled and bolted a spacer which is the plastic box (gotten from half of a power supply box) to the bottom of the ruler or tether. 

Onto the spacer I bolted another short piece of acrylic ruler which will serve as the mount for the leg pivot point.

Step 2: Build Leg Pivot Point

The robot leg pivot point is just a u-shaped piece of metal (or could plastic or wood) that holds a bolt onto which the leg will be bolted.
The second picture shows that the leg is a slotted piece of ruler which will slide up and down on the pivot point or pivot bolt.

Step 3: Add Leg Drive Bolt to Wheel

Drill a hold in the wheel and bolt on a 1.5 or 2 inch bolt to which you will attach the leg. This is what drives the leg up and down and forward and back when the wheel turns. Note: the leg needs to be able to rotate on this bolt so do not make the nuts too tight.
I put the outside nut on loosely and then added a glob of hotglue so that the nut would not come off or tighten itself up.

Step 4: Build the Leg

The leg itself is about an 8 inch long piece of acrylic ruler. 

Drill one hole towards the top end to mount to the motor wheel.

Cut a slot in the leg about the length of the wheel diameter. To do this I just drilled a bunch of consecutive holes, took an old soldering iron and melted the holes together until I got a rough slot. Frankly, I am amazed this thing works as well as it does with such a crude slot. It could be more efficient if I smoothed out the slot so that the pivot bolt would slide back and forth more easily.

Putting washers on both sides of the leg, mount it to the wheel and to the pivot point bolt.

Step 5: Build the Tether System

Since it is a single leg, you need something to support it which is an acrylic ruler with the motor attached at one end and a counter-balance at the other end.

The first picture shows a piece of jump rope handle that I cut and then put a bolt through to attach the ruler that supports the motor.
Attach this plastic bearing about the middle of the support tether ruler.

The second picture shows a piece of old metal mop handle that will sit inside some weights to hold it vertical.

The plastic bearing sits inside the piece of metal mop handle and allows the tether to rotate.

The third picture shows how it should all look put together.

Add some counterweight to the other end of the tether to take some weight off the leg and to make smoother rotation of the tether.

Step 6: Add a Support Leg (peg-leg)

Finally you need a support leg to hold up the motor and tether while the leg is in transition forward or backward.
For this I used a hollow piece of fishing pole cut  a little shorter than the fully extended leg. I put some wire shrinkwrap on the bolt so that the support leg would fit tightly but so that I could take if off if I need to disassemble the unit.

And that is it, your robot leg is complete and ready to operate

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    20 Discussions


    4 years ago on Introduction

    could you tell us actually what to get at the store so we can build it? it's kind of hard to make it if you don't know what to get at the store don't you think

    2 replies

    Reply 4 years ago on Introduction

    No offense taken. The problem is that I just built this with junk I had lying around so is difficult to say to go buy this and then buy that. Hopefully you can see the principles involved and make it with junk you have lying around. Mostly I used acrylic rulers, the motor and wheels from a toy truck, some chopsticks and nuts and bolts.I
    As I said in the instructable, it is just a generalized build, your parts will vary from mine.


    6 years ago on Introduction

    nice work ^_^.its possible on two robot leg two but its very hard to maintain its balance a fan of your work bro thank you for this concept ^_^

    1 reply

    Reply 6 years ago on Introduction

    Thanks for the kind words. Yeah, it would be difficult to implement. I lost interest as soon as I got this far and am now working on some other stuff. I like to build and fly foam rc planes and that takes a lot of my time, plus teaching plus always thinking about how to use my junk box parts to build something else.
    Am more interested in mechanical motion than actually utilizing it for some specific purpose.


    6 years ago on Introduction

    I have a couple more projects in mind but I am waiting for a couple of 360 degree - heavy duty servos. Hope they pan out.


    Reply 6 years ago on Introduction

    You are very welcome, and keep up the great work, or should I say Fun?


    6 years ago on Introduction

    Thank you. I don't know how useful it is - I am trying to use it to build a little radio controlled bot but without using the pegleg for support, it moves pretty unsteadily. Am working on some different ideas - also rather useless - but interesting.


    6 years ago on Introduction

    Sorry, don't know what the deal is - I posted my video to because I am in a country currently that has blocked the usual video sites. And now this site has suspended me for some reason. Will try to post video to youtube if I can ever get there.


    6 years ago on Introduction

    Neat simple design. I can't view the video (if it is a video) from site5 though. Can you put up a link to see it in action?


    Reply 6 years ago on Introduction

    Thank you. I really was too lazy to take it back apart for really clear pictures so I am happy you got something from it the way it is.


    6 years ago on Introduction

    Back when Robotwars was on the TV in the UK, I designed a robot that used this type of leg to take advantage of the 150% weight limit allowance for walking robots. Same general idea, but instead of a wheel I used a cam with arms at 0,120,240' so it had three legs to one motor, though thinking back perhaps six legs each side at 60' turns would have given a smoother run. I was going to give it a flywheel like Hypnodisc, either drop more weight into that and the drive motor, or use two flywheels that rotate in opposition to counter the 'hit and fling' effect of a flywheel with a blade on the edge.
    Noone ever 'got it' when I describe this simple little leg to them.

    1 reply

    Reply 6 years ago on Introduction

    I looked around to see if others had used this design and I did see two similar examples: one where the wheel is at the bottom and the sliding shaft pivot is on the wheel. This gives a pendulum motion that slowly moves the leg back and forth but still causes the scuffing and dragging feet problem. The other used the wheel to drive 3 legs on each side but I didn't see a video to see how well it worked.
    I really see this leg motion as just a curiosity and don't know how useful it can really be but hope someone can make a useful robot using it or if someone can post an example of a working robot already using this type of motion.


    Reply 6 years ago on Introduction

    Thank you - I'm not much of a hardware programmer type so lean towards the simple and reuse of stuff around the house.

    JimRDM.C. Langer

    Reply 6 years ago on Introduction

    Thanks for watching. I will be posting another video sometime that shows the same mechanism skipping for lack of a better word.