Insect Walker


Dive in

Download the design in Ldraw here. I made it on a Mac with Bricksmith, but the file should work with other Ldraw software too. The model is also on Rebrickable.

I made the design with parts predominantly from the Mobile Crane Mk II (42009) and the Mindstorms NXT 2.0 (8547). You’ll still need chain-links, small Technic turn tables and some friction-less pins. Although I used the Mindstorms set, the Insect walker does not use any of the typical robotic parts like the computer brick, sensors or motors. I haven’t checked, but perhaps you don’t need the Mindstorms model at all if you’re willing to compromise or improve on the color scheme.

More details about this model below these images.

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How does it work?

Perhaps a better name for this walking machine would be crank walker since the two major mechanisms that make it move are both based on cranks. One moves the legs back and forth, and the other moves the feet up and down. There are three cranks to move the six legs, and each leg has one to move its respective foot.

To synchronize the movements of the legs, the walker has a power axle along the entire length of the body. It has a worm gear and a 24 tooth gear for each leg. Synchronizing the leg and feet movements is easy because when the legs go through one cycle of back and forth movements, the feet go through one up and down cycle. This can be achieved by connecting the two cycles through pairs of same-size gears.

The last essential part of the mechanism is the exact timing between leg and foot cycles. After some experimenting this turned out simpler than I initially thought: when the legs move through their neutral point (when they make a 90 degrees angle with the body), the feet should be halfway the the ‘down’ part of their cycle or halfway the ‘up’ part. If one follows the building instructions precisely, then the timing is close to optimal.

At the bottom of this page, you’ll find a video with the moving details of the cranks.

Three problems and their solutions

For those interested, the design had some puzzles and solutions that I would like to share here. First, there was the problem of the different swing speed of the legs. That problem is not solved in the final version, but got so strongly reduced that it is not bothering me anymore. Secondly, the up-and-down movement of the feet was far from satisfactory: Instead of holding the feet down during a full leg swing, the machine would lift them immediately after it had put them down. Thirdly, there were these little ticks that I could sometimes hear. Not so problematic in themselves, but occasionally they turned into big ticks and downright hold-ups which caused damage.

Problem 1. Difference in swing speed of the legs

After I had tested the first walking model a couple of times, I noticed that the legs did not move back and forth at the same speed. It took a while to notice because the funny bit is that they all arrive at the same moment at their furthest-out position. If you look at the following video, you can see that the when the legs turn with the clock, they seem to move faster than when they move against the clock.

Video 1. Insect walker with legs swinging at different speeds

If you look more precisely, you’ll see that when they start their movement with the clock they start slowly and then accelerate. They move the fastest when going through the ‘neutral’ point. Then they slow down, come to a stop at their furthest position and move back, i.e. against the clock. When they move against the clock, their turning speed is much more continuous.

Why is this not good? As you may have realized, the reason why the machine keeps upright is that it is supported by feet at opposite sides of it’s length axis. At most times it is supported by one foot on its left side on the middle set of legs and two feet on its right side on the outer sets of legs (i.e. the front and hind legs). And vice versa. (Only at the hand-over moment is the machine supported by six feet at the same time) This means that two of the feet supporting the machine are attached to legs (the outer legs/ the front and hind legs) that move at a different speed (either faster or slower) than the leg that support the third foot.
In other words, the machine will try to move at different speeds at the same time. This is a loss of energy, it may cause the feet to drag or slip, or if they do not because they are stuck in sand or if friction does not allow them to slip, it will cause stress on the machine and things may break.

How to solve this problem? For that I had to find out what causes the difference. You may have discovered it immediately, but I could not. So I did some experiments. First I wondered if things would differ if two pairs of legs would be connected to the same crank?

Video 2. Insect walker with two sets of legs on one crank

Nope, that was not it. Next, I wondered if it had to do with the distance between the gliding axles that connect the cranks with the legs. For the middle set of legs these axles are further apart. And because I first only noticed the strange behavior with the middle set of legs, I tried to see if it would change if I separated the axles even further.  As you can see in the next video, that made no difference, either.

Video 3. Insect walker with different distance between gliding axles

But hold on… For some reason, all three sets of legs moved in sync. I finally noticed it was not just the middle set that moved faster with the clock than against. Then it dawned upon me: it had to do with the distance between the crank and the legs. To save space, I had minimized that distance as much as I could. Have a look again at the last video and focus on the set of legs at the top. The crank for that set of legs is the black rotating plate just above it. It turns with the clock. At the end of it you see two yellow connectors that connect it to the gliding axles that in turn move the legs. Look at the crank and those yellow connectors. When the crank moves from the 11.00 o’clock position to the 1.00 o’clock position you will notice a lot less movement of the legs, compare to  when it goes from 5.00 to 7.00. When the crank moves past the 6.00 o’clock position, its endpoint is only 3 studs away from the legs’ pivot point. Because of those 3 studs distance, the crank’s movement is somewhat reduced. However, when the crank goes through the 12.00 o’clock position, the distance from its endpoint to the legs’ pivot point is 7 studs, and the movement reduction is far bigger. It is a matter of angles, which I have tried to draw this in the following figure. It’s not exactly high-quality graphics, but hopefully, you get the picture.

Image 1. Insect walker. Effect of crank rotation on nearby legs

My solution is to increase the distance between the crank and the set of legs. In the design that you see in the videos above, the distance between their pivot points is – you guessed it – five studs. In the final design it is 7 studs. There still is a difference in the legs’ swing speeds back and forth, but I find it acceptable. Increasing the distance even more will make the difference even smaller, but it has a downside, which is that the total length of model also increases. This will require a stronger frame, which is also heavier. And at some point it will look ridiculous.

Problem 2. Keeping the feet down

In the following video, the Insect walker is lying on its back with the feet up and moving. You see the problem illustrated by the feet with the wheels and my solution illustrated by the foot without the wheel.

Video 4. Insect walker with two designs for the feet.

The feet of the first design were based on simple cranks. If you focus on the foot to the left and closest to the camera, you will see that it basically makes a diagonal movement : from bottom-left to top-right and back. It actually works, meaning that the machine could walk and because of simplicity and lightness it could go quite fast even by speeding up the movements. But because the feet would hand-over to soon to their opposing counterpart, part of the swing of the leg was lost. That is, if it could hold its feet down (i.e. up in the video) during the entire swing it could walk even faster.

It took me some puzzling until I realized that a thing that I learned from the problem with the swing speeds of the legs could be the solution to this problem: when a crank makes a full turn and its movement is transferred to a lever like with the legs, not all equal parts of the turn cause equal movement of the lever.

Let me try to explain and have a look again at figure 1. When the crank moves from 11:00 to 1:00, it turns 1/6th part of a full circle (or 60º) and the legs make a move of, say, 1/12th of a circle (or 30º). When the crank goes on to move from 1:00 to 5:00, it will have made a 1/3 of a full circle (or120º, so twice as much as from 11:00 to 1:00) whereas the legs moved about 1/12 of a circle (so the same as – or  perhaps even less than – when the crank moves from 11:00 to 1:00) !! The picture is not very precise, but hopefully precise enough to convince you.

1/12 of a circle is not a lot, so if we manage to make that the movement of the foot while the leg swings, then it will stay down, or at least mostly down. And this is exactly what you see in video 4. If you look at the top of the foot without the wheel, you see that it moves in a kind of flat loop from right to left. Another demonstration is video 5. (Notice also that the feet move up faster than they move down, which is the same effect as in problem 1)

Video 5. Insect walker with  feet moving up and down in different speeds

So, I solved the problem of the feet movement, but it is a heavier solution. Also the overall frame needed some reinforcements elsewhere, which made the final design walk slower after all.

Please note:
Now that I have written all this down, and am looking at the figure and video 4, I am beginning to think that the new foot design was not necessary. Probably the only thing it would have taken, was to change the synchronization between the movement of the foot and the movement of the leg. That would get rid of the heavy foot structure and make the Insect Walker a lot lighter and thus faster.

Unfortunately, it will be a couple of months (until May 2018) before I can test my hypothesis. So I will continue with this design, and warn you that the feet could be a lot simpler. I am not sure though that the lighter design will be better overall, because one of the things that the current foot design also seems to do is to take some of the pressure, generated by the weight of the machine, from the mechanism that drives the feet. So, perhaps in the end the current design still has a benefit.

Problem 3. Getting rid of the ‘Little ticks’ that turn into big problems

Once I had the first version up and walking, I noticed little ticking sounds in the mechanisms. Some more regular than others. Ticking sounds come from parts hitting somehow against each other. Tiny hooks of irregularities in the parts somehow bump into each other. This causes resistance. Most of the time, the parts continue to move on immediately, but sometimes they really get hung up upon each other and that is not good. The rest of the mechanisms continue moving and this can cause great stress on certain parts and irregular functioning of the machine.

Worm wheel ticks

The most subtle little tick that I discovered was caused by a seem from the mold on a worm gear on the central power axis. The seem ticked against the teeth of the 24 tooth wheel. Initially, I was surprised that it happened, but considering that worm wheels constantly glide against the teeth of the connecting gears, it makes sense. It never turned into a big problem, so I left it as it was. Also, I do not think I would have been able to remove that seem without causing even greater irregularities. Perhaps Lego will make a better mold some day.

Beams sliding against each other with their pin-hole sides

Another little tick was caused by beams sliding against each other in the foot design. In the picture below, the mechanism on the left has a foot made of yellow beams, which slide against the vertical dark-bluish-grey supports.

Image 2. Lego Insect walker, foot-moving mechanism v3 (left) and v4

In this construction, it are the sides with the pinholes that slide against each other. These sides have many edges. You hardly notice it when you rub two beams against each other like that by hand, but there is some friction because of slight irregularities in the shapes. In the insect walker the bars occasionally could get stuck against each other, which could clearly be heard by the whining of the motor and the shock when the bars would finally let go. By then some of the gears would have skipped a tooth or two which caused the feet to be out of sync with their legs and each other. Very bad.

I solved it – initially – by changing the orientation of the beams in the foot, which you see on the right side of the picture above. Here, the smooth side of the white bars would glide against the pin-hole side of the vertical supports. However, it introduced friction between the white beams and the horizontal support beams.

In the final design, the foot was made out of axle instead of a set of bars. I had to reduce weight , and the feet were an excellent target for that because they contain many beams. I realized that the 7 Kg Millenium Falcon is standing on 7 axles which seem totally unfit for that purpose but they do it easily. So, 6 axles could carry the insect walker. Unfortunately, I can not show you a picture, but you see it in video 5.

PS. The picture also shows gears to the sides of the feet. Those also did not make it into the final design. They made it possible to move the attach-point of the mechanism down, so that the body of the machine would be lowered. It lowers the gravity point of the machine, which makes it more stable. Also, it looks better. However, the gears added extra friction and backlash. The backlash in turn made the synchronization of the movement of the feet with the swinging of the legs more difficult.

Irregular shapes that are not supposed to touch

In the final design, you may notice that the connection between the crank of the middle set of legs and the gliding axles seems overly complex compared to how it is done in Video 1 above. The problem with that simpler design is that the connectors brush against the beams of the machine’s lengthwise frame. They are not supposed to touch, but do so anyways. This causes ticks and sometimes holdups. Since there is a lot of torque in the crank’s axle, these holdups were really not good, so I replaced the connectors with the smooth curves of liftarms.

All in all, the first version of the Insect walker often got crippled by ‘little’ ticks, but as far as I can tell, the final design runs like clockwork, provide you build it with attention.

Gears skipping

Last but not least: one of the most ‘horrible’ little noises that one can hear in Technic Lego is the skipping of gears. Because of resistance in the mechanism and ‘bad’ construction, two gears may bend away from each other and loose their grip on each other: the teeth skip over. This may cause damage to the gears, to such an extent that they can not be used any longer. This happened in the original design as well. As explained above, a foot would stop sliding, which caused the set of double-bevel gears that you find at the pivot point of the legs to skip. Gears got damaged, but also the synchronization between foot and leg movements would change which made the feet move down and up at the wrong moments.

The solution to such skipping is to make sure that the axles that go through the gears are supported (by beams or lift arms) on both sides of the gear and preferably directly next to the gears (‘adjacent’ in proper English) . This way, they can not bend away or slide away.

The problem with the gears at hand is that they are at a 90 degree angle and their axles make a T-shape (upside down T shape, to be precise).

Image 3: Unsupported T-crossing of axles.

Image 4. Supported T-crossing axles

As you can see in image 3, there is very little space to build support on both sides of the gear on vertical axle. To my knowledge, there is no ready-made part that can be used here – and if it does exist, it is not included in the two sets used to construct this model. So I invented something that seems to work. I should stress ‘seems’ because I also removed a main cause of resistance (the ), so perhaps, there is far less stress on the this part of the mechanism than before.


Finally, it does not look that cool

Although, I am happy with the mechanism, I am not particularly happy with the machine’s appearance. Or better, for a mechanism, it looks fine, but for a Lego model, there is a lot of unfinished business. It should have some plating and details. I am thinking, a light and dark bluish grey, space look. It would be a kind of carrier machine on mini-figure scale with the cargo hanging underneath the machine. I have the pieces, but I would need to disassemble the Falcon. That will have to wait until after summer, so I’ll leave it at this.