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I once tried to make a 4 wheeled Lego robot using a pivot arm suspension system. The robot was motorized using 2 Mindstorm (2.0) motors (one one each side). I can't find and original picture but it looked like this big black one:

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I used a Lego Technic turntable as the single point of rotation between the two robot sides bu the robot sagged a lot and there was so much pressure on that piece that it would not rotate well.

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Q: How would you build a 4 wheels rover pivot arm suspension system? Considering that it needs to be motorized thus adding the weight of two motors.

Note: The pivot arm design can be one of those two: enter image description here

Edit 1: To give an idea of the size, I'm using this Technic wheels. The final model is about 20cm long and 20cm wide.

enter image description here

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Some versions of the Technic turntable have more friction than others. If yours doesn't rotate freely, chances are that it needs some surgery. Unfortunately I'm not sure what exactly you need to trim, but I know you'll need to open if first. –  Joubarc Oct 29 '11 at 16:46
    
@Joubarc I tried two different Lego turntables with the same results. To be sure, I'll look into the part like you're suggesting. Still, I think that there was way to much torsion (perpendicular to the turntable plane) applied to it and this is why I'm asking around for other design ideas (before I do this with non-Lego parts). –  pcantin Oct 29 '11 at 17:08
    
Yes, another design might be better anyway, I was mentionning this just in case. –  Joubarc Oct 29 '11 at 17:11
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If you're into robotics, why not join the Robotics Proposal? –  Rocketmagnet Oct 6 '12 at 20:19
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3 Answers

up vote 5 down vote accepted

I got inspired by the idea of a vertical Slewing Bearing. I made this “horrible” prototype to demonstrate the principles.

enter image description here

One side of the assembly has a fixed wheel attached to it using multiple pins. The other side has multiple bearings. The idea is that the vertical weight is shared between the bearings (on one side) and between the pins supporting the wheel (on the other side). This way, the axle supports only a part of the weight. In the final design the axle should be much longer and use many bushes preventing the two sides from separating.

enter image description here

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In the end I will build a system like this one. When get to it I'll post a link to pictures. –  pcantin Nov 8 '11 at 23:45
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This may sound like a naive approach, but isn't the turntable overkill in this case?

Did you try a simple Technic pin peg to connect both parts if the robot, or do you actually need some transmission to go from one half to the other?

If a Technic peg isn't robust enough, maybe an axle would do the trick. I'd suggest using one Technic Axle 8 with Stop (55013 — Technic Axle 8 with Stop), on which you first slide 3 regular Technic, Brick 1 x 2 with Hole (3700 — Technic, Brick 1 x 2 with Hole) which will be attached to one half of your model, then 5 Technic, Brick 1 x 2 with Axle hole (32064 — Technic, Brick 1 x 2 with Axle hole) which will be attached to the other. Maybe the strain on the axle will be too much (I read it's prone to breaking), but it could be worth trying.

And if that's not solid enough, build a staple of 2x2 round plates around an axle, fix that to one half of the model, and build a hole on the other half to have it slide in (a square hole is fine and easy to build: just a two bricks wide, two bricks high gap and you cover the studs underneath with a tile. Use archs if you want for the higher half, but don't bother for the bottom one). Of course you have to finish the pseudo-axle with something bigger than 2x2 to lock it in place.

And if THAT's not enough, there are 4x4 round plates now with which you could do the same.

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+1 you solution using round plates on a axle sounds really nice (the 1 axle- or peg-solution is so easy i can't belive pcantin hasn't thought of it himself) –  oezi Oct 29 '11 at 18:12
    
This was my first option but it would have snapped under the weight of the two sides. I've added a picture of the wheel size that I'm using. Imagine that plus a motors and gears on each side plus the structure. The single axle design would not survive (unless it supplemented with some other support). –  pcantin Oct 29 '11 at 20:47
    
I was thinking along the lines of a Slewing Bearing. I've seen designs like that used for massive Lego tower cranes. This gives me an idea [off to the Lego boxes]... –  pcantin Oct 29 '11 at 21:09
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Spread the weight using gears.

Essentially you'll have 4 segments connected to a center gearbox. Use 9 bevel gears in the gearbox, with 6 on the outside.

enter image description here

(Yellow=bricks, Black=rods, Grey=gears)

For strength you can have reinforcements going from the gearbox to the other side of the legs to box it in.

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I'm not sure I understand your design. Would this behave like a rocker-bogie suspension? (like the MER rovers). I don't see which gears are really connecting. Can you clarify the sketch? –  pcantin Oct 29 '11 at 22:26
    
@pcantin I don't know how to explain it other than the axles on one side spin the opposite direction of the other. –  Pubby Oct 29 '11 at 23:57
    
would you mind trying to design it on LDD and posting pictures? –  J. Walker Feb 1 '13 at 0:02
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