One Motor, One Turntable — My LEGO Engineering Solution

 

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LEGO motors are expensive. And my brain is an engineer’s brain. These two things are incompatible.

When I was growing up, the only LEGO electronics I had were for train wheels and one motor that we bought for the Emerald Night. So, after taking inspiration from some LEGO GBCs (Great Ball Contraptions), I realized that you could do a lot with one motor, if you gear everything together right. This GBC module in particular was the main inspiration. So, I want to share one of my designs with you. Unfortunately, I don’t have pictures for all of the parts, but I’ll try to give you an example of each one. In the future, I’d like to explore a few more things you can make as well.

 

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The Turntable

This design was intended to turn LEGO trains around. Most LEGO turntables I’ve seen have two components - something to turn the tracks, and something to lock the turntable in place so the tracks are aligned when the trains travel over them.

This is the one we’re about to build:

And this is the core mechanism I designed for it. We’ll go over it in detail.

 

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How to Get the Turntable to Turn

So, the first question is how to get the turntable to turn. For this demonstration, we need to be familiar with 3 types of gears: spur gears, bevel gears, and worm gears.

Spur: Bevel: Worm:

First, we need a gear running at the base of the turntable and we need to have it transmit power along an axle from the power source. To connect the base to the axle, we’ll use a worm and a spur gear. This means the worm gear can rotate the track around the base, but power cannot be transmitted in the opposite direction. This worm gear will also be important later.

 

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Let’s put our power source in the operator’s box on the track portion for now. We’ll have the motor face down so it’s easiest to hide its shape. The motor will spin a spur gear that connects to a pair of bevel gears that will be able to transmit the power in a perpendicular direction from the motor to the axle. Where the minifigure with the blue hat is standing is where the motor would be, facing down.

Top-down view

Bottom-up view

 

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The Locking Mechanism

Now comes the trick - we want the motor spinning in one direction to rotate the turntable and it spinning in the other direction to lock the turntable into place. How do we do it? With the worm gear, of course!

We’re going to leave some space on either side of the worm gear available, but use spacers with friction to make sure the position of the worm gear on the axle doesn’t change. Because the worm gear will move in the direction of least resistance, and turning the turntable creates more resistance, turning the motor on when there is space for the worm gear to move side to side will make it move laterally rather than turning.

When the worm gear moves the axle in and out, it will slide into and out of a hole underneath the connecting tracks on stable ground to lock the turntable into position so it cannot rotate. When we’re ready to move it again, we’ll turn the motor in the other direction and the worm gear will first slide the axle out of the lock, and once it can no longer move right to left, it will begin turning the turntable.

 

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Taking It a Step Further: Train-Powered Turntable

So, what if we take it a step further? Make the train power the turntable.

By putting a little bump on the turntable, we can prop the train up so its wheels don’t make contact with the track and it can’t move forward once it reaches a certain point. We’ll also change the direction of our original bevel gears so that they create a gear train going up to the powered train wheels, so as soon as the train is propped up, it will engage with the gear train. And lastly, we’ll put a gear on the outside of the train wheel so it will connect with the gear train. Now, when the train approaches the turntable, it will be forcibly stopped, unlock the turntable, and start rotating the turntable. When the train is ready to leave, it will stop rotating the turntable, lock it into place, and then it will pull itself off of the turntable when there is too much resistance.

 

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Testing the Design

I was able to test this design a bit, but it was over 10 years ago so proceed with caution because some of the resistances, like the spacers and gears against the axles, may be weaker or stronger than the torque required to move the turntable. But the general principle has been tested and works.

 

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Final Thoughts

So, just to reiterate, here’s our finished product!

I hope you can see in this design that complex gearing can be quite versatile, and it turns out complex mechanisms can turn into a type of mechanical computing - programming a certain outcome without any software. A lot of these designs would later influence robots I made in college to be able to pick up and move objects on a certain path, and again when I was optimizing my 3D printer to be able to clean itself and move objects into packaging without adding any additional motors.

So I guess the moral of the story is to give it a shot!

Alex