A Technic Train

Sivan Toledo, April 2007

A Technic train with a NXT motor The new Lego remote-controlled (RC) trains use train wheels with Technic axle holes (only in the wheels connected to the motor, unfortunately). This creates an opportunity to power a train using essentially any Technic or Mindstorms motor, not just the highlyl-specialized train motors.

This is my first attempt to build a Technic-powered train. The main construction is the powered bogie truck. It is about the same size as the RC train motor, and in particular, the distance between the axles is exactly the same. It is seven Lego units wide, one unit wider than most Lego trains, but a width that makes sense for studless construction, since most of the studless beams have odd lengths. The height of the bogie is one plate higher than the height of the RC train motor. The construction is high enough off the track to allow it to move freely, even across switch points.

This bogie truck has an axle protroduing up. This axle both powers the wheels, and pulls the engine (the locomotive). The top of this truck is pretty flat, allowing it to rotate freely under the engine.

The powered bogie truck The axle of the powered bogie powers all four wheels, just like the Lego train motors. This is important, since the wheels do not have good traction, so if you power only some of them, the train will be less powerful. It was an interesting challenge to build the driving mechanism of this bogie. In the end, I ended up using a subassembly that is not connected at all to the rest, but which cannot move at all. This subassembly is offset by half a unit relative to the rest of the bogie. I did not find a way to securely connect the two pieces together in such a tight space. Initially I used elastic bands to hold them together, but that did not work so well (the rubber bands got in the way, were difficult to put in place, and still allowed the subassembly some freedom of movement. The locking solution works much better. The pictures below constitute complete building instructions.

I used a standard Lego train bogie on the other side of the engine platform. If you have enough train wheels with axleholes, you can build instead a simple un-powered Technic bogie. I did't have enough wheels with axleholes, so I used a standard bogie.

The engine platform is pretty simple, except for the use of a half-beam to connect the standard bogie. The bogie plate has a pin that looks like half a technic pin, but is a bit too short. Therefore, it cannot lock into a standard Technic beam. You must use either a special train base plate (but it wouldn't accomodate the 7-wide powered bogie), a Technic plate, or a Technic half beam. I used a half beam. It is connected rigidly enough to the rest of the platform.

A Technic train with an RC-Buggy motor The construction works well with both the RC-Buggy Technic motor and with the NXT servo motor. The NXT motor rotates quite slowly, so when it is connected directly to the driving axle, the train runs fairly slowly. The RC-Buggy motor rotates very quickly, producing a lot of slippage when the train starts and high speed afterwards. I suppose that pretty much any Technic motor would work here.

To power the motor, you can mount a battery box in the same engine (it would be pretty large), or in a truck towed behind the engine (as in the 4.5V trains). You can also use a NXT or an RCX to power and control the engine.

The main technical defect in this construction is that the same axle powers the wheels and pulls the train. In a non-Lego construction, it would make more sense to enclose the driving axle in some sort of a sleeve. The sleeve would pull the train, and the axle would rotate freely inside the sleeve. One way to do this with Lego is to use a Technic turntable, but it is both large and has a lot of friction. I opted for the simple solution in which the driving axle is also pulling the train.

Building Instructions

building step Step 1. This will eventually form the body of the bogie.

building step Step 2.

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building step Step 4.

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building step Step 6.

building step Step 7. We move to another sub-assembly.

building step Step 8.

building step Step 9.

building step Step 10.

building step Step 11.

building step Step 12. The 1x2 liftarms should point up (towards you). This sub-assembly is now complete.

building step Step 13. This will connect to the first sub-assembly.

building step Step 14.

building step Step 15.

building step Step 16. Connect the two blue liftarms; the sub-assembly with the two 9-units beams should rotate freely.

building step Step 17. Simply insert the sub-assembly (the one with the four red 1x2 liftarms into place. It does not connect to anything, but it will get locked in place. The rotate the two 9-units long beams to the right (clockwise) and lock them in place with the two blue liftarms.