As the temperatures start to drop and we approach the holidays, we are starting to feel somewhat nostalgic at Imagination-3D. So, what better way to ring in those holidays than to build a train to go around the Christmas Tree. A project like building a train set, or even an engine (as is the subject of our article) is too large to fit into a single article. We are going to cover creating track, engines, controls, cars and everything in between in this series.

“Let’s Build a Train! Part 1: GP38 Gearbox” will be our starting point, as you need an engine to pull a train.

The engine of our first build is the totally awesome GP38 diesel engine designed by Depronized of . He truly produces some incredible designs.

But we needed to create a gearbox that would work with our specific motor. We also found some little things along the way (with the original design) which caused us problems. Solving those problems with our own version of a gearbox did the trick.


So, lets get started!

Here is a list of things you will need:

  1. Depronized’s GP38. You can download it on .
  2. Some PLA filament. We used Solutech Silver Metal  and Real Blue.

3. Some 2mm brass rods. We used these from Amazon.

4. A 28mm high speed motor. We used this one from Amazon .

5. M2 sized screws (4mm and 6mm in length). We chose this from Amazon because it gives us a good assortment.

6. Two M2.5 screws (no longer than 6mm). These will be used to hold the motor to the gearbox. If they are too long, they will hit the internal components of your motor. We had some of these on hand. But did find these online:

Or these :

7. Our gearbox for the GP38. You can download it here.

Getting Started

The first thing you should do is print out all the components of our gearbox. We recommend using 100% infill as most of these parts will be taking some abuse and stress. So, you want to make them as tough as possible.

Mounting the Motor

Once you have all the components printed, the first thing to do is mount your motor. To mount the motor, you will need 2 M2.5 x 6mm or shorter screws. This is what the front of the motor requires.

You just push the motor thru the motor mount and secure it with these screws like this.

If you have not used the same motor we did from Amazon and you encounter any problems at this point with lining things up, we suggest you stop and remix this portion of the motor mount. The reason is, if your motor does not fit correctly, it will affect everything “downstream”. Your gears will not mesh properly or worse, it just will not run at all.

Once your motor is mounted, you can move on to installing the first gear, the pinion gear.

Mounting the Pinion Gear on the Motor

You will notice that we have included 4 different pinion gears. Each have the same number of teeth and is the same outside diameter. What is different, is the hole for the motor shaft.

What we found when trying to utilize the original pinion gear is that it was just too snug and impossible to install on our motor of choice. So, we decided to include 4 different ones for you so you could decide for yourself which one to use. Once you have made your choice, you want to push the pinion gear on until it is almost flush (but not quite) with the motor mount as shown here.

Once you have installed the pinion gear, you will need to cut some shafts for gears 2 and 3. After the shafts are installed, it’s time to move on to the gears.

Shafts for Gears 2 and 3

Our gearbox uses 2mm diameter brass rods for shafts. You will need to cut two shafts 16mm long out of the brass rods specified in the things you will need list.

Once they have been cut you can install them in the motor mount holes as shown below.

Note: You want to make sure that any sharp or jagged edges on each of these shafts has been filed or sanded smooth. Jagged edges will cause friction and may cause your motor to seize up after a period.

Installing the Gears

Installing each of the gears is straight forward. First you want to install gear #3 then gear #2 as shown below:

We designed our gears to “float” on the shafts. We also made them smaller than the original gearbox called for in the GP38. This eliminated many issues for both creating a properly running gearbox and when it came time to mesh the gearbox with the gears in the truck. (a subject of a different article).

Look closely at this photo of how the gears are aligned and how they mesh with each other. Yours should look similar.

If your gears are meshing this way, you are going to have trouble. Notice in the photo below, gear 2 tilted away from the pinion gear. This can be caused by using a motor with different mounting requirements. If you see this STOP and fix the problem by remixing the motor mount or reducing the size of the gear.

Once the gears are installed, you can install the gear protector.

Installing the Gear Protector

The gear protect is held in place by two m2x6 screws at the top of the gear box (as shown below) and two m2x4 screws at the bottom. The gear protector should just slide easily over the gears.

At this point, you should be able to rotate gear 3 (the bottom gear) and make the entire assembly rotate. In the photos shown above, we had a problem. If you look closely, you will see gear 2 is not meshed properly causing incorrect clearance to one side of the gear protector. The assembly as shown here would not rotate.

After you have installed the gear protector, it is time to install the gear bushings.

Gear Bushings

In our design, since gears 2 and 3 “float” we came up with some simple spacer bushings to keep the gears properly engaged.

The 2mm high bushing goes on the end of Gear # 2 as shown below.

Then you need to install the 1mm tall bushing on gear #3 at this location.

Once the bushing2 have been installed you can install the gearbox end plate.

Installing the Gearbox End Plate

To install the gearbox end plate correctly, you need to make sure that both 2mm shafts are properly engaged in each axle shaft hole. The photo above shows the hole for gear # 2. If you look at the other side of the gearbox end plate you can see both holes clearly.

Once both the shaft are fully engaged in the gearbox end plate, it will be flush with the gear protector and you can secure it. The end plate is held in place using four m2x4 screws.

Testing it out

With your assembly complete, you should still be able to rotate gear #3 and get the entire gear box to work by hand. If it does, now is the time to test your gearbox by applying some power to it. We will be uploading a video of our gearbox soon, so you can see how it runs.

This completes the gearbox. Stay tuned for the next installment: building the truck assemblies! If you have not already done so, we recommend that you check out our article on “Bridging the Gap” before printing out Depronized’s body components of the GP38.

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