As well as replacement gear sets I also do a lot of replacement driveshafts and this week I have a 3D print project to share with you that repairs an old Athearn HO DD35A.

These big locomotives have two motors, just like the real thing. Each motor has a drive shaft that connects to the top of the truck and, through a set of gears, drives the wheels. But as the truck swivels the drive shaft has to adjust in direction and length. This is done by the use of universal joints and a sliding driveshaft. The ends of the universal joints are press-fitted onto the metal shafts of the motor and truck. And it’s at this point where they fail; the plastic cracks with age and no longer grips the metal shaft, preventing it from turning. Below is an original complete drive shaft with two universal joints and a sliding centre shaft.

You can just make out the cracks in the plastic in the image below on the right. Oddly the metal shaft on the motor (the universal on the right) is larger than the truck as you can see by the different hole sizes.

The sliding shaft has a keyway along its length; you can see it below on the top of the shaft.

The end of the universal this connects with has a slot for the keyway to fit into. This means the drive shaft will always turn the universal but can slide in and out.

The universal itself consists of a ball and socket configuration and press-fits together.

Using the original parts I was able to 3D model both ends of the universal, also taking into account the different metal shaft sizes. I needed to design the joint so it clips together and won’t easily pull apart, but is not such a tight fit that it cracks on assembly or prevents it from moving freely.

The 3D printed parts were printed in Shapeways Smooth Fine Detail material because it’s very accurate and hard, which is ideal for this use.

The two halves of the universal pushed together with a satisfying click and held in place, but allowed full movement. The ones shown here were the first test print and the tabs that hold the ball were a little weak. One did break so I thickened this up for the second test print.

The original driveshaft also fitted smoothly with its keyway.

The last thing to do was fit the new universals to the locomotive. As with all my gears and parts that press-fit, the holes in the ends needed to be cleaned of 3D print residue before fitting. If this is not done the chance of cracking the new parts is high as they’re a tight fit by design. These were reamed out with a drill bit slightly smaller than the metal shaft. If it’s the same size the part will no longer be a press fit. You might be able to see in this photo that the tabs holding the ball are larger as these are the second test print.

The loco now runs as good as new and is ready to go back to the layout.

A set containing enough parts to replace all four universal kits can be found here.

If you have a part like this on a locomotive that you can’t find a replacement for, please get in touch and we’ll discuss making a new one for you.