A few months ago I shared with you the replacement drive shaft I 3D printed for my N Scale MDC/Roundhouse 2-8-0 steam locomotive, you can find the post here.  Since then I have picked up an N Scale Atlas Pennsylvania Railroad 4-4-0 with a broken drive shaft. The intention was also to 3D print a repair.  In this post I will show you what I did.

The Atlas 4-4-0, as pictured below, was introduced in 2013 and is a fantastic model.  Given the size and limited weight it has a lot of tractive effort.  This, as with Roundhouse’s 2-8-0, is because the motor is located in the tender and connected to the all metal locomotive body by a plastic drive shaft.

Each end of the plastic drive shaft has a ball which fits into a socket, giving the necessary movement to allow the tender to navigate bends as the drive shaft is turning.  Each ball has two pegs protruding at 90° to the drive shaft, the sockets have slots to receive the pegs.  Thus, as the shaft rotates the pegs rotate the sockets.

With this particular locomotive its the plastic socket part that has broken.  This part has the ball socket and slots at one end and a hole through the other.  The part is press fitted onto either the metal drive shaft coming out of the tender or the metal drive shaft entering the back of the locomotive.  As the hole in the part is ever so slighty smaller than the metal drive shaft it is held in place by the natural clamping and friction caused by the undersized hole.  For some reason the plastic socket has split in two, as shown below.  The metal drive shaft entered from the left of the socket.

To repair this locomotive all I really needed was one new plastic socket part however it occurred to me that this may not be the only example of this happening so I drew up a replacement pair of plastic drive shafts and sockets as shown in the rendering below.  This means all bases are covered and there is a spare drive shaft incase a peg breaks off the first.  Given that the Shapeways FUD material is more brittle than regular injection molded plastic it is possible that by removing and inserting the drive shaft many times into a socket, a peg could break off.

The parts arrived from Shapeways and after cleaning looked like this.

Comparing the new parts to the orignal, as pictured below, you can see the new drive shaft is just a fraction longer.  This didn’t have an effect on the locomotive but it has been corrected in the 3D model.

When I modeled the plastic socket part I intentionally made the hole for the metal drive shaft too small.  Once the part had been cleaned and all the excess powdery material had been removed I enlarged the hole using a 1.6mm Ø drill in a pin vice.

This hole was still very slightly smaller than the metal drive shaft so I was very careful not to over do it when I pushed it onto the shaft.  I used a pair of needle nose pliers to apply the pressure whilst securing the locomotive between my fingers.

I resembled the locomotive using a new 3D printed drive shaft and it all fitted perfectly.  I then removed the new drive shaft and installed the original as this was already black in colour.  This also fitted perfectly into the new 3D printed socket.  I didn’t paint the 3D printed parts, however as long as you don’t apply the paint too thickly inside of the socket or on the ball it should not be a problem if you do.  If you do paint the parts I recommend lubricating the ball joints with a non corrosive grease such as Labelle Lubricants No. 106.  This will prevent ware and reduce any resistance caused by the paint, resistance will drain power from the motor.

To see how well the repaired locomotive worked I took it down to my local club, the Poole & District Model Railway Society, to run it on their test layout.  Don’t forget, on the videos you can turn up the quality by clicking in the gear wheel at the bottom right of the window once the video has started.

Here we see the little 4-4-0 pulling a local passenger train.  If you look closely at the first video you can see the white of the 3D printed socket just under the cab floor.

And here it is again with a lumber train, these log cars are also 3D printed, they where special cars built only for Yosemite Valley Railroad.  You can read more about the log cars here.

The slow motion of the locomotive was also unaffected, I havent added any lubricant yet but if I do it can only be an improvement.

My biggest concern was that under a heavy load the traction tires would prove to be stronger than the friction between the replacement 3D printed parts and the metal drive shaft, causing the 3D printed socket to spin on the metal drive shaft.  But my mind was put at rest when the little 4-4-0 trundled off with a train of 13 box cars and a caboose!

The repair is a success and I you need to repair your Atlas 4-4-0 drive shaft can get the repair kit here.

I leave you this week with one more short video of the repaired Atlas 4-4-0.  This locomotive will shortly be converted to DCC, re-spayed and re-numbered to be Sierra Railroad No.7 , sorry Pennsylvania Railroad fans. But you may be pleased to here I will be brining out a much bigger Pennsylvania Railroad locomotive very soon.