Repairing Bachmann split gears/axles

[peteski]

Repairing cracked gear/axle tubes in vintage Bachmann locos

Those cracked axles are very common to many older Bachmann locos.  2-axle (like in GP50, U36B), and 3-axle (like in SD40-2 or E60) trucks.  Even the steam locos from that period (like 2-8-2 and 2-8-0) which utilize white-colored Nylon axles/gears are susceptible to cracking.  Even when they later started molding axles from softer black material (POM or Delrin maybe?) those were also susceptible to cracking.

Easiest way to fix the problem would be to purchase replacement wheelsets or even complete trucks from Bachmann, but those are long out of production. I had an idea for how to fix these for a long time, but never followed up on it  But with the recent interest in the vintage N scale models by some of the Railwire members, I decided to take a stab at this.


BachmannCrackedAxles01.jpg
Here is an example of cracked axle/gear in both, white and black material. These wheelsets are from a U36B loco.

Sometimes just the axle tube cracks. That can cause the metal axle to slip in the plastic tube, and that results in poor pulling ability.  But even more often the crack also extends to the gear. Cracked gear is distorted, causing rough running and clicking noise, in addition to poor pulling ability. Sometimes the metal axle is so loose in the cracked tube that the wheel will actually fall out of the truck.

There could be several reasons why these crack, but one I can easily see contributing to the problem is sloppy manufacturing tolerances!  I measured the metal half-axles which get inserted into those axle tubes and I found them to vary in diameter from 0.061" all the way to 0.065"!  That is quite a range.  The thicker the metal axle is, the more stress it will produce inside the plastic axle, usually cracking it after some time.

My idea for the fix involves installing a thin metal (brass) collar over the cracked tube. That will close the gap in the tube, and also in the cracked gear and keep it closed even after the metal axle is installed.



This is what a repaired wheelset looks like:

BachmSplitGearRepaired01.jpg
The axle tubes are 3.00mm (0.118") in diameter. I was not able to find any thin-wall brass tubing which would have 3.00mm ID (inside Diameter).  I might have been able to find stainless (hypodermic) tubing, but I didn't want to deal with such a hard material, plus I already had some 0.125" (1/8") diameter thin-wall (0.006") brass tubing I bought years ago from Special Shapes Co.  SSC has been acquired by K&S Metals, and while K&S discontinued most of Special Shapes product, the thin-wall brass tubing is still available.

But the ID of that brass tube is 0.114", while the axle tube is 0.118".  To deal with that I ended up reducing the diameter of the plastic axle tube by 0.004" to create a tight fit for the brass collar. Having a lathe in my workshop makes this job easy, but I suppose that chucking the axle tube in a Dremel, and using a file to reduce the diameter should work too.  Chucking the axle tube in a chuck for turning/filing also closes up the gap.




BachSplitAxleCollar02.jpg
First I made the collars.  They have to be 0.050" long (slightly shorter than how much the axle tube protrudes from the gear). I sliced those off from the brass tube using my lathe and a cutoff tool.




BachSplitAxleCollar03.jpg
Before cutting each collar off the tube, using a hobby knife (while the lathe was running), I deburred the inside edge of the tubing, and chamfering the opening to make installing it on the axle tube easier.  In this photo, the left collar has that chamfered end up.




BachSplitAxleCollar04.jpg
I made enough collars for the job, plus few extras in case I lose few.  These are shown before the other end was deburred.  To deburr such short pieces I chucked them in a pin vise (with 1/8" chuck) and then used a hobby knife to clean up the edge. Deburring bit would also work.




BachmSplitGearFix02.jpg
I then chucked the split axles in my lathe, and reduced their diameter by 0.004" I made sure to install them in the 3-jaw chuck in such a away that the pressure from the chuck would close the gap.




BachmSplitGearFix03.jpg
Next, I pushed the collar onto the axle tube.  It should be a tight fit. Or using a pin vise,  press down on the gear.Repeat for the second collar.




BachmSplitGearFix04.jpg
Now chamfer the edge of the hole in the axle tube to ease the re-installation of the metal axles.  I simply chucked the assembly in a pin vise and used a hobby knife to trim away the plastic.




BachmSplitGearFix05.jpg
Next, reinstall the metal axles/wheels back into the axle tubes.  It should now be a tight fit.




BachmSplitGearFix06.jpg
If  (due to the loose tolerance when the metal axles were machined and the metal axle diameter being too thick) the metal axle will now not fit into the collared axle tube, take a 1/16" (0.0625") drill bit and drill out the hole in the axle tube. Since each wheel/axle seems to be slightly different, I like to reassemble the wheelsets using all original parts. I make sure to keep track which axle/gear mates with which wheel/axle pair.




BachmSplitGearRepaired02.jpg


BachmSplitGearFix07.jpg
That's it - once everything fits, disassemble the wheelsets (again keeping tracks of which wheels go with which axles) and reinstall them in the truck frame.

 No more falling out wheels or clicking noise while running.

I also came up with an alternate collar construction for the cracked axle tubes with offset gear (like the ones used in Bachmann steam locos).  I used this method to fix Lee's 2-8-2 loco.  This method can also be used to fix the above mentioned axle tubes without need for turning their diameter down.




Bachm2-8-2GearRepair01.jpg
I still used the same 0.125" thin-wall brass tube, but instead of reducing the diameter of the axle tube, I "stretched" the tubing to 0.118" (3.00mm) diameter.
To enlarge its diameter I sliced through the tube, then spread it, and soldered a patch to fill the gap.

1.  The gear to be repaired.  Actually it is not the original Bachmann gear, but a 3D printed replacement gear. Looks to be printed in FUD material. But (probably due to to the loose Bachmann tolerances), the metal axle was overly thick, and split this replacement axle tube.

2.  Collar cut from 0.125" tubing, ready to be sliced. Notice that I sanded the outside surface (before cutting it off tubing stock) to prepare it for soldering.

3.  Collar with a slice through it,

4.  Collar stretched to its final size.  I stretched it by carefully sliding it up a conical mandrel (made from a dental tool).  Using the mandrel assured that the collar would remain undistorted circular shape, even when stretched.  If I just spread it at the slice, it wouldn't have retained circular shape.

5.  Strip of 0.002" tempered copper sheet.  It is actually copper cladding material I peeled from a PC board. I cut a strip of it (on the right), then scraped it with a hobby knife to remove the adhesive and make it ready for soldering.  I wrapped it around a drill bit shank (smaller than 0.125") to form into a curvature which will  match the curvature of the brass collar (the strip will spring out slightly after being formed).

6. Slice of the copper strip which will be soldered to the collar.

7. Hole drilled into piece of hardwood which will hold the collar and copper strip for soldering.




Bachm2-8-2GearRepair02.jpg
Couple of collars after soldering.  I used TIX Flux (zinc chloride) acidic flux because it works really well on these non-electrical parts.  The collars were then washed to move remains of the flux and the inside-facing solder joint was smoothed out with a jeweler's rat-tail file.




Bachm2-8-2GearRepair03.jpg
On the left is a collar installed on a cracked axle tube.  On the right is one that is not cracked, but will receive a collar to prevent it from  cracking in the future.  For these collars I did apply some CA glue in between the collar and axle tube - just in case . . .

These repairs should be permanent. No more split axles!

[narrowminded]

That's one of the ways to do it.  Nice job.

In the course of this did you find that it's more likely that a too tight fit to the axle rather than poor gear material is the main culprit in those failures?  And how about that FUD for gears?  Can't imagine it, myself.

[u18b]

Wow Pete.
Amazing and creative.   Thank you.

I wonder about the status of a gear where the crack extends into the gear itself and thus the clicking sound can be heard because of the gears cogging a bit.   Would your method pull the gear together tight enough such that the split in the gear itself disappears and thus becomes functional again?

[Mark5]

Clever. 

[SP-Wolf]

Outstanding!!

Wolf

[Rob B]

WOW! That should do the trick. Now all I need to do is buy a lath to fix a $25.00 locomotive.

[cjm413]

Looks like Bachmann's F7 wheelsets also work on the F9, and if so, should also work on the U36B, etc that used the same powered trucks as the F9.

However, my understanding was that the white gears (as shown in the picture) should be treated as a red flag...

https://estore.bachmanntrains.com/index.php?main_page=product_info&products_id=265

[cjm413]

To any extent that: 1) F7 gears work in the F9, U36B, etc; and 2)the white gears in the picture are different from the ones that are prone to cracking:

https://estore.bachmanntrains.com/index.php?main_page=product_info&products_id=10143

[peteski]

Wow Pete.
Amazing and creative.   Thank you.

Thanks!

I wonder about the status of a gear where the crack extends into the gear itself and thus the clicking sound can be heard because of the gears cogging a bit.   Would your method pull the gear together tight enough such that the split in the gear itself disappears and thus becomes functional again?

Absolutely. The gap is closed on the entire assembly. I wouldn't have bothered otherwise.   

I did mention that in the write-up. "No more falling out wheels or clicking noise while running.". I now changed it to a larger font, so it stands out

[peteski]

Looks like Bachmann's F7 wheelsets also work on the F9, and if so, should also work on the U36B, etc that used the same powered trucks as the F9.

However, my understanding was that the white gears (as shown in the picture) should be treated as a red flag...

https://estore.bachmanntrains.com/index.php?main_page=product_info&products_id=265

Bachmann used the gears I covered in this fix in thousands of various models made in the late 70s and 80s.   The wheelset you pointed out is different, newer design. Hopefully by the time this one was made, they have improved their manufacturing quality and these will not be prone to splitting.  But if they do split, you might be able to use this method to fix it (although with the smaller gear being offset, it might not work as well if the gear is split too.

Bachmann's way to address the problem was to redesign the wheelsets by making the metal axles which plug into the axle tube smaller diameter, thus increasing the wall thickens of the plastic axle tubes (making them stronger). I'm not familiar with those and haven't heard much about them splitting.  This fix is for the original, vintage wheelsets.  Like I mentioned in the write-up, I think the main reason for the splitting was loose tolerances when machining the metal axles.  Some were so large that they created too much stress on the plastic tubes, splitting them.  But other things (like quality of the Nylon used) could be in play too.

[peteski]

WOW! That should do the trick. Now all I need to do is buy a lath to fix a $25.00 locomotive.

Every well-equipped modeler should own a lathe!  Just kidding. 

I did mention some ways that this fix can be done without a lathe, but it gets a bit more difficult.  Plus this is really geared (pun intended) to people (like Lee) who want to bring back they youth, and run vintage models at any cost. 

[peteski]

That's one of the ways to do it.  Nice job.

In the course of this did you find that it's more likely that a too tight fit to the axle rather than poor gear material is the main culprit in those failures?  And how about that FUD for gears?  Can't imagine it, myself.

Well, I can measure the poorly manufactured axles, but I have no way to evaluate quality of the plastic. I think both contribute to the splitting. But if the axle is made 0.003" too large for the specs, I suspect that splitting is unavoidable.

[mmagliaro]

Pete,
This is a great fix for these cracked tubes.  I remember Victor Miranda doing the same thing to fix Kato GS-4 cracked axle tubes (the new generation ones that had that problem).

This is just a hunch, but I bet you could do this without a lathe (to those who lamented that they don't have a lathe).  I would bet that one could chuck the plastic gear in a Dremel, and just buzz down the plastic a few thousandths with a diamond file.  The plastic collar next to the gear doesn't really have to be perfectly round within thousandths of an inch.  Once you press the brass collar over it, it's going to close that crack.

[peteski]

Pete,
This is a great fix for these cracked tubes.  I remember Victor Miranda doing the same thing to fix Kato GS-4 cracked axle tubes (the new generation ones that had that problem).

This is just a hunch, but I bet you could do this without a lathe (to those who lamented that they don't have a lathe).  I would bet that one could chuck the plastic gear in a Dremel, and just buzz down the plastic a few thousandths with a diamond file.  The plastic collar next to the gear doesn't really have to be perfectly round within thousandths of an inch.  Once you press the brass collar over it, it's going to close that crack.

Good idea Max.  My original post states: Having a lathe in my workshop makes this job easy, but I suppose that chucking the axle tube in a Dremel, and using a file to reduce the diameter should work too.
I guess one has to read through the whole long post to know.  I'm also guilty of skimming through other's posts.

[cjm413]

Bachmann used the gears I covered in this fix in thousands of various models made in the late 70s and 80s.   The wheelset you pointed out is different, newer design. Hopefully by the time this one was made, they have improved their manufacturing quality and these will not be prone to splitting.  But if they do split, you might be able to use this method to fix it (although with the smaller gear being offset, it might not work as well if the gear is split too.

Bachmann's way to address the problem was to redesign the wheelsets by making the metal axles which plug into the axle tube smaller diameter, thus increasing the wall thickens of the plastic axle tubes (making them stronger). I'm not familiar with those and haven't heard much about them splitting.  This fix is for the original, vintage wheelsets.  Like I mentioned in the write-up, I think the main reason for the splitting was loose tolerances when machining the metal axles.  Some were so large that they created too much stress on the plastic tubes, splitting them.  But other things (like quality of the Nylon used) could be in play too.

I do have an older run U36B I bought for my kids that is completely dead, but I haven't taken it apart to account for the different trucks where I'll keep that in mind.   However, if the motor is shot, I'll probably cheat and look for a newer one that runs for dirt cheap and swap shells....