Back in 2018 I shared with you my design for replacement axles and gears for a HO Bachmann 4-8-4 Northern. You can find the post here. That turned out to be quite the adventure, as simply creating replacement parts didn’t work out so well! Due to the thinness of the original components I had to engineer a much thicker gear which could handle the force applied from the motor. This turned out to be a success. But this gear and axle set only worked for the first generation of the Bachmann HO 4-8-4. As it turns out, although Bachmann redesigned and improved the second generation, it too has problems with split axles and gears, and I’ve regularly been asked if I could produce a set for this locomotive. Well, I finally have and in this post I’ll share the design with you.
To start, I want to explain the difference between the three generations that are commonly available. I’m not going to go into any of the body or cosmetic differences, just the drive train so you can tell which generation you have.
The first generation, as I’ve already covered in other posts, has a pancake motor which drives the rear axle. Below is an exploded schematic drawing, courtesy of John on the Tyco Depot Train Collectors & Model Railroading Forum. You can just about see the gear on the rear axle.
Looking at the underside of the locomotive there’s a bump over the rear axle to make way for the gear.
This is the main clue that it’s a first generation. This locomotive also has round wheel stubs and holes in the axles. That’s what makes quartering the valve gear tricky on this locomotive. ‘Quartering’ means setting the piston rods and side rods on each side of the locomotive at 90° to each other; as the wheel stubs are round it’s easy to get it in the wrong place meaning the loco won’t run smoothly.
The second generation had a lot of improvements such as a can motor and thicker gears and axles. But most importantly the axles and wheel stubs also have square stubs and holes, making quartering far easier to do. You can just about make out the cracks in the axle and gear.
Below is an exploded schematic drawing, also courtesy of John.
The motor this time drives the third axle and there’s no bump in the base plate; this along with the lack of wires running to the tender tells you it’s a second generation.
The third generation, which I believe is the version currently available, had a total redesign. It still has a can motor which drives the third axle, but everything else has changed. Looking at the exploded schematic drawing below, again courtesy of John, you can see the axles are a different shape.
To my knowledge, these don’t fail. These are normally marked as DCC ready and will have wires connecting the tender to the locomotive; the DCC socket and speakers for sound are located in the tender.
Looking inside the second generation you can see the axles and drive gear on the third axle. The second axle is clearly split and once I removed the others I discovered they had split too. As the wheel stubs are square they don’t need to be such a tight fit as the round ones on the first generation.
Interestingly the axles are almost identical to the axles used on the UK Bachmann (former Mainline) designed locomotives that I have 3D printed replacements, for as shown below. These were for a GWR Hall class and can be found here.
However, upon measuring the US 4-8-4 axles and gears I found them slightly different, so a new design was needed. These have been designed to be 3D printed in Shapeways Smooth Fine Detail Plastic.
The kit I’ll supply will contain three axles and a main drive gear. The idler gear between the worm and axle gear to my knowledge never breaks so I haven’t supplied that; if yours has, please let me know and I will add it to the kit.
These have now been sent off for a test print and I’ll see how well they work. As with the first generation, and the UK Bachmann locos, I’m restricted by the size of the axle box and the size of the wheel stub. This means the axle itself nearest the wheel is still not as thick as I would like it to be, and it’s possible to crack the new axles if not cleaned properly before use. Once I’ve test fitted a few, if it appears too easy to crack the new ones I will include a few spares in each kit.
So now I’m waiting for the 3D printed parts to arrive, and I’ll share them with you when they do.
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