Replacement Gears

What I do is either count the teeth and try to buy that gear only, or buy an assortment pack.

Example, searching for "assorted plastic gears" gave me 4 different packs, ranging from $6-$13 delivered that I would bet the bank contain that gear.

These are common mass produced motors and thus the gears that fit the shaft and common gear pitch are all commodity parts. Yes, on occasion there is a gear that is specialized, but buying a bulk pack is both cost effective and good to have in this hobby long term.

Maybe ask one of the 3D printers to fabricate one for you?

Point taken; "assorted pack" is a good call.  All things considered - sure to find something that'll work either on this project or at some point down the road.  Appreciate the suggestion.

@Mark V. Spadaro posted:

Maybe ask one of the 3D printers to fabricate one for you?

The problem with that is understanding the failure in the first place.

The problem is that as you get smaller and smaller in gear size for a given bore diameter there is less plastic or "meat" of the gear making the strength around the bore to grip the shaft. In other words, there is a recommended minimum tooth count for a gear and shaft size. Going less or under that again, the tensile strength of the material is being called into play, but also the fact that gear teeth and even more so- splines or knurling of the shaft before press fitting the gear on leads to stress riser cracks in the plastic. Again, the fundamental problem is that the designers are not using proven known rules around gearing and choosing ratios and teeth count of gears around what they want to fit VS what works best from an engineering reliability and best practices standpoint.

Conversely, 3D printing, because it depends on a bond of the current layer to the previous layer is almost never as strong as a good injection molded part. In fact, often weaker by some margin.

If the original molded gear cracked because it's at or below the minimum teeth ratio to bore size not to mention the press fit dimension ratio, and if the shaft was machined with splines, knurling, ridges or other sharp features. Again, if we are prone to cracking, then a 3D printed gear could be worse and not even make it past pressing onto the shaft.

I'm not saying 3D printing never works or cannot work for gears. I'm saying complete analysis of the failure and gear specifics should be done before evaluating the solution for the replacement.

As an example, let's take the Lionel Intermodal Crane and it's well know to split gears. Well, when I put on a replacement, I drill out the bore of the replacement gear on purpose to reduce the press fit pressure. I also may ever so slightly sand down the sharp edges of splined shafting- again with the goal of not splitting the brand new gear just like the old gear. Obviously there is a balance and we cannot have the new gear loose on the shaft, but even then I often use CA glue and accelerator catalyst to lock the new gear to the shaft given I've loosened up the press fit ratio.