28/01/2026
Here's an FYI post which I thought I'd share.
It's mainly regarding Bachmann class 37's & 47's, which are the most common class of loco that I see in works for this type of repair. This is likely to be because they are the most ubiquitous of the loco classes.
This type of repair is starting to increase, as I suppose locos become older and have become subject to more work on layouts.
This issue is not exclusive to Bachmann class 37's & 47's, by any means, as I also see many other classes of Bachmann loco's which share the same type of worm drive assembly. Heljan loco's are worse, as they have had a production issue over many years, resulting in fractured drive shaft retaining cups.
The 2 photo's show the worm gear drive which is at the top of the bogie tower. As you can see from the 1st photo, the drive shaft retaining cup has moved up its shaft right up to the inner worm gear, which allows the drive shaft to become detached from the retaining cup, as there is now a longer distance between the motor and the retaining cup, or it will allow the retaining cup to spin on the worm gear shaft and not fully engage, which reduces reactive effort.
The solution....
Once you've removed the bodies and bogies, and then the worm gear assembly from its position at the top of the bogie tower, it's just a case of slipping the retaining cup back to it's normal position as in photo 2. To ensure that there is no repeat of this issue, I remove the cup fully from its shaft and check the cup for any signs of fractures, which is very common in Heljan loco's. If no fractures are found then I run a very small amount of superglue around the serrated end of the shaft, and push the cup back on to the shaft, making very certain that no glue seeps down the shaft and onto the inner worm drive bearing, which would not be a good outcome.
Once I'm happy with the positioning, I then drop a tiny amount of glue inside the centre of the cup, ensuring no glue is deposited on the cup sides. Once left to dry for 10 minutes I rotate the inner worm drive bearing to ensure it moves freely, and then reassemble.m, making sure you clean and debris from the worm drive, as I did after the photo's were taken, and while you've got it all sort give the drive and gears some lubricant if necessary.
As stated above, a noticeable reduction in tractive effort with locos with this type of worm drive assembly is the most obvious symptom of this issue, as is a drive shaft which pops out of the motor or bogie retainers.
I hope this helps if you are having this issue, or at the very least it'll arm you with what to look out for to diagnose this issue.
