The Mainline J72 0-6-0 has been around for many years; it first appeared in the Mainline catalogue in 1976, and it was also my very first electric model train.  However this locomotive suffers from the same problem as most of the other Mainline locos; split gears! So in this post I’ll show you my solution for fixing this problem.

The J72 has a motor, which fills the cab and drives the rear axle.  The two forward axles are driven from the side rods so only the rear axle has a gear.

The wheels and side rods are handed, that is to say they are not the same on each side but mirrored.  If you look closely you can see a small section of metal above each pin where the side rod connects to the wheel.  This is to represent the oiling point for the bearing which the real locomotive would have had.  It also helps us to work out which side the wheel sets go.  The side rods are also in two pieces and the two parts connect at the centre wheel.  The rear side rod fits over the front.  It’s important to get this the right way around as the axle spacing is different with the front two being closer together.

The original rear axle with the gear and two regular axles are made from a black injection modelled plastic and looking closely you can see they have all split.  This causes the wheels to spin freely in the axle and consequently jamming the side rods.

To solve the problem I have designed a 3D printed gear and axle set to be a direct replacement.  These have been 3D printed in Shapeways Fine Detail Plastic because of the material’s accuracy and toughness.

They are all 3D printed on a solid bar to keep them all together but the axles are loose. When the bar is cut they will fall off.

The original main gear I designed was a direct copy of the Mainline part, but because of the recess to reduce the amount of material used, this caused a week spot in the 3D printed gear.  Although the Fine Detail Plastic is tough, it’s also brittle at thin areas.  When the wheel was pushed into the gear it broke. So the gear was redesigned to be a bit thicker and the recess was omitted.

When fitting the new axles it’s vital to get the quartering correct.  A description of what this means can be found in my post about ‘Bachmann (Mainline & Replica Railways) Split Chassis Axle Repairs‘.

It’s also important to get all the wheels aligned, as shown below.

The new axles will press-fit onto the wheels and should be pushed all the way on.  I get the axle started then double check it’s at the right position before pushing it all the way.  It’s possible that the axles may have some 3D print residue left inside.  This can easily be removed by reaming the hole with a 1.9mm drill.  If you use a larger one you’ll open the hole up too much and the axle won’t grip the wheel.  It’s also worth double checking you have the geared axle on the right wheel set and the right way around as it’s not symmetrical.  In the image below you can also see the thicker gear from the new design.

Assuming the quartering is correct the repaired wheel sets should drop into the chassis returning it to a working locomotive.  If, once all reassembled, the locomotive lurches or makes a thumping sound at the same point in every wheel revolution then the quartering is out on one of the wheels.  This should be visible be examining the wheels; one will be at a slightly different rotation to the others.  It’s possible to adjust this without taking the locomotive apart.  Hold the correct wheel tight with your fingers and carefully rotate the wheel on the other side,  It should slip in the axle.  Only make small adjustments and retest each time.

The new set of axles & drive gear for the Mainline J72 is available here.

In next week’s post I’ll have part two of the post for the replacement gears for the Bachmann N Scale 4-8-4.