Bachmann (Mainline & Replica Railways) Split Chassis Axle Repairs

One of the popular British outline manufacturers in OO Gauge is Bachmann and many of their older models, which were originally sold under the Mainline or Replica Railways banner, utilize the split chassis design. In this post I will share with you my replacement parts to repair the most common fault these locomotives suffer from, split axles.

A split chassis means that the chassis is split in two halves vertically with the motor sandwiched in the middle.  Because each half conducts power from each set of driving wheels directly to the motor there is no need for any wires, the design is fairly simple.  The axles of the driving wheels sit directly in the chassis to give the best electrical contact which means the center of the axles need to be electrically isolated with a plastic axle to prevent a short.  But, due to the type of material used, this plastic axle is notorious for splitting.  This means that lots of these great locomotives became redundant as Bachmann moved away from this type of design and spare parts were no longer available to repair them.

In this post I will show how to fix a typical locomotive with split axles, in particular a GWR Hall Class, 6990 Witherslack Hall.  Interestingly the real 6990 Witherslack Hall has has been preserved and sees regular service on the Great Central Railway.

These models come apart fairly easily.  There are three screws holding on the chassis base plate.  One is under the cab, in the hole to the rear of the photo below, which also releases the body shell.  The other two are either side of the clearance bump for the drive gear.

With the screws removed the chassis plate lifts off and the front truck comes away with it, leaving the drive wheels in the chassis.  Below you can see the split chassis construction. The plastic screw locators also act as spacers to hold the chassis halves apart and in the right place.

On this particular model the center drive axle is also the main gear which is driven directly from the worm gear on the motor.  The other axles are driven via the side rods so there are no other internal gears.  In the photo below you can see the original drive gear and an axle on the left with a new 3D printed set on the right.  Each driving wheel has a square axle which fits into the square hole, this makes quartering the wheel sets much easer to do than on other locomotives.  Quartering refers to the position of the left hand driving wheels relevant to the right and I will explain why this is important when I reassemble the axes later.

The original main drive axle has split along the weakest point which is the corner of the square hole due to the plastic being thinnest here.  This is probably caused by the motor turning the gear but something is stopping the wheels from turning.  The force of the square metal axle pushing against the plastic of the square hole causes the gear to split.  The square hole then opens up and allows the wheel axle to rotate by 90° throwing that wheel out of quarter.

Once one of the axes is out of quarter the side rods will bind up putting too much force on the other axles and in turn they will also split.  The original axle in the photo below has completely split in two.

The new gear and axles have been 3D printed in Shapeways Fine Detail Plastic, formally known as FUD, because it is very accurate and has proven to be hard-wearing which is ideal for gears.

For the test print I made a set of several axles and gears with fractionally different sizes for the square hole.  This was to ensure the new parts were a good tight fit but not too tight that it split the new parts when the wheels were pressed in.

The axles simply press fit over the square drive shaft.  There is no need to clean these parts, in the same way you need to if you intend to paint them, because they will be hidden from view once the chassis base plate is refitted.

It takes a fair amount of force to squeeze both wheels onto the axle.  But don’t simply fit them all together because it’s at this stage that quartering becomes an issue and you need to pay attention as to which wheels you connect and at what angle. I normally start with the drive axle, fitting one wheel on, then adding axles onto the other wheels on the same side.

Then I position the wheels so all the side rod connecting points are at the same place on each wheel.  Note the rectangular section above the side rod connecting pin.  This represents the lubricating point and will always be on the top.  Also the spacing between the axles is different, the front two are closer than the rear two so in the view below the front set of wheels are on the left.  Another way to tell which is the front is the rectangular section, or lubricating point, on the connecting rod which will also be on the top.

Ensuring the wheels don’t move too much, turn the wheel set over and position the wheels on the other side at a rotation of 90° or one-quarter different from the other.  You should end up with all the connecting rod points in a line on both sides but quartered.  The square holes and axles make this easy to get right, locomotives with round axles can take ages to get right.

Why do steam locomotives quarter their side rods?  If you imagine when the side rod connecting point is at 9 o’clock on the wheel face the piston in the steam cylinder will be at its furthest position from the center or at full stroke and this is where the steam chamber opens to allow the steam to exhaust.  If the connecting rod on the other side was in the same position or at 180° (3 o’clock the wheel face) the other piston would be at the other end of the steam chamber which is also where the steam chamber opens to allow the steam to exhaust.  If the locomotive stopped at this point it would be impossible to get it going again as the steam would simply exhaust rather than push the piston.  But by having the wheels quartered only one piston can be at the full stroke point at a time which means the locomotive can always get going, even if only on one cylinder to start with.

Getting back to the model.  Now the axles have been properly quartered then can be pressed together. It’s important that they are pressed all the way otherwise the wheels will be too far apart and will not fit on the tracks correctly.  If the wheels are too far apart it may be because of some 3D print residue inside the square hole but as the hole runs all the way through there is room inside for the excess material and the axle the wheels can be pushed in with force.  But make sure the force is applied onto the center of the wheel otherwise you risk twisting the new axle and cracking it.  I normally squeeze the two wheels onto the axle between my finger and thumb.

The locomotive can now be reassembled and it’s ready to use.

I have made a set with a replacement drive gear and two axles available which can be found here.

Most of the older Backmann models which were former Mainline or Replica Railways locomotives had the same or similar plastic gears and axles.  Some had a smaller drive gear and some had a few different axles for the pony truck at the front.  In a later post I’ll share these parts as well.