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.

EMD DD35 With Body Mount Couplers – Part 2

In last week’s post I shared with you my design and 3D print of an N Scale EMD DD35 with body mounted couplings.  You can find the post here.  In this week’s post I’m going show how well it worked.

The new EMD DD35 shell, as shown below, is sat on a modified Bachmann DDA40X chassis which has been shortened and had its pilots cut off.

The 3D printed pilots have pockets to receive a Micro-Trains body mount coupling.  This can either be a Type 1015 (Short shank) or a 1016 (Medium shank) and there’s a 3D printed hole in the pilot to receive the mounting screw.

I’ve used the 1016 as the extra length will help with the curves.  Because the coupling rotates slightly off of the screw, the longer arm will mean the coupling can swivel closer to the center of the tracks, which is the ideal location.  The further away the coupling gets from the center the greater the risk of it pulling the train off the tracks.

On our layout ‘Solent Summit’ the tightest radius is in the yards at 16″.  Below you can see the new DD35 coupled up to two originals with the truck mounted couplings.  The three run around the 180° bend with ease and there’s still slack in the couplings.

The middle DD35 has the standard McHenry couplings as supplied by Bachmann.

The McHenry sits a little high compared to the micro trains but the connection is good under tension.  Because the couplings naturally spring straight they will not couple up on the bend, they are way too far out of line, but they don’t seem to be affected once coupled.

In order to test the couplings properly I assembled a train powered by a GP35, GP20, GP7, the new DD35, a dummy DD35, a original powered DD35 and another GP20.  All followed by 42 cars and a caboose.

Apart from being lots of fun, the idea behind all the motive power, some 23,000 horsepower with the new DD35 in the middle, was to see how the couplings worked with pulling and pushing forces. The train, comprised of a lot of older rolling stock, had a lot of drag which added to the draw bar pull.  The big train made its way around the layout, through s bends and the 16″ radius yard curves, several times with no problems at all.

But as the other DD35s had truck mounted couplings, the GP locos being short and the box cars in the train also being short, all their couplings were close to the center of the track.  To make this a decent test the new DD35 needed to be connected to other long locomotives and freight cars with body mounted couplings.  And luckily there was one on the layout.  The train in the video below, built by my fellow modeller Chris, has two Kato SD80MAC locomotives pulling a long line of Atlas 85′ trash cars.

Both the SD80MACs and the trash cars have body mounted couplings so they will swing out further on the bends.

The trash car has Atlas Acumate couplings which as you can see work well with the Micro Train couplings.  There’s some swing on the Atlas coupling but it’s rotating about the end of the car, not the truck center point.

The Kato coupling seemed a little low, or the DD35 body may have lifted and I didn’t notice untill I got home and looked at the photos but it didn’t cause an issue.  The Kato coupling rotates about the end of the loco.

Leaving the East yard the train runs through an s bend, around at tight corner and out onto the layout and the DD35 with its body mounted couplings did this with ease.

It’s possible the shorter 1015 coupling will also work and if the tightest curve is 18″ or 20″ radius then it certainly will.  But I think 16″ is about the smallest radius for the new DD35.

I have a few other things to check and then I’ll make the new DD35 shell kit with pilots and body mounted couplings available to buy.

EMD DD35 With Body Mount Couplers – Part 1

This week I have a modified shell to share with you for my N Scale EMD DD35 project.  The new shell option incorporates body mounted couplers rather than truck mounted.

My DD35 3D printed shell is designed to fit onto a modified Bachmann DDA40X chassis which has truck mounted couplings.  Only the shell and fuel tanks are 3D printed, the trucks and pilots come with the chassis.  You can find the kit here.

The real DD35, and the DDA40X, has body mounted couplings, or rather chassis mounted, which allow the trucks to rotate freely under the chassis.  I originally decided to leave the truck mounted couplings on the model, simply because of the length of the locomotive.  As it’s so long, body mounted couplers will cause a problem with tight curves.  As the locomotive navigates the tight curve the coupling moves too far away from the center of the tracks and can pull the connected rolling stock off the rails or derail the locomotive. That’s also why Bachmann built the DDA40X model the way they did.

But some layouts have larger radius curves than others and I was asked if I could produce an extra part to allow body mounted couplers to be fitted.  So I did and they looked like this.

These came in the form of a pilot section with a cutout for a body mount coupling which, with a bit of modification, could be fixed to the underside of the shell.  You can read my post about them here and they can be found here.

But the ideal situation is to have the pilots 3D printed as part of the shell and that’s what I’ve done as you can see in the renderings below.

The new pilot section has the pocket and screw hole for Micro-Trains body mounted coupling.  The problem comes with fitting the new one piece 3D printed body section onto the chassis which is now too long.  As the pilot sections tuck under the chassis this makes it impossible to simply drop the body down onto it.

The original modified chassis, as shown below, has the pilots attached to the trucks and the chassis stops roughly where the pilots start.

To make the new shell fit, the first thing to do is remove the existing pilots.  These are held on with two screws which release the coupling and pilot.

The pilot mount is plastic and projects from the truck frame.

This needs to be cut off and that can be done with pair of side snips.

The chassis also had to be shortened by filing the ends.  From point to point the chassis needs to be 150mm (5.906″) long in order to fit inbetween the new pilots on the 3D printed shell.

With the chassis reassembled it now looks like this.  I also filed a chamfer on the four corners to ensure the shell was a good fit.

One other modification I made was to file off the four locating bumps on the sides of the chassis.  These normally located the DDA40X shell which has matching holes on the shell.  As the DD35 shell doesn’t have these holes and is held in place by the length of the chassis they are not required.  They will also cause the shell to spread if left in place.

The new shell, which is 3D printed in Shapeways Fine Detail Plastic, fitted onto the chassis and clipped into place, as did the fuel tank.

Once the shell has been painted I will fit the body mount couplers and get some videos of the DD35 traversing curves with its body mounted couplings. I’ll share that with you in another post.

Alco C-855 R-T-R Build – All Together

Happy New Year!

2019 is here and what better way to start than to see a project completed.  My C-855 Ready-To-Run set of N Scale A-B-A Alco C-855 locomotives have been a challenging build but fun to do and I think the outcome is very good. This set are now on their way to their new owner.

The complete How-To series for the build of this A-B-A set can be found here.

Looking forward I have some more projects which need to be wrapped up and the next big one is the Union Pacific Rotary Snow Plow 900081.

I also have some updates for the DD35 as well as several replacement parts to share with you once we get stuck into the year.

So for now it’s back to the digital drawing board and on with 2019.