New Axles for A Bachmann HO 4-8-4 Northern

I often get asked to have a look at damaged locomotives and see if there’s anything that can be done to repair them.  And I’m happy to say most of the time there is.  So this week I have another 3D printed part made specifically to repair a locomotive to share with you.

Bachmann make model locomotives in many scales and I normally work in N Scale but this time it’s HO and it’s a lot bigger than N!

This locomotive has a motor in the rear of the boiler which drives the rear axle.  The other wheels, just like the real thing, are driven by the connecting rods on the side.

However this particular model suffers from cracked axles causing an issue with the quartering. But what does that mean?

Well, as each driving wheel picks up power from the rails the axle needs to be electrically isolated to prevent it from shorting and this is done with a plastic axle.  Each metal wheel has a peg at the center which fixes into the plastic axle.  Below you can see the chassis upside-down with the base removed.  Between each driving wheel you can see the plastic axle and between the rear wheel set is an axle with a gear which is driven by the motor.

This all works well untill the plastic becomes weaker with age and the pressure of the wheels turning causes it to crack.  In the photo below you can see the crack line running through the original axle.

When it’s cracked like this the peg on the wheel will not be fixed tightly into the axle and the wheel can move differently to the wheel on the other side of the axle.  And it’s this which causes the quartering issue.  Quartering is a name given to the positioning of each wheel relative to the cylinder and piston.  In the image below you can see all the wheels are connected to the connecting rod at the same point.  Half way around the wheel on the left hand side, or at the 3rd quarter point.  And the piston will be all the way to the front of the cylinder.

At the same time on the other side of the locomotive the wheels are all connected at the top of the wheel or the 1st quarter point.  And the piston will be in the middle of the cylinder.

All steam engines are offset like this, although some are a bit different if they have more cylinders, but it’s this offset which ensures one cylinder can always push on the wheels no matter where the locomotive stops.  If both side rods were in the same position and the locomotive stopped with the cylinders in the middle of a stroke, it would be nearly impossible to get it going again.  So either side of an axle a driving wheel is positioned a quarter of a turn apart.   But if the axle doesn’t grip the wheels then they get out of sync, the side rods get jammed up and the locomotive stops moving.  And that is exactly what has happened to this locomotive.

However, there is a simple solution.  I have drawn a replacement set of axles and 3D printed them in Shapeways Frosted Ultra Detail material which is accurate and hard-wearing, so ideal for this replacement part.

Yes, I know the original had a square section in the middle but out of the three I’ve changed only one of them did and I don’t know why. The other two had round sections and as I could see no reason for it being square I made them all round. If anyone does know why, please get in touch.

And as you can see in the image of the chassis the axles fit well.  The wheels push in with a tight fit and stay at the correct quarter spacing.

I’ll be making these axles available soon in my Shapeways shop so if you also have a Bachmann HO 4-8-4 with spit axles you’ll be able to fix it and keep it running.

No News This Week

My apologies but there will be no post this week aside to say that today we lost a fellow modeler, so I know you’ll excuse me if I take this evening off.

Look after yourselves, and I’ll catch up with you all next week.

Following up on Messages

My website has now been up and running since November 2013 and in that time many people have contacted me with all sorts of questions which I try my best to answer, although sometimes I do get a bit behind.

However, every now and again I get a question come through via my contact page which I simply can’t answer.  This is not because I don’t know but because there is an issue with a mis-spelling of the supplied email address and I’m unable to send a reply.

So to help stop this I have added another line to my contact form asking you to re-enter your email, just as a check, which should help with the problem.

Alternatively you can always email me directly at  I do try to answer all emails, even if I can’t get to them straight away, but if you haven’t had a response and you think this might have happened them please get in contact again, I’m always pleased to hear from you.

How to Fix Runaway Locomotives on a DCC Layout

When running your layout on DCC power have you ever had the problem of trains suddenly rocketing off down the track at full speed for no apparent reason?  Well a fellow modeler had just this problem this weekend.  So in this post I will explain what was causing his issue and what you can do to avoid it.

Before I can say why there’s a problem I need to explain a bit about how DCC works.  DCC powered trains all have a decoder inside which receives power and instructions through the track.  This combined supply is a 12V to 16V AC (Alternating Current) signal.  The decoder splits this into two separate parts.  The first part is the AC power which runs through a bridge rectifier.  This converts the AC power into 10V to 12V DC (Direct Current).  The DC is used to power the decoder and any outputs, such the motor and lights.  The second part takes the instructions, which are carried in the AC Bi-polar Square Wave as packets, and feeds them into the decoder processor.

(A Bi-polar Square Wave is not the same as a Sine Wave which you may have seen on an Oscilloscope screen trace; one is a series of square shaped variable width pulses and the other is smooth curved [Sinusoidal] and has a constant period time-base. The DCC signal as well as being square in shape has a variable time-base. By varying the width of each square wave pulse, a digital binary data bit can be transmitted. A binary 1 or a binary 0. It is the pattern of ones & zeros that define the DCC command being sent.).

The instructions will be things like increase speed or turn on light.  The DCC command station sends out many packets every second, that’s why the decoder can do many things at once.

A lot of decoders have the ability to run on traditional DC powered (Analog) layouts as well as DCC.  This is achieved by the processor understanding what type of power it’s receiving.  For example, if a locomotive with a suitable DCC decoder is put on a DC layout there will be no power applied until the DC throttle is turned on.  As the processor starts to receive a DC power supply but no information packets it realizes it’s on a DC controlled layout; this takes barely a second.  So it bypasses all of its complicated circuits and sends any DC power received directly to the motor and lights.  This makes the locomotive behave just like a normal DC locomotive.  It repeats this every time it’s moved on a DC layout.

The next time the locomotive is put on a DCC layout the second it receives an information packet it knows it’s on a DCC supply and returns to normal.

In an ideal world this works well and there should never be an issue, but things can go wrong and the primary cause of locomotives rocketing off down the track is short-circuits.  These are usually caused by derailing trains or when you’re putting rolling stock onto the layout whilst the track power is on.  Especially steam engines with lots of wheels!

So why does a short-circuit cause an issue?  When a DCC command station detects a short it turns the power off.  Some will keep trying to turn it back on or will require you to do it manually.  Situations where you have several quick short circuits, for example putting on a steam locomotive, can cause the command station to repeatedly start up and sending out its packet information as it turns the power back on.  If the decoder in the locomotive doesn’t receive a full packet it ignores it.  If this happens too many times on start-up it may get confused and think it’s receiving no packets of information and switch itself to DC.  The problem now is that it will bypass its processor and feed the full 10V to 12V DC from the bridge rectifier directly into the motor and the locomotive rockets off.

This situation can also happen if a train runs into a point or turnout which is set against it.  The system shorts, you change the point, the trains moves forward and shorts again as some wheels have derailed, you lift the derailed item, it shorts again but re-rails itself, the power comes on and other locomotives on the layout rocket off on a joy ride.

So what can you do to stop this? My advice would be to turn the DC running option off on all of your decoders.  This does mean they simply won’t work on a DC layout so bear that in mind if you run them on both.

So how do you do this?  If you have a computer connected to your layout or programming track it should be fairly easy.  Each brand of software is different but the principle is the same.  I use Decoder Pro from JMRI for my programming and the very first screen when you start programming a decoder looks like this.

Below the locomotive address options is the switch for turning off the DC operation.  In the advance setting or Comprehensive Programmer the option is in the basic tab and there is often a tab dedicated to just Analog Control.

But what if you don’t have a computer connected to your programming track?  The option to turn the DC on and off is contained within the CV (Configuration Variable) settings: CV no 29 controls this.  But it also controls the locomotive direction, the speed step settings, Railcom Settings, Speed Curve Settings, long address option and sometimes more, depending on the decoder.  So to work out what number to set CV29 to there are several calculators available on-line to work it out.  This page on Digitax’s website has several CV calculators and the second one down is for CV29.

If you are programing this CV change on an existing locomotive in your collection, rather than a brand new install, it’s a good idea to read CV29 first and see what the value is.  Then replicate this value in the calculator before making the change.  That way you won’t be changing something you don’t want to.

The 2mm Scale Association also has a good calculator here.

Some of the more expensive decoders are smart enough not to suffer from this but I tend to always turn DC off on them all, just to be safe.  Plus if you intend to install any Stay Alive systems to your locomotives you will need to turn it off anyway as a Stay Alive delivers DC power only and it could confuse the decoder again.

With all your locomotives set this way you should have a rocket free layout!