Bachmann N Scale 4-8-4 Replacement Gears – Part 2

In February of this year I shared with you my set of replacement 3D printed gears for the Bachmann N Scale 4-8-4, 3rd Generation.  You can find the post here.

At the end of the post I needed to make some modifications to the gears as the axels were still a little too loose on the wheels and the twin transfer gear was way too loose.  These changes were made and another set was 3D printed.

This time all the gears fitted well into the chassis, but I think I still have a problem with the twin gear as the motor struggles to drive all the gears.  Either the larger set are oversize, causing resistance between the gear and the worm, or the smaller set don’t have a deep enough trough between the teeth, which means the axel gears push the twin gear up into the worm.

With the original twin gear fitted, and all the other 3D printed gears fitted, the assembly runs smoothly.   Below you can see the axels fitted onto the wheels with the chassis plate installed.

With the entire chassis assembled I started testing the gearing and discovered that there was a bind at the same point in every rotation.  After a little adjustment I was able to get it to run much smother.  However, on reflection the next time I do this when fitting the gears to the first set of wheels, as shown above, I’ll attempt to get the gears positioned at the exact same point on each wheel, as I think it was this that caused the issue.  If, as with diesel locomotives, there are no side rods, the position of the gears is not so critical as they will find their place.  Or if there are no internal gears, simply side rods as with the HO 4-8-4, then it’s just the quartering which needs to be correct.  But as this loco has both internal gears and side rods, the quartering needs to be correct as does the gear positions relative to each other.

Below is a quick video of the chassis running with power supplied direct to the motor.

I’ll make the adjustments to the twin gears and do another test print, but in the meantime if you’re keen to get your N scale 4-8-4 back on the road and are happy to use all the other gears then a set is available here. In most cases, these 3rd generation 4-8-4 locos only need the axel gears to make a full repair as it’s the axels that split.

Once the new set arrives I’ll update you with the progress in a later post.

Fixing a Mainline OO Gauge J72

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.

Bachmann HO US 4-8-4 Replacment Axle Shafts & Gear

My apologies for not posting last Monday, unfortunately a loved one was involved in an accident and we didn’t leave the emergency room untill 9pm. All’s well and they’re on the mend so let’s get back to normal.

Last year, coincidently about this time, I shared with you my designs for replacement axles for the HO Bachmann 4-8-4.  You can find the post here.  This week I’ve updated the 3D printed model to include the main drive gear.

The gear is less likely to split than the axles, but sometimes it does, so a set with everything in makes sense.

All the parts are joined on a 3D printed continuous bar so they’re all one part; this makes them cheaper to print, but the bar doesn’t actually touch the parts so there’s nothing to be cut off or trimmed as with injection molded parts.  This is one of the many things which makes 3D printing great.

The parts are also available separately and they can all be found in my Shapeways Shop or via the links below.

Axles & Gears

Axles Only

Gear Only

This coming weekend, 1st to 3rd March 2019, is the N Trak convention in Bournemouth, England and again it’s at The Trouville Hotel.  You can read a bit more about the 2017 convention here and the 2014 convention here.  I’ll be there over the weekend if you’re in the area and want to come by and say hello.

Replacment Gears For A USA Trains G Scale 0-4-0

In a similar post to last week I have another replacement gear to share with you.  This time it’s much larger and for a G Scale 0-4-0 Pennsylvania 20-Ton Diesel Locomotive; made by USA Trains.

Despite being a small locomotive it’s a big model, G scale has a model ration of 1:22.5, and is heavy.  This also means that the motor has lots of power and the transmission needs to be able to withstand the forces applied.

Both of the axles are powered from a worm gear on each end of the central motor. The worm gears drive plastic gears which also form the axles and isolates the two metal wheels from each other.  As you can see in the image below the plastic axle has split.  This means the motor drives the gear which turns but the gear doesn’t turn the wheels.

As replacement parts for this locomotive were not available I drew one exactly the same size as the original and 3D printed it in Shapeways Fine Detail Plastic, formally known as FUD.  As I’ve said in previous posts this material is ideal for gears as it’s hard-wearing.

The original gear was also a bit mangled as the softer plastic tends to get damaged by the metal worm gear when under heavy loads such as starting and stopping suddenly.  The new gear with the harder plastic can withstand this without being damaged.

The axles have grooved ends to enable them to grip the inside of the plastic axle.  With some evenly applied pressure they can be pushed into the new axle.  In the image below they need to be pushed in a bit further but you can see the grooves in the metal.

The locomotive motor is in the box between the axles and when the lid or base is screwed on the new gears are pressed up to the metal worm gears.  The motor box is not connected to the body but clamps to the axles which sit in sprung axle boxes.  This allows the locomotive to navigate bumpy track and gradient changes without losing traction.

Once the motor base is replaced the locomotive is ready to go back out onto the railroad.

The replacement drive gear can be found here.

They are also available in a pair here.

Next week I’ll have even more 3D printed gears to share with you but this time it will be for  N Scale.

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.

Dapol N Gauge Class 73 Replacement Drive Shafts

This week I have another new replacement part to share with you, this time for an N Gauge Dapol Class 73.

The Dapol Class 73 (Pre DCC-ready version), as shown below (stock Dapol photo), has a central motor which powers both trucks or bogies.  This is now common practice on all N gauge diesels.  Because the motor is fixed to the chassis and the bogies rotate a linkage or drive shaft is needed to transfer the power.

The drive shafts connect the brass fly wheels on each end of the motor to worm gears on top of the gear towers in the trucks.  The drive shafts have a ‘cross’ on each end which fit into slots in the fly wheels and worm gears.  This arrangement allows the gear towers to rotate while still being driven.  Below you can see one of the original drive shafts with the crosses on the ends.  The particular model I’m working on came to me with a missing drive shaft.

The drive shaft was then drawn up for 3D printing and designed for Shapeways Smooth Detail material (formally known as FUD).  I use this for its accuracy and strength as it’s a hard acrylic plastic.  As there are two drive shafts in each model I made a set joined together by a figure 8 loop to keep them together while in production.  The loop is not actually connected to the drive shafts but the holes are not big enough for the crosses to pass through.  The loops can simply be cut off.

The test print came out very well with nice crisp detail.  As the material is translucent its somewhat hard to photograph.

With the loops removed you can see the drive shafts are almost identical to the original injection molded part.

The new shafts can then be fitted into the locomotive.  As their the same shape and size their a direct replacement and work perfectly.  You can see the drive shaft replacement on the right is also sloping down.  This shows how this method of connecting the drive shafts allows of a lot of flexibility in the alignment of the two ends.

A pair of drive shafts for the Dapol Class 73 are available here.

Normally drive shafts are different lengths for different locomotives even though they are made by the same manufacture.  This is because when locomotives have a different wheel base its easier to change the length of the drive shaft than alter the motor design.   If you have a locomotive which needs a replacement drive shaft and you can’t find one send me a message via the contacts page and I can probability help.