How to … change split gears

The below article is taken from the n-gauge society website;

The Problem

The black plastic gears on many UK (Poole production) and some Chinese production Graham Farish locomotives have an unfortunate tendency to split or crack between the metal axle and the root of a gear tooth. They will then no longer transmit drive properly and can cause the gear train and other transmission components, such as the spring drive on those chassis where a spring replaces a worm, to jam or lock up. If not identified quickly, the motors can burn out, or the spring drives will become damaged, often beyond repair. The stronger fatter gears from China are much less likely to split, although they can also split in the same way. Other manufacturer’s locomotives do not seem to be affected by this phenomenon quite as frequently, but it has been known to occur. A hairline crack, as shown on the photograph, is normally all that is visible (Figure 1).

Figure 1. A split gear on a drive axle and a 12 tooth intermediate gear

Why does it happen?

Originally, Graham Farish used brass drive gears from 1977 up to about 1984/85 and then changed to white nylon gears. It is believed that this first change in materials was due to variations in the hardness of the brass used. Brass is an alloy of copper and zinc, and the ratio of the metals determines the hardness of the finished material. At the time, Graham Farish discovered that steam locos in particular were stripping the brass drive worm on the armature or centre drive axle. Consequently they changed to white reinforced nylon (the same material used in the bearings). This was very effective and we have seen few if any examples of nylon gears failing, but this material is very expensive (20 times more than the plastic material in use today) and due to commercial pressures, the company had to look for a different material and selected the plastic which led to the phenomenon of split gears.

The first evidence of split gears was seen in diesel models in approximately 1996/97. Graham Farish was subsequently sold to Bachmann in August 1999, and the factory was moved to China in January 2000. The new owners changed the design of the wheel sets and gears which has improved things considerably.

Although the problem first became evident on Poole production stock, the first imports from China used gears of the same dimensions which were also susceptible. The Chinese production of Poole designed Class 20s and some early Chinese production Class 158 and 170 series DMUs remain susceptible, because they retained the same design of bogies, and used small toothed gears in the bogie towers. We also hear of failures from the newer Blue Riband designs and so the problem does not appear to have been completely eradicated. Some steam locomotives are also susceptible.

The most common factors which determine the likelihood of failure are the age of the model, how long and where the item has been stored, and in what situation it is used.

The gears are designed for a tight interference fit onto a brass axle. Changes in temperature cause the axles to expand at a different rate to the plastic. Storage of the loco often plays quite a significant role in failures, particularly when stored in lofts, garages and any other places that can be affected by temperature.

Another problem that can cause split gears is the use of silicon or other heavy grease as both lubricants attract unwanted materials like ballast grit or hair/fibres and allow these foreign bodies to work their way into the gears which in time jam in between the teeth of the gears, again causing stress related cracks.

On steam locomotive models produce in China using the Poole designed chassis, you may come across undue wear on the main drive gear. This is likely to be caused by either adverse pushing of loco on the track if it should stall, or the armature getting hot at some time which can cause the bearing at the brush end of the motor to fail, which allows the armature and worm to spin out of true and wear off the teeth of the drive gear.


A locomotive or multiple unit may start to make a noticeable clicking sound, be very jerky when starting, run very slowly or not at all in one direction, or, in the case of those mechanisms with a spring drive in place of a conventional worm, a bogie may lock and force the end of the spring out of the chassis. In short, any locomotive or unit which starts to misbehave should be stopped, removed from the track and checked for split gears.

Which gears split on diesel or electric locomotive models?

On diesel and electric locomotive models, almost any plastic gear mounted on a metal axle is vulnerable. You may find that the drive gears on powered axles are the most common to fail, but also any intermediate gear in a bogie tower may fail, especially the smaller 12 tooth intermediate gears in the bogies and the reduction gears of the original design Poole and China production Class 20 diesels, and some DMUs.

How to Check

Remove the bogies one at a time from the chassis. Roll the uppermost gear with the ball of a finger for at least one full revolution in each direction. The gear train should turn without any stickiness, ‘grinding’, or restriction other than the normal friction of the gear train. Even bogies which appear to run freely in one direction may not run as smoothly in the reverse direction. The slightest restriction is a good indication of a split gear. Don’t be tempted to push the bogie along the track as the pressure of you pushing it can override the friction drag that the damaged gear may have and you may not be able to tell unless it locks up straight away.

If you suspect something is wrong, unclip the moulded bogie frames, taking care not to lose the coupling and spring. Remove each wheel set in turn and attempt to push the gear sideways on the axle using your fingernail. If it moves, the gear is split. On three-axled bogies this is the usual failure point. On two-axled bogies which have 12 tooth intermediate gears, the likelihood is that if one or more of the drive axles have split gears, then so will the 12 tooth intermediate gear. If all the powered axles are fine but the problem persists then the only thing to do is to remove the intermediate gears in the tower in turn and examine them under a magnifier. Even after finding one gear with a fault continue checking all the rest on a loco or unit as there may be more than one failure.

The Solution

Firstly, it is not a good long term solution simply to remove a gear, or relocate failed axles to get over the problem, as all it will do is hasten damage due to additional stress on the remaining gears. The only solution is to replace the offending wheel set or gear.

When replacing split gears in the bogie train (not those on the final drive axles), they do not need to grip the axles so fiercely. It is possible to open out the axle hole by a fraction of a mm, by reaming the axle hole with a jeweller’s broach to reduce the stress on the gear. If done carefully, and the hole is not made so large that the gear fails to remain aligned with the others in the drive train, this simple action will almost completely eliminate the possibility of this gear splitting again. We do not recommend any attempt at re-gluing a failed gear, as the tooth alignment will never return to the original shape.

On Chinese production diesel and electric classes, if you have the skill, you have the ability to replace the damaged gear on a drive axle, if it should occur, without having to replace the whole axle, unlike the Poole made wheel rims, which were locked on in manufacture. One rim  is designed to be pulled off, which allows you to replace the gear without replacing the whole wheel and axle. However, this is not easy, and if you are a novice engineer, it is easier to replace the entire wheel set.

Complete replacement of all the gears, just because one has failed, is not necessary, but you should be aware that the problem could arise again even in a locomotive that you have recently replaced a split gear on.

Spares for original Graham Farish designs

Replacement gears for the original Graham Farish (Poole) designs are available from the NGS Shop, and from Bob Russell at BR Lines. Please check what you require and stock availability before ordering from either supplier, and note that the NGS Shop cannot answer questions regarding what gears are needed if you have not been able to identify your problem. Figure 2 shows examples of the most common 12, 16 and 25 tooth gears as fitted to Poole and Chinese production versions of the original Graham Farish (Poole) designs.

The drive axles themselves are only available from BR Lines. They are no longer available in the original Graham Farish (Poole production) bright finish with deep flanges, so if you have to replace any of these, you should upgrade them with a complete set of Bachmann low profile blackened finish wheel sets (Figure 3). These have significant benefits over the older Poole made axles. The new low profile rims are chemically blackened and will retro fit to older locos of Poole production, immediately upgrading their appearance.

You should not mix Chinese drive axles with Poole made drive axles on the same locomotive.

Figure 2.  Examples of a 12 tooth, 16 tooth   and 25 tooth gear, to help you identify what you need to order.

Figure 3.  Examples of drive axles in Poole Production style (bright finish) and current Bachmann style low profile wheels.

Spares for Bachmann Blue Riband designs

Previously, all Graham Farish diesels had one standard driving wheel diameter and all multiple units a smaller diameter driving wheel. Blue Riband locomotives can now have slightly different diameter driving wheels depending on the class of locomotive. Since production moved to China, new chassis designs have become unique to individual locomotive classes, as correct wheel diameters and bogie spacing are modelled. This has introduced additional variations above and beyond the three common gear sizes previously used. For instance, in addition to the 12, 16 and 25 tooth gears used in earlier designs, 14 tooth gears and two different sizes of 24 tooth gears have also been introduced. These and other unique parts are not available from the NGS Shop.

Because spares availability may be restricted due to the Chinese production of these models, we recommend your individual needs for all spares for Blue Riband designs are confirmed by a phone call first, and then obtained either from Bachmann Industries or from Bob Russell at BR Lines.

Bachmann Industries:

Web :

BR Lines:

Tel: +44 (0) 1287 633036 during normal office hours only.



Being able to identify the locomotive class, and if the locomotive is a Poole production or China production locomotive is important, as this will ensure the correct diameter drive wheels and gear sets are ordered. There are examples of locomotives that have been completely retooled since production moved to China (e.g. Class 20/37/47/Black 5), the chassis of these are totally different, and parts are not interchangeable.

This version of the Split Gears article is June 2013 © N Gauge Society.