Split gears
The Problem
The black plastic gears on many UK 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, and the spring drives will become damaged, often beyond repair. The earlier white nylon or brass gears on Poole production models do not suffer from the problem whilst the stronger fatter ones 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 photographs is normally all that is visible (Figure 1)
Figure 1. A split gear on a drive axle
Why does it happen?
In general, it is believed that because the plastic gears are a very tight interference fit on the axles, they suffer stress and ultimately fail. The experience of many members in the Society has shown that failures have occurred in new models straight from the box, those that have run many track miles, still others that have been in storage, and so on. In short there seems to be no way of predicting when or if it might happen. Oil has been suggested as a contributory factor as it could cause the plastic to become more brittle over time, but there is no conclusive evidence either way.
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 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 have also heard of occasional failures from the newer Blue Riband designs and so the problem does not appear to have been completely eradicated.
Symptoms
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 ones split?
Almost any plastic gear mounted on a metal axle is vulnerable. You may find that the drive gears on powered axles as illustrated are the most common to fail, but also any intermediate gear in a bogie tower may fail, especially the smaller intermediate gears in the bogies and the reduction gears of the original design Poole and China production Class 20 diesels, and some DMUs. The drive gear on the Jinty and 57xx tank engines also suffer but otherwise the steam locos seem to be free from the problem.
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. Another approach is to run the bogie along a length of track. 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 will suggest that there is a split gear.
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. If it moves the gear is split. If all the powered axles are OK 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 others may be 'hiding'.
The Solution
In the short term, you can remove a split gear from the bogie tower and the remaining axles becomes unpowered. It is also possible to remove the split gear on a wheel set and swap the unpowered axle on a Co-Co or A1A-A1A arrangement locomotive with an undamaged centre axle. You can then refit the now unpowered axle in the centre position with very little loss of performance, although it may be necessary to elongate the axle hole vertically to give it a little more upward float.
There is no other long term solution other than to replace the offending wheel set or gear. It is possible to remove a wheel from an axle and fit a replacement gear but it is not easy without specialist tools. It is therefore easier to replace the wheel set.
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.
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
Replacement gears and drive axles are available from the NGS Shop, and from Bob Russell at BR Lines. Please check what you require and stock availability before ordering.
|
Item |
NGS Shop Catalogue Number |
BR Lines Catalogue Number |
|
Replacement Gears for Farish Locos (mix of gears) |
SSP 008 |
Not available |
|
Replacement Gears - Bulk Pack |
SSP 009 |
|
|
12 tooth gear |
SSP 011 (pack of 10) |
GF 2503 (pack of 4) |
|
16 tooth gear |
SSP 012 (pack of 10) |
GF 2504 (pack of 6) |
|
25 tooth gear |
SSP 013 |
GF 2502 (pack of 2) |
|
Drive Axle for Class 20/25/ 31/33/37/40/42/44/45/46/47/ 50/52/55/57/87/90/91/HST |
Not available |
GF 7101-1 (Farish Poole production bright finish style) (specify pack of 4 or 6) |
|
B 7101-1 (Bachmann style) (specify pack of 4 or 6) |
||
|
Drive axle for GW Railcar/ Class 101/158/159/170 and also some (early) Class 56 |
Not available |
GF 7101-2 (Farish Poole production bright finish style) (pack of 4) |
|
B 7101-2 (Bachmann style) (pack of 4) |
Figure 2. Examples of a 12 tooth, 16 tooth and 25 tooth gear, to help you identify what you need to order.
The drive axles are only available from BR Lines. They can be supplied in either the original Graham Farish (Poole production) bright finish with deep flanges (so you can replace your old Graham Farish wheels with matching replacements), or in the current Graham Farish by Bachmann blackened low profile finish (Figure 3).
Figure 3. Examples of drive axles in Poole Production style (bright finish) and current Bachmann style low profile wheels.
The wheels and gears are a direct replacement, like for like, so you can upgrade Poole production locomotives with Bachmann low profile wheel sets, but you should ensure that all drive axles on the same locomotive match.
If you do not know what the problem is, it is possibly best to contact Bob Russell at BR Lines to discuss your requirements, as he may be able to understand your problem and guide you to what gears to purchase. The NGS Shop cannot answer questions regarding what gears are needed, if you have not been able to identify your problem. BR Lines can be contacted as follows:
Phone: +44 (0) 1287 633036
Email: brlines@aol.com