Sometime a go I designed the `Micro Drive’ creating a drive with a coreless motor and 54:1 gearing. It was aimed at providing a replacement drive for the Agenoria Quarry Hunslet kits I own and sell. It was necessary for two reasons. The first was that the Mashima motors that the kit was designed for are no longer available. The second that coupled with the 30:1 gears the high revving small Mashima wasn’t ideal for a quarry shunting locomotive as it was too fast a combination.

As soon as this was available one of my regular customers started musing on whether the `Micro Drive’ would fit in a Skylark kit he was slowly building (his building speed sometimes makes continental drift look nippy). The short answer was no! The distance from the firebox, where the motor lives, to the driving axle is a lot longer in this loco. The result was the `Skylark Drive’ this uses the same basis as the `Micro Drive’ but adds and idler gear to increase the length of the drive.

Fast forward a few months. Also in the Agenoria range are kits SL2 Jubilee 1897 and SL7 Lilla. These are considerably larger engines that the quarry Hunslet. Lilla hasn’t been available for awhile due to issues with the whitemetal castings in the kit being unobtainable (a saga in itself) but after quite a of work remastering patterns and upgrading the castings to brass it’s been relaunched.

But with what shall I motor it I don’t hear you ask. Well I asked myself and thought that surely one of the existing drives would fit. Silly me, of course it didn’t. Micro Drive was too short, Skylark too long. The new Lilla drive was born and it seemed sensible to try an make it fit Jubilee as well.

The design process started with  a load of measuring which then morphed into an outbreak of chassis building. Plan A was to superglue a Jubilee and Lilla chassis together using the spacers for a 16.5mm gauge model and then to break it with a little heat and build them both with solder to 14mm gauge. That went a bit wrong when I found I already had a part built 14mm gauge Lilla and I ended up building 14mm & 16.5mm chassis for both kits. Part of Plan was to do the gearbox design just by measuring but then as I don’t have a complete mode of either for myself I kept going.

The new design needs a smaller idler gear in it to reduce the length of the gear train but its also been worked on to get the motor to sit as low as possible.

This picture is the first test. Several additional changes have been made since

The prototype has allowed a test fit in a Jubilee 1897 body showing that it fits. there is also scope to open out the cab front to match the arc of the firebox which would allow a larger flywheel to be fitted if so desired.

A trial in a Lilla has also revealed that it fits. In this instance whilst there is room to the side for a larger flywheel there isn’t behind the motor between it and the backhead.

The CAD design did reveal a problem though. I usually use 2mm shafts for the gears that are bought with a 2.1 bore so they spin freely but with a 14tooth idler the 27 tooth reduction gear just impinges on the shaft for the idler. In this image your looking down the hole that would have the idler shaft in it.

For the testing I have been using a stub axle to carry the gear and a stop to hold it in place. For the production units I will change to a 1mm shaft with gears with that size hole in the middle. The gears are on order but will take about three weeks to arrive.

When they get here and final tests with them are done the gear drive along with dimensioned drawings will be added to the NGtrains website