Double Fairlie locomotive in 7mm scale / 0-16.5 by caderifor

Last crawled date: 3 years, 3 months ago

This is a model of Merddyn Emrys of the Ffestiniog Railway, a Double Fairlie design of articulated steam locomotive. The model has been designed to be powered by two Hornby Railways Caledonian Pug 0-4-0 mechanisms (cut down a bit) The Hornby "smokey Joe" is much derided but I decided to give the pug a try, since other mechanisms made the double loco project rather costly. Two new locos from eBay came to less than £50 and they seem to run smoothly and at a sensible speed.
The design started from an 009 design of a different double Fairlie, but ended up being a virtually new design. The loco is split into several parts for printing on an Ender 3 FDP printer with the minimum of support structure to remove afterwards.
The motorised chassis is allowed to swing by means of a front section (including cylinders) that is fitted in to the cut down front of the chassis, and includes an arm that goes up and backwards, just above the worm gear, to a pivot that matches one in the body approximately level with the front of the side tanks. (see photos) This allows most of the mechanism movement to be within wide part of the body, and hopefully minimise the appearance of excessive swing on smaller radius curves. (Not yet confirmed in practice)
The file, "Front truck wider" has the cylinders wider spaced to accomodate the rod spacing of the Hornby chassis. The holes through the cylinders are for 1.6mm nickel silver rod.
The stl file (and Tinkercad screenshot) illustrate my method of making a crosshead and spindle to clear the chassis parts while keeping the cylinders as close in as possible. This is not to scale and not for 3D printing.
The locomotive is symetrical in its two halves except that the driver's side has toolboxes on top of the tanks and the fireman's side has coal bunkers. Thus one body file can be used twice but there are two tank tops (in one file) I have put firehole doors in the firebox that will have to be on the fireman's side. (Maybe to include firebox glow LEDs)
The cylinder spindle, crossheads and slidebars missing from the Hornby mechanism have yet to be designed and built (but not 3D printed) I have placed the cylinders closer to the chassis than the Hornby, with its very pronounced joggle in the connecting rod, but the cylinders could be moved out to the Hornby position and even slotted to use the Hornby rods unmodified. Most of the design was done from a couple of photographs, but the Railway Modeller magazine, January 2021, arrived just in time to allow corrections to be made. (The drawings are of David Lloyd George, that is actually a foot shorter)
Great dimensional accuracy is not claimed for this design.
Photos of assembled loco promised ASAP. Chassis modifications pix added 30/12/20