Double Fairlie locomotive in 7mm scale / 0-16.5

thingiverse

NOTE The model has progressed to making cab fittings, with some photos from the nice people at the Ffestiniog. But this revealed that I have made the firebox(s) too big, so crew and fittings do not really have enough room. I will not be correcting my model now, but you might want to. Try the Fairlie firebox reduced.stl file. Also, I decided the roof was to flat and have redesigned it. This is a model of Merddyn Emrys belonging to the Ffestiniog Railway. It is a Double Fairlie patent 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 run smoothly and at a sensible speed. Hornby appear to have quietly redesigned this mechanism. 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. I have fabricated crossheads from nickel silver sheet and 1.6mm dia rod but do not claim to have found an ideal plan. The difficult bit of fitting crossheads is clearance between the back of the crosshead and the crankpin of the leading axle. I fixed the crosshead to the connecting rod with an M2 screw and nut, but I had to grind down the screw head and crankpin as thin as I dared. This would be easier if the front truck was modified to be wider still, but this spoils the narrow gauge front view of the loco. "Fairlie truck braced" replaces earlier truck designs and includes a bracing arm that contacts the motor and prevents the weight of the body pressing the pivot into contact with the worm gear. 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 and have included firebox glow LEDs (2 in parallel - in opposite polarity - plus current limiting resistor) These are connected to the wires connecting the two motors to give current pick up to both engines on all eight wheels. 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. WARNING This set of files does not include all the parts for adding slidebars and crossheads to the Hornby chassis. My construction (in nickel silver) involved trial and error, and considerable fettling. There is very probably a better solution.