Building a new model of an old loco

A render photo of a type 16 loco of the NMBS dating from 1908.

Designing the geartrain

One of the first things on starting a new design is finding out which motor will fit and what geartrain you will need. The gearing depends on one side on the motor and on the other side on the required speed of the loco. A motor with high RPM's requires a larger ratio but this can be compensated by the maximum required speed. Simply said a small shunter has different requirements than a loco for express trains. And on top of that it all has to fit in the available space, preferably be invisible in order not to spoil the looks. To decide if it all fits a simple sketch with the outline of the loco in 2 cross dimensions is sufficient. Actually the minimum requirement is a piece of paper and a calculator is helpful but not necessary. This will lead to paper noteblock designs like that on the following photo:

The old fashioned but still feasible way of designing things on a piece of paper using available components

In the paperless world a 2D line drawing looks neater but still serves the same purpose.

Any 2D drawing program suffices to make a simple sketch with cross sections in one or more dimensions. Good 2D drawing programs can be found as free download on the web at various firms, anno 2009 there is no need to spend any money on those. A view like the above can be used for determining which gear train fits best by drawing some rectangles and circles with the dimensions of the gears. In this case the point of interest is the visibility of the first stage which may be visible as a part of it can just stick in front of the tank below the boiler. In this case the required reduction of the gear train is calculated to be around 1:42. This is based on the rpm speed of the motor under load together with the size of the wheels and the maximum speed of the real loco. Such a gear train can be derived at using different combinations of gear wheels in various Modules and teethnumbers. In this case the choice is between M 0.2 and M 0.25 for worm and wormwheel and M 0.4 for the remaining gears. This leads a to set of options as published in the graph on the right. In this case the wormwheel should be below a certain size to keep hidden behind the tank in side view. The second option with M0.2 gears comes out closest, with the third one with M0.25 gears as second, but this combination needs a smaller size worm than standard available. However this won't form any problem to lathe owners.

The selected option with lowest visibility and required gearing.

Actually the earlier 2D view was made with CAD program with 3D capability, this graph gives an 3D impression

However better impressions are realised by 3-D programs. Above we see the outline of the loco using dimensions taken from the fiches. The advantage of the 3D approach is that you can see into the model and inspect clearances. Here some parts have transparency switched on. The size of the flywheel is a trade-off between visibility, space and smoothness in running.

Here we see into the model with most parts made invisible.

Using 3-D it is very easy to play around where the different cut-outs should be made out for optimal result. For instance above we see the footplate and the inside of the tanks. In this case the out of scale flanges of 0.5 mm give tight clearance on the front of the tank and the cut out in the footplate was filled in again as hole appeared by taking the clearance initially too large. Reducing the flanges to 0.3 mm will make it easier to built it. You also see the difference in sort of image produced by different software, CAD vs visualisations made in Cinema4D. The next picture gives an impression of the complete motorisation. The green block represents the size of DCX75 decoder, a SL74 sound decoder won't fit but there is space for separate sound decoder on top.

click for a larger view.

Motorisation using a 8mm Faulhaber with flywheel and gear train on the front axle. To get it around .8 m bends the bogie needs to have its point of rotation above the second axle and the frame spacers need to be a touch narrower at the front axle to give enough clearance.

The bottom side with various details added.

click for a larger view.

first impression in plain 3D looks.

You see the loco developing when adding more and more parts to the drawing. Here we see it still in plain looks which shows nice shadow effects on the smokebox door. Unfortunately that sort of detail is no longer noticeable when you add dark colours to the model.

More or less complete now including some first order surface detail on the tank and some paint defects, this time a view of the other side with the Westinghousepump

a real photo of the last survivor for comparison

to be continued...
date:25 april 2009