I worked out that I roughly need a 10:1 gearing, next I need to work out options for the Burrell 10:1 gearing. The gears are going to be 0.5 module with a tooth height of 1.1mm that would be 22mm at full scale.
After some browsing I found a set of 0.5 module gear cutters at a reasonable price. The only thing that I can see strange is they appear to be numbered back to front. Hence N8 is for 12-13 teeth. The hole is 13mm in diameter and so I need to make a mandrel for them.
The overall gearing will be achieved in two stages with the intermediate gear having the differential included. This means that this intermediate gear needs to sit on a layshaft that traverses the cabin and provides drive independently to both wheels.
The location of this layshaft is tricky as there are a lot of items that could be in it’s path:
- shaft needs an outer cover and clearance
- firebox door needs to clear the shaft cover
- steering shaft needs to clear the layshaft
- position needs to also match ~10:1 overall ratio
- gears on drive wheels cannot be more that ~65mm in diameter
Quite a few requirements here, this is system engineering and I’m solving it the old fashioned way. I’m not using CAD. As a starting point I need a rough location and some dimensions.
This gives me some idea as to the centre to centre distance for each stage of the gearing.
OK, not quite so old fashioned. I used a simple excel sheet to allow me to look at options for this gearing.
This calculation is included in the workshop calculations, a free excel workbook download.
This shows that I have a stage 1 with 27 teeth on the crankshaft and 96 teeth gear on the layshaft. Then stage 2 has 41 teeth on the layshaft and 117 teeth on the drive wheels. An important point to check now is whether you can index for these gears. An easy thing to miss.
Locating the Layshaft
I’ve only made a few drawings of this engine, however, this elevation drawing allowed me to check the gear meshing and centre-to-centre distances for each stage. The arcs show the crossing point and hence location for the layshaft.
The stage 1 arc is 30.75mm = 6.75 + 24mm, this is drawn with the compass centred on the crankshaft. The stage 2 arc is 39.5mm = 10.25 + 29.25mm and is drawn from the centre of the rear wheels.
Where the two arcs cross is where the layshaft is centred to engage all of the gears.
Now this is assuming the gears are perfect at the pitch circle radius, in reality they will want a tiny amount of play. This will be small and my plan is to mark the location of the layshaft once I have made the gears and know exactly how they mesh.
I now have the basic system for a 10.15:1 gear set.