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A gearbox for a little locomotive

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This is about a little gear box for a next project, another small gauge locomotive powered with a 4 st engine...

this gearbox looked quite simple, simple enough to be made very close to the original, and if the rail gauge is 45mm, the spur gears are between 38 & 15 mm, easy to cut with a module between 0.5 and 0.75.

it is something that I thought to be fairly simple, but I underestimated the loss of gray cells when getting older,
and I can't get the succession of gear wheels involved in each position of the 2 selector forks...
how to obtain low and high gear together with for and backward run, as moving either of the lever leads to engage the clutch ?
between idle and leading wheels, i'm out of touch in fact !

Roger B:
As I see it the engine drives the large gear on the first shaft which then drives the small gear on the second shaft. The shafts will then turn in opposite directions at different speeds.
The speed is selected by engaging one of the shafts with one of the equal size gears in the middle. These are free running on the shafts.
The direction is then selected by engaging one of the smaller pinions with one of the equal size gears (which will be turning in opposite directions) which engage with the large gear below.

I think that I have seen a model similar to this in Maschinen im Modellbau  :thinking:

I see Roger just beat me to the answer.

The important thing to understand is that the two equal sized gears in line with the center crank line are freewheeling on the shafts (like the two smaller ones just below), until one of them are locked to the shaft with the two moving gearless wheels on top of the draving (connected to the High/Low selector handle). And the shafts are locked to the gear wheels on the bottom.

Funny - I can't find any clutch on the drawing.


Roger B:
I believe that the clutches are the fast and slow gear selectors. I have driven some German narrow gauge locomotives with this system. In the cab picture the hand wheel operates a screw mechanism to engage either clutch. The hand lever in the gear diagram is not correct.

Roger B:
This was the build I was think of:

If I remember correctly the ignition System was in the log wagon.


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