I've done some work using openSCAD to replicate the gearbox used on the Greenly starter loco.
The initial brief I gave myself is to create a clone of the gearbox, not necessarily identical but close enough that it can be used as a replacement in the loco.
The aim was to allow myself to experiment with different gear profiles and ratios, primarily the use of herringbone gears rather than the straight spur gears used in the original.
As herring bone gears of the same modulus as the original are slightly larger than straight spur gears, the idler gear shaft has to be offset from the line between the crown wheel and the output axle shaft. This is so that the relative position of the motor and axle shaft can be kept the same.
The original gearbox uses modulus 1 gears between the motor and crown gear, and modulus 0.75 between the other gears.
I'm aiming the provide 3 different gearboxes initially:
- A clone of the original
- As the original but using herringbone gears and Zerol gears for the gearbox and crown wheel.
- As above but using mod 0.5 gears for the herringbone gears.
Other's may follow.
All use the same shafts as the original. The original crown wheel called for a rz73 bearing. In many of my implementations I've replaced this with a 3 x 8 x 3.5 thrust bearing. I got these from Amazon:
https://www.amazon.co.uk/dp/B0FD2ZYG3Y
At first it may seem counter intuitive but the part with the larger hole fits into the crown wheel and the part with the smaller hole bears on the frame which is fixed to the shaft.
Please let me know if there are any problems with the fit of any of the gears.
These are still experimental so are subject to change and I can't guarantee they will work or fit properly.
All were printed on a Qidi Q1 pro using 0.2 layers PETG
Folder : Clone of the original gearbox
This folder contains the gears and gearbox for the clone of the original gearbox. It uses straight spur gears in a straight line.
3D Model
3D Model
3D Model
3D Model
3D Model
3D Model
3D Model
Folder : As original but using zerol and herringbone gears instead
This collection uses the same gear modulus as the original. As the spiral of the herringbone hears changes to effective pitch diameter of the gears, the idler gear shaft needs to be moved from in-line with the crown wheel and output axle.
Coming soon...
Folder : Herringbone gear version using mod 0.5
This version of the gearbox uses herringbone gears for the "plain" gears and zerol gears for the crown wheel.
The zerol crown gear has been left at mod 1 and the other gears have been reduced to mod 0.5. To get a similar size of gear, the gear ratios have been adjusted to compensate.
Original New
10 -> 15
15 -> 22
10 -> 15
25 -> 37
As the herringbone gears effectively self centre against each other the gear widths need to ensure that there is not overlap of the next gear that interferers with a larger gear. i.e. the output gear width needs to be narrower than the small gear on the idler gear and the large gear on the idler wheel needs to be narrow than the herringbone gear on the crown wheel.
When the gear ratios and types are changed the location of the idler gear shaft will move so as to preserve the location of the crown wheel and the output axle.
The magnet collar from the standard gearbox can be used with this gearbox.
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