Thing about servos, I think is that maximum angle of movement of each join is 90 degrees. Only way to get greater movement with one bit is something like coupling rods on loco, outside frame. Even then it is not an even movement in one direction. For those building their own mechanism, who are comfortable doing it, this is fine, but I wanted something that was easier to install, and everything is included. As far as I can tell the plastic is strong enough, and I can't see it getting worn out quickly. If that does happen, spares can be bought easily, like all 3D printing.
As I designed my system I realised it was probably more familiar to those designing locking mechanisms. The door locks on my French house have a double action, so to move bolt across you have to turn key twice. This is not much different to my design, just other way round. Each movement of the 'bolt' moves the 'key' through 90 degrees.
I am also looking to see if it could be used to operate small turntable, as my current design for that only allows 90 degree turn, which is enough for most things.
It was only when I showed my test model to friends that t was confirmed to be a simplified rack and pinion. Standard rack and pinions have lots of teeth, but not only would this be more complex to design, but tolerances/variations in 3D printing result in small sections being more difficult to create accurately. They would also be weaker, unless I made gear wheel of much larger diameter, which I did not.
The standard design is set to operating at 90 degrees or parallel to door. I can easier design versions of the gear with different angles.
I am also considering making a version that can fit a signal wire, so it could be used to operate signals easily. That would use the window/shutter type version as this has control arm rotated through 90 degrees. If the control arm is thought to be in xy plane, then the gear wheel is in yz plane. I have a test sample of the window mechanism, which I will photo.
Note the gear on right is actually in wrong position. On the new version, there is a cover so gear won't slip out of position so easily, and also the square sections which go through gear wheel will be longer. They are a lot stronger, yet still flexible enough.
I have separated out the mechanism part so it can be used on different window sizes. This will also form basis for the signal operating mechanism.
The shutter version sits below the window, and on outside edge, and the offset shutter spindle will move through 180 degrees. I had to increase width of window so there was enough room for control bars to move, having tries various options of one being in parallel plane to other, but there was always something in the way.
In theory, the mechanism should work other way round. rotating the gear should move the lever, but there is too much slack, so it would require some more design, possibly a bigger gear to give it more strength. Not impossible, but does need some thought.