I've been doing some reading into spinning wheels and old-fashioned sewing machines and so far as I can tell, they use a system similar to a piston in an engine. The main difference being, the oscillation is coming from a treadle. It got me wondering about something though. With the way the linkage works, rotation to oscillation should work fine at any speed. That being said, couldn't oscillation to rotation be messed up if the oscillations are happening too slowly?
If the connecting rod doesn't pass the middle point (where the rod is angled at 0°), another oscillation might cause the connection rod to push backwards, reversing the rotation. Also, if the rod is angled at 0° (almost exactly), the linkage might jam because the oscillation isn't being directed one way or another? If that is the case, the crank/wheel would need to have enough momentum to pass that spot when the next oscillation happens?