Do crank-sliders require some minimum amount of momentum to work in linear-to-rotary convertions?

Question

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?

Answer 1

I've actually sat at a treadle sewing machine and made seams.

Yes, if they're going too slow they stop. They have flywheels that lower that speed, but it's still there.

Note that with a treadle machine you rock the treadle, which can both push and pull on the connecting rod. With some pedal-operated machines (such as the spinning wheels I've seen) you only push on the pedal, and the connecting rod only pulls. That works on a spinning wheel because you generally want the wheel going at a constant and fairly rapid rate. With a sewing machine, you need finer control of the speed, and you want to be able to start at (almost) all positions and to go slower for delicate work.

With a treadle sewing machine, when you're ready to sew you use the hand-wheel on the machine to get it started (and over the top), then use the treadle to continue. Starting it by hand means that you can feel which way you need to rock the treadle to continue.

At least for the electric sewing machines that were in use through the 1970's, you did the same thing: the motors are generally poor at going slowly, and if you're doing delicate work you want direct control over the thing anyway -- so you use the wheel to get things started right, then the motor once you're sure you're ready to go.