Discontinuous Drive Mechanism

By Steve Baker
I've been trying to build a mechanism that takes a continuous rotational input on one axle and generates a discontinuous rotation on an output axle - such that the output stays still for some large percentage of the time - then moves rapidly over the short remaining time.

I asked a lot of people - without success - but in the end, the answer was quite simple:

You rotate one of the two 40t gears which drives a little 8t gear (it's hard to see behind the red beam in my photo) - which in turn drives the other 40t gear. So, the red beam stays horizontal as it travels around in a circular path. Every revolution of the beam rotates the beige 8t gear at the bottom by exactly one tooth - or an eighth of a revolution.

So, for every revolution of the input shaft, the output will rotate for an eighth of a turn and remain stationary for the remaining seven eighths.

If you move the rack and the 4x1 plate one stud to the left, the output will rotate for a quarter of a revolution and stay still for the remaining three quarters.

If you move the rack two studs to the left, it will occasionally jam the mechanism - so don't do that.

The only remaining problem I have is that the output shaft is not held still during it's stationary periods. Hence if you drive this mechanism too fast or if there isn't much friction on the output shaft, it will rotate a little more (typically one tooth more) with each revolution.

Because of this, it's not really precise in 'counting' the number of input revolutions - which is a shame because that's what I need.