# Gear train with fixed average speed but irregular instantaneous speed

Is it possible to create a gear train whose average speed is fixed, but whose speed constantly fluctuates? I'm imagining something that would ideally always complete a single revolution in fixed time, but move irregularly along the way.

My interest in this problem is from the writings of Terry Pratchett, whose character Lord Vetinari has a clock which ticks irregularly, but keeps correct time. Numerous versions of this clock have been made using electronic control, but I'm interested in the mechanical possibilities of it.

• See youtube.com/watch?v=WYcqJ5HdxA4 for some ideas. Sep 19, 2019 at 21:31
• Irregular ticking but ultimately keeping the correct time is a different problem. Eccentric gears are certainly a thing, but driving an escapement with eccentric gears wouldn't do what you wanted. OTOH, if you could build an escapement that only "ticks" (and takes energy from the clockspring) when the pendulum swing has diminished below a certain point, then you could use that in conjunction with irregular gears driving the escapement to get irregular ticking. And in centuries of clock-making, such escapements have probably been made. Sep 20, 2019 at 0:11
• And note that it wouldn't keep as good of time as a regularly-ticking clock: a driven pendulum is oh-so-slightly faster than an undriven one. Having an irregular impetus to the pendulum can't be good for accuracy. Sep 20, 2019 at 0:13
• This is literally the Corpus Clock in Cambridge, UK. The mechanical clock is only accurate every five minutes (by design). In between, it erratically goes faster or slower: "this erratic motion reflects life's 'irregularity'". Maybe there's a description of that clock somewhere.
– Wasabi
Sep 20, 2019 at 2:04