When I was at my parents for Christmas, we were visiting one of their neighbors who owned an old style mechanical clock. It was simple, a small box with the clock face, and the pendulum and weights coming out the bottom of the clock. It got me thinking that I could build one with the same principles in LEGO Technic. Here are the goals I had for the clock, in priority order:
- Accurate, within a few seconds per hour.
- All of the key operating mechanisms should be visible at the front of the clock.
- Be able to wind the clock without interrupting the operation.
- Be able to change the time easily.
- The pendulum should have a period of 1 second.
- It should run for at least one hour.
After a few weeks of tinkering, I have met all the above goals, and have the clock running for 4 hours!
The heart of any mechanically driven clock is the escapement. This is the part of a clock mechanism that makes the 'tick tock' sound. The escapement is responsible for allowing a particular amount of rotation per swing of the pendulum, and is also responsible for giving the pendulum a slight push to overcome the losses in energy from friction. The pendulum regulates how quickly the escapement is allowed to turn, by exploiting the fact that the period of a pendulum for small swings is a constant amount of time. With the axle of the escapement spinning at a fixed speed, the rest of the clock is just simple gear ratios to drive the hands.
The clock is designed to be perfectly accurate if the pendulum has a period of exactly one second. To get the pendulum to have a period of one second with good accuracy, the adjustment screw on the pendulum moves the weight consistently and precisely along the pendulum. It is very similar to the adjustable bobs on real pendulum clocks.
Two other mechanisms are important for realistic operation: the ratchet for winding, and the switch to set the time. In order to be able to wind the clock while the clock is running, I used a differential and a ratchet to prevent the winding wheel from being turned the wrong way, either by the drive weight or by hand. The differential allows the force of the drive weight to be transmitted to the escapement even while the clock is being wound up. The switch for setting the time is built using the gear shifting parts, where putting it in neutral disconnects the escapement from the hands.
Tuning the clock is when this project turned really nerdy. In order to get precise timing measurements from the clock, I rigged up a headset on the clock so I could record the sound the escapement was making. This way I could see whether the escapement needed balancing (the tick-tocks were evenly spaced) and whether it was running fast or not (the weight was in the right place on the pendulum).
The major problem the rig indicated is just how important it is for the drive weight to be exactly the amount of weight required to keep the clock going and no more. If it was too heavy, the escapement would push the pendulum too much and it wouldn't behave like an ideal pendulum. I knew this from earlier experimenting with the clock, but I did not expect it to be as sensitive to the drive weight as it is.
Unfortunately, the string I was using was quite thick. This might seem innocuous enough, but when the clock was fully wound up, the string was piling up on itself on the spool, increasing the effective radius. This meant that when the clock was fully wound up, the weight was pulling harder on the drivetrain than when it was after the spool unwound for a bit. The effects of this were significant, if I tuned the pendulum to have exactly 60 seconds per minute when the clock was fully wound up, by the time the clock unwound it was running at 59.5 seconds per minute, almost 1% different! To solve this problem I switched the string out for some thin kite string, which can wind the weight all the way up with only 2 full layers on the spool.
After tuning and tweaking, here are the final stats for the clock:
- Accurate to +/- 2 seconds per hour
- Runs for over 4 hours
- Efficiency of .57 hr/(ft·lb)
- 1 lb drive weight
Finally, here is a video of the clock in action:
Here are some other LEGO clocks on the web: