LEGO Clock

By | January 15, 2011
LEGO Clock

LEGO Clock

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:

  1. Accurate, within a few seconds per hour.
  2. All of the key operating mechanisms should be visible at the front of the clock.
  3. Be able to wind the clock without interrupting the operation.
  4. Be able to change the time easily.
  5. The pendulum should have a period of 1 second.
  6. 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!


Close up of escapement

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.

Adjustable pendulum bob and drive weight

Adjustable pendulum bob and drive weight

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.

Clock internals

Clock internals

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 Rig

Rig for tuning the clock using audio

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.

Clock audio

Audio after 0, 1, 2, and 3 hours with thick string

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:

7 thoughts on “LEGO Clock

  1. Connor

    Can you get a better picture of the top of the pendulum on your LEGO clock? I'm drawing it up in LDD and it seems that long brick won't line up with the grey 1/2 cam piece at the top. It's wracking my brain D:

    1. Dillon Post author

      Hey Connor,

      Part of what might be tricky is there is a single one stud block near the weights on the pendulum, so the pendulum beam is a total of 43 studs long. From the top, I used the following beams: 16, 16, 1, 10.

      Btw, I'm very curious about your LDD work. Would you be willing to post the file when you are done? I will give you any pictures you need to do this!

      Have you tried to build the internals, or are you planning to do that? I can't imagine you'll be able to do it without more pictures...

      (I moved your comment to the LEGO Clock post)

  2. Justin Williams

    Really cool, I'd have never imagined that kind of accuracy would be possible with injection molded plastic...

  3. JulesVerne

    Could you make building instructions? I have been trying to find a good lego clock, and yours is the best one.

  4. Rob Torok

    I'd definitely be interested in building instructions (or at least enough photos to reproduce the clock). Have you, or anyone else, come up with any?

  5. Brendan

    Great! Just Great! I recommend this to anyone wasting their time in the virtual world on minecraft! 🙂

  6. Pingback: Clock Number 1 | Our Lego Blog

Leave a Reply

Your email address will not be published. Required fields are marked *