[MOC] Auto-rewind Clock

2GodBDGlory · January 29, 2022

This is a fairly simple Technic clock I built recently for the challenge of it. For those of you who are familiar with Lego clocks, most of its features will be familiar, except for the auto-rewind, which used a mechanism I hadn't seen before.

Features:

Auto-rewind
Hour hand
Minute hand
Second hand

The basic clock mechanism was based on a long one-second period pendulum, which operated an escapement I copied from this video:

after which I modified it to increase reliability. Mine looked like this:

Because it works on a 40T gear, some gearing was needed to get the correct motion of the second hand, so I did a 16:24 gear ratio using a chain to make one tick of one second equal one sixtieth of a rotation. The second hand was driven by its axle, the minute hand was driven by a differential housing, and the hour hand was driven by a 56T turntable. The weight used was a disposable water bottle, since water has the two crucial characteristics of being cheap and heavy. The weight ran a winch through the spider gears of a differential to run the clock, while a PF L-motor was hooked up to the carrier gear of the differential to allow for auto-rewind. Originally I had planned on using a PU L-motor and a Pybricks program to set the program to rewind the weight every x minutes, but this wasn't a very elegant or accurate solution, so I looked into some other things. The solution I set on involved having a PF switch on the side of the clock and a string tied from it to the weight. This meant that the switch got pulled on by the weight whenever the string became taut, causing the rewind to start. I then needed a way to shut off the motor at the right time, which was done using another string which went down from the weight, around a pulley, back up above the switch, around another pulley, and then downwards to the switch. This caused the string to become taut and pull the switch back up when it reached the top of its travel. The clock's biggest failings were in accuracy. I had a small linear actuator in the pendulum to adjust it's period minutely, but I couldn't get it to give me accurate results. Additionally, I found that while the motor was winding up the weight the pendulum would start swinging faster, throwing off the time. At the end of the day, it doesn't really affect me because I took the clock apart almost immediately (I needed my 11x15 frames to build a copy of the Sian), but it was still disappointing.

Here's some extra pictures. I'm pretty tight for space in my dorm room, so this was about the only place I could have it set up, resulting in a rather messy background for the pictures.