Practical LEGO Pendulum Clock

By | November 4, 2017
LEGO pendulum clock

LEGO pendulum clock

This is another version of my mechanical LEGO pendulum clock. This clock is truly practical: it runs for 35 hours, it is very easy to rewind, and it is accurate to within a few seconds per day, or just about 1 minute after running for 2 weeks straight. It uses a Galileo escapement for efficiency, although it is built in a different configuration to accommodate LEGO constraints, and to make it easy to see at the front of the clock. It is driven by a chain that can simply be pulled through the clock to rewind. The pendulum bob can be precisely positioned with a screw, allowing it to be very accurate.

Here is a video of the clock in action.

LEGO pendulum clock

LEGO pendulum clock

The basic operating principle behind a clock like this is to have a source of energy driving a high ratio drivetrain of gears, connected to the hands of a clock. One of the gears in the drivetrain is the escapement wheel, the rotation of which is regulated by pendulum. With a small swinging range, the period of the pendulum will be nearly constant, being only a function of the length of the pendulum. The escapement has an additional responsibility of giving the pendulum a small amount of energy with each swing to keep it going. The gears connecting the escapement to the hands then just need the appropriate ratios to show the correct time.

Escapement closeup

A closer look at the escapement geometry

This clock uses a number of techniques to meet the goals of being practical. The escapement is a Galileo escapement, with very precise adjustability to minimize wasted energy. Here is a slow motion video of the escapement. The escapement wheel is a 40 tooth gear. It would have been convenient (and probably more efficient) to make the pendulum have a period of 1.5 seconds (to give one revolution per minute of the escapement wheel), but such a pendulum is very long, I prefer the pendulum to have a period of one second.

Escapement closeup

Escapement release pallet and adjustment

The pendulum is suspended on a knife edge to minimize friction. This drastically reduces friction compared to mounting the pendulum on an axle.

Escapement closeup

Pendulum pallet and release counterweight

In addition to the pendulum suspension and efficient escapement, the rest of the clock is built to minimize friction. All axles pass through exactly two beams, with lots of extra bracing and support beams to minimize flex under the load of the drive weight. The minute hand has a counterweight to balance the force of gravity.

It was also helpful to select the individual parts used for the escapement wheel and pallets to ensure they were free of blemishes. There are at least two different molds for the 1x1 ramp pieces used as the pallets, one mold has the injection site very close to the top of the ramp, which adds a sticking point with the escapement wheel. The other mold has the injection site closer to the base, where it does not interfere with the escapement.

All of these efficiency features enable the clock to operate with the weight falling 2.59 cm/hour, using a roughly 600g weight. This is over 6 times more efficient than my previous clock! The weight is 11 LEGO boat weights at 53g each, plus a few parts to hold the weights together. The clock currently has enough chain to run for 35 hours; it is mounted high enough such that with more chain, it could run for 60 hours.

Adjustable Pendulum Bob

Pendulum Bob with adjustment screw, and the drive weight and chain

The pendulum bob is mounted on a screw to allow fine tuning the length of the pendulum; this enables the clock to be very accurate. This clock should be accurate to within a minute per day. Without a seconds hand, it will take quite some time to tune the pendulum bob weight position.

Clock drivetrain

Drive, differential, and ratchet mechanism for rewinding (back)

To make it easier to rewind, the clock is driven by a weight hanging on a chain (LEGO tank treads). To rewind the, simply pull the chain back through the clock. The chain drives a differential, with one side of the differential providing power to the clock, the other is a ratchet allowing the chain to be pulled back through the clock freely. The chain is a loop connected back to the bottom of the weight, to balance the weight of the chain as the weight falls.

If you want to see more LEGO clocks, check out Ben Van de Waal and KEvronista on YouTube. The idea for a knife edge pendulum suspension comes from their videos, and they have a lot more cool clock videos to see.

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