Grandfather Clock

Functional Requirements

Clocks generally need the following to function

1. An oscillator to generate a consistent time base.

2. A power source to sustain operation.

3. A display that presents the time to the user.

4. A mechanism that ties everything together.

Since this is my first design, I set a general goal of matching the performance of a mass-produced household grandfather clock. Those generally use a pendulum as an oscillator, a weight (reset weekly by a human) as a power source, clock hands moving about a face as a display, and a wheel train to tie everything together. 

1. Pendulum

This project started out with designing the pendulum (or perhaps you could say designing the pendulum started this project). On the right is the derivation of the equation of motion. However, this math negates pivot friction and air resistance. For the equation to accurately model the system, I had to ensure both are minimal. To negate pivot friction, I ended up just using a ceramic bearing for V0. Ball bearings aren't anywhere near ideal for a pendulum pivot but I decided to go the easier route and will revisit this later. For air resistance, the target frequency can be lowered as air resistance squares with velocity and the pendulum body was designed to be airfoil-like. I decided to design for a ~1-meter pendulum with a 2-second period. 

I ended up deciding to make it out of a carved 2x4 for simplicity. To lower the pendulum's frequency I needed to increase its moment of inertia by placing weight further from the pivot. I chose to accomplish this by drilling two 1/2" holes around 12" deep (the longest I could find a bit for). In those channels inexpensive steel round bar can be inserted to act as a static weight. Clock pendulums also have to be adjustable to account for environmental factors. Being that I was uncertain of how spot on I would be with a design I designed the pendulum to accommodate a large weight on a lead screw allowing for a lot of after-the-fact adjustment. 

Spreadsheet that converts design parameters into resulting characterisitics.

2. Weight

To allow the weight to interface with the clock I chose a chain and designed a ratcheting sprocket. I decided not to set a target mass for the weight and instead set it experimentally once the clock is built due to the number of unknowns.

3. Display

For the face, I wanted to do something interesting. Taking inspiration from Against the Run a clock in Kendal Square where the entire clock rotates in its housing as the hour ticks I decided to do something similar. For this clock rather than hands moving to tell the time the rings of numbers for the hours and minutes rotate instead with a space at the bottom where one can read the time. This is similar to how the date window would work on a traditional watch except everything is in plane view. In addition, the rings are clear allowing you to see into the mechanism. A piece of metal on the bottom highlights the current time.