My name is Andrew Noyes and this is my first successful attempt at making a lego clock. It keeps accurate time within a few minutes per day. Standing 3'8" tall, it can operate for a little more than 24 hours before it needs to be wound. It is powered by a weight that falls at a rate of ~1.25 inches per hour. It requires no lubrication. I realize that my clock is not 100% lego, but the weight, the string, the clockface, and the stand are the only nonlego elements. The weight weighs approximately 5 pounds, the string was taken from a kite, the clockface is available at one of the links at the bottom of the page, and the stand is made of knex. The clock is shown with another clock for comparison.
This is the top-down view of the inner workings of my clock. The minute hand protrudes from the center of a turntable. The minute hand's axle has an 8 tooth gear connected with a 16 tooth gear. This axle has a second 8 to 16 tooth connection to a third axle, which has an 8 tooth gear connected to the 24 tooth gear inside of the turntable. That is how I concentrically mounted the minute hand and the hour hand. 2 * 2 * 3 = 12, yielding the desired 12:1 gear ratio necessary for a clock. My original design had a dark grey differential that you see towards the left. It is used so that the escapement does not have to be disengaged to wind the clock. I later tried a new mechanism to accomplish the same goal, but using less gears to reduce friction. Here is the new mechanism. The new mechanism proved unreliable, so i switched back to the differential.
My first breakthrough was the implementation of a compound pulley. The compound pulley allows a much heavier weight to be used, because the weight is spread over at multiple points, reducing the stress on any one lego piece. The compound pulley simulates the weight falling 8 times farther than it actually does (2^3). As a result, a gear ratio of 1:125 between the weight and the escapement is sufficient, greatly reducing friction. Through experimentation, I found that the friction in a gear ratio of 1:125 (5*5*5) is negligible, while the friction of anything more, because of the extra axle, is significant. Amnon Silverstein had the brilliant idea of using turntables as large gears(see the link below), which would allow for a gear ratio of 343:1 over 3 axles, but I only have 1 turntable. I used a gear ratio of 8 teeth(the escape wheel) to 3 5:1 ratios, to the power source, to a 2:5 ratio, to the minute hand, for a total gear ratio of 2500:1. This requires a a pendulum with a period of 1.44 seconds.(3600 seconds per hour divided by 2500 = 1.44)
This was my next big breakthrough. After several futile experiments with large, bulky, and unreliable escape wheels, I happened upon an ideal, small, and reliable escape wheel while visiting Eric Harshbarger's page. The small, 8 blade propeller that you see here and the two flagpoles mounted 5 holes apart seem to make an ideal escapement. This piece is somewhat difficult to come by. I had to order mine on ebay.
My final breakthrough concerns how the pendulum is attached to the clock. The standard method of suspending the pendulum on a peg has a significant amount of friction, limiting the pendulum to being small, imprecise, and with a fast period. By suspending the pendulum with two universal joints, I eliminated most of the friction, allowing for a long, reliable pendulum. A longer pendulum can be adjusted more precisely. Also, the design pictured can adjust the location of the two pallets (the flagpoles) precisely on two axes, allowing the escapement gear and the flagpoles to be oriented close to perfectly. When the periods between the ticks are uniform, the influence of the escapement on the pendulum can be minimized.
A view from the side.
A view from the bottom.
A view of the pendulum. The rate of the clock can be tuned by adjusting the length of the pendulum.
I am now implementing a longer pendulum to make my clock more accurate. The gear ratios have been adjusted accordingly. This also makes the clock more efficient, with the weight falling a smaller distance per hour.
A more detailed view of the weight. This is the power source for the clock.
I am now experimenting with a digital clockface. The middle dial rotates once per hour, exactly like a minute hand. The left dial has a gear ratio of 1:12 to the middle dial (2*2*3), and the right dial has a gear ratio of 6:1 to the middle dial (2*3).
Lego Clock Links
Clock Face
Amnon Silverstein's page
Eric Harshbarger's page
