A weight-driven clock can track the sun without electricity, but only on a polar-aligned axis, one parallel to the Earth’s axis, where a steady 15° an hour or one turn in 24 hours follows the sun; on an ordinary up-and-down, left-and-right mount the required rate keeps changing. A balanced dish needs about 6.3 J a day to turn, so a small weight is enough, but a gust can load the axis with about fifty times the clock’s torque, so the drive must be self-locking or the dish stowed in wind.
Part of Tracking Systems
A gravity-driven clock tracker turns a concentrator about a polar axis at the Earth’s rate using a falling weight and a clock escapement, with no electricity.
Why It Must Be A Polar Axis
The sun’s daily motion is a rotation of 15° an hour about the Earth’s axis. A mount whose main axis points at the celestial pole (tilted from the horizontal by the latitude, pointing true north in the northern hemisphere) follows the sun by turning at exactly that constant rate. Seasonal changes in the sun’s height then only need a second, slowly adjusted axis (Seasonal Angle Adjustment). On an altitude-azimuth mount (the usual dish mount), both axes must change speed through the day, which a simple clock cannot do. The parent page’s 15° an hour is correct for the polar axis only.
How Little Energy It Takes
Inputs, assumed: a balanced dish, so only axis friction resists, with a friction torque of 1.0 N·m; rate 15° an hour = 0.262 rad per hour.
- Work per hour = torque × angle = 1.0 N·m × 0.262 rad = 0.26 J.
- Work per 24 hours = 6.3 J.
- A weight of 20 N (a mass of 2.04 kg, since 20 ÷ 9.81 = 2.04) acting on a drum of radius 0.05 m gives a torque of 20 × 0.05 = 1.0 N·m.
- Energy 6.3 J ÷ 20 N = 0.31 m of fall in 24 hours at the drum, before any gear reduction.
So a weight of about 2 kg falling 0.3 m can drive a balanced dish for a day, in principle. The real difficulty is not energy but control.
What Defeats It: Wind And Timekeeping
Wind. For the 1.8 m dish of the conversion page, a 30 km/h wind puts about 160 N on the dish (Satellite Dish Conversion). With the centre of pressure 0.3 m off the axis (assumed), that is 48 N·m, about fifty times the 1.0 N·m a clock supplies. A clock cannot hold a dish against a gust: the drive needs a high gear ratio that cannot be back-driven, such as a worm gear (Gear Ratios), a lock, or you stow the dish edge-on in wind.
Rate. To keep the focus within 1 cm over a 4-hour run of 60° of travel with f = 0.6 m, the allowed pointing error is atan(0.01 ÷ 0.6) = 0.95°, which is 1.6% of 60°. So the clock must hold its rate within about 1.6%, which any decent pendulum or governor does easily.
Equation of time. The sun runs up to about 14 minutes behind the clock in February and about 16 minutes ahead in early November (recalled astronomical values). 16.4 minutes at 15° per hour is 4.1°, which moves a 0.6 m focus by 0.6 × tan 4.1° = 4.3 cm. Re-synchronise the drive to the sun every few days with a shadow check.
Gear ratio. A clock hour hand turns once in 12 hours; a polar axis turns once in 24. A 2:1 reduction from the hour-hand shaft gives the right rate.
What You Need
- A polar axis mount (axis tilted by your latitude, pointing true north in the northern hemisphere), strong and balanced about the axis
- A weight, drum and gear train, with a pendulum escapement or governor
- A worm drive or lock against wind
- A winding method and a reset to start each morning
Method: Set It Up
Step 1: Align the polar axis with the shadow stick method to true north (Shadow Stick) and tilt it to your latitude.
Step 2: Balance the dish about the axis so friction is the only load.
Step 3: Wind and set it each morning to the sun’s position, not the clock time.
Step 4: Check the focus every half hour and re-sync.
Common Mistakes
- Fitting a clock drive to an altitude-azimuth mount.
- An unbalanced dish. The weight must then also lift the dish.
- No wind lock.
Safety
A drive that runs on its own leaves a focus sweeping unattended
A tracker that works hides the hazard: the dish stays aimed all day with nobody near, so anything that wanders into the focus ignites or burns, and an eye that looks at the focus is permanently and painlessly damaged. Fence off the cone and focus, keep a cover ready and use it when the drive stops, and stow the dish in wind. The falling weight can drop; guard its fall.
What’s Next
- Learn next: Seasonal Angle Adjustment — the second axis
- Combine with: Gear Ratios — reduction trains