Weight-Driven Clock Drop Time Calculator

Weight-Driven Clock Run Time

A weight-driven clock runs as long as the weight has further to fall. The run time depends on:

1. Drop distance: how far the weight can travel
2. Gear ratio: how slowly the weight is paid out by the going train
3. Pulley arrangement: whether the cord is single-strung or doubled (compound)

The basic formula: Run hours = (Drop distance × Gear reduction) / (Pulley factor × cord-out per hour)

Or more practically: Run hours ≈ Drop in inches × hours-per-inch-rate

Typical hours per inch of cord drop (single-strung):

• 30-hour movement: 1.5-2 hours per inch (rapid drop)
• 8-day movement: 5-7 hours per inch (slow drop)
• Month-running movement: 18-22 hours per inch
• Year-clock movement: 200+ hours per inch

Pulley factor:

• Direct cord (no pulley): 1.0×
• Single pulley (weight on a hook): 2.0× longer run for same drop
• Compound pulley setup: 3.0×+

So an 8-day clock with a 24-inch drop and a single pulley: 24 in × 6 hours/in × 2 = 288 hours = 12 days of run time per wind.

Practical considerations:

• The weight must be fully wound to top before counting drop
• Most movements stop with 1-2 inches of cord still wound (safety reserve)
• Friction losses reduce run time 5-10% from theoretical
• Heavier weights don’t extend run time: they only ensure adequate driving force

Common case lengths:

• Wall cuckoo: 30-40 inch drop → 24-30 hour run
• Mantel / shelf clock: 15-20 inch drop → 8-day run with proper pulleys
• Grandfather (longcase): 60-72 inch drop → 8-day run direct, monthly with pulley

If your clock stops short of expected duration, check: weight properly wound to top, no friction drag in the cord/chain, no slipping at the click ratchet.