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Balance Wheel

Made of

Metal (General) Steel Brass Spring Steel

Brief description

The balance wheel is the timekeeping element used in portable mechanical timepieces, such as watches and marine chronometers. It acts as a harmonic oscillator, swinging back and forth, analogous to a pendulum in a stationary clock.

Use / Function

  • Portable Timekeeping: Allows clocks to function correctly while moving, tilted, or being carried (unlike pendulums which require a fixed, level position).
  • Regulation: Determines the accuracy of the watch by oscillating at a constant frequency.

Operating principle

The balance wheel works in conjunction with a hairspring (balance spring).

  1. Oscillation: The wheel rotates in one direction, winding the hairspring.
  2. Restoring Force: The tension in the spring stops the wheel and pushes it back in the opposite direction.
  3. Inertia: The wheel’s momentum carries it past the resting point, winding the spring in the other direction.
  4. Isochronism: Ideally, the time it takes to complete one swing is constant, regardless of how far it swings (amplitude).

How to create it

  1. Wheel: Fabricate a lightweight, perfectly balanced wheel. The rim is often heavier to increase inertia.
  2. Hairspring: Coil a very fine strip of spring steel or bronze into a spiral. Attach the inner end to the wheel’s axis (collet) and the outer end to a fixed stud.
  3. Poising: Test the wheel on knife-edges. Remove material from heavy spots to ensure gravity does not affect its rate when the watch is in different positions.

Materials needed

  • Wheel: Brass or Steel.
  • Hairspring: Spring Steel, Phosphor Bronze, or specialized alloys resistant to temperature changes.
  • Pivots: Hardened Steel running in Ruby bearings.

Variants and improvements

  • Bimetallic Balance: The rim is made of two metals (brass and steel) fused together. It curls as temperature changes to compensate for the hairspring’s changing elasticity.
  • Glucydur Balance: Made of a beryllium-bronze alloy that is hard, non-magnetic, and stable.
  • Free Sprung Balance: Regulated by adjusting inertia weights on the rim rather than changing the effective length of the spring.

Limits and risks

  • Temperature: Heat makes the spring softer and the wheel larger, slowing the watch. Compensation is required for high accuracy.
  • Magnetism: Steel hairsprings can become magnetized, causing the coils to stick together and the watch to run wildly fast.
  • Shock: The pivots are extremely thin (like a hair) and can break if dropped. Shock protection systems (spring-loaded jewels) are needed.