Interesting Facts About “Fast Beat” Movements…

Forum: TimeZone – Advanced Forum Date: Sat, 20 Dec 1997 08:24:09 GMT

From: Walt Odets

…some interesting facts:

  1. The beats per hour (BPH) are the number of half swings the balance wheel makes in an hour (i.e. from fully counter clockwise rotation to fully clockwise, or vice versa).
  2. The “standard” beat movement is considered to be 18,000 bph, or five beats per second. The true fast beat is 36,000 bph, or 10 beats per second.
  3. The advantage of a fast beat movement is that for a given percentage change in torque or power input, there is a small percentage change in balance amplitude (i.e. the “length” of the swing. For chronographs, fast beat movements allow the measure of smaller increments of time (at least theoretically).
  4. A fast beat balance dissipates more energy and thus requires more torque in the form of a thicker spring. The thicker spring, however, has less even torque output over its wind. This offsets some of the advantage of the fast beat.
  5. Favre-Leuba (working with a group of other manufacturers) experimented with a modified fast beat movement: the escape wheel had 21 teeth instead of 15; and the spring barrel had no cover allowing a taller (wider) spring. The balance was of reduced thickness (cross section), only 0.35mm. This watch provided accuracy comparable to an Accutron tuning fork watch. (Barrels with only a lower bearing and no upper bearing, with or without covers, are called “floating” barrels.)
  6. Testing 400 otherwise identical movements supplied by Girard Perregaux, Favre-Leuba obtained the following comparative figures on 21,600 and 36,000 bph movements (200 at each speed): Mean variation: 1.1 and 0.7 (21.6 and 36.0 respectively) Greatest Departure: 2.4 and 1.6 Positional Variation: 4.8 and 2.1  
  7. The superiority of the fast beat movements is attributed partly to a smaller drop in amplitude when moving the watch from an edge position (e.g. crown right) to dial up or down. In moving the watch from an edge position to dial up the following changes in amplitude were found. Fully wound: 33 and 20 degrees (21.6 and 36.0 respectively). 24 hours down: 26 and 17 degrees.
  8. In actual use on the wrist, error introduced by rapid arm movement (inertial forces) are lower in the fast beat movement.
  9. Favre-Leuba suggested that over two year tests, no abnormal wear was found in the fast beat movements. They suggested that as long as proper lubrication was maintained wear rates would remain comparable.

So, although a fast beat movement may not need more frequent servicing than a slower beat movement, it will be less tolerant of lubrication failure (due to aging, dissipation, etc.).