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Accurate Clock Pendulums$
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Robert J. Matthys

Print publication date: 2004

Print ISBN-13: 9780198529712

Published to Oxford Scholarship Online: January 2010

DOI: 10.1093/acprof:oso/9780198529712.001.0001

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Variations on a Riefler bob shape

Variations on a Riefler bob shape

(p.79) Chapter 12 Variations on a Riefler bob shape
Accurate Clock Pendulums

Robert James Matthys

Oxford University Press

Riefler's bob shape consists of two truncated cones, back-to-back. It is a shape with low air drag, and works well inside the clock case, where front-to-back space is frequently limited. A 1988 study on the air drag losses of various bob shapes found that the football and oblate spheroid shapes had even less drag than Riefler's two truncated cones. However, they require more front-to-back space, assuming equal volume, and are harder to make. Reducing a bob's air drag is worthwhile for two reasons. First, it reduces the weight needed to drive the escapement's gear train, which in turn reduces the wear on the gearing and the escapement's pallets. Second, the less you disturb a pendulum, the more accurate it becomes. For the lowest air drag, Riefler bobs should have an edge angle of 50-75 degrees and a diameter-to-thickness ratio of 2. Such bobs need 2% less drive force than a sphere of equal volume. This chapter also includes information on bob aerodynamics.

Keywords:   Riefler bobs, bob shape, edge angle, air drag, pendulum, aerodynamics, gearing, escapement

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