Lever vs. Detent Escapement

[Andy Dervan]

Someone mentioned that a true chronometer should have either a fusee or detent escapement.

I checked DeCarle's Enclopedia at a detent chronometer escapement, and it looked like a circle with one tooth that contacted the escape wheel.

Two questions - could someone explains how a detent vs. lever escapement works and why is it more accurate than a lever escapement?

Thank You.... Andy Dervan

[Andy Dervan]

Someone mentioned that a true chronometer should have either a fusee or detent escapement.

I checked DeCarle's Enclopedia at a detent chronometer escapement, and it looked like a circle with one tooth that contacted the escape wheel.

Two questions - could someone explains how a detent vs. lever escapement works and why is it more accurate than a lever escapement?

Thank You.... Andy Dervan

[doug sinclair]

Andy,

Check out the following site: http://www.daniels-london.co.uk/escapements.html

Then if you have any questions, let us know. A picture is worth a thousand words.

Regards,
Doug S.

PS A thankyou to whoever it was that made us aware of the above some weeks ago. It is great!

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[John Cote]

Andy,

One thing that you don't get from the pictures is that the detent escapement does not require oil whereas the lever does. As the viscosity of the oil changes, or as dust gets in the oil etc. etc. the drag between the escape wheel and the pallet jewels changes and the motion...timing of the watch changes. This is especially true at sea, in the salt air.

John Cote

[Tom McIntyre]

The pictures may not make it obvious that the lever has a relatively long engagement as compared to the chronometer. i.e. the pallets stay on the escape wheel and the fork stays on the roller jewel during the impulse period. In the chronometer, it is a true impulse with essentially a blow being delivered by the escape wheel to the impulse jewel.

Essentially all current chronometers have Earnshaw's form of the escapement. In the Arnold form, the escape wheel teeth are curved and the impules is slightly longer. The Arnold form requires light oil on the teeth of the escape wheel. Arnold's has the detent spring in tension and Earnshaw's has the detent spring in compression.

These two escapement designs are a wonderful example of the "cut and try" approach vs. the "analytical approach." Arnold's design was analytical and allowed for contingencies that never really arose in practice. Earnshaw's design was the simplest possible arrangement and actually worked just fine even though it might theoretically have had problems with the detent buckling and impact damage from the sharp blow between the escape wheel and the impulse jewel.

The second problem actually showed up in practice, but was alleviated by a small change in the impulse angle.

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Tom McIntyre
President, NAWCC Chapter 174
Pocket Horology Web (http://www.pocketwatch.org)
Tommy the JOAT's Web (http://www.mcintyre.com/McIntyre)