Earnshaw Escapement

 This is a demonstration model of the chronometer escapement modified in 1783 by Thomas Earnshaw with a spring detent. It eventually proved to be extremely accurate and for this reason became the escapement of choice for marine chronometers. 

Below is the general layout of a detent escapement.

The roller table rotates backwards and forwards under the influence of the balance wheel and hairspring. On going anticlockwise the release pallet impacts with the passing spring and lifts the detent arm and locking pallet. This releases the escape wheel which accelerates into the impulse pallet giving the rollertable/balance wheel a push. The detent arm falls back with the release pallet brushing past the passing spring but the locking pallet remains locked. Thus the balance wheel is thus given only one impulse per cycle. In between impulses the balance wheel is swinging  freely and has no effect on the escape wheel. It is known as a detached escapement. The unlocking of the pallet can only occur during impulse because at other times the locking face of the roller table guards against unwanted movement of the escape wheel such as may occur from a sudden jolt.

Below is the general layout of my model showing the balance wheel. hairspring and the first wheel in a drive train.

All parts are laser cut from 3mm plywood including the hairspring. The hairspring seems to work very well but could perhaps benefit for a coating of varnish. Sundry other parts are miniature bearings, 4mm steel rod for arbors and 5mm brass tubing.

The Wooden Model

The Wooden Hairspring

Close up of the locking Pallet

Close up of the release pallet and passing spring

The 5p coins in the balance wheel give mass at the rim of the wheel which reduces its oscillating frequency.

The video below shows the escapement in action. In this model it is powered by the tension in the rubber band on the right. A small electric motor (running at 2.5volt 20mA) keeps the band in tension. The drive is transferred to the escapement by a rubber band drive belt. Between them they absorb the intermittent action of the escapement.