Escapements

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W A T C H E S C A P E M E N T S

Words in italics (which should also display in red) are defined in more detail in the Glossary.

An escapement is that part of a clock or watch movement which converts the raw power of the driving force (mainspring, weight or electric motor) into regular and uniform impulses.

Most escapements are variations on the theme of an arm or shaft carrying two tongues, called pallets, which alternately engage with the teeth of a wheel (the escape-wheel). The pallets are moved to and fro by the oscillating medium, or controller (balance-wheel or pendulum); the escape-wheel is part of the train of wheels and pinions driven by the spring, weight or motor. In each cycle of the controller there is always a moment when both pallets are out of contact with the escape-wheel; it is only then that the wheel itself is free to turn. As it does so it again comes up against one of the pallets, to which it ‘imparts impulse’ (in plain language, gives a push); this constant renewal of impulse is necessary to keep the controller from drifting to a halt. The pallets alternate between providing impulse and performing a locking function (stopping the train until the swing of the controller brings round the next period of release). The ideal of an escapement is to provide this alternation of impulse and locking whilst interfering as little as possible with the free movement of the controller.

Some sophisticated escapements do away with one or both pallets, using instead special configurations of the balance-wheel or its staff, or the escape-wheel, or both.

V E R G E

The verge is the oldest known form of escapement, undoubtedly in use by 1350 and probably invented in Italy in the previous century. It was initially made of iron and used for large church-tower clocks, but the arrangement was virtually identical to that of the watch escapement shown in this side view between the plates of a full-plate watch. The name is derived from the Latin virga, a rod or staff.

The balance-wheel (A - shown without the hairspring) is about to start its anti-clockwise swing. Its staff or arbor carries two pallets, B and C. Pallet B is engaged with a tooth of the escape-wheel D (sometimes called crown-wheel because of its shape) and is receiving impulse from it. As the balance-wheel turns, pallet B will disengage from the crown-wheel, which will then turn until pallet C (currently almost hidden behind the balance-staff) comes up against a tooth at the bottom of the wheel. By this time the balance-wheel will be ready to swing back and the process will begin again on its return journey. The crown-wheel arbor, or verge-staff, carries a pinion (E) which engages with the contrate-wheel F, the fourth wheel of the train; this wheel can be seen on the edge of the movement and provides an easy way to identify a verge watch. G is a brass block called a potence (shown partially cut away), screwed to the underside of the plate H and drilled for the pivots of the balance-staff and verge-staff.

The verge is usually considered a crude device because of the large amount of friction between pallets and wheel; yet makers like Tompion achieved excellent results from it. John Harrison used a version of it in his last two chronometers. It was the only escapement in general use for more than 300 years and survived in cheap Swiss watches until the 1880s.

C Y L I N D E R

The cylinder – also called the ‘horizontal escapement’ because the escape-wheel lay parallel to the watch plates instead of standing upright between them – was the first serious rival to the verge. George Graham developed it in the 1720s from a prototype by Thomas Tompion. It has no pallets. The cylinder itself, a cut-away drum initially made of steel (which wore particularly badly) and later of ruby, forms part of the balance-staff. Wedge-shaped teeth, mounted on posts which stand around the perimeter of the escape-wheel, successively engage first with its outside and then with its inside face, delivering impulse on entering the cylinder and again on leaving it; their shape is calculated to deliver the thrust in exactly the right direction.

In the illustration (viewed at an angle from above), the balance-wheel – shown cut away – is not quite half-way through its anti-clockwise swing. Tooth A has enjoyed the first of its two moments of freedom whilst travelling across the interior of the cylinder C; it is now held against the inner wall, but presently, as the balance continues to turn, it will escape past the end of that wall, delivering an impulse as it does so, and there will be a second period of free movement, ending when tooth B comes up against the outside face of the cylinder and the sequence begins again.

The cylinder (in steel form) was particularly popular in 19th-century Swiss watchmaking; it was liked for its compactness and quiet running, both arising from the fact that it had few moving parts. It was, however, prone to wear and especially vulnerable to poor lubrication.

L E V E R (English, or Side)

The first lever watch was made by Thomas Mudge in 1759. Oddly enough he thought little of his invention, and it was not followed up until Josiah Emery applied it in the 1780s. Emery used an ‘anchor’ layout like that of a modern mechanical watch, but later English makers (from about 1810) turned to the ‘side’ layout seen here, with the pallet-carrier (shown in white and dotted) mounted beneath the lever and in line with it. This layout was used throughout the 19th century, the only change being that the lever grew slimmer, with indented sides, and the prongs of the fork were splayed outwards.

A is the lever itself, pivoted centrally; B and C. in red, are the pallets (usually jewelled). D is the ruby impulse-pin, mounted on the roller E (a steel disc with a cut-out section on the side nearest the pin) which is a push-fit on the balance-staff. Just now pallet B is holding a tooth of the escape-wheel F. As the balance-wheel (G) turns clockwise, the impulse-pin will push on the left-hand tine of the fork at the inner end of the lever; the outer end will therefore swing to the right, allowing the tooth to escape past the end of B to which it will impart impulse as it goes. The period of free movement or ‘drop’ will end when another tooth comes up against pallet C which now lies in its path. H indicates the banking-pins, soldered to the bottom plate. I is a steel pin on top of the lever, engaging in the cut-away sector of the roller.

R A C K L E V E R

Originally devised by the 75-year-old Abbé Jean Hautefeuille in 1722, the rack-lever was revived and patented by Peter Litherland of Liverpool in 1791 and was largely exploited by his fellow-townsmen (notably the firms of Tobias and Roskell as well as Litherland's own company) between then and 1825, after which it rather abruptly disappeared.

In layout the rack-lever closely resembles the English detached lever (above), which indeed it probably influenced. At E the roller is replaced on the balance-staff by a steel pinion of eight leaves; this engages with a curved quadrant, the rack itself, on the inner end of the lever. The pallet-arm is mounted close to the outer end of the lever A, allowing the rack to swing over a slightly wider arc. The lever has a crescent-shaped counterweight at its outer end to balance the weight of the rack. The sequence of operation is exactly the same as in the detached lever.

The gearing between rack and pinion meant that the balance was given a very wide swing and its momentum was correspondingly high – high enough to overcome the friction between these components which might have been expected to result in very inefficient running. In fact, as Britten observes, the rack-lever worked much better than in theory it had any right to do.

D U P L E X Animated image: please be patient

The duplex escapement is so called because the escape-wheel has two sets of teeth, long radial ones for locking and short upright (contrate) ones for impulse. Its invention is attributed to Pierre le Roy (c. 1750). It is based on the cylinder, but the ruby drum is now almost completely closed, with only a narrow slit. A long tooth enters this as the balance nears the end of its clockwise swing. The balance changes direction, the long tooth escapes (giving a tiny impulse as it goes) and immediately afterwards a short tooth comes up against a steel comma-shaped cam mounted above the cylinder on the balance-staff. This contact between short tooth and cam is the main source of impulse. The balance continues anti-clockwise, releasing the short tooth, and the escape-wheel leaps forward until the next long tooth meets the outside surface of the cylinder; there it is held motionless until the slit comes round to meet it and the cycle begins again.

Despite its origin, the duplex found favour chiefly in England, where it is found in high-class watches between about 1790 and 1860. Its low level of friction made it a great improvement on the verge, but rapid wear on the tiny ruby cylinder eventually caused it to lose ground in favour of the English lever.

The escapement is seen as if from the back of the watch. The balance-wheel (shown in outline) would be closer to the eye than the other components and separated from them by the back plate. The hairspring (under the balance) is omitted.

The best general guide to watch escapements known to me is Escapement - Available in the Market by Frederick Choi (Hong Kong, published by the author, 2003). The author, a Chinese collector, gives brief descriptions of 31 different types of escapement, both familiar and obscure, with diagrams and (in all but a few cases) very detailed colour photographs of the escapements and the watches in which they appear. The book also includes brief details and portraits of 58 notable watchmakers, inventors and theorists. The book is available from the author ( frederickchoi@hotmail.com or [website] http://www.escapement.info ).