Straight edge conservation: A chronometer escape wheel

By JC Li, MA Conservation Studies student, specialising in Clocks

This 1830s English carriage clock by Arnold & Dent has had issues with one of its most crucial components: The escape wheel. As power runs from the mainspring through to the train of wheels, the escape wheel is the final step before reaching the balance. Each oscillation of the balance releases power from the escape wheel one tooth at a time, which in turn impulses the balance to maintain consistent oscillations. The correct operation of this mechanism – called the escapement – is paramount to good timekeeping.  

Figure 1: The movement in its case

This particular type of escapement is known as the ‘spring detent’ and was originally developed for precision timekeeping in marine chronometers: A cutting-edge navigational tool for long voyages at sea. Why then, you may ask, is this one in a domestic carriage clock? It’s a bit like putting a 300bhp engine in a Volkswagen Golf: Utterly unnecessary but you can show it off to your friends and the retailer can charge you double for it. There was no doubt an element of prestige of decorating one’s home with the same device that helped Britain rule the waves. Since a great deal of value was placedÔÇ»on the functionality of this escapement, which is still in many ways the main attraction of the object, its owner wished to have it restored to working order. 

Figure 3: A bent tooth under magnification

As is often the case at West Dean, a previous student had already done some work on this project before it arrived on my bench. The escape wheel had been written off as scrap and the process of manufacturing a new one had already been started.  

Close inspection of the escape wheel confirmed the damage; each of the teeth were bent outwards at their fragile tips. This was likely the result of a previous repairer’s mistake of removing the balance with power still in the mainspring, causing the escape wheel to spin freely and knock against the locking jewel. In any case it was not the result of normal wear and tear. The clock ran terribly, with poor locking and low amplitude.  

Nevertheless, I felt it a shame to discard a nearly 200-year-old component that was otherwise in good shape. These are no longer made in any significant numbers today and so there will only be fewer surviving historical examples in the future. The chronometer as a navigational instrument became obsolete ever since the sun set on the British Empire, and modern quartz technology surpassed mechanical timekeeping. As a relic of a bygone era, the socio-historical significance of this particular example is further enriched by its presence in a domestic clock, for no apparent purpose aside from ‘bling’. It’s the living culmination of values and features that its contemporary stakeholders considered to be desirable enough to warrant a high price tag, and prestigious enough to brandish the names Arnold & Dent – a joint business venture by two well-known chronometer makers.  

Wearing my conservation hat, I decided to attempt to save the wheel before diving straight into producing a new one. It is technically challenging to achieve the latter, and I also suspect that damaged chronometer escape wheels outnumber horologists that possess the required manufacturing facilities. It might therefore prove useful for other horologists to design a possible method to fix the issue.  

The key points determining the success of this treatment are: 

  • The restoration of the original geometry of the teeth 
  • The preservation of historic material.  

With this in mind, the following method was designed: 

  1. File a brass block to fit exactly within the gaps between the teeth, matching the front, back, and circumferential profiles as shown in fig. 4.
  2. Polish with lapping paper to avoid scratching the teeth. 
  3. File the brass flush with the side profile of the teeth. 
  4. Make a burnisher with a mild steel taper pin, polished longitudinally with 1000 grit emery paper.  
  5. Hold a tooth with the brass support block in a watchmaker’s vice, using masking tape on the jaws to prevent scratching. 
  6. Holding the burnisher in a pin vice, gently roll it along the back taper of the tooth starting in the direction as shown by the arrow in fig. 5.  
  7. Repeat 4 and 5 until all teeth are straightened.  

This straightforward method worked surprisingly well. My main fear was that the tips would break under pressure, being extremely thin and made of hardened brass. This didn’t turn out to be the case. A very gentle rolling action on the tips supported by the brass block was all that was needed to straighten the teeth and restore their original profile, with no loss of material. This method also has the advantage of requiring little equipment other than a small vice and some files. With strong magnification and some patience, it is achievable even within the most rudimentary workshop. Once this was done, I double checked the locking jewel for damage and repositioned it to a shallower depth in relation to the escape teeth (it had been adjusted by a previous repairer to overcompensate for the bent teeth), and the clock ticked happily ever after. 

It is important that both the brass support and the teeth are of the exact same depth so that they can both be held firmly in the vice jaws. I also suggest sticking a wide piece of paper or something similar underneath the vice to catch any pieces in case they fall. Next time I would use a round bar instead of a taper pin for the burnisher as it is more difficult to apply even pressure across the width of the tooth with the taper. I would also polish the burnisher with a slightly higher grit to reduce the chances of abrasion against the brass teeth. 

Figure 6: Teeth post-treatment

Straightening the teeth was one of several things that needed doing on this clock. It is a reminder of the many invisible tasks that conservators do to preserve the integrity of objects that come into our hands. In this case, shifting a nearly imperceptibly small amount of brass across a few micrometres has had the dramatic effect of restoring the object’s value to its stakeholders. 

Figure 7: Complete escapement in situ, top view

Sources and further reading

Appelbaum, Barbara. (2010) Conservation Treatment Methodology. Abingdon: Routledge. 

Betts, Jonathan. (2017) Marine Chronometers at Greenwich. Oxford: Oxford University Press. 

Britten, F. J. (1896) The Watch and Clockmaker’s Handbook. London: E. F. & N. Spon, p75. 

Gould, Rupert T. (2013) The Marine Chronometer: Its History and Development. Woodbridge: Antique Collectors’ Club. 

Mercer, Vaudrey. (1972) John Arnold & Son. London: Antiquarian Horological Society.  

The Institute of Conservation. (2023) ICON Ethical Guidance. https://www.icon.org.uk/resources/resources-for-conservation-professionals/standards-and-ethics/icon-ethical-guidance.html (Accessed: 1 January 2023). 

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