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Originally published in the Horological Science Newsletter – Issue 2001-2 pages 2-4 A fresh look at W5 Philip Woodward Good news has lust reached me concerning W5, my fifth and final attempt at making an all-mechanical precision clock1. It was completed in 1985 and has served as the workshop regulator of Anthony Randall, the marine chronometer specialist, since 1994. Its timekeeping has improved rather than deteriorated over the years, probably due to the ageing of the invar rod of its master pendulum. After having warned me beforehand that W5 was doing remarkably well, Anthony wrote to me as follows in a letter dated 11th April 2001. I am really delighted to advise you that the 100 days are up! The seconds hand of W5 has not strayed away from the starting zero second division by more than +1/4 or -1/ 2. It has, therefore, shown GMT to within these limits during the entire period of elapsed time without any correction, the application of a rate or any other kind of applied correction, manipulation or subterfuge of any kind whatsocver2. The test started on the 27th of December last. The seconds hand showed -1/8. Today it shows -1/2. The only misfortune that has happened during the intervening 105 days is that on the 3rd March I let the clock stop by failing to wind it. However, I was able to start it again with the hand showing the same small error as for the previous three days. WS was completed in 1985. Since then it has been running almost continuously and with no maintenance other than lubrication of the count wheel, not counting a couple of breaks for transport, first to the Royal Greenwich Observatory where it was a candidate for public exhibition. At the crucial time, however, the clock galleries were to be given a new look and the number of clocks on permanent exhibition greatly reduced, the better to highlight the observatory's most important pieces. In consequence, and after a brief sojourn in a cellar (the wont of museums) W5 was de-selected. The original plan had been for W5 to serve as an educational exhibit illustrating the master-and-slave principle of "free pendulum" clocks, notably the Shortt clocks used for standard time in Great Britain during the inter-war years. I believe it would have demonstrated the principle more clearly than the Shortts themselves, which rely on two seemingly unconnected pendulums, one hidden in a vacuum cylinder and the other in an ordinary Synchronome cabinet, physically separate though electrically connected. From ________________________________________________________________________ 1 This clock is described and illustrated in My Own Right Time (Oxford University Press 1995). 2 The writer is evidently well aware of the devious methods of horological scientists! 1 Originally published in the Horological Science Newsletter – Issue 2001-2 pages 2-4 personal experience I know that, even in its running state, the principle of a Shorts clock can be obscure to museum visitors. Jonathan Betts, Curator of Horology at Greenwich, regarded W5 as a potentially useful exhibit to model the Shorn principle because both of the pendulums are in the same cabinet where their interaction can easily be observed. No electricity is used, and the fact that the pendulums swing with different periods of vibration serves to excite the attention of the viewer. Public exhibition was not to be, and in the event I removed WS into the private custody of Anthony Randall. That a horologist of his stature should have been willing to set it up as his workshop regulator did me even greater honor than had already been paid by Jonathan Betts, whose original plan had failed through no wish of his own. I am now old enough to view all these matters without any trace of modesty, false or otherwise, and can only say that I enjoy a sense of satisfaction that W5 should be appreciated by two professional horologists of world renown. For practical use, Anthony Randall expects a workshop regulator to indicate the right time to within a fraction of a second without the need for making allowances of any kind. (He checks his clocks daily by eye and ear to a nominal accuracy of an eighth of a second using the radio time signal transmitted from Rugby England.) However, clocks being what they are, something has to give and for W5 it cannot be the hands. Altering the seconds hand upsets the relative phase of its pendulums and causes the clock to stop or even to 'crash'. Any waywardness, therefore, must be corrected by altering the weight on the master pendulum's regulating tray. Whenever necessary, Anthony effects these adjustments by adding or subtracting small pieces of metal foil from the tray. It is unfortunate that \R'5 should require winding before the lapse of seven and a half days. In the actual data which Anthony has recorded daily for over six years, there are necessarily some gaps, and some stops and restarts caused by absences of longer than a week. Truth to tell, the case was designed for another clock and is slightly too small for W5. It takes eight days for the weight to bottom in the case, but to guard against a gradual fading of the driving power as the springs of the Harrison maintaining power relax, I incorporated stopwork which abruptly disconnects the impulsing mechanism for both pendulums half a day earlier. Without this device, there would be a danger of the escapement remontoire acting sluggishly, which could result in fairly severe damage (broken impulse roller pivot or cracked jewels). The same danger is avoided in the Shortt clock by 'give' in the bracket for the impulse roller, but I lacked either the patience or the skill to implement this solution. For the record I show, below, data for the 15-week period from December 27th 2000 to April 11th 2001 inclusive, extracted from Anthony Randall's log sheets XXII and XXIII. Squares () indicate days on which no observations were made, and the triangle (∆) is the break on March 4th referred to earlier in his letter. This table should be read horizontally, each line representing two weeks of data, and the units are seconds. 2 Originally published in the Horological Science Newsletter – Issue 2001-2 pages 2-4 Errors of the seconds hand of W5 observed daily by AGR over a 15-week period Note: these figures are not rates W T F S S M T W T F S S M T -1/8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -1/8 -1/8 -1/8 -1/8 -1/8 -1/8 -1/8 -1/8 0 0 0 0 0 0 0 0 0 0 0 0 0 +1/8 +1/8 +1/8 +1/4 +1/4 +1/4 +1/4 +1/4 +1/4 +1/4 +1/4 +1/4 +1/4 +1/8 0 0 0 0 0 0 0 0 0 -1/8 -1/8 -1/8 ∆ -1/8 -1/8 -1/8 -1/4 -1/4 -1/4 -1/4 -1/4 -1/4 -1/4 -1/4 -1/4 -1/4   0 0 0 0 0 0 0 0 0 0 0 -1/8 -1/4 -1/4 -1/4   -3/8 -3/8 -3/8 -1/2 -1/2 -1/2 I do not kid myself that this is a very great achievement, as it took six years to turn up! Like so many other hopeful enthusiasts, I did originally set out to break all records for accuracy - or at least to achieve something surprising - and was crestfallen when, after so much thought and work, the timekeeping of W5 seemed to be no great shakes, though it has steadily improved with age. With a rod of fused silica in place of invar for the master pendulum I am inclined to think that the story would have been very different, and I have been living in hopes that some younger enthusiast might follow the design principles of W5 with just such improvements in mind. It cannot be too difficult to make, for otherwise I could never have made it with a drill press, a micrometer and a watchmakers' lathe with no slide rest. All the lathe work was done with a hand graver. Here I find myself in a slight quandary. Horologists do not like copying a design, screw for screw, and I would be the last to wish it. To my way of thinking, that would be model engineering. However, one enthusiast in Australia (with family roots in Ipswich, England) did ask me for detailed drawings of W5, which I could not immediately supply because I did not have any such drawings myself. As he was prepared to commission a set, I did the best I could with Indian ink on squared paper and sent the drawings off, keeping only poor photocopies for myself (on which the squares did not come out). They were received, but the gentleman concerned found them too challenging and I have been unable to re-establish contact with him. I fear he may now have passed away. In writing the book called MORT, my aim was to try a different tack by giving what I thought to be sufficient information for anybody to make their own version of W5 with all the improvements that would spring from the application of fresh minds. Several readers have expressed an interest in just such a project, but have told me the information provided in the book is not really sufficient. I am not at all clear where the middle course lies. I can only say that the harnessing of a slave pendulum is fun to make, fun to watch when made, and celebrates the principles of the most accurate pendulum clocks which were ever used for observatory timekeeping. 3 ~- ,ii \ I I \ \ / ( c· )----- --- \. .• -- CLOCKS 47 A horological adventure Part2 Philip Woodward describes his double gravity escapement and mechanical remontoire My AIM, as readers of Part I will remember, was to design and build a clock for the < home which would model the ( principles of the famous Shortl frcc- J pcndulum clock. I would sacrifice the vacuum - and with it moSt of the pendu- c lum's 'freedom· -and I would deny myself -c the use of electricity. Some people take kindly to electricity in clocks. but I am not of their number. I like to be able to see what is going on. At first glance, the unusual feature of my clock is the sight of two pendulums swinging side by side in different rhythms. The master pendulum makes seven swings to every ten of the slave, which is a half-seconds pendulum. You can tell visitors with some truth that the shorter pendulum controls the seconds hand, and the longer one the minute hand. Every half a minute they have to be synchronised. The real purpose of the slave is to release a gravity arm at regular intervals so that the Skeleton dial of Philip Woodward's clock. master pendulum receives its impulses without having to ask for them (figure I). carry on and automatically unhook itself. of lowering the gravity arm has impulsed The interval between impulses is tradition- The soft landing on the roller jars the the slave! The slave unhooks itself, the arm ally and' conveniently 30 seconds. I have master pendulum very little indeed. I have does a shorl dead run, impulses the master made clocks with longer intervals, even as calculated that it is equivalent to a free fall and drops away. One gravity arm for both long as four minutes, but this time I had of just two thousandths of an inch, and pendulums, and all the clockwork is left to decided to bow to convention. without the need for any critical adjust- do is to operate one remontoire. But first I And now come the real questions. How ments. But -eureka! -this is not the only had to be sure that the hook could be lifted will the gravity arm be reset when it has merit of the arrangement, for the very acl into engagement with the gravity arm in t> fallen off the master pendulum's impulse roller? And how will the remontoire be combined with the clockwork which drives Figure 1: Impulse roller the slave? It is perfectly fc<lsible to use two and gravity arm. separately wound movements, but I sought a simpler solution. I hit on it while thinking about another aspect of the design, the initial drop of the gravity arm on to the impulse roller. In the Shortt clock, the arm is unlatched on receipt of an electrical sig- 0) nal from the slave,and/a//.ron to the roller. -- I had never cared much for this method of \ initiating the impulse. With the slave pendulum only inches away. would it not be possible to hook up the gravity arm to the slave pendulum and have it swung gently into position? The idea is sketched in figure 2. The hook is anachcd to the slave pi.:ndulum by a pivot. As soon as the arm lands on the roller. it will be brought momt:ntarily to rest, and the slave will 48 CLOCKS response to a trigger of some kind from the slave's count wheel. Experience has taught me to be wary of count wheels, so I resolved to 1ake great Figure 2: Sketch or gral·ily arm pains over this one. It is desirable that most shared between two pendulums. of the resistance to pendulum motion should come from the air, which is a good arc stabiliser. If the count wheel is not to dissipate much more energy than the air, its construction has to be extremely delicate. ff it cannot be small in diameter, it must at least have a low moment of inertia and low pivot friction. fn relation to a hair-seconds pendulum, the diameter of my count wheel had to be rather large (l.125in), so r decided to make if of Perspex. In a different clock, I have had a Perspex count wheel running for over ten years with no signs of wear or deterioration. Such a wheel, well crossed out, is highly functional and beautiful in its own way, but it does not blend well with traditional materials, to my eye at least. I have therefore hidden it behind the silvered brass seconds hand dial. The wheel, shown in figure 3, weighs just half a gram and is mounted ona sturdy arbor of diameter Y1in. This is supported in the notch formed by two closely spaced overlapping discs. These anti-friction rollers support the count wheel assembly under its centre of gravity, and two highly polished grooves (hidden from view in the photograph) keep the arbor property the sensing arm has passed the pin and has located. The back end of the arbor has an fallen back upwards (!) leaving the hook ordinary pivot. TI1is is little more than a engaged with 1he gravity arm. The signal positional constraint, as the whole box is simply a pair of brass plates pegged assembly is almost balanced without it. and screwed to a spacer block clamped and positively located on the pendulum rod. The block also carries a steel tab made or There arc two ways of propelling a clock spring bent and snapped, which acts count wheel. Shortt's slave uses as the catch for the hit-and-miss synchro- the conventional backstop roller. niser yet to be described. with the active pawl gathering the The power of the clock train musl now ratchet teeth by their tips only. This scheme be harnessed to reset the gravity arm after is well-suited to heavy pendulums driving its fall from the master impulse roller. The wheels with comparatively few teeth ( 15 for mechanism which does this is usually called Synchronome). With a wheel of 30 teeth. I a remontoire, from the French 'to go back opted for the system used with such bravura up'. Casting all modesty aside:, I must own figure 3: Count wheel assembly. by Bulova in the Accutron tuning form to taking great pride in my remontoire. As watch of 1960, which had no fewer than 300 far as I am aware, ii isa completely original detent falls away. As the scape wheel has teeth! Pawl and backstop remain in perm- invention, though owing much in concept more than enough power to lift the gravity anent contact with the wheel, and the to the thinking of John Harrison. It works arm, it rapidly recovers its sense of direc- balance or frictional forces can be import- in a thoroughly reliable way, and is illus- tion and performs the lift. As it docs so. a ant. The active pawl must not inch the trated in figure S. pin on the car resets the detent and locks wheel ahead prematurely, but must slide The gravity arm is pivoted at G and has the escapement. The action is intriguing to fully into the notch first. At the end of the pivoted upon it a triangular 'ear'. The pin watch because it happens so fast. stroke, on the other hand. it must stay in its for the slave hook is at ff. Figure S(a)shows A special feature of this rcmontoire is the notch and rec'oil the wheel until the back- the rcmontoire in its normal position after variable ratio between the movement of the stop is fully home. The system works extra- having been reset. The detent pivoted at D scape wheel and that of the gravi[y arm. If ordinarily well provided that the pressures keeps the seven-pin scape wheel locked. the scape wheel is turned steadily by hand. arc correctly matched to the wheel and the When His engaged by the slave pendulum the arm rises more and more slowly as the teeth are sufficiently sloped. Bulova adop- towards the end of its swing to the right, the motion of the scape pin becomes tangential ted it because it permitted wide variations in remainder of the swing pulls the ear free of lo that of lhecar. Consequently. the torque amplitude (theoretically three to one). In the scape pin so that it drops to its banking demanded of the scape wheel is greatest at my clock. the rndial pressures on the count in the neck of the gravity arm. As the slave the slart or the lift. and diminishes on the wheel are 50 milligrams for the pushing returns to the left. it lowers the gravity arm way up. This results in a good positive pa\ :lnd 20 milligrams for the backstop. on to the master roller and unhooks itself. action. Once begun. the rescuing will A~ 1.:an be seen from figure 4. lhe pawl P This position is shown in figure S(b). After certainly becomptc1ed.and the arm cannot is pivoted in a little 'signal box'on the slave impulsing the master pendulum. lhe arm get srnck half way up. Further. although poendulum rod. The uppermost arm senses falls to the position shown in figure S(c). the ear is brought to rest very abruptly al a pin in the count wheel. which is denccted where the car h~isjust struck the nl!'Xt scape the end of the lift. the main body of the arm . eYery half a minute as the pendulum swings pin. The inertia or the falling arm causes is stopped comparatively gently. This is to the right. Before this swing is compli:te. the scape wheel to recoil :1 trille. so th;ll the extremely important, for if the arm has CLOCKS 49 degrees. The fly is constructed out of a strip of aluminium, thin edge to the wind and with weights of l.25g on each end. This is loosely mounted on the scape arbor along- side a helical compression spring between collars, outboard at the back of the clock. I have no doubt at all that in my case the fly' is an inertial device which would operate just as well in a vacuum. It holds fast during steady acceleration, reducing it consider- Figure 4: Signal ably, but refuses to transmit shocks. [tslips box on slave during the initial recoil of the scape wheel, pendulum rod and slips again (by less than JO degrees) with front pivot- when the wheel is locked by the de tent. This plate removed. slippage is essential as a means of dissi- pating the surplus energy of the remontoire. All danger of tripping is now eliminated, and the sound of the remontoire is reduced to an acceptable domestic level. At dinner, one cannot hear ic. The purpose of the hit-and-miss synchroniser is to convey the master pendulum's time to the slave and hasten its current swing if necessary. Knowledge of master time comes from the fall of the gravity arm from the impulse roller. As 0 soon as the arm bottoms on the scape pin, the detent falls and actuates the synchroniser. The arrangement can be understood from the diagram, figure 6. The spring which supplies the hastening force for the slave is not on the pendulum itself, as in the Shortt clock, but is pivoted on the clock frame to be flicked into engagement with the stiff tab fixed on the pendulum bracket. The engaging ends of the spring and tab are gathered so much angular momentum that honed to a sharp edge to make it difficult it cannot stop, the ear falls again and the for the synchroniser to sit on the fence. The whole system goes into oscillation (with two surfaces arc polished to reduce the interesting consequences). Initially, this is amount of brushing friction in a miss egxuaacrdtleyd w ahgaati dnsidt iht ainp ptheen dtoe smigen, ,a In Cde lat sr aI thhaedr (figTuhree 7m)o. dus operandi of a hit-and·miss perplexed. All was gloom until I synchroniser is fairly obvious and well remembered that Lord Grimthorpe had a known. The losing rate of the slave must be cure for the ailment. such that a hastening tug will be needed on Up to this moment in my horological about half of the occasions. Quite a weak activities, I had paid scant attention to the spring is sufficient, my own having a force 0 S'fulyd'd ienn Glyr iimt btheocarpmee's agnr auvrigtye nets csaupbejemcet nftosr. rate of l.5g/mm. This is strong enough to control a half-seconds pendulum weighing investigation. three quarters of a kilogram, even though To steady its ac1ion, all the remontoirc its rate were to Ouctuate as much as two needs is a damper. There used to be some minutes a week in either direction. discussion in the horological literature on the question of whether a fly works by air C are must be taken when starting resistance or simply by inertia. (Unfortu- up a master and slave system, as nately I have been unable to trace it.) In a the synchroniser may not be able: striking train, I have always assumed the to pull the slave into synchronism fly to be an air fan, because a steady from a starting position which is too far terminal speed is reached and you can hear adrift. My own synchroniser as described it purr. How it works in Big Ben's escape- can cope perfectly well if the slave is started ment I cannot say. In a regulator clock with up too far in advance of the master, because his four-legged gravity escapement, Grim- misses will simply occur until it has lost thorpe[I] refers to the fly as a 'common enough time to need its first hit. After that, fan-fly' even though its 2in blades turn it will settle down to its normal routine of through 45 degrees only. I cannot believe roughly alternate hits and misses. But if the that was a fan. In my own remontoire,cach slave is started up more than a small frac- arm is I.Sin long and the angle is about 52 tion of a second behind its correct time, the synchroniser will miss when it should have hit. What happens is that it tries for a hit on the very first occasion, but cannot wait long 0 Figure 5: Escapement remonloire (a) arter enough for actual engagement to take place. reset, (b) afler slave impulse, and (c) arter The spring falls back before the slave has master impulse. arrived. C> CLOCKS The master pendulum weighs only I .2kg altogether. I am as yet far from convinced that very great weight is essential for precision timekeeping. High density and low air resistance are the factors which make a high Q, but even this is not necessarily the most important parameter in practice. Swinging without impulse, my pendulum's Q is 10,700. When the losses associated with the impulse roller are included, it drops to about 9000. In other words, about one sixth of the energy delivered to the pendulum is wasted in the very act of de- livery. This will have to be improved, though not primarily to increase Q. The essential thing will be to reduce the vari- ability of impulse associated with frictional losses, which is, after all, the rationale behind the use of a gravity arm in the first place. My impulse roller cries out for jewel- ling, but it would be hard indeed for an amateur to approach the refinement of the Shortt clock in this crucial part of the Figure 6: Plan of mechanism. escapement, showing As for performance, with beginner's luck fly (F), safety horn (C) and a favourable cycle of indoor tempera- and synchronizer tures, the clock stayed within a tenth of a spring (S). second of UT for a period of 23 days in February and March 1984, and might have done so for longer had it not been taken down for finishing. When finished, it was never able to repeat it. By keeping careful records and adopting a strictly scientific approach, one can diagnose the more Main plate of obvious causes of irregularity and correct movement, wilh dial them, but it all takes time. I still hope to and hands removed. achieve what I call the 'horological ton', an The rod of lhe master error chart which stays within one second pendulum is on the lert and that of the slave to the right of centre. Further right is the winding square, drive line and stopwork. It is dangerous for a master and slave acceptable part of the dCcor. system to go out of synchronism unless In keeping with the present trend, the special precautions have been taken, such 'works' are exposed to view, except for the as spring loading the bracket on which the train of precision spur gears. These were master's impulse roller is pivoted. A head- ordered from a gear manufacturer, and on collision could damage the roller, break although far too good for a gas-meter, they its pivot, strain the pendulum suspension are not in keeping with horological and damage the gravity arm. I feel it is tradition. It is as well to hide them from the equally dangerous to let the clock run eyes of Mike Harding! There are four of a straight line throughout a period of down, as the Harrison maintainer will give meshes in the power train, with tooth JOO days. I have done it for 50, but to rise to a slow loss of torque through the counts from barrel to scape of 192:12. qualify in the precision stakes, the whole train, with uncertain consequences. To 80:20, 72:24 and 40:28, all at diametral ton is called for, preferably starting on a remove this wofry. I have fitted stopwork, pitch 48. The bearings -dare I say it -are date chosen by someone else. so that when a week plus a period of grace all ballraces. During construction, I had In conclusion, my thanks go to Mr has elapsed, the count wheel back stop is the immense pleasure of noticing that a 2in Martin Burgess for his constant encourage- nipped out of engagement. This at once winding crank, which had been left on the ment, and for sponsoring what turned out stops all further impulses. winding square with its arm horizontal, to be the overall winning entry in the 1985 Mike Harding has pointed out[2) that was able to turn the whole train under its British Horological lnstitute's annual there is more to making clocks than engin- own weight. The actual driving weight is a competition for horology of outstanding eering, for otherwise 'they might just as hefty 4.2kg falling 20in in eight days. interest. CJ well be gas-meters for all the individuality Remontoires are, of their very nature, and feeling they contain'. This brilliant extravagant in power consumption. The References observation is one I shall never forget, but I force of the gravity arm on the impulse I Lord Grimthorpe, Clocks. Watches and must leave others to judge what my own roller of the free-pendulum is 2g. equiva- Bells. 8th edition, London 1903 (re- creation most closely resembles. As the lent to 4g at the scape pin, as this is twice as printed by EP Publishing, 1974). See attentive reader will have gathered, my close to the fulcrum. The remontoire p.123. clock is in the dining room, where few opposes this with a force of IOg, a safety 2 Mike Harding, 'Skeleton clocks: Prob- would regard the Shortt Clock as an factor of 2.5 to be paid for in extra lead. lems of design', Clocks, May 1983.