Table Of ContentOriginally 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
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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 dif-
ferent 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 de-
cided 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 synchro-
niser. 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 preci-
sion 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.
