THE MOTION WORKS



by Walt Odets

The motion works, as the name implies, is the
mechanism that gives motion to the hands of the watch. In other words,
it is the motion works that translates the unwinding of a mainspring,
the rotation of a gear train, and the reciprocating action of an escapement
into the meaningful information that makes a watch a watch–rather than
simply a sprung mechanism. The motion works is also the mechanism that
allows manual setting of the watch’s hands without rotation of the entire
movement.

THE BASIC COMPONENTS

Caliber 351 motion worksIn the conventional wristwatch, the motion works is
located entirely on the bottom plate (dial side) of the movement.
In this traditional form, the motion works consists of only three parts.
The heart of the mechanism is the cannon pinion, shown at
right
at the red arrow. The inset,
at upper left, shows the cannon pinion in its entirety. (In the main
photograph, it is largely hidden under the hour wheel.) The cannon pinion
is slid over the extended pinion (or shaft) of the movement’s center
wheel (tip visible at the the yellow arrow), and carries the
minute hand of the watch.

The second component, the hour wheel (blue
arrow
), sits on top of the cannon pinion and carries the hour hand.
The third component is the minute wheel (small green arrow),
which is driven by the cannon pinion (see inset, white arrow)
and, via its pinion (large green arrow) drives the hour wheel.
In other words, the movement drives the cannon pinion and attached minute
hand, and the cannon pinion drives the hour hand (via the minute wheel).
(The illustration also reveals the lower Incabloc shock absorber of
the balance wheel at the white arrow, and the intermediate wheel
driven by the castle wheel of the keyless works at the orange
arrow
. The keyless works is discussed in an accompanying article.)

HOW THE MOTION WORKS WORKS

Cannon pinion illustrationThe arrangement of the critical cannon pinion is shown
in the illustration left. The extended pinion (or shaft)
of the center wheel (2, blue) rises through the bottom plate,
and the cannon pinion (3, pink) is placed on top of it. As illustrated
(blue arrows), a crimp in the cannon pinion tube creates the
friction that allows the center wheel pinion
to drive the cannon pinion (and the minute hand attached to the top
of the cannon pinion tube). The amount of friction supplied by the crimp
is critical. There must be enough friction to properly drive the cannon
pinion, minute wheel, and hour wheel (4, green); but there must
also be enough slippage to allow the cannon pinion to rotate on the
center wheel pinion when the hands are manually set by means of the
keyless works (without turning the entire movement of the watch). The
cannon pinion is thus a clutch that must remain engaged during normal
operation, but disengage during hand setting. As will be later discussed,
improper friction at the cannon pinion is a common source of problems
in the watch. This illustration also shows the seconds pinion
(1, orange) of a center seconds watch. The seconds pinion rises
through the hollow center wheel pinion. The hour wheel (4, green)
rotates freely on the cannon pinion.

THE POWER FLOW

Motion works power flow diagramThe power flow from the movement and through the motion
works
is shown in the illustration at right. The sequence of power
flow is indicated in red numbers, while the components are designated
with letters. A is the plate of the movement (the top
surface, as illustrated, is behind the dial). B is the center
wheel. As indicated at 1, the mainspring
barrel drives the center wheel. By means of the extended pinion, the
center wheel drives the cannon pinion (D) as indicated at 2.
The cannon pinion carries the minute hand at 3,
and also drives the minute wheel (F) at 4.
Via its pinion, the minute wheel drives the hour wheel (E) as
indicated at 5. The hour wheel carries
the hour hand at 6. In an indirect
center seconds watch, as illustrated, the seconds pinion (C)
is (usually) driven by the third wheel at 7.
The tip of the seconds pinion emerges above the end of the cannon pinion
and carries and drives the center seconds hand at 8.

THE MOTION WORKS AND THE KEYLESS WORKS

The primary purpose of the cannon pinion is to allow
manual hand setting. Without its slip-friction fit on the center wheel
pinion, setting the hands of the watch with the crown would require
rotation of the entire gear train of the watch, which would be impossible
because of the pallets locking action on the escape wheel. The pallets,
and possibly the escape wheel, and pallet lever would be destroyed.
The Intermediate wheelkeyless
works (covered in an accompanying Horologium article) connects the castle
gear
(or clutch, red arrow, illustration left) to
the intermediate wheel (green arrow), which rotates the
minute wheel (blue arrow) and cannon pinion (yellow arrow).
As the minute wheel is rotating the cannon pinion, its pinion (black
arrow
) is simultaneously rotating the hour wheel (removed from over
the cannon pinion in this illustration). Thus, as compared to normal
operation, the power flow between cannon pinion and minute wheel is
reversed during setting (from minute wheel to cannon pinion during
setting). In both operations–normal running and hand setting–the minute
wheel pinion drives the hour wheel. During the setting operation, the
cannon pinion slips on the center wheel pinion and the movement continues
to run normally.

VARIATIONS ON THE THEME

The modern, “Zenith” layout of the gear train–introduced
by Zenith in 1948, and now commonly seen in most contemporary, direct
center second automatics–places the “center” wheel out of the center
of the movement. The fourth wheel often occupies the center of the movement
so that its extended pinion can directly drive the center seconds hand.
Some movements, like the Jaeger LeCoultre 889 use a similar layout,
but with indirect center seconds, because the arrangement allows
space for larger mainspring barrels and balance wheels. In watches with
a Zenith construction, the conventional cannon pinion cannot, of course,
be attached to the pinion of the center wheel because the center wheel
(or great wheel) is not in the center of the watch and therefore
cannot carry the minute hand. Thus, the cannon pinion clutch function
is sometimes provided in an offset pinion attached to the pinion of
the out-of-center great wheel. The wheel and pinion actually carrying
the minute hand is driven from this offset pinion through one or two
gears, often called coupling wheels. This train is sometimes
on the top plate (not on the dial side). Such watches may also have
a cannon pinion in the conventional
position, but clutch traction may be provided on its backside, or elsewhere
in the gear train between the great wheel and minute hand pinion.

Center seconds layoutIn the illustration at right, the following
components can be seen: Mainspring barrel (1); great wheel (2,
formerly the “center wheel”); third wheel and third wheel pinion (3
and 3A); fourth wheel (4); coupling wheel (5);
cannon pinion (6); hour wheel (7); and minute wheel (8).
In the watch illustrated, the third wheel pinion drives the coupling
wheel, which drives the cannon pinion via clutch friction. Friction
is provided at the red arrow, rather than on the front of the
cannon pinion. The fourth wheel pinion (green arrow) extends
through the cannon pinion and directly drives the seconds hand. The
hour and minute wheels are conventional.

(The illustration is based on Henry B. Fried, “The
Watch Repairers Manual,” American Watchmakers Institute Press, Cincinnati,
1986).

CARING FOR THE MOTION WORKS

The motion works is accessed manually through the
keyless works, and the caveats that apply to using the keyless works
apply here. (See The Keyless Works in an accompanying Horologium
article.)

The largest issue with the motion works is the cannon
pinion. If the friction is too high (because of lack of lubrication
or over-tightening during service), hand setting will be stiff and ultimately
impossible without turning the entire movement and thus damaging it.
The cannon pinion should never provide enough friction to turn the movement
and damage the pallets or escape wheel.

Cannon pinion on staking toolThe
more usual problem is a cannon pinion with too little friction. This
occurs because of normal wear, a lack of lubrication leading to abnormal
wear, or excessive hand setting. Cannon pinion friction often needs
to be raised by slightly renewing the friction crimp during service,
as shown in the illustration at left. A support is normally inserted
through the pinion to avoid collapsing the tube (in this case a round
burnishing broach, normally used to smooth holes). Traditionally, a
jewel press or staking tool is used with a special stake
(the upper bit) and stump (lower support) designed for the purpose.
The tool illustrated (below right) is a K&D watchmaker’s staking
tool which is provided with 100 different stakes and 35 stumps.

KD staking toolWhen
there is too little friction at the cannon pinion, hand setting will
seem excessively light. Low friction may also create slippage between
cannon pinion and the great wheel pinion during normal running to the
point that timekeeping appears to be erratic: the movement may be running
at regular rate, but the cannon pinion–and thus the minute hand, minute
wheel, hour wheel, and hour hand–will not remain with the movement.
Neither a center seconds nor subsidiary seconds hand will be affected
by this slippage. Thus, correct seconds indication with a drifting minute
hand is a sure indication that the problem is cannon pinion slippage
rather than a true rate problem with the movement.

Care of the cannon pinion is simple. Keep the watch
well lubricated and adjusted, and avoid truly unnecessary hand setting.
When resetting the time, it is usually best to move the hands either
forward or backwards–which ever involves less travel. (Hands must not,
however, be turned backwards through the period of engagement of complications
such as a calendar mechanism. Always known the restrictions for a particular
watch when setting Omega caliber 351 settingthe
hands backwards. If in doubt, set the hands forward and then correct
the date or other complication manually.)

For those who must set their watches to the second,
the cannon pinion may usually be used for “hacking” when the watch does
not provide a true balance-interference hack lever. By setting the watch
to the time desired and then applying a small amount of back pressure
on the crown (i.e. in the direction used to reverse the hands), the
cannon pinion will bring the movement to a stop. Simply releasing the
crown will allow the movement to restart. If the watch cannot be stopped
with this technique, a slight tightening of the cannon pinion is required.
Such use of the cannon pinion for hacking will not normally add to the
wear of the pinion, because, with the movement stopped, there is no
slippage between cannon pinion and the great wheel pinion.

 
© 2012 Bourne In Time Inc.