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06
Basel/SIHH Archive

05
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04
Basel/SIHH Archive

Industry News
Archive

THE VALJOUX 7750 CHRONOGRAPH

BY WALT
ODETS

If there is a single movement ubiquitous
in modestly-priced contemporary chronographs, it is the
Valjoux 7750. Introduced in 1974--five years after the
famed Zenith El Primero automatic--the movement has been
supplied in both 21,600 and 28,800 beat
versions. It uses an automatic-winding module
attached to the top of the movement, winding in one
direction by means of a single double-click
wheel. Initially using 17 jewels, current versions
run in 25. The 7750 is 13.25 ligne (30 millimeter)
design, 7.9 millimeters
thick.

THE CONCEPT OF COULISSE-LEVER DESIGNS

Like many contemporary chronographs, the Valjoux 7750
dispenses with the traditional column wheel for switching
functions. Instead a heart piece limiter
(left) is used to coordinate starting, stopping,
braking, and reset functions, usually by means of two buttons
in contemporary designs (the earliest coulisse-lever calibers
used three buttons). The Valjoux 7750 also utilizes a
concept first patented in 1941 by a watchmaker, Henri
Jacot-Guyot. This is a reset-to-zero (or heart
piece) lever which pivots to reassure accurate
reset-to-zero of both the center hand and minute counter.
Previous coulisse lever designs had not been as precise in
this regard.

While the parts count of a coulisse-lever
design is comparable to that of a column wheel design, the
various levers and springs in the lever design require much
less precision in form and materials and are much less
expensive to manufacture. This--economy of manufacture--was
and is the purpose of a coulisse lever caliber.

THE CONSTRUCTION OF THE VALJOUX 7750

For ease of manufacture and service (right), the
Valjoux 7750 is constructed of a mainplate (1), a
calendar plate (2), and a chronograph top plate
(3).

An additional bridge for the automatic winding
system is attached to the chronograph plate on the top (watch
back) of the movement.

THE CHRONOGRAPH PLATE

With the
automatic winding bridge removed, the full chronograph plate
is visible (left). Parts include (1) the
heart piece limiter; (2) the center wheel with heart
piece; (3) the minute accumulator with heart piece;
(4) the Jacot-Guyot pivoting heart piece lever with
self-adjusting hammers (5); the tilting pinion arm
(6).

The tilting pinion drive engagement is a design
going back to the late 19th century. The
design dispenses with the more conventional spur gears
and the arm that carries the intermediate spur
gear. Shown left, the lower (left) end of the
pinion is continuously engaged with the movement's fourth
wheel. When the chronograph is started, the heart piece acts
on the pinion arm to bring the upper (right) part into
engagement with the chronograph center wheel. (This
action will be observed later.)

THE HEART PIECE IN ACTION

The heart
piece is acted upon by one of the two case pushers and rotates
back and forth (unlike a column wheel, which rotates
continuously in one direction). As shown right, the heart
piece is in the zero-reset position.

In this position of the heart piece, the
center wheel brake (1) is clear of the center wheel (to
allow reset to zero). The hammers of the heart piece
lever (2) rest on the flats of the heart cams for the
center (3) and minute accumulator (4), holding
them at zero. The tilting pinion drive is disengaged from
the center wheel (below right).

The heart piece is next rotated clockwise by
the upper case pusher to start the chronograph (right).
The case pusher is indicated at the arrow.

In this position of the heart piece, it can be
seen (right) that both the center wheel brake
(1) and reset hammer (2) are clear of the center
wheel. The tilting pinion is engaged with center wheel
(3).

Finally, the heart piece is rotated
counterclockwise by the lower pusher (arrow, right) to
stop the chronograph.

As illustrated right, the pinion drive
(1) is disengaged from the center wheel, the brake
(2) is applied to the center wheel, and the reset
hammer (3) is disengaged from the heart cam.

Typically the operation of a coluisse-lever
design with heart piece limiter will not be as smooth as that
of a column wheel, and pusher forces between various functions
will vary. Coordination of functions with a modern design
like that of the 7750 will typically be good, though
imprecision of hand start, stop, and reset may be greater than
that of good column wheel designs.

CONTINUE TO PART 2 OF THE ARTICLE.

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