LE PAUTE’S PIN WHEEL ESCAPEMENT.
Probably in no other escapement, except the lever, has there been so
many modifications as in the pin wheel; this is so to such an extent
that it will be found by the student that nearly every escapement of
this kind which he will examine will differ from its fellows if it
has been made by a different maker. They will be found to vary in the
lengths of the pallet arms from three-fourths to one and a half times
the diameter of the escape wheel; some of them will have the longer
arm of the pallets outside and some inside; some will have the lift
for both pallets laid out on one side of the perpendicular P, Fig.
39, while others will have the lift divided, with the perpendicular
in the center. Very old escapements have the pallet center directly
over the escape wheel center, while the pallet arms work at an angle
of 45°, while others have them with the pallet center planted on a
perpendicular, tangent to the pitch line of the escape wheel. Some have
the circular rest or locking faces of the pallets rounded slightly to
hold the oil in position while others have them flat and still others
have them made of hard stone, polished. More than half have the pins in
the escape wheel cut away for one-half of their diameters, leaving the
bottoms round, as shown in Fig. 39, while others use a wider pin and
trim away the bottoms also, as in Fig. 40, leaving the lifting surface
on the pins not more than one-fourth the arc of the circle. This is
especially true of the larger escapements used in tower clocks, though
they are also found in regulators.
[Illustration: Fig. 39. Pin Wheel Escapement.]
[Illustration: Fig. 40. Pin Wheel With Flattened Teeth.]
In view of the wide variation in practice, therefore, we have
endeavored to present in Fig. 39 a conservative statement of the
general practice as found in existing clocks. We say existing, because
very few of these escapements are made now—none at all in America—and
those in use are generally in imported regulators, which have come from
Switzerland or Germany. The Waterbury Clock Co. at one time made this
escapement for its regulators and the Seth Thomas Clock Company made a
number of its early tower clocks with it, but both have discontinued
it for some years, and it is safe to say that any movement coming into
the watchmaker’s hands which has this escapement is imported; or if
American, it is out of the market. Le Paute claimed as an advantage
the fact that the impact of the escape wheel teeth is downward on both
pallets, whereas in the gravity and recoil escapements one blow is
struck upwards and the other downwards. He claimed that by this means
a better action was secured after the pivot holes began to wear, as
there was less lost motion with both blows in the same direction and
any shake would not affect the amount of impulse given to the pendulum.
The difference is more theoretical than practical, however, and the
escapement possesses one serious fault, which is that the pins forming
the escape wheel teeth conduct the oil away from the pallets, so that
the clock changes its rate in from eight months to one year after being
oiled and cleaned. The most effective means of counteracting this is
to round the locking planes of the pallets slightly, so that the oil
will be held on them by capillary attraction. Another method is to turn
the pins so that they are thicker in diameter at the point of contact
with the pallets, but this is seldom tried. The best plan is to keep
the pallets as close as they can be to the face of the wheel without
touching.
TO DRAW THE ESCAPEMENT.—In laying out this escapement the first thing
to consider is the arc of swing of the pendulum, because one-half
of the lift is on the pin and consequently one-half the lift must
equal one-half the diameter of the pin, as shown in Fig. 39. If the
pendulum swings four degrees, then the diameter of each pin must
equal four degrees of the pallet movement. This establishes the size
of our pin; it is measured from the pallet staff hole. There are 30
of these pins for a second’s pendulum, and unless it is a very large
escapement the pins cannot be made less in diameter than one-fourth the
distance between the pins, or they will be too weak and will spring;
consequently 360 ÷ 30 = 12° and 12° ÷ 4 = 3°, so that three degrees of
the pitch line of the escape wheel equals the swing of the pallet fork.
This establishes the relation as to size between the escape wheel and
the opening, or swing of the pallet fork. Draw a perpendicular, P, from
the pallet center and on one side of it lay out the lift lines L, L;
draw a line at right angles to the perpendicular and where it crosses
the inner lift line draw a circle touching the outer lift line. The
diameter of this circle equals three degrees of the circumference of
the wheel, on its pitch line, and this multiplied by 120 gives 360° or
the pitch circumference of the escape wheel. Dividing the sum so found
by 3.1416 gives the diameter of the escape wheel and half of this is
the radius. After finding the radius draw the pitch circle and set out
the other twenty-nine teeth spaced twelve degrees apart, and drawn in
half circles as shown in Fig. 39.
Now to get the thickness of the pallet arms. When the pin shown in
action in Fig. 39 has just cleared the lower edge of the inner pallet,
the succeeding pin should fall safely on the upper corner of the outer
pallet; consequently the thickness of these two arms, the pin between
them, and the drop (clearance between the pin and the lower edge of the
upper pallet) should just equal the distance between two pins, from
center to center, or 12° of the escape wheel. With the first or inner
lift line as a starting point, draw the lower arcs of the pallets and
draw the upper or locking planes from the perpendicular and the outer
lift line. Then draw the lifting planes of the pallets by connecting
the ends of these arcs. The enlarged view above the escape wheel in
Fig. 39 will show how this is done more clearly than the main drawing.
It is best to make the pallet fork of steel, in two pieces, screwed to
a collet on the pallet arbor, as the inner arm must be bent, or offset,
so that it will clear the pins of the escape wheel, and the pallets
should lie in the same plane, as close to the wheel as is possible
without touching it. The pallets are hardened.
In tower clocks the escapement is so large that a pin having a diameter
of three degrees of the escape wheel gives a half pin of greater
strength than is necessary for the work to be done and such pins are
cut away on the bottom, as in Fig. 40. In making the wheel it should be
drilled in the lathe with the proper index to divide the wheel and the
pins riveted in; then the pins are cut with a wheel cutter as if they
were teeth of a wheel. Pins should be of hard brass.
Care should be used in handling clocks with this escapement while the
pendulum is connected with the pallet fork, as, if the motion of the
fork should be reversed while a pin was on one of the lifting planes,
it would bend or break the pin.