THE RECOIL OR ANCHOR ESCAPEMENT.
This escapement, always a favorite with clockmakers, has had a long and
interesting history and development. Because it started with a suddenly
achieved reputation, and because it is adapted to obtain fair results
with the cheapest and consequently most unfavorable working conditions,
it has won its way into almost universal use in the cheaper classes of
clock work; that is to say, it is used in about ninety per cent of the
pendulum clocks which are manufactured to-day.
It achieved a sudden reputation at its birth, because it was designed
to replace the old verge, which, with its ninety degree pallets close
to the arbor, and working into the crown wheel, required a very large
swing of the pendulum. This necessitated a light ball, a short rod,
required a great force to drive it, and made it impossible to do
away with the circular error, while leaving the clock sensitive to
variations in power. The recoil escapement was therefore the first
considerable advance in accuracy, as its use involved a longer and
heavier pendulum, shorter arcs of vibration and less motive power than
was practicable with the verge; and as the pendulum was less controlled
by the escapement, it was less influenced by variations of power.
In the early escapements the entrance pallet was convex and the exit
pallet concave. Escapements of this description may still be met with
among the antiquities that occasionally drift into the repair shop.
Later on both pallets were made straight, as shown in Fig. 41. It will
be seen by studying the direction of the forces that the effect is to
wear off the points of the teeth very rapidly, and for this reason the
pallets were both made convex (See Fig. 42), so as to bring the rubbing
action of the recoil more on the sides of the teeth and do away to a
large extent with the butting on the points which destroyed them so
rapidly.
[Illustration: Fig. 41. Recoil Escapement with Straight Lifting Planes.]
The rather empirical methods of laying out the recoil escapement, which
have gained general circulation in works on horology, have had much
to do with bad depthings of this escapement and the consequent undue
wear of the escape wheel teeth and great variation in timekeeping of
the movements in which such faulty depthings occur, particularly in
eight-day movements with short and light pendulums. The escapement
will invariably drive the clock faster for an increase of power and
slower for a decrease; an unduly great depthing will greatly increase
the arc of vibration of the pendulum, as the train exerts pressure on
the pendulum for a longer period during the vibration; the consequence
is that instead of the pendulum being as highly detached as possible,
we have the opposite state of affairs and a combination of a strong
spring, light pendulum and excessive depthing will easily make a
variation of five minutes a week in an eight-day clock.
The generally accepted method of laying out this escapement is shown
in Figs. 41 and 42, as follows: “Draw a circle representing the escape
wheel; multiply the radius of the escape wheel by 1.4 and set off this
as the center distance between the pallet and escape wheel centers.
From the pallet staff center describe a circle with a radius equal to
half the distance between escape wheel and pallet centers. Set off
on each side of the center line one-half the number of teeth to be
embraced by the pallets and from the points of the outside teeth draw
lines tangent to the circle described from the pallet center. These
lines would then form the faces of the pallets if they were left flat.”
We wonder how much information this description and the drawing conveys
to the average reader. How long should the pallets be? What is the
drop? How much will the escape wheel recoil with such a depthing? What
arc will the pallets give the pendulum? Why should the center distance
always be the same (seven tenths of the diameter of the wheel) whether
the escapement embraces eight, or ten, or six teeth? As a matter of
fact it should not be the same. We could ask a few more questions as
to other details of this formula, but it will be seen that such a
description is practically useless to all but those who are already so
skilled that they do not need it.
[Illustration: Fig. 42. Recoil Escapement with Curved Lifting Planes.]
[Illustration: Fig. 43. Drawing the Lock Lift and Recoil of the Usual
Form.]
Let us analyze these drawings. A little study of Figs. 41, 42 and 43
will show that there is really only one point of difference between
them and Fig. 32, which shows the elements of the Graham, or dead
beat. The sole difference is in the fact that there are no separate
locking planes in the recoil, the locking and run taking place on an
extension of the lifting planes. Otherwise we have the same elements in
our problem and it may therefore be laid out and handled in the same
manner; indeed, if we were to set off on Fig. 32, the amount of angular
motion of the pallet fork which is taken up by the run of the escape
wheel teeth on the locking planes, by drawing dotted lines above the
tangents, T, we should then have measured all the angles necessary to
intelligently set out the recoil escapement. We should have the lock
at the tangent, T, the lift and the run (or recoil) being defined
by the lines on either side of it, and the length of our running and
lifting planes would be found for the entering pallet by drawing a
straight line between the points of the two acting teeth of the escape
wheel and noting where this line cut the lines of recoil and lift.
A similar line traced at right angles to this would in the same way
define the limits of run and lift on the exit pallet. It will therefore
be seen that our center distances for any desired angle of escapement
may be found in the same way (Fig. 28), for either escapement, and
thus the method of making the pallets for the ordinary American clock,
Fig. 43, becomes readily intelligible. The sole object of curving the
pallets, as explained previously, was to decrease the butting effect of
the run on the points of the teeth. This is accomplished in Fig. 43 by
straight planes on the pallets and straight sides to the teeth with 20°
teeth on the escape wheel; merely inclining the plane of the entering
pallet about six degrees toward the escape wheel center, thus serving
all purposes, while the gain in the cost of manufacture by using
straight instead of curved pallets and wheel teeth is very great.
[Illustration: Fig. 44. Recoil with Curved Planes.]
[Illustration: Fig. 45. Showing the Usual Position in Cheap Clocks and
the Verge Wire.]
One factory in the United States is turning out 2,000,000 annually of
two movements, or about 1,000,000 of each movement; there are four
other larger factories and several with a less product; so it will
readily be seen that any decrease in cost, however small it may be on
a single movement, will run up enormously on a year’s output. Suppose
the factory mentioned were enabled to save only one-eighth of a cent on
one of its million movements manufactured last year, this would amount
to $1,250 per year, a little over $100 per month. Thus it will be seen
that close figuring on costs of production is a necessity.
[Illustration: Fig. 46. Drum Escapement.]
Fig. 44 shows the method of drawing the escapement according to the
common sense deductions given above. As the methods of laying out the
angle of escapement, lock, lift, and run, were given in detail in Figs.
28 to 32, they need not be repeated here.
Fig. 46 shows the escapement frequently used in French “drum” clocks
and hence called the “Drum” escapement. These are clocks fitted to
go in any hole of the diameter of the dial and hence they have very
short, light pendulums. An attempt is made to gain control over the
pendulum by decreasing the arc of escapement to not more than two and
sometimes to only one tooth. This gives an impulse to the pendulum
only on one-half of the vibrations, the escape wheel teeth resting and
running on the long circular locking pallet during alternate swings
of the pendulum. The idea is that the friction of the long lock will
tend to reduce the effect of the extra force of the mainspring when
the clock is freshly wound. Such clocks often stop when the clock is
nearly run down, from deficiency of power, and stop when wound, because
the friction of the escape wheel teeth on the locking plane is such as
to destroy the momentum of the light pendulum. All that can be done in
such cases is to alter the locking planes as shown by the dotted lines,
so that the “drum” becomes virtually a recoil escapement of two teeth.