THE GRAHAM OR DEAD BEAT ESCAPEMENT.
This escapement is so-called because the escape wheel remains “dead”
(motionless) during the periods between the impulses given to the
pendulum. It is the original or predecessor of the well known detached
lever escapement so common in watches, and it is surprising how many
watchmakers who are fairly well posted on the latter form exhibit a
surprising ignorance of this escapement as used in clocks. It has like
the latter a “lock,” “lift” and “run”; the only difference being that
it has no “draw,” the control by the verge wire rendering the draw
unnecessary.
It may be made to embrace any number of teeth of the escape wheel,
but, owing to the peculiarities of angular motion referred to in the
last chapter, see Fig. 26, B C, D E, the increased arcs traveled
as the pallet arms lengthen introduce elements of friction which
counterbalance and in some cases exceed the advantage gained by
increasing the length of the lever used to propel the pendulum.
Similarly, the too short armed escapements were found to cause
increased difficulty from faulty fitting of the pivots and their holes,
and other errors of workmanship, which errors could not be reduced in
the same proportion as the arms were shortened, so that it has been
determined by practice that a pallet embracing ninety degrees, or
one-fourth of the circumference of the escape wheel, offers perhaps
the best escapement of this nature that can be made. Therefore the
factories generally now make them in this way. But as many clocks are
coming in for repair with greater or less arcs of escapement and the
repairers must fix them satisfactorily, we will begin at the beginning
by explaining how to make the escapement of any angle whatever, from
one tooth up to 140 degrees, or nearly half of the escape wheel.
It is quite a common thing for some workmen to imagine that in making
an escapement, the pallets ought to take in a given number of teeth,
and that the number which they suppose to be right must not be departed
from; but there seems to be no rule that necessarily prescribes any
number of teeth to be used arbitrarily. The nearer that the center of
motion of the pallets is to the center of the escape wheel, the less
will be the number of teeth that will be embraced by the pallets. Fig.
28 is an illustration of the distances between the center of motion of
the pallets and the center of the wheel required for 3, 5, 7, 9 and
11 teeth in a wheel of the same size as the circle; but although we
have adopted these numbers so as to make a symmetrical diagram, any
other numbers that may be desirable can be used with equal propriety.
All that is necessary to be done to find the proper center of motion
of the pallets is first to determine the number of teeth that are to
be embraced, and draw lines (radii) from the points of the outside
ones of the number to the center of the wheel, and at right angles
to these lines draw other two lines (tangents), and the point where
they intersect each other on the line of centers will be the center of
motion of the pallets.
[Illustration: Fig. 28.]
[Illustration: Fig. 29. Note the difference in length of arc for the
same angle.]
It will be seen by the diagram, Fig. 28, that by this method the
distance between the centers of motion of the pallets and that of the
scape wheel _takes care of itself_ for a given number of teeth and that
it is greater when eleven and one-half teeth are to be embraced than
for eight or for a less number. These short pallet arms are imagined
by some workmen to be objectionable, on the supposition that it will
take a heavier weight to drive the clock; but it can easily be shown
that this objection is altogether imaginary. Now, bearing in mind
the principles of leverage, if the distance between the pallets and
escape wheel centers is very long, as in Graham’s plan, in which the
pallets embraced 138° of the escape wheel, the value of the impulse
received from the scape wheel and communicated through the pallets to
the pendulum is no doubt greater with a proper length of verge wire,
for, the lifting planes being longer, the leverage is applied to the
pendulum for a longer arc of its vibration, yet we must not suppose
that from this fact the clock will go with less weight, for it is
easy to see that the longer the pallet arms are the greater will be
the distance the teeth of the escape wheel will have to move (run) on
the circular part of the pallets. See Fig. 29. The extra amount of
friction, and the consequent extra amount of resistance offered to the
pendulum, caused by the extra distance the points of the teeth run on
the circular locking planes of the pallets and back again, destroys
all the value of the extra amount of impulse given to the pendulum
in the first instance by means of the long arms of the pallets. The
escape wheel tooth resting on the locking plane of the pallet is quite
variable in its effective action, and since it rests on the pallet
during a part of each swing of the pendulum and the pendulum is called
on to move the pallet back and forth under the tooth, any change in the
friction between the tooth and pallet is felt by the pendulum and when
the clock gets dirty and the friction between the tooth and pallet is
increased, the rate of the clock gets slow, as the friction holds the
pendulum from moving as fast as it would without friction. Now, as
this friction increases by dirt and thickening of the oil, all these
forms of escapements are subject to changes and so change the clock’s
rate. An increase of the driving weight, or force of the mainspring, of
clocks with dead-beat escapements always tends to make their rate slow,
from the action mentioned.
It is for this reason that moderately short arms are used in clocks
having dead-beat escapements of modern construction. Most of the
first-class modern makers of astronomical clocks only embrace seven
and one-half teeth, on a 30-tooth wheel, with the centers of motion
of the pallets and scape wheel proportionately nearer, as it can be
mathematically demonstrated that with the pallets embracing an arc of
90° the application of the power to the pendulum is at right angles to
the rod and therefore is most effective.
TO DRAW THE ESCAPEMENT.—In order to make the matter clearer we show
in Fig. 30 the successive stages of drawing an escapement and also the
completed work in Figs. 32 and 33 embracing different numbers of teeth.
Draw a line, A B, Fig. 30, to serve as a basis for measurements. With
a compass draw from some point C on this line a circle to represent
the diameter of our escape wheel. Now we shall require to know how
many teeth there will be in our escape wheel. There may be 60, 40, 33,
32, 30, or any other number we desire to give it; seconds pendulums
generally have 30 teeth in this wheel, because this allows the second
hand to be mounted directly on the escape wheel arbor and thus avoids
complications. We divide the number of degrees in a circle (360) by the
number of teeth we have selected, say 30. 360 ÷ 30 = 12° for each tooth
and space. One-fourth of 360° equals 90° and one-fourth of 30 teeth
equals seven and one-half teeth; each tooth equaling 12 degrees, we
have 12 × 7 = 84°, which gives us six degrees for drop, to ensure the
safety of our actions.
[Illustration: Fig. 30.]
We now take 90° and, dividing it, set off 45° each side of our center
line and draw radii, R, from the center to the circumference of our
circle; this marks the beginnings of our pallets. Now to find our
pallet center distance we draw tangents, T (at right angles), from the
ends of these radii toward the line of centers. The point where they
intersect on the line of centers is the pallet center.
Now we must determine how much motion we are going to give our
pendulum, so that we can give the proper lift to our pallets. Four
degrees of swing is usual for a seconds pendulum, so we will take four
degrees and, dividing it, give two degrees of lift to each pallet. To
do this we draw a line two degrees _inside_ the tangent, T (towards the
escape wheel center), from our pallet center on the entering pallet
side and another line from the pallet center two degrees _outside_
of the tangent, T, on the exit pallet side. Next, from the pallet
center we draw arcs of circles cutting the tangents, T, and the radii,
R, where they intersect; this gives us the locking planes on which
the teeth of the escape wheel “run” (slide) during the excursions of
the pendulum, if the escapement is to have unequal lockings; if the
lockings are to be equidistant (if the pallet arms are to be of equal
length) the arc for the entering pallet is drawn three degrees below
(outside) the radius, R, while that on the exit pallet is drawn three
degrees _above_ (inside) the exit radius. Finally the lifting planes
are drawn from the intersection of the arcs of circles struck from
the pallet center with their tangents, T, to the lines, marking the
limits of the lift, two degrees away. These lifting planes should be
at an angle of 60° from the radii, R, and as a tangent is always at
right angles (90°) to its radius, they are consequently at 30° to the
tangents running to the pallet center. Thus we can measure these angles
from either the escape wheel or the pallet center, as may be most
convenient.
When making a new pallet fork, it is most convenient to mark out the
lifting planes on the steel at 30° from the tangents, T, as we then do
not have to bother with the escape wheel further than to get its center
distance and the degrees of arc the lifting planes are to embrace. The
workman who is not familiar with this rule is apt to have his ideas
upset at first by the angles of inclination toward the center line
which the lifting planes will take for different center distances,
as owing to the fact that the tangents meet on the center line at
different angles for different distances, the lifting planes assume
different positions with regard to the center line and he may think
that they do not “look right.” They are right, however, when drawn at
30° to their tangents. Fig. 31 shows several pallets with different
arcs arranged in line for purposes of comparison, each being drawn
according to the above rule, as measurements with a protractor will
show.
[Illustration: Fig. 31.]
We have now arrived at the complete escapement, having finished our
pallets. We have, however, nothing to hold them in position; they must
be rigidly held in position with regard to each other and the escape
wheel, consequently we will make a yoke to connect them to the pallet
arbor out of the same steel, giving it any desired shape that will not
interfere with the working of the clock. Two of the most usual forms
are shown at Figs. 32 and 33.
[Illustration: Fig. 32.]
[Illustration: Fig. 33.]
Let us see how this rule will work in repairs. Suppose we have a clock
brought in with the pallet fork missing, and that the movement is one
of those in which the pallet arbor is held by adjustable cocks which
have been misplaced or lost, so that we don’t know the center distance
of the pallet arbor and escape wheel. We shall have to make a new part.
Measure the escape wheel, getting its diameter carefully, take half
of this as a radius, and mark out the circle with a fine needle point
on some copper, brass or sheet steel, drawing the escapement as
detailed in Figs. 30 and 32. Then measure carefully the angles made
by the tangents with the center line; take the steel which is to be
used in making the pallets and fork; draw on it a center line; lay
off the tangents and the lift lines; draw the locking arcs and the
lifting planes carefully from the tangents and give the rest of the
fork a symmetrical shape. Use needle points to draw with and have your
protractor large enough to measure your angles accurately. Then drill
or saw out and file to your lines, except on the locking and lifting
planes; leave these large enough to stand grinding or polishing after
hardening. Harden; draw to a straw color and polish the planes. Your
verge will fit if it has not warped in hardening. If this is the case,
soften the center, keeping the heat away from the pallets, and bend or
twist the arms until the verge will fit the drawing, when laid on top
of it. In grinding the pallets the fork should be mounted on its arbor
and the latter held between the centers of a rounding-up tool while the
grinding is done by a lap in the lathe. This insures that the planes
will be parallel to the pallet arbor and hence square with the escape
wheel teeth, so that they will not create an end thrust on either
escape or pallet arbor. It is also the quickest, easiest and most
reliable way of doing the job. When clocks come in with the pallets
badly cut; soften the center of the fork, place the ends between the
jaws of a vise, squeeze enough to bring them closer, mount in the
rounding-up tool and lap off the cut planes until they are smooth and
stand at the proper angle; then polish. This is done quickly.
[Illustration: Fig. 34. Drawing escape wheel to fit a tracing from a
pallet fork.]
Can we work the rule backwards? Suppose we get a clock in which we
have the pallet arbor adjustable as before, and we have the pallet
fork all in good shape, but we have lost the escape wheel, or it has
been butchered by somebody before coming to us, so that a new one is
required.
Take off the pallet fork; lay it on a sheet of brass and trace around
it carefully with a needle point. Fig. 34. Mark the center carefully
at the pallet arbor hole and measure carefully the distance between
the pallets and mark that center. Draw a center line cutting these
centers and extending beyond. Now draw the tangent from the beginning
of the entering pallet (as shown by the tracing on our brass) to the
pallet center; do the same with the exit pallet. Now take a metal
square and place it on one of the tangents exactly, with the end at
the beginning of the entering pallet; trace a line cutting the line
of centers and we have the radius of our escape wheel. Trace a circle
from the intersection of the radius and the center line and we have
the circumference of our escape wheel. This circle should also cut
the intersection of the tangent and radius on the other side if it is
drawn correctly; if it does not do this an error has been made in the
drawing.
[Illustration: Fig. 35. Drawing an escape wheel to cut. The last
drawing shows the complete wheel.]
Having found the diameter and circumference of our escape wheel it may
be sawed out and mounted for wheel cutting; or, if we have no wheel
cutter and must make the wheel, we must draw it on the brass by hand
with a fine needle point before proceeding to saw it out by hand, Fig.
35. Say that the wheel is to have thirty-two teeth, which is a common
number; then 360° ÷ 32 = 11¼° as the space between the points of our
teeth. Take a large protractor, one with the degrees large enough
to be divided (I use a ten inch); place its center on the center of
our escape wheel, set off 11¼° and mark them on the brass with the
needle point, at the edge of the protractor. Then take a straight edge
and draw a radius from the center to the circumference; change the
straight edge to the other mark and mark the point where it crosses
the circumference; set your dividers accurately by this mark and
space off the teeth on your circumference. If they are set at eleven
degrees and fifteen minutes they will come out exactly at the end.
Now take your protractor and with its center at the junction of the
radius and circumference set off ten degrees and draw a line past the
center of the wheel; set off twenty degrees and draw another line the
same way. From the center of the escape wheel draw two circles just
touching these lines. Outside of these draw two circles defining the
inner and outer edges of the rim of the wheel. With the straight edge
just touching the inner circle draw in the fronts of the teeth; these
will all be set at ten degrees from a radius, so that only the extreme
points will touch the locking planes of the pallets and thus reduce the
friction during the run. The backs of the teeth are marked out in the
same way from the twenty-degree circle. The hub is made to coincide
with the ten-degree circle; the spokes are traced in and we are ready
to begin sawing out.
If the workman has a wheel cutter the job is much simpler. A piece
of brass is mounted on a cement brass with soft solder, faced off,
centered and the pitch circle, inner and outer edges of the rim and
the hub are traced with the T-rest and graver. The extra metal is then
cut away and a suitable index placed on the spindle and locked. The
wheel cutter is set up with a fine-toothed, smooth cutting saw on the
spindle, horizontal, with its upper edge at the line of centers of the
lathe. It is then run out to the circumference of the wheel, turned
upwards ten degrees and the wheel cut around, Fig. 36. This makes the
fronts of the teeth. Turn the saw ten degrees more and cut the backs
of the teeth. Then turn the saw so that it will reach from the front
of one tooth to the root of the back of the next one, without touching
either tooth, and cut round again; this cuts out a triangular piece of
waste metal between the teeth. Turn the saw again so that it reaches
from the bottom of the front of a tooth to the top of the back of the
next one and cut around again, thus removing another portion of the
waste metal, and leaving only a small triangle between the teeth. Lower
the saw its own thickness and cut around the wheel again, repeating the
operation until the waste metal is all removed and you have a smooth
circular rim between the teeth, Fig. 36.
[Illustration: Fig. 36. Making an escape wheel with a saw, showing the
successive cuts.]
Set the saw horizontally at the lathe center; raise it one-half the
thickness of the spokes; set the index pin of the lathe head firmly at
O; feed in the saw the thickness of the wheel and make straight cuts
across from the circle of the inner rim to the circle marking the hub,
but not cutting either; set the index pin at 30 and repeat; next lower
your saw and cut the other side of the spokes the same way.
Next you can mount a lap in place of the saw and smooth the fronts and
backs of the teeth and if you have a rather thick disc the outer edge
of the rim, between the teeth, may also be smoothed.
If you have a good strong pivot polisher, mount a triangular end mill
in the spindle, lock the yoke, and cut the arcs of circles of the hub
and rim from edge to edge of the spokes, feeding carefully against the
mill with the hand on the lathe pulley.
Put on your jeweling tailstock and open the wheel to fit the pinion,
collet, or arbor, if there is no collet.
You now have the wheel all done, except facing the side that was
soldered to the cement brass and trimming up the corners of the spokes
at the rim and hub, and you have got it round, true and correct in much
less time than you could have done in any other way, while an immense
amount of work with the file and eye-glass has been avoided. It is true
because it was soldered in position at the beginning and has not been
removed until finished.
Sometimes what are known from their appearance as club-shaped teeth are
used in the wheels of Graham’s escapements. Pendulums receive their
impulse from escapements made in this manner partly from the lifting
planes on the pallets, and partly from the planes on the scape wheel.
The advantage gained by this method is, that wheels made in this
way will work with the least possible drop, and consequently, power
is saved; but the power saved is thrown away again in the increased
friction of the planes of the wheel against those of the pallets, which
is considerably more than when plain-pointed teeth are used on the
escape wheel.
Clock pallets are usually made of steel, and on the finer classes of
work jewels are often set into them to prevent the oil from drying,
after the same fashion as jewels are placed in steel pallets in a
lever watch; but it is obvious that stone pallets made in this way
have to be finished with polishers held in the hand, and that, except
in factories, they cannot be made so perfectly regular, especially
that pallet that is struck downwards, as the particular action of a
fine Graham escapement requires. When great accuracy is required, the
pallets are usually made of separate pieces, and the acting circles
ground and polished on laps, running in a lathe. This method of
constructing pallets also allows a means of adjustment which in some
particular instances is very convenient.
There is also a plan of making jeweled pallets adjustable, which is
practiced on fine work, such as astronomical and master clocks. The
pallet fork consists of two pieces of thin, hard, sheet brass, cut out
in the usual form and two mounted on one arbor. Circular grooves are
cut in the sides of both plates, at the proper distance, and of the
proper size to receive the jewels which are the acting parts of the
pallets. When jewels cannot be made of the desired size, pallets of
steel are made, and the jewels are then set into the steel large enough
for the teeth of the wheel to act upon. The two parts of the fork are
fastened at a given distance apart, and the jewels, or pieces of steel,
go in between them, and, after they have been adjusted to the proper
position, are fastened by screws that pull the frames close together
and press against the edges of the jewels. Pallets made in this manner
have a very elegant appearance. Another method is to have only one
frame, and to have it thick enough, where the jewels have to be set in,
to allow a groove to be cut in its side as deep as the jewels (or the
pieces of steel that hold the jewels) are broad, and which are held in
their proper position by screws. This system of jeweling pallets is
frequently adopted by the makers of fine mantel clocks.
[Illustration: Fig. 37. Brocot’s visible escapement, escaping over 120°
with pointed teeth. Dotted lines on pallets show where they are cut to
avoid stopping.]
BROCOT’S VISIBLE ESCAPEMENT.—Fig. 37 represents a system of making
and jeweling pallets much used by the French in their small work,
especially in visible escapements. The acting parts of the pallets
are simply cylinders, generally of colored stones, usually garnets,
one-half of each cylinder being cut away. These cylinders extend some
distance from the front of the pallet frame, and work into the escape
wheel the same as the pallets of a Graham escapement—the round parts
of the pallets serving as impulse planes. The neck of the brass pallet
frame is cut up in the center, and the width between the pallets is
sometimes adjusted by a screw, sometimes by bending the arms.
Clock movements with this escapement, of a careful construction, will
frequently come for repairs, accompanied by the complaint of constant
stopping and that no attempt at closely regulating can succeed with
them, although they appear to have no visible disturbing cause. In such
cases the depthing of the escapement is generally wrong. With proper
depthing the point of the escape wheel tooth should drop on the center
or a little beyond the center of the pallet stone. If it is set in
this way the clock will stop when wound, especially if it has a strong
spring, as the light pendulum will not then have momentum enough to
unlock it against the full power of the spring. If the pallets are set
shallow, in order to avoid this difficulty, then, the pendulum will
take too short a swing and thus the clock will have a gaining rate.
Generally the pendulum ball cannot be made enough heavier to correct
the defect.
[Illustration: Fig. 38. Brocot’s visible escapement escaping over 90°
with a small lift on the escape wheel teeth.]
In these movements, in which the length of the pendulum does not
exceed 4 inches, the pallet fork embraces, generally about 120°, or
the one-third part of the wheel; it will be seen that unless there are
stop works on the barrel of the main spring no manner of regulating
is possible with these conditions, in view of the considerable
influence exercised by the mainspring through the train on the very
light pendulum, and by replacing this unduly high anchor by a lower
one, I have always been able to produce a very satisfactory rate with
movements having pendulums of three and a half to four inches. Fig. 38
shows a 90° escapement with a small lift on the escape wheel teeth.
In spite of its incontestable qualities, the visible escapement
possesses one inherent fault. I refer to the formation of its pallets,
the semi-circular shape of which renders unequal the action of the
train in giving impulse to the pendulum exceeding 50 centimeters (20
inches), since to make it to describe arcs of from one to two degrees
only, with pendulums of from 60 centimeters to one meter in length,
it became necessary to make the anchor arms extremely long, which
considerably impeded the freedom of action, especially when the oil
became thick, and this disposition would, therefore, stand in direct
contradiction with the principles of modern horology. Both stopping and
the irregularity of rate can be obviated by changing the semi-circular
form of the pallets for one of an inclined plane, either by grinding a
new plane or turning the stones in such manner as to offer an inclined
plane to the action of the wheel, analogous to that of the Graham
escapement. See Fig. 37, the dotted lines on the pallets showing the
portion to be ground away.
The importance of this transformation will readily be understood; it
suffices to give to these planes a more or less large inclination in
order to obtain a greater regularity of lifting, and, at desire, a
lifting arc more or less considerable without being compelled to modify
the proportions of the fork or to exaggerate the center distance of
wheel and pallet arbor.
In adjusting an escapement, perhaps it may be advisable to mention that
moving the pallets closer together, or opening them wider, will only
adjust the drop on one side, while the other drop can only be affected
by altering the distance between the centers of the pallets and scape
wheel. This is accomplished in various ways. The French method consists
of an eccentric bush, riveted in the frame just tight enough to be
turned by a screwdriver. Another plan, common in America, is simply
pieces of brass (cocks) fastened on the sides of the frames. The pivots
of the pallet axis are hung in holes in these cocks, and an adjustment
of great accuracy may be quickly obtained by loosening the clamping
screws. Lock, drop and run should be of the same amount on each pallet.
However, we do not approve of adjustments of any kind, except in the
very highest class of clocks, where they are always likely to be under
the care of skillful people, who understand how to use the adjustments
to obtain nicety of action in the various parts.
In making escapements, lightness of all the parts ought to be an object
always in view in the mind of the workman, and such materials should be
used as will best serve that purpose. The scape wheel, and the pallets
and fork, should have no more metal in them than is necessary for
stiffness. The pallet arbor, and also the escape wheel arbor, should be
left pretty thick when the wheel and pallets are placed in the center
between the plates, to prevent their springing when giving impulse to
the pendulum. We have often been puzzled to find out the necessity or
the utility of placing them in the center between the plates, as they
are so generally done in English clockwork. The escapement acts much
more firmly when it is placed near one of the plates, and it is just as
easy to make it in this way as in the other.
It is often assumed that the friction of the teeth on the circular
part of the pallets of a dead-beat escapement is small in amount and
unimportant in its value. With respect to its amount, we believe it
is often not far short of being equal to one-half of the combined
retarding forces presented to the pendulum; and with respect to its
being unimportant, this assumption is founded on the supposition that
it is always a uniform force, when it is easy to show that it is not
a uniform force. It is very well known that the force transmitted in
clock trains, from each wheel to the next, is very far from being
constant. Small defects in the forms of the teeth of the wheels and
of the leaves of the pinions, and also in the depths to which they
are set into each other, cause irregularities in the amount of power
transmitted from each wheel to the next; and the accidental combination
of these irregularities in a train of four or five wheels, makes the
force transmitted from the first to the last exceedingly variable.
The wearing of the parts and the change in the state of the oil, are
causes of further irregularities; and, from these causes, it must be
admitted that the propelling power of the scape wheel on the pallets is
of a variable amount, and a more important question for consideration
than it is usually supposed to be. To avoid the consequences of this
irregular pressure of the scape wheel on the pallets being communicated
to the pendulum, is a problem that has puzzled skillful mechanicians
for many years; for, although we find the Graham escapement to be
pronounced both theoretically and mechanically correct, and by
some authorities little short of perfection, we find some of these
same authorities—both theoretically and practically—testify their
dissatisfaction with it by endeavoring to improve on it. In Europe the
experience of generations and the expenditure of small fortunes, in
pursuit of this improvement, through the agency of the gravity, and
other forms of escapements, proves this fact; while of late years,
in the United States, much time and money has been spent on the same
subject, and results have been reached which have raised questions
that ten years ago were little dreamed of by those clockmakers who are
generally engaged on the highest class of work.
While considering this class of escapements, we would say a few words
in regard to the sizes of escape wheels generally used. Small wheels
can now be cut as accurately as larger ones and there is now no reason
or necessity for continuing the use of a wheel of the size Graham and
Le Paute used, and which has been the size generally adopted by most
European makers who use these escapements. The Germans and Swiss make
wheels much smaller for Graham escapements than the English makers do;
and the American factories make them smaller still. On the continent
of Europe the wheels of Le Paute’s escapement are made much larger
than they are made in England and in the United States. No wheel, and
more especially a scape wheel, should be larger than will just give
sufficient strength for the number of teeth it has to contain, in
proportion to the amount of work that it has to perform. The amount
of work a scape wheel has to perform in giving motion to the pendulum
is of the lightest description, and not more than one-tenth of what
it is popularly supposed to be, which is shown by its variation under
slight increase of friction; therefore we do not consider that we take
extreme ground in recommending wheels for these escapements to be made
nearly half the size their originators made them, and the pallets
drawn off in proportion to the reduced size of the wheel. It is plain
that by reducing the size of the wheel its inertia will be reduced.
When the teeth begin to act on the inclined planes of the pallets, the
wheel will be set in motion with greater ease, as it has a shorter
leverage, and the amount of the dead friction of the scape wheel teeth
on the inclined planes and circular part of the pallets will also be
proportionately reduced by making the wheel smaller. Factory experience
and examination of a large number of clocks in repair shops have also
shown that smaller and thicker escape wheels will wear much longer than
larger and thinner ones, as all the wear is at the points of the teeth
and this is the portion to be protected.