theoretically, the impulse would be transmitted 2° from the bankings.
The narrow ruby pin n receives the impulse on the line _v_, which is
closer to the line of centers than the line _u_, on which the large ruby
pin receives the impulse. Here then we have an advantage of the narrow
ruby pin over a wide one; with a wider ruby pin the balance is also more
liable to rebank when it takes a long vibration. Also on account of the
greater angle at which the ruby pin stands to the slot when the impulse
takes place, the _drop_ of the fork against the jewel will amount to
more than its shake in the slot (which is measured when standing on the
line of centers). On this account some watches have slots dovetailed in
form, being wider at the bottom, others have ruby pins of this form.
They require very exact execution; we think we can do without them by
judiciously selecting a width of ruby pin between the two extremes. We
would choose a ruby pin of a width equal to half the angular motion of
the fork. There is an ingenious arrangement of fork and roller which
aims to, and partially does, overcome the difficulty of choosing between
a wide and narrow ruby pin, it is known as the Savage pin roller
escapement. We intend to describe it later.
If the face of the ruby pin were planted on the theoretical impulse
radius _ii_, Fig. 19, the impulse would end in a butting action as
shown; hence the great importance of distinguishing between the
theoretical and real impulse radius and establishing a reliable data
from which to work. We feel that these actions have never been properly
and thoroughly treated in simple language; we have tried to make them
plain so that anyone can comprehend them with a little study.
Three good forms of ruby pins are the triangular, the oval and the flat
faced; for ordinary work the latter is as good as any, but for fine work
the triangular pin with the corners slightly rounded off is preferable.
[Illustration: Fig. 21.]
[Illustration: Fig. 23.]
[Illustration: Fig. 22.]
English watches are met with having a cylindrical or round ruby pin.
Such a pin should never be put into a watch. The law of the
parallelogram of forces is completely ignored by using such a pin; the
friction during the unlocking and impulse actions is too severe, as it
is, without the addition of so unmechanical an arrangement. Fig. 21
illustrates the action of a round ruby pin; _ii_ is the path of the ruby
pin; _cc_ that of the acting length of the fork. It is shown at the
moment the impulse is transmitted. It will be seen that the impact takes
place _below_ the center of the ruby pin, whereas it should take place
at the center, as the motion of the fork is _upwards_ and that of the
ruby pin _downwards_ until the line of the centers has been reached.
The same rule applies to the flat-faced pin and it is important that the
right quantity be ground off. We find that 3/7 is approximately the
amount which should be ground away. Fig. 22 illustrates the fork
standing against the bank. The ruby pin touches the side of the slot but
has not as yet begun to act; _ri_ is the real impulse circle for which
we allow 1¼° of freedom at the acting edge of the fork; the face of the
ruby pin is therefore on this line. The next thing to do is to find the
center of the pin. From the side _n_ of the slot we construct the right
angle _o n t_; from _n_, we transmit ½ the width of the pin, and plant
the center _x_ on the line _n t_. We can have the center of the pin
slightly below this line, but in no case above it; but if we put it
below, the pin will be thinner and therefore more easily broken.
[Illustration: Fig. 14.]
_The Safety Action._ Although this action is separate from the impulse
and unlocking actions, it is still very closely connected with them,
much more so in the single than in the double roller escapement. If we
were to place the ruby pin at _X_, Fig. 14, we could have a much
smaller roller than by placing it at _P_. With the small roller the
safety action is more secure, as the intersection at _m_ is greater than
at _k_. It is not as liable to "butt" and the friction is less when the
guard point is thrown against the small roller. Suppose we take two
rollers, one with a diameter of 2.5 mm., the other just twice this
amount, of 5 mm. By having the guard radius and pressure the same in
each case, if the guard point touched the larger roller it would not
only have twice, but four times more effect than on the smaller one. We
will notice that the smaller the impulse angle the larger the roller,
because the ruby pin is necessarily placed farther from the center. The
position of the ruby pin should, therefore, govern the size of the
roller, which should be as small as possible. There should only be
enough metal left between the circumference of the roller and the face
of the jewel to allow for a crescent or passing hollow of sufficient
depth and an efficient setting for the jewel. For this reason, as well
as securing the correct impulse radius and therefore angle, when
replacing the ruby pin, and having it set securely and mechanically in
the roller, it is necessary that the pin and the hole in the roller be
of the same form, and a good fit. Fig. 23 illustrates the difference in
size of rollers. In the smaller one the conditions imposed are
satisfied, while in the larger one they are not. In the single roller
the safety action is at the mercy of the impulse and pallet angles. We
have noticed that in order to favor the impulse we require a large
roller, and for the safety action a small one, therefore escapements
made on fine principles are supplied with two rollers, one for each
action.
It may be well to say that in our opinion a proportion between the fork
and impulse angles in 10° pallets of 3 or 3½ to 1, _depending_ upon the
size of the escapement, is the lowest which should be made in single
roller. We have seen them in proportions of 2 to 1 in single roller--a
scientific principle foolishly applied--resulting in an action entirely
unsatisfactory.
When the guard point is pressed against the roller the escape tooth must
still rest on the locking face of the pallet; if the total lock is 2°, by
allowing 1¼° freedom for the guard point between the bank and the roller
the escapement will still be locked ¾°. How much this shake actually
amounts to depends upon the guard radius. Suppose this to be 4 mm.,
then the freedom would equal 4 × 2 × 3.1416 ÷ 360 × 1.25 = .0873 mm.
[Illustration: Fig. 24.]
[Illustration: Fig. 25.]
_The Crescent_ in the roller must be large and deep enough so it will be
impossible for the guard point to touch in or on the corners of it; at
the same time it must not be too large, as it would necessitate a longer
horn on the fork than is necessary.
Fig. 24 shows the slot _n_ of the fork standing at the bank. The ruby
pin _o_ touches it, but has not as yet acted on it; _s s_ illustrates a
single roller, while S2 illustrates the safety roller for a double
roller escapement. In order to find the dimensions of the crescent in
the single roller we must proceed as follows: WA is in the center of the
fork when it rests against the bank, and is, therefore, one of the sides
of the fork angle, and is drawn from the pallet center; V A W is an
angle of 1¼°, which equals the freedom between the guard point and the
roller; _g g_ represents the path of the guard pin _u_ for the single
roller, and is drawn at the intersection of VA with the roller A′ A2 is
a line drawn from the balance center through that of the ruby pin, and
therefore also passes through the center of the crescent. By planting a
compass on this line, where it cuts the periphery of the roller, and
locating the point of intersection of VA with the roller, will give us
one-half the crescent, the remaining half being transferred to the
opposite side of the line A′ A2. We will notice that the guard point has
entered the crescent 1¼° before the fork begins to move.
The angle of opening for the crescent in the double roller escapement is
greater than in the single, because it is placed closer to the balance
center, and the guard point or dart further from the pallet center,
causing a greater intersection; also the velocity of the guard point has
increased, while that of the safety roller has decreased. Fig. 24, at
_ff_, shows the path of the dart _h_, which also has 1¼° freedom between
bank and roller. From the balance center we draw A′ _d_ touching the
center or point of the dart; from this point we construct at 5° angle
_b_ A′ _d_. This is to ensure sufficient freedom for the dart when
entering the crescent. We plant a compass on the point of intersection
of A′ A2 with the safety roller, S2, and locating the point where A′_b_
intersects it, have found one-half the opening for the crescent, the
remaining half being constructed on the opposite side of the line A′ A2.
_The Horn_ on the fork belongs to the safety action: more horn is
required with the double than with the single roller, on account of the
greater angle of opening for the crescent.
The horn should be of such a length that when the crescent has passed
the guard point, the end of the horn should point to at least the center
of the ruby pin.
The dotted circle, _s s_, Fig. 25, represents a single roller. It will
be noticed that the corner of the crescent has passed the guard pin _u_
by a considerable angle, and although this is so, in case of an accident
the _acting edge_ of the fork would come in contact with the ruby pin;
this proves that a well made single roller escapement really requires
but little horn, only enough to ensure the safe entry of the ruby pin in
case the guard point at that moment be thrown against the roller. We
will now examine the question from the standpoint of the double roller;
S2, Fig. 25, is the safety roller; the corner of the crescent has safely
passed the dart _h_; the centers of the ruby pin _o_ and of the crescent
being on the line A′ A2, we plant the compass on the pallet center and
the center of the face of the ruby pin and draw _k k_, which will be the
path described by the horn. The end of the horn is therefore planted
upon it from 1½° to 1¾° from the ruby pin; this freedom at the end of
the horn is therefore from ¼° to ½° more than we allow for the guard
point; it depends upon the size of the escapement and locking angles
which we would choose. It must in any case be less than the lock on the
pallets, so that the fork will be drawn back against the bank in case
the horn be thrown against the ruby pin.
When treating on the width of the ruby pin, we mentioned the Savage pin
roller escapement, which we illustrate in Figs. 26 and 27. This
ingenious arrangement was designed with the view of combining the
advantages of both wide and narrow pins and at the same time without any
of their disadvantages.
In Fig. 26 we show the unlocking pins _u_ beginning their action on the
line of centers--the best possible point--in unlocking the escapement.
These pins were made of gold in all which we examined, although it is
recorded that wide ruby pins and ruby rollers have been used in this
escapement, which would be preferable.
The functions of the two pins in the roller are simply to unlock the
escapement; the impulse is not transmitted to them as is the case in the
ordinary fork and roller action. In this action the guard pin _i_ also
acts as the impulse pin. We will notice that the passing hollow in this
roller is a rectangular slot the same as in the ordinary fork. When the
escapement is being unlocked the guard pin _i_ enters the hollow and
when the escape tooth comes into contact with the lifting plane of the
pallet the pin _i_, Fig. 27, transmits the impulse to the roller.
[Illustration: Fig. 26.]
[Illustration: Fig. 28.]
The impulse is transmitted closer to the line of centers than could be
done with any ruby pin. If the pin _i_ were wider the impulse would be
transmitted still closer to the line of centers, but the intersection of
it with the roller would be less. It is very delicate as it is,
therefore from a practical standpoint it ought to be made thin but
consistent with solidity. If the pin is anyway large, it should be
flattened on the sides, otherwise the friction would be similar to that
of the round ruby pin. It would also be preferable (on account of the
pin _i_ being very easily bent) to make the impulse piece narrow but of
such a length that it could be screwed to the fork, the same as the dart
in the double roller. The impulse radius is also the radius of the
roller, because the impulse is transmitted to the roller itself; for
this reason the latter is smaller in this action than in the ordinary
one having the same angles; also a shorter lever is in contact with a
longer one in the unlocking than in ordinary action of the same angles;
but for all this the pins _u u_ should be pitched close to the edge of
the roller, as the angular connection of the balance with the escapement
would be increased during the unlocking action. This escapement being
very delicate requires a 12° pallet angle and a proportion between
impulse and pallet angles of not less than 3 to 1, which would mean an
impulse angle of 36°; this, together with the first rate workmanship
required are two of the reasons why this action is not often met with.
George Savage, of London, England, invented this action. He was a
watchmaker who, in the early part of this century, did much to perfect
the lever escapement by good work and nice proportion, besides inventing
the two pin variety. He spent the early part of his life in Clerkenwell,
but in his old days emigrated to Canada, and founded a flourishing
retail business in Montreal, where he died. Some of George Savage's
descendants are still engaged at the trade in Canada at the present day.
The correct delineation of the lever escapement is a very important
matter. We illustrate one which is so delineated that it can be
practically produced. We have not noticed a draft of the lever
escapement, especially with equidistant pallets and club teeth, which
would act correctly in a watch.
We have been aggressive in our work and have sometimes found theories
propounded and elongated which of themselves were not right; this may
have something to do with it, that we so often hear workmen say, "Theory
is no use, because if you work according to it your machine will not
run." We say, "No, sir, if your theory is not right in itself, then your
work will certainly not be correct; but if your theory be correct then
your work _must_ be correct. Why? it simply cannot be otherwise." We
will give it another name; let us say, apply sense, reason, thought,
experience and study to your work, and what have you done? You have
simply applied theory.
A theorem is a proposition to be proved, not being able to prove it, we
must simply change it according as our experience dictates, this is
precisely what we have done with the escapement after having followed
the deductions of recognized authorities with the result that we can now
illustrate an escapement which has been thoroughly subjected to an
impartial analysis in every respect, and which is theoretically and
practically correct.
We will not only give instructions for drafting the escapement now under
consideration, but will also make explanations how to draft it in
different positions, also in circular pallet and single roller. We are
convinced that by so doing we will do a service to many, we also wish to
avoid what we may call "the stereotyped" process, that is, one which may
be acquired by heart, but introduce any changes and perplexity is the
result. It is really not a difficult matter to draft escapements in
different positions, as an example will show.
Before making a draft we must know exactly what we wish to produce. It
is well in drafting escapements to make them as large as possible, say
thirty to forty times larger than in the watch, in the present case the
size is immaterial, but we must have specifications for the proportions
of the angles. Our draft is to be the most difficult subject in lever
escapements; it is to be represented just as if it were working in a
watch; it is to represent a good and reliable action in every respect,
one which can be applied without special difficulty to a good watch, and
is to be "up to date" in every particular and to contain the majority
of the best points and conclusions reached in our analysis.
_Specifications for Lever Escapement_: The pallets are to be
equidistant; the wheel teeth of the "club" form; there are to be two
rollers; wheel, pallet, and balance centers are to be in straight line.
The lock is to be 1½°, the run ¼°, making a total lock of 1¾°; the
movement of pallets from drop to drop is to be 10°, while the fork is to
move through 10¼° from bank to bank; the lift on the wheel teeth is to
be 3°, while the remainder is to be the lift on the pallets as follows:
10¼ - (1¾ + 3) = 5½° for lift of pallets.
The wheel is to have 15 teeth, with pallets spanning 3 teeth or 2½
spaces, making the angle from lock to lock = 360 ÷ 15 × 2½ = 60°, the
interval from tooth to tooth is 360 ÷ 15 = 24°; divided by 2
pallets = 24 ÷ 2 = 12° for width of tooth, pallet and drop; drop is to
be 1½°, the tooth is to be ¾ the width of the pallet, making a tooth of
a width of 4½° and a pallet of 6°.
The draw is to be 12° on each pallet, while the locking faces of the
teeth are to incline 24°. The acting length of fork is to be equal to
the distance of centers of scape wheel and pallets; the impulse angle is
to be 28°; freedom from dart and safety, roller is to be 1¼°, and for
dart and corner of crescent 5°; freedom for ruby pin and acting edge of
fork is to be 1¼°; width of slot is to be ½ the total motion, or
10¼ ÷ 2 = 5⅛°; shake of ruby pin in slot = ¼°, leaving 5⅛ - ¼ = 4⅞° for
width of ruby pin.
Radius of safety roller to be 4/7 of the theoretical impulse radius. The
length of horn is to be such that the end would point at least to the
center of the ruby pin when the edge of the crescent passes the dart;
space between the end of horn and ruby pin is to be 1½°.
It is well to know that the angles for width of teeth, pallets and drop
are measured from the wheel center, while the lifting and locking angles
are struck from the pallet center, the draw from the locking corners of
the pallets, and the inclination of the teeth from the locking edge.
In the fork and roller action, the angle of motion, the width of slot,
the ruby pin and its shake, the freedom between dart and roller, of ruby
pin with acting edge of fork and end of horn are all measured from the
pallet center, while the impulse angle and the crescent are measured
from the balance center. A sensible drawing board measures 17 × 24
inches, we also require a set of good drawing instruments, the finer the
instruments the better; pay special attention to the compasses, pens and
protractor; add to this a straight ruler and set square.
The best all-round drawing paper, both for India ink and colored work
has a rough surface; it must be fastened firmly and evenly to the board
by means of thumb tacks; the lines must be light and made with a hard
pencil. Use Higgins' India ink, which dries rapidly.
[Illustration]
We will begin by drawing the center line A′ A B; use the point B for the
escape center; place the compass on it and strike G H, the primitive or
geometrical circle of the escape wheel; set the center of the protractor
at B and mark off an angle of 30° on each side of the line of centers;
this will give us the angles A B E and A B F together, forming the angle
F B E of 60°, which represents from lock to lock of the pallets. Since
the chord of the angle of 60° is equal to the radius of the circle, this
gives us an easy means of verifying this angle by placing the compass at
the points of intersection of F B and E B with the primitive circle G H;
this distance must be equal to the radius of the circle. At these points
we will construct right angles to E B and F B, thus forming the tangents
C A and D A to the primitive circle G H. These tangents meet on the line
of centers at A, which will be the pallet center. Place the compass at A
and draw the locking circle M N at the points of intersection of E B and
F B with the primitive circle G H. The locking edges of the pallets will
always stand on this circle no matter in what relation the pallets
stand to the wheel. Place the center of the protractor at B and draw the
angle of width of pallets of 6°; I B E being for the engaging and J B F
for the disengaging pallet. In the equidistant pallet I B is drawn on
the side towards the center, while J B is drawn further from the center.
If we were drawing a circular pallet, one-half the width of pallets
would be placed on each side of E B and F B. At the points of
intersection of I B and J B with the primitive circle G H we draw the
path O for the discharging edge of the engaging and P for that of the
disengaging pallet. The total lock being 1¾°, we construct V′ A at this
angle from C A; the point of intersection of V′ A with the locking
circle M N, is the position of the locking corner of the engaging
pallet. The pallet having 12° draw when locked we place the center of
the protractor on this corner and draw the angle Q M E. Q M will be the
locking face of the engaging pallet. If the face of the pallet were on
the line E B there would be no draw, and if placed to the opposite side
of E B the tooth would repel the pallet, forming what is known as the
repellant escapement.
[Illustration: Fig. 28.]
Having shown how to delineate the locking face of the engaging pallet
when locked, we will now consider how to draft both it and the
disengaging pallet in correct positions when unlocked; to do so we
direct our attention until further notice to Fig. 28. The locking faces
Q M of the engaging and S N of the disengaging pallets are shown in
dotted lines _when locked_. We must now consider the relation which the
locking faces will bear to E B in the engaging, and to F B in the
disengaging pallets when unlocked. This is a question of some
importance; it is easy enough to represent the 12° from the 30° angles
when locked; we must be certain that they would occupy exactly that
position and yet show them unlocked; we shall take pains to do so. In
due time we shall show that there is no appreciable loss of lift on the
engaging pallet in the escapement illustrated; the angle T A V
therefore shows the total lift; we have not shown the corresponding
angles on the disengaging side because the angles are somewhat
different, but the total lift is still the same. G H represents the
primitive circle of the escape wheel, and X Z that of the real, while
M N represents the circular course which the locking corners of the
pallets take in an equidistant escapement. At a convenient position we
will construct the circle C C′ D from the pallet center A. Notice the
points _e_ and _c_, where V A and T A intersect this circle; the space
between _e_ and _c_ represents the extent of the motion of the pallets
at this particular distance from the center A; this being so, then let
us apply it to the engaging pallet. At the point of intersection _o_ of
the dotted line Q M (which is an extended line on which the face of the
pallet lies when locked), with the circle C C′ D, we will plant our
dividers and transfer _e c_ to _o n_. By setting our dividers on _o_ M
and transferring to _n_ M′, we will obtain the location of Q′ M′, the
locking face when unlocked. Let us now turn our attention to the
disengaging pallet. The dotted line S N represents the location of the
locking face of the disengaging pallet when locked at an angle of 12°
from F B. At the intersection of S N with the circle C C′ D we obtain
the point _j_. The motion of the two pallets being equal, we transfer
the distance _e c_ with the dividers from _j_ and obtain the point _l_.
By setting the dividers on _j_ N and transferring to _l_ N′ we draw the
line S′ N′ on which the locking face of the disengaging pallet will be
located when unlocked. It will be perfectly clear to anyone that through
these means we can correctly represent the pallets in any desired
position.