An Analysis of the Lever Escapement | Page 5

action of the cutters. The truest wheel can be made of steel, as each
tooth can be successively ground after being hardened and tempered.
Such a wheel would require less drop than one of any other metal.
Supposing we have a wheel with a primitive diameter of 7.5 mm., what
is the amount of drop, allowing 1½° by angular measurement?
7.5 × 3.1416 ÷ 360 × 1.5 = .0983 mm., which is sufficient; a hair could
get between the pallet and tooth, and would not stop the watch. Even
after allowing for imperfectly divided teeth, we require no greater
freedom even if the wheel is larger. Now suppose we take a wheel with
a primitive diameter of 8.5 mm. and find the amount of drop;

8.5 × 3.1416 ÷ 360 × 1.5 = .1413 mm., or .1413 - .0983 = .043 mm.,
more drop than the smaller wheel, if we take the same angle. This is a
waste of force. The angular drop should, therefore, be proportioned
according to the size of the wheel. We wish it to be understood that
common sense must always be our guide. When the horological student
once arrives at this standpoint, he can intelligently apply himself to his
calling.
The Draw.--The draw or draft angle was added to the pallets in order to
draw the fork back against the bankings and the guard point from the
roller whenever the safety action had performed its function.
[Illustration: Fig. 5.]
Pallets with draw are more difficult to unlock than those without it, this
is in the nature of a fault, but whenever there are two faults we must
choose the less. The rate of the watch will suffer less on account of the
recoil introduced than it would were the locking faces arcs of circles
struck from the pallet center, in which case the guard point would often
remain against the roller. The draw should be as light as possible
consistent with safety of action; some writers allow 15° on the
engaging and 12° on the disengaging pallet; others again allow 12° on
each, which we deem sufficient. The draw is measured from the
locking edges M and N, Fig. 5. The locking planes when locked are
inclined 12° from EB, and FB. In the case of the engaging pallet it
inclines toward the center A. The draw is produced on account of MA
being longer than RA, consequently, when power is applied to the
scape tooth S, the pallet is drawn into the wheel. The disengaging pallet
inclines in the same direction but away from the center A; the reason is
obvious from the former explanation. Some people imagine that the
greater the incline on the locking edge of the escape teeth, the stronger
the draw would be. This is not the case, but it is certainly necessary that
the point of the tooth alone should touch the pallet. From this it follows
that the angle on the teeth must be greater than on the pallets; examine
the disengaging pallet in Fig. 5, as it is from this pallet that the
inclination of the teeth must be determined, as in the case of the
engaging pallet the motion is toward the line of centers AB, and

therefore away from the tooth, which partially explains why some
people advocate 15° draw for this pallet. As illustrated in the case of
the disengaging pallet, however, the motion is also towards the line of
centers AB, and towards the tooth as well, all of which will be seen by
the dotted circles MM2 and NN2, representing the paths of the pallets.
It will be noticed that UNF and BNB are opposite and equal angles of
12°. For practical reasons, from a manufacturing standpoint, the angle
on the tooth is made just twice the amount, namely 24°; we could make
it a little less or a little more. If we made it less than 20° too great a
surface would be in contact with the jewel, involving greater friction in
unlocking and an inefficient draw, but in the case of an English lever
with such an arrangement we could do with less drop, which advantage
would be too dearly bought; or if the angle is made over 28°, the point
or locking edge of the tooth would rapidly become worn in case of a
brass wheel. Also in an English lever more drop would be required.
The Lock.--What we have said in regard to drop also applies to the lock,
which should be as small as possible, consistent with perfect safety.
The greater the drop the deeper must be the lock; 1½° is the angle
generally allowed for the lock, but it is obvious that in a large
escapement it can be less.
[Illustration: Fig. 6.]
The Run.--The run or, as