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.
CHAPTER IX.
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.
Fig. 39. Pin Wheel Escapement.