An Analysis of the Lever Escapement | Page 8

and tooth with different angles, through every stage of the lift, with both wide and narrow pallets and teeth, in circular and equidistant escapements, will have received an eye-opener. We strongly advise all our readers who are practical workmen to try it after studying what we have said. We are certain it will repay them.
[Illustration: Fig.?2.]
The Center Distance of Wheel and Pallets. The direction of pressure of the wheel teeth should be through the pallet center by drawing the tangents AC and AD, Fig.?2 to the primitive circle GH, at the intersection of the angle FBE. This condition is realized in the equidistant pallet. In the circular pallet, Fig.?3, this condition cannot exist, as in order to lock on a tangent the center distance should be greater for the engaging and less for the disengaging pallet, therefore watchmakers aim to go between the two and plant them as before specified at A.
When planted on the tangents the unlocking resistance will be less and the impulse transmitted under favorable conditions, especially so in the circular, as the direction of pressure coincides (close to the center of the lift), with the law of the parallelogram of forces.
It is impossible to plant pallets on the tangents in very small escapements, as there would not be enough room for a pallet arbor of proper strength, nor will they be found planted on the tangents in the medium size escapement with a long pallet arbor, nor in such a one with a very wide tooth (see Fig.?4) as the heel would come so close to the center A, that the solidity of pallets and arbor would suffer. We will give an actual example. For a medium sized escape wheel with a primitive diameter of 7.5?mm., the center distance AB is 4.33?mm. By using 3° of a lifting angle on the teeth, the distance from the heel of the tooth to the pallet center will be .4691?mm.; by allowing .1?mm. between wheel and pallet and .15?mm. for stock on the pallets we find we will have a pallet arbor as follows: .4691?-?(.1?+?.15)?×?2?=?.4382?mm. It would not be practicable to make anything smaller.
[Illustration: Fig.?3.]
It behooves us now to see that while a narrow pallet is advisable a very wide tooth is not; yet these two are inseparable. Here is another case for a compromise, as, unquestionably the pallets ought to be planted on the tangents. There is no difficulty about it in the English lever, and we have shown in our example that a judiciously planned club tooth escapement of medium size can be made with the center distance properly planted.
[Illustration: Fig.?4.]
When considering the center distance we must of necessity consider the widths of teeth and pallets and their lifting angles. We are now at a point in which no watchmaker of intelligence would indicate one certain division for these parts and claim it to be "the best." It is always those who do not thoroughly understand a subject who are the first to make such claims. We will, however, give our opinion within certain limits. The angle to be divided for tooth and pallet is 10?°. Let us divide it by 2, which would be the most natural thing to do, and examine the problem. We will have 5?° each for width of tooth and pallet. We must have a smaller lifting angle on the tooth than on the pallet, but the wider the tooth the greater should its lifting angle be. It would not be mechanical to make the tooth wide and the lifting angle small, as the lifting plane on the pallets would be too steep on account of being narrow. A lifting angle on the tooth which would be exactly suitable for a given circular, would be too great for a given equidistant pallet. It follows, therefore, taking 5?° as a width for the tooth, that while we could employ it in a fair sized escapement with equidistant pallets, we could not do so with circular pallets and still have the latter pitched on the tangents. We see the majority of escapements made with narrower teeth than pallets, and for a very good reason.
In the example previously given, the 3° lift on the tooth is well adapted for a width of 4?°, which would require a pallet 6° in width. The tooth, therefore, would be ? the width of pallets, which is very good indeed.
From what we have said it follows that a large number of pallets are not planted on the tangents at all. We have never noticed this question in print before. Writers generally seem to, in fact do, assume that no matter how large or small the escapement may be, or how the pallets and teeth are divided for width and lifting angle, no difficulty will be found in locating the