Chapter VIII: The Escapement
75. It is outside the scope of this work to describe the different escapements or to discuss their comparative advantages. We are concerned only with the external parts of the movement which serve the escapement and which hold it rigidly in place.
76. We begin with the cylinder escapement. It always seemed to me that the adjustable bridge (chariot) is a nearly redundant addition. If the distance between the cylinder and escape wheel are correctly set then it is desirable to maintain this invariably, and the mobility of the chariot is a danger for the good operation of the watch. Also, nobody thinks that the correct setting up of a cylinder escapement is a difficult task, but the duplex escapement rarely has a chariot even though it requires greater accuracy because of its fragile nature. Besides, the cylinder escapement is more suitable than other escapements for interchangeable manufacture and we should take advantage of this.
The complete omission of the chariot would make the movement simpler and easier to make, because the lower balance bridge can then be omitted and the balance hole and escape wheel hole, when on the bridge, can be set in the plate. The need for a chariot is just a bias which developed from habit and blind imitation. If a cylinder escapement is correctly set then it will remain that way for all time and no inexperienced hand will be able to change this set-up; with regard to those escapements which are incorrectly set up, they should not have passed examination without being corrected.
77. The cylinder escapement does not require as much area as the lever escapement. Thus the movement is not pushed for space and an advantage is that a better train can be made with a higher numbered centre wheel than in a lever watch of the same diameter (Art. 53).
78. In all countries, France perhaps excepted, the cylinder escapement has been nearly displaced by the lever escapement, and some remarks need to be made about it.
The lever escapement permits a greater diversity of layout and we must firstly consider the question, whether it is to be set in a straight line or at right angles. The latter has been recommended because it saves space or, which means same, it allows a more convenient arrangement of the parts. Thus with it we could make the escape wheel, lever and fork larger in a movement of the same diameter. This may appear advantageous for the reasons I have given for the size of wheels and pinions (Art. 53), but for escapements other aspects must be taken into consideration. In the first place, we must remember that during the interrupted running of a resting or free escapement the inertia of the wheels has to be overcome with each oscillation, and this inertia should be minimised. Over and above this, the sliding friction of the escape wheel on the pallet surfaces is, by its very nature, different from the rolling friction of teeth and it becomes considerably larger as the surface area increases. For these reasons the escape wheel, lever and fork should not exceed certain limits and should be made as light as possible without adversely affecting their strength. The length of the fork must also be limited. I do not want to repeat here what I have already detailed in my prize essay on the free anchor escapement (chapter IX, page 62). The action of the fork and roller is more robust, so that we can make them relatively larger in order to observe their action more easily. (11)
For the same reasons it is not advisable to make the wheel and other parts of the escapement from gold, whose density would be disadvantageous.
79. As we have seen, there is no benefit in terms of saving space from using a right-angle escapement, unless in very complicated work where the available room is limited by other parts of the mechanism. The use of one or other of these two layouts must be considered as little more than a matter of taste. As far as friction is concerned, there appears to be a small difference in favour of the right-angle escapement; however this difference, which was discussed in the prize essay mentioned above, is of no great importance. All straight line escapements must have jewelled lever holes, because the bushing of a lever hole is more vital with the straight line escapement than with the other. The reason is that any deviation from the correct centre distances necessarily causes not only an error in the action of the escape wheel and pallets, but also in the fork and roller.
80. In view of the previous arguments, the diameter of the escape wheel in a lever watch should not exceed 1/5 of the diameter of the pillar plate. A good ratio will be then obtained by making the acting length of the fork, i.e. the distance from the lever centre to the working corners of the fork, equal to the radius of the wheel or 0.1 of the diameter of the plate.
With these conditions the centre of the lever will lie within the circumference of the balance, if the balance is not disproportionately small.
81. We could obtain a slight saving by setting the escape wheel and lever under the same bridge; but then we would have to make do without the advantage of a short fork or we would have to make the escape wheel arbor as short as the pallet arbor, which should be under the balance. We should avoid this, because the stability of the escape wheel pinion is greater if the pivots are further apart (Art. 60); therefore the small amount of work and expense which result from using a separate bridge for the escape wheel should not be regarded as a major obstacle.
82. The depth of the 4th wheel in the escape pinion should not be too great because otherwise the correct operation of the gear, which by its nature is the most sensitive and least perfect of the whole train, is endangered by the smallest change in the steady pins of the escape wheel bridge.
For the same reason this bridge should be set so that a straight line between the pivot hole and the screw hole passes through the centre of the 4th wheel, or not far from it, because the depths will then be less affected by a bent steady pin.
83. The balance cock, in the normal course of production and repair, must be removed and replaced frequently; and it is important to make the steady pins very carefully as much trouble and damage can result if they are badly made. A well fitted cock, especially that of the balance, should slide easily into the steady pin holes to a distance of some tenths of a millimetre from the plate, stand firmly in its place and at the same time hold in such a way that the escapement can be tested safely without screwing the cock on. This can be only obtained by using slightly tapered steady pins.
I cannot recommend the English method of screwing in the steady pins, because they are not easy to make and do not give the same certainty of exact fitting as a pin which is driven into a round hole. For doing this I have always found the following method very good. I take a piece of wire, somewhat thicker than the hole, and file its length slightly tapered (the same as a broach) until it goes approximately half way into the steady pin hole of the pillar plate. Then I take a sharp polishing file (12) and, with the pin in a pinvise resting on a suitable notch in a filing block, I work on the end of it with the polishing file so that somewhat more of its length than amounts to the thickness of the plate is tapered. If this done in the right way the conical part of the pin will fit into the hole of the pillar plate. Then I take a good broach and broach the appropriate hole in the cock until the prepared pin goes in so far that its end is flush with the lower surface of the cock (13). However, this must be left to experience because much depends on the comparative hardness of the cock and the pinwire, as well as on the shape in which the wire has been filed. Then it is cut off leaving a just sufficient length protruding above the upper surface of the cock. Then, after the bottom of the cock has been put on a flat piece of steel with a hole only little larger than the pin, I drive the pin in firmly, frequently testing it in the hole of the pillar plate, until the pin holds the cock firmly. The other pin is made in the same way. A cock which is fitted according to this method goes completely easily into the holes until the pivot is in the jewel hole, and then with the last pressure, which can now be applied safely and without danger for the jewel hole, it holds securely. These conical steady pins offer the further advantage that if they are slightly bent the position of the bridge is not significantly affected; because, due to their conical form, they gain their secure footing in the plate only by the part closest to the bridge, while those parts most likely to be bent are free in the hole.
84. If the pins are well fitted then two are perfectly sufficient and much better than three made in the usual careless way, with which a cock fits tightly at first but nevertheless has shake when it is close to the plate.
The steady pins should not be too long, because they are very easily bent otherwise. Their length may not exceed twice their diameter, and the pinwire should always be drawn as hard as possible. If the steady pins are to do their service in an effective way then they must be placed as far apart as the foot of the cock allows.
85. The balance is a part which has very different relative sizes in different watches and without undertaking a more extensive description I will limit myself to stating that a good value for the diameter of the balance is to multiply the diameter of the pillar plate by 0.4, or to take 4 tenths of it. For a work of 43 mm diameter the balance would then be 43 x 0.4 = 17.2 mm.
86. If the work is to have a compensation balance, great care must be exercised to ensure that the inside and outside of the rim have plenty of room. I have seen many cases where inexperienced workers were driven nearly to despair by watches which were apparently in good condition and went very well, but at the start of the cold season stopped every night. If one was examined, which naturally happened in a warm room, it resumed its usual running without showing the least problem, until it was found that the expansion of the balance caused it to come too close to a bridge or another part.
11 This sentence makes little sense to me in either language. [Trans]
12 Presumably a pivot file [Trans].
13 The hole goes right through the cock and the pin is inserted from the top. [Trans]