53. The Lever Arbor Pivots should also be small, with small shoulders so as to reduce fluid friction to a minimum.
It may be well to add that in all uncapped bearings of pivots in the train, whether they be of jewels or of brass, a slight convex shape can profitably be given to the surface where the shoulder of the arbor, or pinion, touches the bearing, thereby reducing not only the surface of contact at the shoulder, and consequently diminishing the cause of friction (41), but by reducing the distance from the center, at which the friction acts, the retarding effect of the friction is much less (46), thus obtaining a greater effort (25).
54. The Balance Arbor Pivots and Bearings, as well as those of the lever and scape wheel where their pivots run in capped jewels, deserve particular attention. Fig. 14 shows hole and cap jewels in settings, but what applies to them is equally applicable to all capped jewels, with few exceptions.
Fig. 14.
In Fig. 14 all the laws of capillary action are applied. It has been shown (22,8) that, when two watch glasses are fixed rigidly relatively with their convex sides adjacent, if a drop of oil be placed near their centers it can be shaken from its position only with great difficulty.
The jewels, in this instance, present much the same form, though only a minute quantity of oil, instead of a drop, is involved; but the same influences are at work in both cases.
This reservoir, if properly made, will contain enough oil to last a long time; as, when the oil in the center is used up, that which is nearer the settings will be drawn to the pivot. The writer has said "nearer" the settings; but it is very important that the oil should never touch the setting (58).
Both settings are cut away at aa´, in order that as little attractive influence (22) as possible may be exerted on the oil by the metal in the settings.
Where the adjacent surfaces of the hole and cap jewel are flat and parallel the oil will usually have a tendency to be drawn to the setting—the evil effect of which will be shown (58) later—especially if the hole and cap jewel are at any appreciable distance from each other; while if they are too close together, the reservoir will not be sufficiently large.