4. "Bra.s.s pivot holes are much less affected by the drying of the oil than agate holes would be; and, in the absence of experiment, we must a.s.sume that this would be the case with ruby or other jewels.

5. "When the oil is perfectly fresh, agate and steel have a very low coefficient of friction."

How much these results would be altered by the use of a disk of such weight, and pivots of such proportionate size, as to meet the actual requirements in horology, remains to be ascertained.

Certainly the experiments of Jenkin, are _not_ applicable to the pivots of a watch, as stated by E. Rigg; especially are they not applicable to the friction of pivots in the escapement, where the laws of fluid friction are more nearly applicable; and when it is remembered that the weight of the disk was 86 pounds, and the pivots .25 c. m. in diameter, (or about the size of pivot of a large barrel arbor,) it is evident that the solid friction produced was much in excess of that produced in even the heavy part of the train of a watch.

Furthermore, even Jenkin and Ewing, in their paper, state "that, owing to the very great intensity of the pressure on the small bearing surfaces of the axle, the lubricants must have been to a great extent forced out." In a properly made watch, with a good lubricant, this does not occur.

But there can be no doubt that if the apparatus above described were so constructed as to meet the actual conditions in time recording instruments, very valuable data could be thereby secured. This could be done by reducing the weight of the disk, so as to make the weight bear the proper relation to the size of the pivots.

FOOTNOTES:

[7] Thurston. Friction and Lost Work in Machinery.

[8] Philosophical Transactions, 1877, Vol. CLXVII., p. 502.

[9] The Horological Journal, Apr., 1881. Vol. XXIII., page 98.

[10] The writer has italicised this phrase.

CHAPTER IV.

APPLICATION OF THE LAW OF FRICTION AND LUBRICATION IN HOROLOGY.

~45.~ The scope of this work will not permit the discussion of the proper size, shape and construction of each and every part of all the various kinds of time-keeping mechanisms which have been produced. However a number of _representative_ cases of friction and lubrication will be considered, and the laws applying to the same will be demonstrated.

Practical methods of obtaining the best results will be shown and mistakes to be avoided will be pointed out.

The knowledge of what we ought not to do is sometimes of vastly greater importance than it is usually considered to be.

~46. The Proportions of Pivots, Shoulders and Bearings~, where the bearings are not capped jewels, should be such that the coefficient (33) of the combined solid and fluid friction will be a minimum, and such that the lubricant will not be expelled at normal pressure, while the "fit" (37) must be good.

1. _The diameters of all pivots_ should be of the smallest size compatible (43,6) with the foregoing condition, and with the stresses which they are expected to sustain.

2. _The length of bearing surfaces_ is regulated by the pressures which may occur (43) between them, and by the nature of the materials of which they may be composed.

3. Given the diameter and the pressure, the length of the bearing surfaces can be so proportioned as to prevent abrasion and to present surfaces, between which the film of oil is interposed, of such magnitude that the lubricant will not be expelled at normal pressure.

[Ill.u.s.tration: Fig. 13.]

4. In Fig. 13 the length of bearing surface of the pivot is equal to its diameter, but the proportion must be varied according to conditions.

5. The barrel arbor pivots are sometimes necessarily of large diameter, and the bearing surfaces can be made shorter in proportion, as the surfaces will then be great enough to give good results as well as to retain (48) the oil.

6. In the center pinion (49) where the diameter of the the pivots is made small for reasons explained (43, 6), the length of the bearing surfaces must be such that abrasion will not occur, and that the oil will not be expelled.

7. The rest of the train is subject to the same laws. The length of the bearing surfaces of the pivots remote from the motive force can be made shorter in proportion.

8. The diameter of the shoulder S, Fig. 13, is reduced to as small a size as will properly sustain the "end thrust," thus reducing the friction, both solid and fluid, to a minimum, at the same time reducing the distance from the center of the arbor (43, 6) at which the friction acts.

9. The above proportions vary with the nature of the material; where jewels are employed a shorter bearing surface may be used, if it be desired to reduce friction, but the pressure on the oil is the same with jewel as with bra.s.s bearings, so that it must not be made so short that the oil will be expelled.

~47. The Shape of Pivots, Shoulders and Bearings~, where the bearings are not capped jewels, should be such as to produce as little friction as possible. They should be hard, symmetrical, and smooth (30).

_The construction should be such that a considerable amount of oil may be applied without having a tendency to spread._

The advantages of the construction shown at Fig. 13 are:

1. The oil sink _O_ is deep and narrow, rather than wide and flat--thus causing the oil to be drawn towards the apex of the angle, i. e. towards the pivot, with greater force (22, 5) than if the oil sink were wide and shallow, in which case the oil would have a tendency to spread, as too often occurs.

2. The total length of the pivot is to the length of its bearing surface as 5 is to 3, thus further reducing the angle, which produces a greater tendency (22, 5) in the oil to stay in the oil-sink.

3. A circular groove G is cut around the oil sink, which produces a still greater tendency on the part of the oil to stay in the sink, by removing metal which would otherwise exert an attraction (19) on the oil.

4. The beveled portion P is comparatively large--while the shoulder S is relatively small--thus forming the angle O' of about 20 with the flat surface of the bearing. This will cause the oil to have a tendency to flow towards the pivot, for the reason given in considering the oil-sink.

5. The boss B is made to diminish the liability of the oil to spread, by a reduction (18-19) of the amount of metal which would otherwise cause it.

6. The back taper T is made for the same reason. Some watchmakers (?) seem to think this is added only for ornament, but it is a very important factor in producing longevity of the oil.

7. The slight chamfer C, in the bearing, serves two purposes; it becomes a reservoir for oil and removes any burr that might otherwise exist in a metal bearing, without in any way altering its effectiveness.

8. It will thus be seen that the oil reservoirs O, O' and C are made to contain, and retain, the maximum amount of oil, and the supply of the lubricant is thus increased to a maximum length of time.

The application of these principles to each part to which they relate will be considered.

~48. The Barrel Arbor~, with its bearing, should be so constructed that the oil will not spread to the contiguous parts. The oil sink, with circular groove cut around the outside (46-47), both in the barrel and its cover, should not be neglected.

It is well to apply oil to the bottom and on the cover of the barrel, as well as on the coils of the spring; and before putting on the cover, a small amount applied on the arbor nut at the shoulders will a.s.sist greatly in causing the oil to be at once drawn to its proper place.

Care must be exercised while and after cleaning the mainspring, in order that it may come in contact with the fingers as little as possible, as the acids contained in perspiration are liable to be transferred to the spring and so work serious injury by contaminating the oil.

A part frequently neglected is the point of contact of the click spring with the click. If this part be not oiled rust is likely to form, and many instances have occurred where rust has found its way all through the movement from this cause. In fact, this may be said of the point of contact of all springs, with few exceptions, both in plain and complicated work.

If the watch has a chain and fusee, these both should be looked after; the former can be well oiled, and the surplus wiped off so as to leave a minute quant.i.ty in the interstices of the links; while the latter should have oil on its clicks, as well as on the arbor where it pa.s.ses through the wheel. If the ratchet of the maintaining power be of bra.s.s it should not be oiled; while if it is of steel oil should be applied. Its click should have the pivots of its arbor oiled, while what was said of clicks in general will apply here.

~49. The Center Pinion Pivots~, with their bearings, should be very carefully constructed, as this is the vulnerable point of most watches.

With proper precautions (46-47) these parts can be made so as to wear as long as the rest of the watch.

In a high-priced watch the bearings should be jewels; but in a cheap watch, where the price will not warrant correct work and careful fitting, the bearings are preferably of bra.s.s or some other metal.