Friction, Lubrication and the Lubricants in Horology

by W. T. Lewis

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.

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 brass 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 assist 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 quantity in the interstices of the links; while the latter should have oil on its clicks, as well as on the arbor where it passes through the wheel. If the ratchet of the maintaining power be of brass 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 brass or some other metal.

Where the bearings of the center pinions are of brass or nickel, there is little difficulty experienced in making them perfectly "upright"—a condition necessary to produce a minimum amount of friction—while, if the bearings are jewels which are not upright, the friction, and consequent wear, will be increased. Properly jeweled bearings produce a maximum durability, as they cause the least friction; while the coefficient of friction is subject to much less fluctuation on account of the harder, smoother surface of the jewel, (43, 46, 47 and 61).

Where there is a brass bearing for the lower pivot, in watches having a solid center arbor on which the cannon pinion revolves in setting, the length of the bearing may be profitably increased by making a boss on the outer side of the lower plate, provision for which is then made in the cannon pinion by a suitable recess. In either case the laws previously given should be complied with.

A source of mischief in many watches is the manner in which the minute wheel is made; the construction being such that its teeth touch the plate so near the bearing of the center arbor that capillary attraction (19, 22) is produced, which causes all the oil to leave the lower bearing of the center arbor. This can be avoided by cutting off the lower parts of the teeth of the minute wheel; or, by turning a groove in the plate which will be concentric with the minute wheel post, and which will pass under the teeth of the wheel, but not near enough to the bearing of the center arbor to injure the latter.

The oil from the stem wind mechanism, also, sometimes flows under the minute wheel, and from there into the center arbor bearing; and, when the oil is used up in the former place, it is drawn up again out of the latter place leaving it dry. A means of preventing this will be discussed (59) later.

Another and very frequent cause of the lower center pivot cutting, particularly in new watches, is the neglect to remove the polishing material from the cannon pinion where the center arbor is solid.

A small portion of oil should be applied to the bearings of the minute wheel, (where its pinion, or the pivot on which it revolves, is steel), hour wheel, and cannon pinion where the center arbor is solid, and to the set hands arbor where the center arbor is hollow. The safety pinion should always be oiled, as it may not otherwise be of much service.

50. The Third Pinion Pivots are sometimes the source of mischief. When the center wheel is placed above or below the barrel, the upper or lower pivot of the third pinion receives such great stress that the oil is forced out in many cases. By increasing the length of the pivot this could be obviated. The minute wheel is sometimes so close to the lower bearing of this pinion as to absorb the oil. This can be remedied by cutting a recess in the lower side of the minute wheel. Where it is possible to do so the wheels should be so placed on their pinions and arbors, and at such a distance from the bearing surfaces of the latter, that the stress on each pivot—the combined result of the weight of the wheel and the forces acting on it—will be equal.

51. The Fourth Pinion Pivots should follow the same general laws as that given for the rest of the train; but it should be borne in mind that fluid friction acts as a retarding force much more perceptibly in the lighter parts of the train; consequently if no second-hand is to be carried, very small bearing surfaces should be the rule in this case.

52. The 'Scape Pinion Pivots as well as the shoulders should not be too large, while there should be sufficient back taper to insure the oil remaining at the pivots. A very small quantity of oil should be applied, as, when too much is used, it is liable to work up into the pinion where the latter is short, as in very thin watches, thus producing, when very fine dust is added, a mixture that acts much like oil stone power and oil, which cuts away the leaves of the pinion.

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.

The conical pivot shown is the usual form in the finer grades of American watches; and as this form of pivot combines strength with a minimum tendency to attract the oil from the jewel hole, it is to be highly recommended. The back-taper T should never be neglected for reasons previously (47, 6) given. The proportions that should exist between the diameter of the pivot and the length of its bearing surface, as well as the shape of the end of the pivot, cannot be discussed here, as the scope of this work will not permit; but it should be borne in mind that the smaller the pivots, consistent with strength, the less the fluid friction will be. The sides of the pivots should be straight and parallel for a minute distance from their bearing surfaces; while the form of the rest of the pivot should be a gradually increasing curve, terminating at the point where the back-taper begins.

The proper proportion of the diameter of the pivot to the diameter of the jewel hole varies according to conditions; but it has been previously (37) shown in a general way what this should be.

55. The Escapements should be constructed in such a way that a maximum durability of oil may be secured. The acting surfaces of the teeth of the scape wheels should be made as small as possible consistent with durability (43, 8); while enough metal should be left near the acting surfaces to be sufficient to retain the oil and prevent its attraction to the web of the wheel. The teeth of chronometer scape wheels should not be oiled, as it is liable to seriously alter the rate. When the oil becomes viscous by oxidation or by cold it would produce too much variation of fluid friction and so diminish the effort (25) of the mechanism. Some watchmakers oil the fork of the lever in anchor escapements very slightly, by applying oil and then using pith to remove any surplus, while others never oil the fork. The writer has frequently observed ferric oxide or "rust" on the roller, fork, and on the plate or potance; but whether this was the result of not oiling or of oil having been applied which afterward become gummed, or evaporated, it would be interesting to know.

56. The Curb Pins sometimes produce the ferric oxide mentioned by their action on the hairspring. This has been remedied by the same method as used in the fork just referred to, and if a very minute quantity of oil can be applied—such a minute quantity that if the whole spring were equally covered by a coating of oil equally thin, such film being so thin that it would have no tendency to cause the coils to adhere, or to cause small particles of matter to adhere—then it may be that this method deserves notice.

By making a solution of benzine and oil (100 drops of the former with 1 to 10 drops of the latter) and by immersing the hairspring in this solution and on withdrawing it dry it quickly between soft, fine linen, it will be found that the coils of the hairspring do not adhere to each other. The effect that this would produce on the whole spring by way of preventing rust in damp, warm climates, will be stated (78) later.

57. The Application Of Oil must be attended with great care. The shoulders of the barrel and center arbors may be profitably oiled before putting them in their places, applying an additional small amount afterward. The rest of the pivots should be oiled after the movement is set up—except in the case of capped jewels—as if oil is applied to each pivot as the wheel is put in position it would be difficult to keep the oil in good condition and at its proper place if it should be necessary to take the movement apart again for any purpose.

The oil is more evenly distributed on the teeth of scape wheels, where such require lubrication, if a small quantity of oil be applied to each tooth, or every second or third tooth. A small amount added to the surfaces on which the teeth act will in most cases be beneficial. If it be necessary to take the movement partially apart for any purpose, after it has been oiled, care should be taken not to give the train a too rapid motion, as the centrifugal force (23) resulting from the rapid circular motion of the wheels will be liable to cause the oil to leave the jewel holes and spread upon the surfaces of the jewels, and also cause the oil to fly off the teeth of the scape wheel to its determent and that of other parts which are better without oil.

58. The Method of Oiling Capped Jewels has been given by Saunier, as follows:[11] "When a drop of oil is introduced into the oil cup of the balance pivot-hole, insert a very fine pegwood point, so as to cause the descent of the oil. When this precaution is not taken, it frequently happens that in inserting the balance pivot its conical shoulder draws away some of the oil, and there is a deficiency both in the hole and on the endstone." In both the English and American editions, this erroneous method is repeated.

By this means, only an insufficient quantity of oil can be caused to flow into the reservoir, as the pressure of the air inside will prevent the oil flowing in; as, in the case of a glass tube with the upper end sealed up, it has been shown (22, 2) that the water refused to be drawn up the tube, even with the added pressure caused by the lower end of the tube being below the water line. Again, the point of pegwood is liable to have minute fibres of wood adhering to it, which will be incorporated with the oil; and its liability to break off, and remain in the jewel hole, is another reason why pegwood should never be used. The author advances a method, which is not open to these objections, as follows: When about to place the cap jewel in position—after the hole jewel is in place if it be in a setting—a small quantity of oil is placed ON THE CAP JEWEL, as shown at O, Fig. 14, being very careful to allow no oil to spread upon the cap jewel setting. This setting is then carefully placed in position; when the oil, if the operation has been skillfully performed, is seen to be collected in the reservoir R and in the jewel hole. The appearance which it will assume is shown in Fig. 14. The advantages which this method possesses are: the reservoir can by this means be made to contain the maximum quantity of oil; and the oil cup or sink S is left with its surface dry, thereby exposing less oil to the influences of the air; and, at the same time the tendency of the oil to flow towards the shoulder of the pivot is decreased.

Skill is necessary in order to judge of and place the requisite amount of oil on the cap jewel before putting it in position; as, if too much is used it is worse than if too little is employed, because the oil would then flow on to the setting, and from there between the settings at b, when it will rapidly be all drawn from the bearing, leaving it dry, while the settings are copiously supplied. The approximate relative position which the oil should occupy is shown at d, Fig. 14, in section; and this can be seen by looking through the jewels with a double eye-glass, when a true circle, concentric with the jewel hole, will be seen to have formed. This circle represents the limit of the distance which the oil has flowed from the jewel hole. When too much oil has been applied, this limit is not a circle, but represents a U.

In the example given, the upper surface of the cap jewel is made flat, while the lower surface is made convex with a flat space in the center; as a better view of the end of the pivot and the condition of the oil can be thereby obtained.

In no case should the contiguous surfaces of the hole and cap jewel be both made flat; as, when their planes are vertical, the oil will be drawn downwards by gravitation (18), there being no counteracting force (22) to keep the oil in place. The author has remedied this defect, in many instances, by cutting a groove around the jewel, leaving only enough metal near the jewel to hold it, and enough near the edge of the setting to rest solidly against the other setting.

In some watches, particularly those of Swiss make, the jewel bezels—both cap and hole—are brought well up around the jewel, while a groove is cut around the jewel bezel. In this instance the oil may be made to cover the whole inside surface of both jewels, as the groove will prevent the oil from flowing away to parts where it is not required.

The reprehensible practice of replacing a broken cap jewel by cutting away the bezel and placing the new jewel in loosely, cannot be too severely condemned. The new cheap foreign-made watches contain this objectionable feature in many instances.

Where the jewels are in settings, sharp instruments, as tweezers, etc., should never be used to push the settings in place; as the projections produced in this manner would not only injure the appearance of the settings, but would prevent their close contact. Thoroughly clean, well-finished jewel pushers are indispensable; as even pegwood is liable to leave fibres at least.

The shape of the oiler is a matter of some importance; as with a poorly-made oiler it is next to impossible to do work satisfactorily. The tip is preferably of gold, tapering towards the end to about the size of a second's hand pivot of an eighteen size American movement; but at the end it should be about three times as wide and flat. A nickel fastened to the end of a lead-pencil will give the idea approximately. This large end will cause the oil to remain where it may be readily applied to the bearing surface, instead of flowing back on the oiler towards the handle, as it would (22, 7) if the point were tapering.

59. The Stem Winding Mechanism should be thoroughly well made, always keeping in view that the laws of capillary attraction must be complied with.

Wherever an angle can be formed, with its apex pointing towards the place where the oil is required to remain, it should be done.

A very good lubricant for stem wind parts is found in stearine, from which the animal oils are expressed at cold temperatures, as it is very thickly fluid at ordinary temperatures; while an excellent lubricant for this purpose is paraffine—not the wax nor the oil, but that white, soft substance from which both are obtained (13 & 73). Stearine and paraffine both possess great viscosity; and, though the fluid friction is increased by their use, the solid friction is diminished. Then, too, the tendency to spread is very much less.

60. The Pendant is frequently a cause of trouble to the watchmaker. It is very important that the winding stem be lubricated with a substance that will not spread at ordinary temperatures. The lubricant should be applied at all places where steel rubs on steel or other metal. The winding stem and case spring, and the sleeve if present should have as much as can be safely applied; as they are so much exposed that rust often forms, which finds its way down through the movement, frequently resulting in serious damage to the delicate parts. The bearings of collet on stem and the pendant screw should also be lubricated.

Attention to these details will also prevent "that squeaking sound" which, sometimes occurring shortly after a watch has been repaired, causes the owner to believe that the work was not done properly.

The lubricants just mentioned (59) serve admirably for this purpose.

61. The Cause of the Cutting of Pivots, in addition to the effect of friction (32, 1) and other causes which have been mentioned (49), may be that minute currents of static electricity are induced between the surfaces of the pivot and bearing, the oil acting as the electrolyte.

If this be the case, the cause of pivots turning black would appear to be explained—the molecules of iron becoming electrically disassociated from the molecules of carbon, the latter being by their nature black, and being now on the surface in sufficient quantities to make themselves evident, give the surface the black color. Such is the first stage of "cutting."

The molecules of iron, becoming incorporated in the now thick and viscous oil or imbedding themselves in the bearing, act as an abrasive; the black surface is removed, making the pivot again bright, but "ringed." The molecules of iron, uniting with the molecules of oxygen which exist in the oil in its oxidized state, forms ferric oxide.

Ferric oxide is known as colcothar, English-Roth, rouge, crocus, etc.

The above theory is advanced by the author for what it may be worth, as it seems to explain this curious phenomenon.