The less the viscosity, consistently with the use of the oil under the maximum pressure to be antic.i.p.ated, the less is, usually, the friction.

The best lubricant, as a rule, is that having the least viscosity combined with the greatest adhesiveness. Vegetable oils are more viscous than animal, and animal more so than mineral oils. _The fluidity of an oil is thus, to a large extent, a measure of its value._"

The relation between the viscosity and the friction reducing power of oils has been determined by Mr. N. C. Waite[23] and others to be very close.

An oil having little viscosity is suitable for the escapement and lighter parts of the train, but is not a good lubricant for the bearings of the center pinion and barrel arbor and the mainspring, which require a more viscous lubricant; while a still greater viscosity renders it more serviceable on the stem winding mechanism (59) and in the pendant (60).

Again, an oil that possesses sufficient "body," or combined capillarity (32) and viscosity, to resist the tendency to be "squeezed" from between the bearing surfaces in the heavier parts of the mechanism will produce a _great excess of fluid friction_ in the lighter parts of the train and in the escapement.

~81. The Relative Viscosity of Oils~ is determined in several ways.

Various machines have been devised for testing the lubricating properties of oils, but as the cheap ones are of no use, and as those which are reliable are so expensive as to prohibit their general use except in laboratories and large factories, a simple method of ascertaining the relative viscosity of oils is desirable.

The author used a piece of plate gla.s.s of suitable size on which one drop of each oil to be tested was placed near its end. The gla.s.s inclined from the horizontal, longitudinally--the angle of inclination being 6 degrees--and was placed in a constant temperature of 15.5 C. (= 60 F.)

The total distance in centimeters which each had traveled by the end of each day, as well as the appearance of the "track" which it had left is shown in table VII.

TABLE VII.

RELATIVE VISCOSITY AND GUMMING OF OILS.

Temp. 15.5 C. = 60 F. Inclination 6 degrees. Time 7 days.

----------------+---------------------------------------+----------- SYMBOLS | DISTANCE IN CM. | ACCORDING TO | TRAVELED BY OIL AT THE END | WIDTH TABLE III. | OF EACH DAY. | OF | | TRACK.

----------------+----+----+-----+-----+-----+-----+-----+----------- DAYS. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | ----------------+----+----+-----+-----+-----+-----+-----+----------- E. K. w. |16 |18 |Stat.| ... | ... | ... | 18 | Medium.

W. F. N. w. |15 |16.5| 18 | 19 | 20 |Stat.| 20 | "

W. C. w. |17.5|19 | 20 |Stat.| ... | ... | 20 | Narrow.

B. & K. w. |12.5|15 | 17.5| 20 |Stat.| ... | 20 | "

C. L. Co. w. | 7.5|10 | 12.5| 15 | 17.5|Stat.| 17.5| Very wide.

C. L. Co. No. 1.|15 |16.5| 18 |Stat.| ... | ... | 18 | Medium.

Glyc. |15 |16.5| 18 |Stat.| ... | ... | 18 | "

Sp. | 0 | 2.5| 5 | 7.5| 9 | 10 | 11 | Narrow.

Ol. | 5 | 6.5| 7 |Stat.| ... | ... | 7 | "

Table VII not only shows the relative viscosity of the various oils, but also their tendency to gum or dry (79.) The "width of the track" left by the oil is an indication of the cohesion (20) and adhesion (21) which exists, respectively, in the oil and between the oil and the gla.s.s. A narrow track denotes great cohesion and little adhesion; a wide track denotes great adhesion and little cohesion; while a medium track indicates that both properties are more nearly equal.

If an oil possess great adhesion and little cohesion it is more liable to resist the tendency to be squeezed out of bearings, but it is also more likely to spread.

Another test made in the manner just described (table VII) gave results as shown in table VIII:

TABLE VIII.

RELATIVE VISCOSITY AND GUMMING OF OILS.

Temp. 24 C. = 75 F. Inclination 7 degrees. Time 7.3 days.

------------------+------------------------------------------ SYMBOLS | DISTANCE IN CM.

ACCORDING TO | TRAVELED BY THE OIL AT THE END TABLE III. | OF EACH DAY.

------------------+----+----+----+----+----+----+-----+------ DAYS. | 0.3| 1.3| 2.3| 3.3| 4.3| 5.3| 6.3 | 7.3 ------------------+----+----+----+----+----+----+-----+------ E. K. w. |14 |23 |26.5|28.5|29.5|31.5|32.5 |33 W. F. N. w. |12.5|20 |26.5|29 |31 |32.5|33.5 |34 W. C. w. |19 |24 |26.5|28 |29 |30.5|32 |33 B. & K. w. |14 |17.5|25 |27 |29.5|31.5|33 |33.5 S. B. & Co. w. c. |10 |20 |26 |26.5|27 |27.5|28 |28.5 C. L. Co. w. |29 |38 |40.5|42.5|43 |43.5|Stat.|43.5 C. L. Co. No. 1. |17.5|23 |27 |28 |29 |30 |31 |32 Glyc. |17.5|23 |28 |30 |32 |34 |35 |35.2 Alb. f. |15 |20 |29 |33 |35 |37 |38 |38.5 ------------------+----+----+----+----+----+----+-----+------

The author once heard a watchmaker say to a customer, when the latter called for a clock which had been left for repairs, "I have cleaned your clock thoroughly; and, as you are a good customer, I made as good a job of it as I could. _I even oiled it with watch oil._" This watchmaker evidently _thought_ he was right. It is hardly necessary to mention that a stock of oils of different viscosity should be kept on hand and intelligently used; the different bearings in any time keeping mechanism requiring oils of different viscosity. It is not to be supposed that the author means _each_ bearing in a watch is to have a separate oil applied; but a distinction should be made between the light and heavy pressures.

~82. The Effect Of Heat On Oils~ is very marked in all cases; some oils being much more subject to change than others, in viscosity and other properties, under the influence of an increase of temperature.

The lubricating power of an oil is decreased, while its tendency to spread is increased, with a rise of temperature. In order to ascertain the relative values of various oils in this respect the writer used a plate of gla.s.s 28 cm. x 40 cm., placed it flat on a table, and, depositing one drop of each oil near one of its longer edges, allowed it to remain in a temperature of 21 C. (= 70 F.) for 30 minutes. At the end of this time the gla.s.s plate was placed in a vertical position, with its edge near which the drops of oil had been deposited uppermost and horizontal. The time required by each oil to run down to the bottom, a distance of 25 cm., was noted. The width of the track, at a point 3 cm.

from the location of the drop at the start, was measured when the oil had pa.s.sed that point, and again measured _at the same point_ when the oil had reached the bottom.

The same test was repeated, with all the conditions similar except that the temperature of the room was raised to 38 C. (= 100 F.) before the oil was placed on the gla.s.s; but the gla.s.s was allowed to remain in this temperature also for 30 minutes.

The results of both experiments are shown in table IX.

TABLE IX.

RELATIVE VISCOSITY, COHESION AND ADHESION OF OILS.

Temp. 21 C.(= 70 F.) and 38 C. (= 100 F.) Inclination Vertical.

-----------------+----------------+--------------------------------------- SYMBOLS |MINUTES REQUIRED| WIDTH OF TRACK IN MM. AT A POINT ACCORDING TO | TO FLOW 25 CM. | 3 CM. BELOW STARTING TABLE III. |AT A TEMPERATURE| PLACE WHEN THE OIL HAD FLOWED | OF | | +-----------------+--------------------- | |Temp. 21C(=70F.)|Temp. 38C.(=100F.) -----------------|-------+--------+-------+---------+--------+------------ | 21C. | 38C. | | | | |=70F. |=100F. | 3 CM.| 25 CM. | 3 CM. | 25 CM.

-----------------+-------+--------+-------+---------+--------+------------ E. K. w. | 21 | 14 | 5 | 5 | 5 | 5 W. F. N. w. | 18 | 12 | 5 | 5 | 5 | 5 D. C. S. w. | 20 | 13 | 5 | 5 | 5 | 5 D. C. S. ch. | 15 | 10 | 5 | 5 | 5 | 5 D. C. S. cl. | 20 | 11 | 5 | 5 | 5 | 5 W. C. w. | 13 | 8 | 5 | 1 | 5 | 1 B. & K. w. | 13 | 11 | 5 | 0 | 5 | 0 S. B. & Co. w. c.| 15 | 11 | 6 | 6 | 6 | 8 C. L. Co. w. | 17 | 15 | 6 | 7 | 7 | 8 C. L. Co. No. 1. | 15 | 10 | 6 | 6 | 5 | 5 Glyc. | 14 | 10 | 6 | 6 | 5 | 8 Alb. f. | 14 | 10 | 6 | 6 | 5 | 6 Sp. | 10 | 7 | 6 | 1 | 5 | 0 Ol. | 14 | 12 | 5 | 2 | 5 | 1 -----------------+-------+--------+-------+---------+--------+------------

While the relative viscosity of oils in varying high temperatures is shown in table IX, the width of the track indicates the same properties as were explained in reference to table VII. Thus it is seen that the third and fifth columns of figures denote the relative adhesion of the oils, approximately according to the value of the figures; while the fourth and sixth columns exhibit their relative cohesion, and absence of adhesion, approximately according to the inverse value of the figures.

Thus the tendency of the oil to spread, in the warm temperature to which time keeping mechanisms are frequently subjected, is indicated.

~83. The Effect Of Cold On Oils~ is very observable in some varieties, converting them into greases, or even into hard, waxy solids. For out-of-door work unguents must be selected that will "feed" at any temperature to which they are exposed in the working of the bearings to which they are applied.

The author has subjected various oils to a low degree of temperature, using a sufficient number of thin gla.s.s test tubes of 3 cubic centimeters capacity,[24] into each of which 2 cubic centimeters of the oils to be tested were poured. The test tubes were then tightly corked and properly secured to a thin board, and placed in a temperature of -15 C. (= 5 F.) the condition of the oils being noted at various intervals, the result of which is shown in table X.

~84. The Variations of Viscosity of Oils in Varying Temperatures~ always create fluctuations of their friction reducing power; while the variations of fluid friction which result are also of great importance in horology. When it is known that the viscosity and lubricating power of an oil are usually (80) very closely related, it is seen that change of temperature has an exceedingly important effect upon oils, even for general lubricating purposes; but particularly so when they are applied to small and delicate mechanisms.

An oil of the proper viscosity at ordinary temperatures may be very unsuitable in an extreme of heat, or cold, to which timepieces are frequently subjected--on account of being too limpid in high temperatures to properly separate the rubbing surfaces; while in low temperatures it may become so viscous as to seriously impede the motion of the escapement and the lighter parts of the train.

TABLE X.

RELATIVE EFFECT OF COLD ON OILS.

Temp. -15 C. (= 5 F.) Time of Exposure = 6 hours

-----------------+------------------------------------------ SYMBOLS | ACCORDING TO | CONDITION OF OIL.

TABLE III. | -----------------+-------+-------+-------+--------+--------- TIME. |15 MIN.|30 MIN.|1 HOUR.|6 HOURS.|ORDER OF | | | | |VISCOSITY -----------------+-------+-------+-------+--------+--------- E. K. W. w. | ... | ... | ... | ... | 2 W. F. N. w. | ... | ... | t-f. | t-f. | 4 D. C. S. w. | ... | ... | ... | ... | 2 D. C. S. ch. | ... | ... | ... | ... | 2 D. C. S. cl. | s-s. | s-s. | s-s. | s-s. | 6 W. C. w. | ... | ... | ... | ... | 2 B. & K. w. | ... | ... | ... | ... | 2 S. B. & Co. w. c.| ... | ... | ... | ... | 1 C. L. Co. w. | s-s. | s-s. | s-s. | s-s. | 5 C. L. Co. No. 1. | s-s. | s-s. | s-s. | s-s. | 7 Glyc. | ... | ... | ... | ... | 1 Alb. f. | ... | ... | ... | ... | 3 Sp. | s-s. | s-s. | s. | v-s. | 8 Ol. |v-t-f. | s-s. | s. | v-s. | 9 -----------------+-------+-------+-------+--------+--------

T. F. = Thickly fluid; or like honey. V. T. F. = Very thickly fluid; or like jelly. S. S. = Semi-solid; or like b.u.t.ter at 60 F. S. = Solid; or like b.u.t.ter at freezing point. V. S. = Very solid; or like paraffin wax.