Friction, Lubrication and the Lubricants in Horology

by W. T. Lewis

FRICTION—ITS NATURE AND THEORY.

29. Friction. The relative motion of one particle or body in forced contact with another is always retarded, or prevented, by a resisting force called friction.

Friction manifests itself in three ways: Between solids it is called sliding and rolling friction; between the particles of liquids, or of gasses, when they move in contact with each other, or with other bodies, it is called fluid friction. Quite different laws govern these three kinds of friction, as they are quite different in character.

Friction can never of itself produce or accelerate motion, being always a resisting force, acting at the surfaces of contact of the two particles, or masses, between which it occurs, and in the direction of their common tangent, resisting relative motion in whichever direction it may be attempted to produce it. The greatest loss of energy in a timepiece in which all the parts are rigid enough to prevent permanent distortion, is that occurring through friction. Another source of loss of energy is the reduction in elasticity of springs caused by a rise of temperature.

30. The Cause of Sliding Friction is the interlocking of the asperities of one surface with those of another; and only by the riding of one set over the other, or by a rubbing down or tearing off of projecting parts, can motion take place. It follows, then, that roughness is conducive to friction; and that the smoother the surface the less the friction will be.

31. The Cause of Rolling Friction is the irregularity and lack of symmetry of the surfaces between which it occurs. It acts as a resisting, or retarding, force when a smoothly curved surface rolls upon another surface, plane or curved.

Motion is prevented, or retarded, by the irregular variation of the distance between the center of gravity and the line of motion in the common tangent of the two bodies at the point of contact, caused by the irregularity of form, or of surface, in the one or the other body. Rolling friction is small where hard, smooth, symmetrical surfaces are in contact, and increases as the surfaces are soft, rough or irregular.

In a knife edge support, seen in some forms of pendulums, is exhibited a form of rolling friction.

32. Solid Friction, either sliding or rolling, could be overcome if it were possible to produce absolutely smooth surfaces. It is evident, then, that the character of the material, as well as the form of their surfaces, determines the amount of friction.

In all time-keeping mechanism both sliding and rolling friction manifest themselves; the former principally between the surfaces of pivots and bearings and in the escapements, the latter mainly between the surfaces of the teeth of wheels, and to some extent in some of the pivots, and sometimes in parts of escapements. It is not the intention of the author to treat of the proper shape of the teeth of wheels, leaves of pinions, or the proportions of the escapements, the nature and scope of this work not permitting of it; but he will confine his remarks principally to the parts that involve lubrication.

33. The Laws of Sliding Friction, as given by Thurston,[7] with solid, unlubricated surfaces, are, up to the point of abrasion, as follows:

1. The direction of frictional resisting forces is in the common tangent plane of the two surfaces, and directly opposed to their relative motion.

2. The point, or surface, of application of this resistance is the point, or the surface, on which contact occurs.

3. The greatest magnitude of this resisting force is dependent on the character of the surfaces, and is directly proportional to the force with which two surfaces are pressed together.

4. The maximum frictional resistance is independent of the area of contact, the velocity of rubbing, or any other conditions than intensity of pressure and condition of surfaces.

5. The friction of rest or quiescence, "statical friction," is greater than that of motion, or "kinetic friction."

He further states that these "laws" are not absolutely exact, as here stated, so far as they affect the magnitude of frictional resistance. It is found that some evidence exists indicating the continuous nature of the friction of rest and of motion.

When the pressure exceeds a certain amount, fixed for each pair of surfaces, abrasion of the softer surface or other change of form takes place, the resistance becomes greater and is no longer wholly frictional.

When the pressure falls below a certain other and lower limit the resistance may be principally due to adhesion, an entirely different force, which may enter into the total resistance at all pressures, but which does not always appreciably modify the law at high pressures.

This limitation is seldom observable with solid, unlubricated surfaces, but may often be observed with lubricated surfaces, the friction of which, as will presently be seen (41), follows different laws. The upper limit should never be approached in machinery.

The coefficient of friction is that quantity which, being multiplied by the total pressure acting normally to the surfaces in contact, will give the measure of the maximum frictional resistance to motion.

34. Sliding Friction is Proportional to Pressure according to the third law quoted above. This is easily demonstrated by ascertaining what force is necessary to produce, or continue, motion in a body lying on a plane surface; double the weight of the body and the force required to produce, or continue, motion, will have to be doubled. The converse is also true (36).

35. Sliding Friction is Independent of the Area Of Contact, the pressure remaining the same (law 4, 33).

This is accounted for by the fact that if, for example, the area of contact be doubled, though twice the number of asperities will present themselves, each individual retarding force is only half of what it was previously, and the general effect is the same (36).

36. The Intensity of Sliding Friction is Independent of Velocity. (Law 4, 33.) This is explained by the fact that the interlocking of the asperities on each surface has a shorter time to take place in increased speed, and consequently cannot be so effective as with slow speed. But with high speed more asperities are presented than in low speed, so the effect is the same in both cases.

The above (33-36) are not exact, being the statement of experimental laws, and admit of considerable modification when applied in horological science, as will be shown (41-42.)

Fig. 12.

37. The Effect of a Loose Bearing is an increase of friction, and consequently a loss of energy, resulting in the wear of one or both surfaces in contact, according to conditions. In Fig. 12, A is a loose bearing, B a journal at rest and C the point of contact. If the journal be now turned in the direction of the arrow by the motive force, it will have a tendency to roll over a short arc of the bearing to a new point of contact, as at D, when it begins to slide; so long as the coefficient of friction is unchanged it retains this position; but approaches or retreats from the point C, as the coefficient of friction diminishes or increases, continually finding new conditions of equilibrium. The arc of contact is thus too small to withstand the pressure without abrasion of one or both surfaces.

It will thus be seen that the journal, or pivot, should fit its bearing closely; but it should be borne in mind that no tendency to "bind" should be produced, the fitting being such that the wheel will turn readily with a minimum pressure.

The film of oil which must be interposed between the bearing surfaces of the journal, or pivot, and its bearing, will also occupy some space; and this must be remembered, particularly in the case of pivots in the escapement.

38. The Laws of Rolling Friction are not as yet definitely established, because of the uncertainty of the results of experiments, as to the amount of friction due to (1) roughness of surface, (2) irregularity of form, (3) distortion under pressure.

The first and second of these quantities vary inversely as the radius; and the third depends upon the character of the material composing the two surfaces in contact.

It follows, then, that in such minute mechanical contrivances as are used in horology, as the motive force is in some cases very light, the horologist should endeavor to produce, where rolling friction takes place, the maximum—smoothness of surface—regularity of form—adaptation of surfaces (31.)

There are many other points on which the writer would like to dwell, as engaging and disengaging friction, internal friction, etc., etc., but the scope of this paper will not permit.

39. The Friction Of Fluids in horology is of grave importance. It is subject to quite different laws from those met with in the motion of solids in contact. When a fluid moves in contact with a solid the resistance to motion experienced is due to relative motion of layers of fluid moving in contact with each other. At surfaces of contact with a solid the fluid lies against the solid without appreciable relative motion; as the distance from the surface is increased by layer upon layer of the fluid, the relative velocity of the solid and the fluid becomes greater. Fluid friction is, therefore, the friction of adjacent bodies of fluid in relative motion.

While fluid friction acts as a retarding force in mechanism it converts the mechanical energy required to produce it into its heat equivalent, thus raising the temperature of the mass in a greater or lesser degree.

The resisting property which thus effects this conversion, and which is the cause of fluid friction, is called viceosity.

It is thus apparent that a variation of the viceosity of the oil used on a watch would cause a variation of fluid friction and consequently a variation of the effort (11), and would seriously interfere with the rate of the watch. This will be discussed (84) more thoroughly in another paragraph.

40. The Laws of Fluid Friction are:

1. Fluid friction is independent of the pressure between the masses in contact.

2. Fluid friction is directly proportional to the surfaces between which it occurs.

3. This resistance is proportional to the square of the relative velocity at moderate and high speeds, and to the velocity nearly at very low speeds.

4. It is independent of the nature of the surfaces of the solid against which the stream may flow, but it is dependent to some extent upon the degree of roughness of those surfaces.

5. It is proportional to the density of the fluid and is related in some way to its viscosity.

41. The Compound Friction of Lubricated Surfaces, as Thurston terms it, or friction due to the action of surfaces of solids partly separated by a fluid, is observed in all cases in which the rubbing surfaces are lubricated. The solids, in such instances, though partly supported by the layer of lubricant which is retained in place by adhesion (21) and cohesion (20), usually rub on each other more or less, as they are usually not completely separated by the liquid film interposed between them.

Wear is produced by the rubbing together of the two solids; and the rate at which the lubricant becomes discolored and charged with abraded metal indicates the amount of wear.

The journal and bearing are forced into close contact in the case of heavy pressures and slow speeds, as is shown by their worn condition; while the journal floats on the film of fluid which is continually interposed between it and the bearing, in the case of very light pressures, and high velocities; in the latter instance the friction occurs between two fluid layers, one moving with each surface.

With heavy machinery, as the hardness and degree of polish of the surfaces cannot be increased in proportion to their weight, the solid friction is so great that while the interposition of a lubricant between the surfaces adds fluid friction, it also reduces the solid friction; and as the fluid friction is so insignificant as compared to the solid friction, the former is almost completely masked by the latter. In this case the laws of solid friction are more nearly applicable.

But in a delicate machine like a watch, especially in the escapement, where the power is so light, and where the rubbing surfaces are so hard, smooth and regular, the solid friction is so minute as compared to the fluid friction, that the former is relatively very slight, as compared with the latter. The laws of fluid friction are more nearly applicable in this instance.

There are thus, evidently, two limiting cases between which all examples of satisfactorily lubricated surfaces fall; the one limit is that of purely solid friction, which limit being passed, and sometimes before, abrasion ensues; the other limit is that at which the resistance is entirely due to the friction of the film of fluid which separates the surfaces of the solids completely.

42. The Laws of Friction of Lubricated Surfaces are evidently neither those of solid friction nor those of fluid friction, but will resemble more nearly the one or the other, as the limits described in the previous paragraph are approached. The value of the coefficient of friction varies with every change of velocity, of pressure, and of temperature, as well as with the change of character of the surfaces in contact.

For perfectly lubricated surfaces, were such attainable, assuming it practicable with complete separation of the surfaces, the laws of friction, according to Thurston, would become:

1. The coefficient is inversely as the intensity of the pressure, and the resistance is independent of the pressure.

2. The friction coefficient varies as the square of the speed.

3. The friction varies directly as the area of the journal bearing.

4. The friction varies as the temperature rises, and as the viscosity of the lubricant is thus decreased (80).

43. The Methods of Reducing Waste of Energy Caused by Friction in time keeping mechanisms are based upon a few simple principles. It is evident that to make the work and power so lost a minimum, it is necessary to adopt the following precautions:

1. Proper choice of materials for rubbing surfaces (29-32).

2. Smooth finish and symmetrical shape of surfaces in contact (29-32 and 38).

3. The use of a lubricant the viscosity of which is adapted to the pressure between the bearing surfaces (80).

4. The best methods for retaining the lubricant at the places required, and for providing for a continual supply of the lubricant.

5. The bearing surfaces of such proportions that the lubricant will not be expelled at normal pressure.

6. The reducing of the diameters of all journals, shoulders and pivots, to the smallest size compatible with the foregoing conditions, and with the stresses they are expected to sustain, thus reducing the space, through which the fluid friction acts, to a minimum (40); as well as reducing the distance from the axis of the arbor or pinion at which the friction, both solid and fluid, acts. The work done is independent of the length of the journal; except as it may modify pressure, and thus the coefficient of friction.

7. Proper fitting of bearing surfaces (37).

8. The reducing of the rubbing surfaces in escapements as much as the nature of the materials will allow without abrasion in the course of time (55).

44. Friction Between Surfaces Moving at Very Slow Speed, has been investigated by Fleming Jenkin and J. A. Ewing. A contrivance, which would be very excellent with some improvement, for the determination of the amount of friction under such conditions, is given in a paper[8] read before the Royal Society of London.

The arrangement employed by them was composed of a cast iron disk two feet in diameter and weighing 86 pounds. This disk, being turned true on its circumference, was supported by a spindle terminating in pivots 0.25 C. M. in diameter, the pivots resting in small rectangular bearings composed of the material the friction of which with steel is to be determined.

A tracing of ink was produced on a strip of paper which surrounded the disk, the ink being supplied by a pen actuated electrically by a pendulum, as in the syphon recorder.

As the traces thus left on the paper were produced without in any way interfering with the freedom of motion of the disk, they afforded a means of determining the velocity of rotation.

The relative velocities of the pivot to the bearing surfaces varied from .006 C. M. to 0.3 C. M. per second, being the velocities met with in the various parts of time keeping devices.

Experiments were made with the bearing surfaces successively in three different conditions: viz. 1, dry; 2, wet with water; and 3, wet with oil; and gave the following results: