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.