262 FLUIDITY AND PLASTICITY
them several inches high can be raised by lifting the topmost
one. In imperfect lubrication we first have excessive wear, then
scoring of the bearings and finally seizure with a more or less
complete welding together of the surfaces. Thus there is a con-
siderable mass of evidence to prove that whenever two clean
surfaces come together they adhere and thus the conditions for
plastic flow may be reestablished. The problem of lubrication
is therefore to substitute as far as possible fluid friction for the
enormously higher resistance to shear in plastic flow.
According to the above view, " solid friction," as ordinarily
observed, is intermediate between true plastic flow and true
viscous flow. Under favorable conditions it approaches closely
to simple viscous flow, whereas under very unfavorable conditions
it may approach the conditions for plastic flow. It is clear
therefore that the coefficient of solid friction may vary within the
widest limits depending upon the condition of the bearing
surfaces, the temperature, speed, and character of the lubricant.
Thus at the outset we may state that it is impossible to
specify the lubricant that will be most suitable for a given
machine, provided that that machine works at variable speeds,
temperatures and loads, and where the bearings are continually
subject to wear due to defective lubrication. On the other
hand, if bearings are perfectly lubricated and run under constant
conditions, there is practically no wear, so that the problem to
find the most suitable lubricant has a definite solution. With
the steady advance of industrial development, the theory of
lubrication takes on increasing interest.
The laws of solid friction may be stated as follows: (1) When
two unlubricated smooth surfaces slide over each other, the fric-
tioual resistance P varies directly as the load W or
P = fW (111)
11 and the coefficient of friction f is defined as the ratio between the
P !; friction and the load.
| • 2. The force P0 required to maintain an indefinitely small rate
i of shear, the so-called static friction, is greater than when the
| rate of shear is appreciable. The dynamic friction is independent
I of the velocity.
I 3. The friction is independent of the area of the surfaces in