268 FLUIDITY AND PLASTICITY
the point of nearest approach is then on the "on-side." Only
when the bearing is unloaded does the point of nearest approach
coincide with the.middle of the brass, 0. As the load increases
the point G moves from 0 up to a certain maximum value after
which it recedes toward 0, resulting finally in a discontinuity in
the oil just as in the case of a limited supply of oil.
We have considered only bearings of unlimited length, whereas
in practical bearings the lubricant is squeezed out at the sides,
as well as at the ends. Michell (1905) has made a study of the
changes of pressure in the oil film of bearings of various shapes.
Generally speaking the integrations necessary to define the exact
relations between load, speed and the friction have not been
effected.
The theory of lubrication is not inconsistent with the experience
that the friction in limited lubrication is proportional to the
load and independent of the velocity. Increase of load will
result in a diminution of the distance between the bearing
surfaces, a lengthening of the oil-pad, and therefore an increase
in the resistance. Increasing the velocity increases also the
I** resistance, but it also increases the pressure and therefore the
distance between the surfaces, provided that the load is kept
constant, and this produces a decrease in the resistance.
For further details of the development of this very important
subject the reader is referred to the original papers of Petroff,
Tower, Reynolds, Sommerf eld, Michell, Lasche to name but a few.
LUBEICATION AND ADHESION
In the early use of lubrication, fixed oils and greases were
depended upon almost exclusively. The fixed oils, that is
the non-volatile oils of animal or vegetable origin, are expensive,
they may become gummy and rancid, which interferes with
proper lubrication and the acids developed may corrode the
machines. These oils moreover often partially solidify when only
slightly cooled. The range of viscosities obtainable is also
restricted by the small number of oils available in any quantity.
With the advent of mineral oils, these troubles were all overcome,
so the battle which was waged between the mineral and the
fixed or fatty oils was short and apparently decisive. The