278

FLUIDITY AND PLASTICITY

Anti-friction Metals. A bearing in usually made of a different
material from the journal, but the composition of the so-called
anti-friction metals varies within wide limits. It must be soft
enough HO that the bearing may be easily scraped and quickly
run in to an exact fit. During the process of "running in" the
bearing, the particles of metal doubtless servo to wear down the
high spots of the softer metal, leaving the harden* journal in a
highly polished condition. The maximum wear in naturally
whore the journal and bearing are in closest proximity, hence if a
bearing has been run in with motion in a given direction, the
coefficient of friction will bo altered if the direction of motion is
reversed, as observed by "Power. The bearing must bo hard
enough to (tarry the load without flow of the metal. It seems
probable that the material of the bearing should have aa small
an adhesion for the metal of the journal an practicable and in cane
of necessity the material of the bearing should bo capable of
acting as a lubricant. At any rate it should not tend to seize
the journal even when molten. lee may bo regarded as the
oldest anti-friction material, and from certain points of view
it in ideal Since it molts tinder pressure, it furnishes its own
lubricant and adhesion does not occur due to pressure. A
sleigh standing on moist ice may become frozen in which is
evidence*, that adhesion in not impossible even between ice and
steel. Adhesion between unlike materials is less serious how-
ever because of their different coefficients of expansion.
Tin is a common constituent of anti-friction metals and
there was a serious shortage of tin during the late war. The
lack of tin ores in the United States makes very desirable the
knowledge of alloys whieh do not contain tin and at the same
time are useful for bearings. Experiments) indicate that lead
containing a very low percentage of metallic calcium is very
satisfactory.
Whether the alloy should have a certain metajlographic
structure, as for example, crystals of comparatively hard material
imbedded in a softer amorphous solid is a moot question.