CHAPTER XI
LUBRICATION
When- a solid substance Is subjected to a stress,
ife undergoes plastic flow if the stress is greater the
value of the material. In this process of shear, lateral
arise and if the material is not supported laterally by
pressure, rupture of the material will finally result.
surfaces formed by rapture slide over each other to the
laws of solid friction stated by Coulomb. The surfaces are
separated for the most part by a layer of fluid which may be air,
water j oil, a layer of oxide, etc. So two surfaces formed by a
rupture as, for example, two broken pieces of porcelain do
adhere together firmly even when they seem to fit together very
nicely. So also the resistance to movement between
smooth, surfaces is far less than the resistance to low.
If, liowever, sufficient force is brought to bear between
sliding surfaces of similar material, there will occur, far
the molting point of the substance, a welding together of
surfaces into a more or less compact whole, there is
present some substance which prevents such welding. Two
surfaces of glass ordinarily touch each other at very few
and thiey do not adhere strongly, but when the two
are ground to an optical surface and cleaned, it Is difficult to
separate the two surfaces without tearing them, after they
been brought together. A motor bearing wMch has care-
fully fitted by "lapping la'7 may be ruined completely by a
slight turn with the hand after the surfaces have
and again brought together. Powdered metals adhere strongly
"when, subjected to heavy pressures, even at temperatures con-
siderably below the melting point. The Johannsen blocks
in gage testing are made of hardened steel with are
•exceptionally true. When these blocks are placed one on top of
the otter, the adhesion between them Is so great that a of