LUBRICATION 283

**E>parent contact, within wide limits. The

^v^er, be large enough so that the surfaces

Since it is impracticable to obtain a pair of
"Unlxibricated surfaces, it is needless to say are

irxexact. As already intimated, well-fitting and of

similar material would probably seize of

flow, which are very different from the
have, however, both historic interest

Just as the laws of solid friction are
to the laws of plastic flow, so these laws are in
to the laws of viscous flow which apply to
With well-lubricated surfaces we have the

where S is the area of surface in contact, dv is the velocity and dr
Is the thickness of the oil film. According to
1 . The frictional resistance P is independent of the
2. The friction is directly proportional to the is
"tbterefore zero when the velocity is zero.
3. The friction is also directly proportional to the of
faces in contact.
In view of the absolute antithesis between sets of
laws, it is not surprising that the results of the study of
as recorded in the literature are often contradictory. We
liowever, state broadly that slow-moving, poorly
faces follow approximately the laws of friction,
rapid-moving and hence necessarily well-lubricated
such as electric dynamos and motors, follows the of
friction. Most bearings are imperfectly lubricated
neither set of laws exactly.
Petroff (1887) seems first to have applied the of
friction to lubricated bearings testing out Ms views by
Most important in its relation to the development
of lubrication is the experimental work of
C 1885-4), undertaken at the instance of the of
Mechanical Engineers. His experiments were
extreme care and under varied and well-chosen
His results, as obtained under ordinary conditions of hibri-