consumes the least power and produces the least
molecules together and thus prevent motion of the
amount of heat.
fluid. The adhesive forces of the molecules of the
Several factors determine the effectiveness of
fluid cause the fluid to adhere to the paddle and
oil film lubrication, such as pressure, temperature,
thus create friction between the paddle and the
viscosity, speed, alignment, condition of the
fluid. Cohesion is the molecular attraction
bearing surfaces, running clearances between the
between particles that holds a substance or body
bearing surfaces, starting torque, and the nature
together; adhesion is the molecular attraction
of the lubricant's purity. Many of these factors
between particles that causes unlike surfaces to
are interrelated and interdependent. For example,
stick together. In lubrication, adhesion is the
the viscosity of any given oil is affected by
property of a lubricant that causes it to stick
temperature, and the temperature is affected by
(or adhere) to the parts being lubricated;
running speed; hence, the viscosity is partially
cohesion is the property that holds the lubricant
together and enables it to resist breakdown under
dependent on the running speed.
A lubricant must be able to stick to the
pressure.
bearing surfaces and support the load at operating
Different materials have cohesion and adhesion
speeds. More adhesiveness is required to make a
in widely varying degrees. In general, solid bodies
lubricant adhere to bearing surfaces at high speeds
are highly cohesive but only slightly adhesive.
than at low speeds. At low speeds, greater
Most fluids are highly adhesive but only slightly
cohesiveness is required to keep the lubricant from
cohesive; however, the adhesive and cohesive
being squeezed out from between the bearing
properties of fluids vary considerably.
surfaces.
Lubrication reduces friction between moving
Large clearances between surfaces require high
parts by substituting fluid friction for sliding or
viscosity and cohesiveness in the lubricant to
rolling friction. Without lubrication, moving a
ensure maintenance of the LO film. The larger
100-pound weight across a rough surface is
the clearance, the greater must be the lubricant's
difficult; however, with lubrication and proper
resistance to being pounded out with consequent
attention to the design of bearing surfaces, a
destruction of the LO film.
l-million-pound load can be moved with a motor
High unit load on a bearing requires high
that is small enough to be held in the hand. By
viscosity of the lubricant. A lubricant subjected
reducing friction, lubrication reduces the amount
to high loading must be sufficiently cohesive to
of energy required to perform mechanical actions
hold together and maintain the oil film.
and also reduces the amount of energy that
dissipates as heat.
LUBRICATING OILS
Lubrication is important throughout the ship-
board engineering plant. Moving surfaces must
Lube oils approved for shipboard use are
be steadily supplied with the proper kinds of
limited to those grades and types that provide
lubricants. Lubricants must be maintained at
proper lubrication under all anticipated operating
specified standards of purity and at designed
conditions. In the following paragraphs, we will
pressures and temperatures in the lubrication
discuss the classification and properties of LO.
systems. Without adequate lubrication, many
The Naval Ships' Technical Manual, Chapter 262,
units of shipboard machinery would quite literally
"Lubricating Oils, Greases, Hydraulic Fluids, and
grind to a screeching halt.
Lubrication Systems," is a good source for
A liquid is used for most lubrication
additional information (not covered in the
requirements because, in an enclosed space, a
following paragraphs) on LO.
liquid is incompressible. Were it not for this
incompressibility, separating moving metal
Classification of Lube Oils
surfaces from each other (preventing metal-to-
metal contact) would be impossible. As long as
The Navy identifies LO by symbols. Each
the lubricant film remains unbroken, fluid
identification number has four digits and, in some
friction replaces sliding friction and rolling
cases, appended letters. The first digit shows the
friction.
class of oil according to type and use; the last three
In any process involving friction, some power
digits show the viscosity of the oil. The viscosity
is consumed and some heat is produced. Over-
digits show the number of seconds required for
coming sliding friction consumes the greatest
a 60-milliliter (ml) sample of oil to flow through
amount of power and produces the greatest
a standard orifice at a certain temperature.
amount of heat. Overcoming fluid friction
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