FRICTION AND LUBRICATION.
QUESTION 358. _What is meant by friction?_
_Answer._ Friction is the resistance between two bodies in contact
which opposes the sliding of the one on the other. Thus if a brick is
placed on a board with a slight inclination, it will not slide because
the friction between them, or the resistance opposed to motion, is
greater than the force exerted by the weight of the brick to move
it downward. If, however, the inclination of the board is increased
sufficiently so that a larger proportion of the weight of the brick
urges it downward, then the friction will be overcome, and it will
slide. When the brake-blocks of a car are pressed against the wheels,
they produce friction, which resists the revolving motion of the wheels
and will ultimately stop the car; and when the weight of an engine
is supported on the driving-wheels and they rest on the rails, the
friction between them, as has already been pointed out, resists their
slipping on each other, and thus enables a locomotive to exert tractive
force. Friction also resists the turning of an axle on its journal and
therefore makes the tractive force of the locomotive necessary to move
a train of cars.
QUESTION 359. _On what does the amount of friction depend?_
_Answer._ The amount of friction of two bodies in contact depends (1)
UPON THE PRESSURE OF THE ONE ON THE OTHER, AND IS INDEPENDENT OF THE
AREA OF THE SURFACES IN CONTACT; (2) ON THE NATURE OF THE MATERIALS
IN CONTACT; (3) ON THE NATURE OF THE SUBSTANCE, SUCH AS OIL OR OTHER
LUBRICANT, WHICH IS INTERPOSED BETWEEN THEM. Thus, a brick will slide
down an inclined board as easily if it is laid on its broadest side as
it will if placed edgewise; and if a cast iron plate, say 10 inches
square, is planed and scraped, so as to be as nearly a perfect plane
surface as it is possible to make it, it will, if loaded with say a
hundred pounds weight, slide on a similar true surface as easily as
another plate with half as much area and loaded with the same weight. A
shaft resting against a long bearing will require no more power to turn
it than would be needed if the bearing was short.[83]
[83] In fact, ordinarily less power is required to turn it if the
bearing is long than if it is short, the reasons for which will be
explained hereafter.
QUESTION 360. _What is meant by the “co-efficient of friction?”_
_Answer._ It is the proportion which the resistance to sliding motion
bears to the force pressing the surfaces together. Thus a smooth, clean
and dry cast iron plate loaded with 100 pounds will require a force of
about 15 pounds, or fifteen one-hundredths of the weight or pressure of
the plates, to slide them on each other. The _co-efficient of friction_
is therefore said to be 0.15, and with any other weight or pressure
on the plates we could determine the force required to slide them on
each other by multiplying the pressure by the co-efficient of friction.
Thus, if the plates were loaded with 250 pounds, the force required to
slide the one on the other would be equal to 250 × .15 = 37.5 pounds.
The co-efficient of friction, however, varies for different materials.
Thus, while the co-efficient of friction between two pieces of smooth,
clean and dry cast iron is 0.15, that of a piece of brass on cast iron,
under similar conditions, is 0.22, and of two pieces of wood about 0.4.
QUESTION 361. _What is the effect of introducing some unguent or
lubricating material, such as oil, between the surfaces in contact?_
_Answer._ The co-efficient of friction is very much reduced thereby.
Thus the co-efficient of friction of the cast iron plates, if their
surfaces are greased with tallow, is 0.1; if lubricated with lard 0.07,
with olive oil 0.064, and with lard and plumbago 0.055, thus showing
that the amount of the friction depends very much upon the nature of
the lubricant which is used, as well as on that of the materials in
contact.
QUESTION 362. _What effect on the amount of friction has the manner of
applying the lubricating material to the surfaces in contact?_
_Answer._ The more perfect the lubrication the less will be the
co-efficient of friction. It has, for example, been found by
experiments made with cast iron shafts turning on bearings of the
same material that when the lubricating material was applied so that
the surfaces were only “unctuous,” that is slightly greasy, the
co-efficient of friction was very little less than when they were dry,
that is when there was no lubricating substance between them, and
that when they were greased “from time to time” the co-efficient was
reduced to 0.07 and 0.08; but when they were continually oiled it
averaged 0.05, and sometimes fell as low as 0.025, showing that with
the best lubrication the friction was only one-sixth what it was when
the surfaces were only “unctuous.” Between these two limits there is
every degree of frictional resistance, according to the condition of
lubrication. This shows how important it is that the oiling fixtures
should be kept in the most perfect condition and the utmost care be
exercised in keeping every part of a locomotive thoroughly lubricated.
QUESTION 363. _What effect does the pressure per square inch of the
surfaces in contact have upon the lubrication?_
_Answer._ The tendency is, when this pressure becomes excessive, to
press out the lubricant which is between the two surfaces, and ordinary
experience proves that the greater the weight or the force per square
inch with which two bodies are pressed together, the greater is the
difficulty of keeping them perfectly lubricated.
Thus it is easier to keep the journals of a car well lubricated when
it is empty than when it is heavily loaded, and the guide-bars of a
locomotive are more liable to be cut when the engine is pulling a heavy
load than with a light one.
QUESTION 364. _What effect has the velocity of the surfaces in contact
on the friction and lubrication?_
_Answer._ With the surfaces in the same condition, the friction
is independent of the velocity of motion of the surfaces against
each other, but perfect lubrication becomes more difficult as the
velocity increases, so that an increase of velocity will often
increase indirectly the amount of friction. Thus, taking our previous
illustrations, it is more difficult to keep the journals of a car or
engine well lubricated when running fast than when running slow, and
the same thing is true of the guide-bars.
QUESTION 365. _What considerations should govern the proportions of
frictional bearings for locomotives and other machines?_
_Answer._ The dimensions to be given them should not be determined
from a consideration solely of their resistance to rupture,[84] but
they should be made so large that the pressure they must bear will be
distributed over so much surface that the proportion borne by each
square inch will be comparatively small, thus making good lubrication
much less difficult, and consequently reducing the co-efficient of
friction.
[84] Morin’s Mechanics.
QUESTION 366. _Is not the amount of energy required to overcome the
friction on a journal of large diameter greater than would be required
if the journal was smaller?_
_Answer._ If the co-efficient of friction in the two cases is the same,
undoubtedly the large journal will require the greatest expenditure
of energy to turn it, because its periphery moves further than that
of the small one; but the advantage attributed to large journals is
that they can be lubricated more perfectly, because their surfaces
being larger the pressure is not so great per square inch, and thus
the gain from the reduction of the co-efficient of friction is greater
than the loss attributable to the increase of the diameter of the
journal. Thus if a car journal is 3¹⁄₄ inches in diameter × 5¹⁄₂ inches
long, the available surface exposed to friction is equal to that of a
longitudinal section of the journal, or 3¹⁄₄ × 5¹⁄₂ = 17.875 square
inches.[85] Supposing now that the journal is loaded with 5,000 pounds,
and the average co-efficient of friction is 0.085. In one revolution
of the wheel the journal will move 0.85 of a foot, and therefore 5,000
× .085 = 361¹⁄₄ foot-pounds of work. If now the journal is made, as
has been proposed, 3³⁄₄ × 7 inches, then its effective surface will
be equal to 26¹⁄₄ square inches, but the journal will move 0.98 of a
foot in one revolution. If, however, the lubrication is improved by the
increased area of the journal so that the co-efficient of friction is
reduced from 0.085 to 0.07, then the energy consumed in one revolution
will be equal to 5,000 × 0.7 × .98 = 343 foot-pounds, or less than
was consumed with the small journals. The co-efficient of friction is
assumed, and could only be determined by experiment, but the assumption
shows how the resistance of the large journals may be less than that
of the small ones. Of course it would be better to give the increased
bearing surface by adding to the length of the journal, but nearly all
locomotives and car journals must be increased in diameter as well
as in length when they are enlarged, in order to have the requisite
strength to carry the loads they must bear.
[85] The reason for this is that the effective surface of the
journal _A_, fig. 209. which resists the pressure of the bearing, is
equivalent only to the horizontal area represented by the dotted line
_a. b._ just as the surface which resists the pressure inside of a
boiler is equivalent to the diameter multiplied by its length, as was
explained in answer to Question 99.
QUESTION 367. _Is the law that_ FRICTION IS IN PROPORTION TO THE
PRESSURE ON EACH OTHER BY THE SURFACES OF CONTACT _true under all
circumstances?_
_Answer._ No; there is a limit to the exactness of the above law, when
the pressure becomes so intense as to crush or grind the parts of the
bodies at and near their surfaces of contact. At and beyond that limit
the friction increases more rapidly than the pressure;[86] and the
friction then becomes very irregular.
[86] Rankine.
QUESTION 368. _In what cases is the limit referred to probably reached?_
_Answer._ Probably in some locomotives the pressure of the
driving-wheels on the rails is sufficient to partly crush the latter.
QUESTION 369. _What effect has the nature of the materials in contact
on the friction?_
_Answer._ The amount of friction and also the lubrication is very
much influenced by the nature of the bearing surface and also by the
material used as a lubricant. Some metals, such as brass and other
alloys, are much less liable to abrasion and seem to retain lubricants
on their surfaces better than other metals, and are therefore much used
for journal and other bearings. Some substances, especially oils, are
good lubricants, while other materials of apparently similar nature
are not. The reason why these materials possess these properties while
others are without them is not known, and the value of any material
as a lubricant, or the degree to which another will resist friction
without abrasion, can only be tested by experiment.