BEARINGS
In the general meaning of the term, a bearing is any part that carries
weight or pressure and at the same time rubs over another surface.
According to this definition, the portion of the cylinder walls
traversed by the pistons are bearings, and that is in reality the case,
but the term has come to be applied more specifically to the part of
the machine in which another part _revolves_, either continuously or
intermittently. Thus the portions of the crank shaft on which it is
supported and the parts of metal in which they revolve combine to form
the crank shaft bearings. The shaft or stud on which a gear or wheel is
mounted and on which it revolves is the bearing of that gear or wheel.
Although they are concealed, as some six-cylinder motors may be
provided with as many as three dozen, or more, bearings--if we consider
those on which the cam, pump, and magneto shafts and the gears are
mounted--but what descriptions, rules, and precautions apply to all
hold true in the largest sense when the crank shaft, connecting rod,
and wrist pin bearings only are considered. It is on this latter class
that the greatest wear of the motor is concentrated, and the owner who
understands and inspects these need fear no trouble from the cam shaft
and gear bearings.
The expert will judge of the condition of a motor by the wear that has
occurred in the bearings rather than by any exhibition of temporary
power that it may develop in a short test, and it is for this reason
that the "general public" runs a risk whenever it buys a second-hand
car that has not been thoroughly overhauled by a reputable factory or
inspected by a competent engineer. The bearings are in reality the
vitals of the motor, and when these are worn beyond the point of easy
adjustment or renewal, the repairs necessary to place the machine in
good condition would oftentimes cost more than the entire engine is
worth. But even in a badly-worn motor, the bearings may be "taken up"
and "doctored" so that, for a while at least, the engine will seem to
run perfectly and develop its full power. This will not be for long,
however, and soon the motor will begin to pound, knock, and rattle
until an examination will bring to light the true condition of the
bearings.
In no machine are the bearings subjected to more severe usage than in
the automobile motor. In order that the motor car power plant shall
be light in weight and occupy but a small amount of space, the power
must be transmitted at high speeds. In many an automobile motor, the
pressure imparted to a single bearing during a certain portion of its
revolution may frequently be well over two tons, and in this same
bearing, the "speed of rubbing" may approach eight or nine hundred feet
per minute. In other words, at normal speeds of the motor, about a
sixth of a mile of steel surface will rub over a certain point in each
crank shaft bearing during every minute that the engine is running.
When properly lubricated, an iron or steel shaft will run in almost
any kind of a metal bearing that is sufficiently strong to carry the
weights and pressures imposed upon the shaft. The friction generated
between two different metals that rub against each other, however,
varies according to the composition of those metals, and consequently
it is advisable to employ some material for a bearing that will offer
a minimum resistance to the turning of the shaft. Friction must be
reduced between all moving surfaces in order that the mechanical
efficiency of the machine shall be high, and it is in the bearings that
a large amount of power may be absorbed.
But even between the best-lubricated surfaces, employing the most
efficient metal as a bearing, some wear is bound to occur. The crank
shaft of a four- or a six-cylinder motor is forged or sawed from
one piece of steel, and with the accurate machining, finishing, and
grinding to which it is subjected, it becomes an expensive part of
the engine. Consequently it is advisable that the wear of bearings
of such parts shall be restricted to the "boxes" or surrounding
stationary metal in which the shaft revolves at these points. In order
that all wear shall occur here, rather than in the shaft, the boxes
are made of or lined with a softer metal. If the crank shaft is of
hard steel, the bearing metal may be of brass or bronze, but it has
been found that babbitt metals give the most satisfactory service for
such conditions--particularly as a sufficiently hard crank shaft is
difficult to produce commercially.
Not only is a babbitt metal softer than the steel of the shaft and
consequently receives practically all the wear of the bearing, but it
has the added advantage of melting at comparatively low temperatures.
At first thought, this may seem like a doubtful advantage, but in case
of a failure of the oil supply to that bearing, this characteristic may
be the means of saving the crank shaft, and possibly the crank case,
cylinders, and connecting rods, from rack and ruin.
The purpose of lubrication is to reduce friction between the two
surfaces in contact. Friction generates heat, and consequently the
temperature of a bearing to which a sufficient supply of oil is not
delivered will be raised to a very high point. This high temperature
will cause both parts of the bearing to expand, with the result that
the fit becomes very tight and the shaft binds or "seizes" in its box.
This is the familiar "hot box," so often the bane of railroad men, and
if the shaft is still run under these conditions, the bearing material
will be torn out and the surface of the shaft, axle, or whatever the
revolving portion happens to be, will be cut and abraded, oftentimes
beyond the possibility of repair. It is such accidents as these that
are prevented by the use of an easily-melted babbitt metal.
If the oil supply becomes insufficient so that the temperature of the
bearing is raised above a certain point, the babbitt metal will be
melted and will run out of its container before any damage can be done
to the shaft. Efficient running cannot, of course, be obtained with the
bearing "burned out" in this manner, but the babbitt is quickly and
easily renewed and serves as a sort of fusible safety valve that saves
many an expensive crank shaft replacement.
Babbitt metals may be of various compositions and proportions and many
contain lead, but those which have been found to give the best results
for use on the crank shafts of automobile motors are composed only of
tin, antimony, and copper. If lead is used at all for this purpose,
it should not appear in proportions above one per cent of the total
composition. Inasmuch as a babbitt metal will fuse at a comparatively
low temperature and is much softer than steel, it is obvious that such
a material will not withstand heavy pressures unless reinforced and is
unsuited for structural purposes. Consequently the babbitt is placed
in the bearing box in the form of a thin lining within which the shaft
revolves.
When the shaft is "lined up" in the box, the hot babbitt metal may be
poured in until the space is entirely filled. When the babbitt cools,
the shaft may be turned, and when lubricant has been introduced in
the oil grooves which should have been provided for the purpose, the
new bearing will be ready for use. It is not to be expected that the
majority of motor car owners will rebabbitt the crank shaft bearings
themselves, but it is necessary to understand the general principles of
such bearing design in order to inspect the motor intelligently and to
determine upon the repairs needed.
The above method of renewing "burned out" bearings applies to babbitts
in general, but the severe usage that automobile engine crank shaft and
connecting rod bearings are called upon to withstand necessitates the
exercise of a certain amount of additional care. It is necessary that
the box shall fit the shaft perfectly, so that there can be no "play,"
and yet the shaft must be allowed to turn easily within its surrounding
babbitt metal.
As was stated above, the shaft may be easily loosened from the babbitt
metal after the latter has cooled, and this would form a satisfactory
type of bearing were it not advisable that some means be supplied by
which the wear could be taken up without renewing the entire babbitt
lining. The bearing boxes of the crank shaft are each made in two
halves, the lower portion being cast integral with the crank case,
while the upper half is in the form of a separate cap that may be held
in place by two or four bolts. In this case, it is necessary that
the boxes shall be in two sections, for the shape of the crank shaft
prevents it from being slid into place lengthwise, and consequently it
must be placed on its bearing from the top. In some designs of motors
the bearing caps form the lower half of the box, but as in this case
the base of the motor must be inverted in order to remove the crank
shaft, the caps will still be considered as the "top" halves of the
boxes.
There may be dove-tail grooves cut in the inside of the halves of the
boxes to retain the babbitt metal after it has been poured in place.
Consequently, in order to remove the cap after renewing the babbitt
lining, the babbitt metal must be cut in two at the joint between the
two halves of the box. The two halves of the box, instead of fitting
closely together, are separated by thin strips of copper or fiber known
as "shims" that serve to relieve the shaft from the pressure of the
bolts when the bearing cap is screwed in place. In other words, the
two halves of the box must be held tightly in place by means of the
bolts and nuts, but none of this pressure should rest on the revolving
shaft, as this would bind it and prevent it from turning easily.
Consequently by "building up" the space between the two halves with
these thin shims the proper adjustment may be obtained.
These shims provide the method of taking up the wear in the babbitt
that will eventually result. By loosening the box retaining bolts and
removing the required number of shims, the halves of the box will be
brought closer together. When the bearing cap is screwed securely in
place, the shaft should be able to revolve freely without binding, and
yet the fit should be sufficiently tight to prevent any "play" at right
angles to the length of the shaft.
The pressure of a shaft should not be concentrated in one place, but
should be distributed over as large a surface of the babbitt metal as
is possible. A few years ago, when renewing or repairing a bearing, it
was considered sufficient to pour in the molten metal or to remove the
proper number of shims--and the bearing was then said to be ready for
its work. But even though no play was apparent, it was possible that
the shaft rested on only a few portions of the bearing surface; and
the increased attention that is now paid to the details of automobile
construction is no better exemplified than in the fact that nearly
all bearings are "scraped" in. This operation is simple and consists
merely in removing any slight excess babbitt metal so that the lining
fits the shaft throughout its entire length and circumference. The
babbitt is sufficiently soft to enable it to be peeled or scraped with
a sharp tool provided for the purpose, and no great degree of skill is
necessary in obtaining the required fit.
In order to determine at exactly what portions of the babbitt lining
the pressure is too great, a dye or paint known as "blueing" is used.
The bearing portion of the crank shaft is painted with this, and the
cap is then screwed in place. If the crank shaft is then turned and the
cap removed, it will be found that the blueing has been transferred
from the bearing to the portions of the babbitt metal on which the
pressure is the greatest. These portions should then be shaved with the
tool mentioned above, and the same test repeated. As the excess metal
is removed, it will be found that the blueing gradually is deposited
over a larger area of the babbitt, but it is not to be supposed that
the fit can be made so perfect that the color will be distributed
evenly over the entire surface. Care should be taken to screw the
bearing cap onto the shims as tightly as possible each time the blueing
test is to be made.
There is nothing that will heat a bearing so quickly as a poor
alignment of the shaft supported by it. For this reason gasoline engine
crank shafts are made exceptionally strong and heavy, especially those
that are supported only at their extremities, or at these points and in
the center of their length. A shaft that is bent or twisted to even the
slightest degree will soon "burn out" all of its bearings, regardless
of the amount of oil that may be fed to them. This is because of the
unequal pressures on the different sides of the bearing that allow no
room for the admission of the film of oil or other lubricant that is
necessary in all cases to prevent a "hot box."
On the other hand, the bearings must all be in perfect alignment, for
to set one slightly "off" would produce the same result as though the
shaft were bent. It will be seen that the use of babbitt produces a
"self-aligning" bearing, for the straight shaft may be set in its
proper position and the molten metal poured around the interior of the
boxes.
As it is highly important that the cap screws or nuts holding the
bearing cap in place should remain set as tightly as possible,
precautions must be taken to prevent any of these from working loose.
This may be done by means of a cotter pin that passes through a hole in
each bolt and through a pair of corresponding notches cut in the top of
opposite faces of the nut. A notch is generally cut in the top of each
face of the nut in order that the latter may be held securely in place
in any position. A continuous wire passing through all of the bolts and
nuts is sometimes used instead of the individual cotter pins.
Many modern automobile motors are designed with the crank shaft running
in ball bearings. The type generally used consists of a row of balls
set between the inner and outer edges of two concentric rings. The
inside of the outer and the outside of the inner ring are grooved,
constituting the ball "race" which forms the surface upon which the
balls roll and which, at the same time, serves to hold them in place.
Each ball of the same bearing must be made of exactly the same size as
its companions--or at least within one or two ten-thousandths of an
inch--and each one must be large enough and of sufficient strength to
withstand, by itself, the entire pressure in that bearing. The inner
ring slips over the bearing portion of the shaft with a comparatively
tight fit, while the outer ring remains stationary in its bed in the
crank case.
The inner ring turns with the shaft, thus causing the balls to roll in
their race. Each ball rolls about its own axis, and the entire series
describes a circular motion in the same direction as that taken by
the shaft, but considerably slower. Consequently there is no rubbing
in such a bearing, all the motion being of the rolling type, and as
this reduces friction to a minimum, the balls may be run without oil,
although lubrication of the proper kind would certainly not harm the
bearing. Ball bearings are adapted only for a two-bearing crank shaft,
for inasmuch as the rings must be slipped over the shaft, it would be
manifestly impossible to provide a ball bearing in the center, or in
any other portion beyond a crank.
Next in importance to the main bearings of a crank shaft are those by
which the connecting rods communicate their motion to the cranks. These
are known as the crank pin bearings or the "big end" of the connecting
rod bearings. But inasmuch as the upper, or smaller, end of the
connecting rods are termed the wrist-pin bearings, the other end may be
called simply the connecting rod bearing.
The connecting rod bearings are similar to the main bearings described
in the foregoing pages and are renewed and adjusted in the same manner.
It is probable, however, that these receive a greater amount of wear
than do the main bearings, inasmuch as the former obtain the direct
impact of the force of each explosion. Furthermore, the box of the
connecting rod bearing describes a complete circle with each revolution
of the crank shaft, in addition to the "internal rotation" of the
crank, while an alternate push and pull is delivered to it by the
connecting rod on its various strokes.
Consequently it is the connecting rod bearings that will become loose
and require "taking up" before any attention need be bestowed on the
main bearings. The wear will increase in the connecting rod bearing as
the play becomes greater, and if matters are not remedied, the box may
eventually be broken, with the result that the end of the connecting
rod thus freed will start on the "rampage" and will punch several
pieces out of the bottom of the crank case.
Brass or bronze bearings may be used at the big end of the connecting
rods, but the large majority of motor car engines are provided with
babbitted bearings at these points. It is especially necessary that
these bearings should be scraped to a perfect fit and that the shims
should be adjusted properly so that no side play will be apparent
when the connecting rod is moved transversely to the length of the
crank shaft. When renewing the babbitts of connecting rod bearings
care should be taken to allow the connecting rod to swing free before
the molten metal is poured in. If this is not done, the connecting
rod may be forced slightly to one side or the other and will be held
permanently in this position when the babbitt cools. This will induce
a slight side thrust in the connecting rod, which will be communicated
to the piston, with the result that the side of the latter and of the
portion of the cylinder wall against which it moves will be scored and
worn unduly.
Inasmuch as the connecting rod bearings are subjected to such a variety
of strains, and as looseness at these points will result in serious
wear, it is doubly necessary that the nuts and bolts holding the
bearing caps in place should be securely wired or held tightly by means
of the previously-mentioned cotter pins. It is evident that the base of
the large end of the connecting rod forms the upper half of the bearing
box, while the cap constitutes the lower end and is attached from the
bottom.
The connecting rod bearings on some motors are hinged at one side so
that the cap may be turned away from the crank shaft when it is desired
to remove the connecting rod. In this case the hinge replaces the one
or two bolts or nuts on one side of the box and is held in the proper
position by those on the other side. While it may be easier to adjust
a bearing provided with such a cap, the results obtained can hardly
be expected to be as satisfactory for high-grade service, as is the
case when the shims may be used on both sides of the two halves of the
bearing.
The wrist-pin bearing is located at the upper, or small, end of each
connecting rod, and, although it also carries the full force of each
explosion, it is not subjected to as great wear as is the bearing at
the other end of the connecting rod. The reason for this is that this
bearing does not revolve and its friction surface is reduced to the
comparatively small arc through which the connecting rod swings. Wear
can occur here, however, and because this bearing is more inaccessible
than is the crank shaft or connecting rod bearing, trouble at the wrist
pin is often overlooked.
The wrist pin can only be reached by the removal of the piston and
connecting rod. In the majority of designs the wrist pin is placed
in the sides of the piston and is held stationary by small keys or
by set screws. In this case, the bearing surface is formed by the
wrist pin and the small end of the connecting rod, at which point the
greatest wear occurs. This bearing is never babbitted, but in order to
reduce the wear on the wrist pin--which is generally made of hardened
steel--the circular opening in the upper end of the connecting rod is
lined with a bronze or brass bushing that forms a bearing fit over
the wrist pin. It is this lining, or bushing, that will wear rather
than the hardened steel wrist pin, but as the former is easily removed
and is not expensive to replace, the renewal of this bearing is a
comparatively simple matter.
In other types of wrist pin bearings, the pin is held stationary in the
connecting rod opening and turns with it as the connecting rod swings
through its arc on each stroke of the piston. With such a design, the
bearing surface is formed by each end of the wrist pin and the openings
in the sides of the piston walls in which the wrist pin rests. In order
to form an easily-replaced bearing surface, these openings in the
piston walls are lined with brass or bronze bushings that receive the
major part of the wear, as has been described in connection with the
bushings fitted to the opening at the small end of the connecting rod.
There is nothing complicated or mysterious connected with the renewal
or repair of bearings, but the man who makes such replacements or
adjustments must be an accurate and careful worker, and while he
need not be a "born machinist," he must at least possess the "knack"
of handling tools properly. And he must, above all, realize that
the designers and manufacturers of his motor have been dealing in
measurements of the thousandth part of an inch and that too great care
cannot be taken in the repair of bearings to obtain a perfect fit.
If he is renewing a connecting rod or a wrist pin bearing, he must
also remember that the piston has formerly been traveling over a
certain area of cylinder surface that has not varied in length the
ten-thousandth part of an inch between one stroke and the next.
Consequently, the babbitts or bushings should be so replaced that the
piston shall occupy the same position relative to the cylinder walls at
the top and bottom of its stroke that it did formerly. In other words,
by varying the thickness of the top of the babbitt he is replacing, he
may change the "center" of the bearing so that the piston will start
on its upward stroke from a different point than was previously the
case. Thus, while the length of travel of the piston will be the same,
it will traverse a slightly different portion of the cylinder walls
under the new conditions, and this will have the effect of changing the
compression and, possibly, of wearing the piston and rings unduly.