LUBRICATION
Lubrication is a subject that should be studied by every gas-engine
user. So far as the piston is concerned it is a matter of the utmost
importance. The piston does its work under very peculiar conditions. It
is driven at great linear velocities; and it is, moreover, subjected to
high temperatures which have nothing in common with good lubrication if
care be not exercised.
The piston is the essential, vital element of an engine. Upon its
freedom from leakage depends the maintenance of a proper compression,
and, consequently, the production of power and economical consumption.
As it travels forward and as it recedes from the explosion-chamber, it
uncovers more and more of the frictional surface constituting the
interior wall of the cylinder. This surface, as a result, is regularly
brought into contact with the ignited, expanding gases after each
explosion. For this reason the oil which covers the wall is constantly
subjected to high temperatures, by which it is likely to be volatilized
and burned. Therefore, the first condition to be fulfilled in properly
lubricating the piston is a constant and regular supply of oil.
=Quality of Oils.=--For cylinder lubrication only the very best oils
should be used; perfect lubrication is of such importance that cost
should not be considered. Besides, the surplus oil which is usually
caught in the drip-pan is by no means lost. After having been filtered
it can be used for lubricating the bearings of the crank, the cam-shaft,
and like parts.
Cylinder-oil should be exceedingly pure, free from acids, and composed
of hydrocarbons that leave no residue after combustion. Only mineral
oils, therefore, are suitable for the purpose. Those oils should be
selected which, with a maximum of viscosity, are capable of withstanding
great heat without volatilizing or burning. The point at which a good
cylinder-oil ignites should not be lower than 535 degrees F.
Whether an oil possesses this essential quality is easily enough
ascertained in practice without resorting to laboratory tests. All that
is necessary is to heat the oil in a metal vessel or a porcelain dish.
In order that the temperature may be uniform the vessel is shielded from
the direct flame by interposing a piece of sheet metal or a layer of dry
sand. As soon as gases begin to arise a lighted match is held over the
oil. When the gases are ignited the thermometer reading is taken, the
instrument being immersed in the oil. The temperature recorded is that
corresponding with the point of ignition.
For cylinder lubrication American mineral oil is preferable to Russian
oil. The specific gravity should lie somewhere between .886 and .889 at
70 degrees F. Oil of this quality begins to evaporate at about 365
degrees F. Ignition occurs at 535 degrees F. The point of complete
combustibility lies between 625 and 645 degrees F. Oil of this quality
solidifies at 39 or 41 degrees F. Its color is a reddish yellow with a
greenish fluorescence. Compared with water its degree of viscosity lies
between 11.5 and 12.5 at a temperature of 140 degrees F.
Before lubricating other parts of the engine with oil that has been used
for the piston, heavy particles and foreign matter, such as dust,
bearing incrustations, and the like, should be filtered out. The
piston-pivot and the connecting-rod head are preferably lubricated with
fresh oil, because their constant movement renders inspection difficult
and the control of lubrication irksome. A good, industrial mineral oil
of usual market quality will be found satisfactory. In order to bring
home the importance of employing good cylinder-oil and of proper
lubrication the author can only state that in his personal experience he
has frequently detected losses varying from 10 to 15 per cent. in the
power developed by engines poorly lubricated.
=Types of Lubricators.=--Among the more common apparatus employed for
automatically lubricating the cylinder, the author mentions an English
oiler of the type pictured in Fig. 67 which is driven simply by a belt
from the intermediary shaft, and which rotates the pulley _P_ secured on
the shaft _a_ of the apparatus, at a very slow speed. The shaft _a_ is
provided at its end with a small crank, from which a small iron arm _f_
is suspended, which arm dips in the oil contained in the cup _G_ of the
oiler. When the shaft _a_ is turned this arm, as it sweeps through the
oil-bath, collects a certain quantity of oil which it deposits on the
collector _b_. From this spindle the oil passes through an outlet-pipe
opening into the bottom of the oiler, and thence to the cylinder. The
entire apparatus is closed by a cover _D_ which can be easily removed in
order to ascertain the quantity of oil still remaining in the apparatus.
Many other systems are utilized which, like the one that has been
described, enable the feed to be controlled. Often small force-pumps are
employed as cylinder-lubricators. Whatever may be the type selected,
preference should be given to that in which the feed is visible (Fig.
68).
[Illustration: FIG. 67.--An automatic English oiler.]
If the oil be fed under pressure the cylinder is more constantly
lubricated. Pressure-lubricators are nowadays widely used on large
engines. It is advisable to add a little salt to the water contained in
sight-feed lubricators so that the drop of oil is easily freed.
These oil-pumps are provided with small check-valves at their outlets as
well as at the inlets of cylinders. In order that pressure-lubricators
may operate perfectly they should be regularly inspected and the
check-valves ground from time to time.
The lubrication of the crank-shaft and of the two connecting-rod heads
should receive every attention.
[Illustration: FIG. 68.--Sight-feed lubricating-pump.]
[Illustration: FIG. 69.--Method of oiling the piston and end of the
connecting-rod.]
Lubricating devices should be employed which, besides being efficient,
do not necessitate the stopping of the engine in order to oil the
bearings. The foot of the connecting-rod at the point where it is
pivoted to the piston is generally lubricated with cylinder-oil which is
supplied by a tube mounted in the proper place across the piston-wall
(Fig. 69). This arrangement may be adequate enough for small engines;
but it is not sufficiently sure for engines of considerable size. An
independent lubricating system should be employed, lubrication being
effected either by a splasher mounted in front of the cylinder or by a
lubricator secured to the connecting-rod by which the pivot is
lubricated through the medium of a small tube supplying special oil
(Fig. 21). The head of the connecting-rod where it meets the crank, must
also be carefully lubricated because of the important nature of the work
which it must perform, and because of the shocks to which it is
subjected at each explosion. For motors of high power the system which
seems to give most satisfactory results is that illustrated in Fig. 70.
The arrangement there shown consists of an annular vessel secured at one
side of the crank and turning concentrically on its axis; the vessel
being connected with a long tube extending into a channel formed in the
crank and discharging at the surface of the crank-pin within the bearing
at the head of the connecting-rod. An adjustable sight-feed lubricator
conducts the oil along a pipe to the vessel. Turning with the shaft, the
vessel retains the oil in the periphery so that the feed in the
previously mentioned channel in the connecting-rod head, is constant.
[Illustration: FIG. 70.--Method of oiling the crank-shaft.]
The main crank-shaft bearings are more easily lubricated. Among the
systems commonly used with good results may be mentioned that shown in
Fig. 71, in which the half section represents a small tube starting from
the bearing and terminating in the interior of an oil recess or
reservoir cast integrally with the bearing-cap. This reservoir is filled
up to the level of the tube opening. A piece of cotton waste held on a
small iron wire is inserted in the tube, part of the cotton being
allowed to hang down in the reservoir. This cotton serves as a kind of
siphon and feeds the bearing by capillary attraction with a constant
quantity of oil, the supply being regulated by varying the thickness of
the cotton. When the motor is stopped, the cotton should be removed in
order that oil-feeding may not uselessly continue. Glass, sight-feed
lubricators with stop-cocks, are very often used on crank-shafts. They
are cleaner and much more easily regulated. Of all shaft-bearing
lubricators, those which are most to be recommended are of the
revolving-ring type (Fig. 72). They presuppose, however, bearings of
large size and a special arrangement of bushings which renders their
application somewhat expensive. Furthermore, the revolving-ring system
can hardly be used in connection with engines of less than 20
horse-power. Since the system is applied almost exclusively to
dynamo-shafts, it need not here be described in detail. As its name
indicates, it consists of a metal ring having a diameter larger than
that part of the shaft from which it is suspended and by which it is
rotated. The lower part of the ring is immersed in an oil bath so that
a certain quantity of lubricant is continually transferred to the shaft.
[Illustration: FIG. 71.--Cotton-waste lubricator.]
The revolving ring bearing should be fitted with a drain-cock and a
glass tube in order to control the level of the oil in the bearing.
Many manufacturers have adopted lubricating devices for valve-stems, and
especially for exhaust-valves. The system adopted consists of a small
tube curved in any convenient direction and discharging in the
stem-guide. The free end is provided with a plug. A few drops of
petroleum are introduced once or twice a day.
[Illustration: FIG. 72.--Ring type of bearing oiler.]
The lubrication of an engine entails certain difficulties which are
easily overcome. One of these is the splashing of oil by the
connecting-rod head. In order that this splashed oil may be collected in
the base of the engine a suitably curved sheet-metal guard is mounted
over the crank. A more serious difficulty is presented when the oil from
a crank-bearing finds its way to the hub of the fly-wheel, whence it is
driven by the centrifugal force to the rim. The oil is not only splashed
against the walls of the engine-room, but it also destroys the adhesion
of the belt if the fly-wheel be employed as a pulley. In order to
overcome this objection the oil is prevented from spreading along the
shaft by means of a circular guard (Fig. 73) mounted on that portion of
the shaft toward the interior of the bearing.
[Illustration: FIG. 73.--Shaft with oil-guard.]
The problem of lubrication is of particular importance if the engine is
driven for several days at a time without a stop. This happens in the
case of mill and shop engines. Lubricators of large volume or
lubricators which can be readily filled without stopping the engine
should be employed.