Constructional Details of Pistons--Aluminum Cylinders and
Pistons--Piston Ring Construction--Leak Proof Piston Rings--
Keeping Oil Out of Combustion Chamber--Connecting Rod Forms--
Connecting Rods for Vee Engines--Cam-Shaft and Crank-Shaft
Designs--Ball Bearing Crank-Shafts--Engine Base Construction.
CONSTRUCTIONAL DETAILS OF PISTONS
The piston is one of the most important parts of the gasoline motor
inasmuch as it is the reciprocating member that receives the impact of
the explosion and which transforms the power obtained by the combustion
of gas to mechanical motion by means of the connecting rod to which it
is attached. The piston is one of the simplest elements of the motor,
and it is one component which does not vary much in form in different
types of motors. The piston is a cylindrical member provided with a
series of grooves in which packing rings are placed on the outside and
two bosses which serve to hold the wrist pin in its interior. It is
usually made of cast iron or aluminum, though in some motors where
extreme lightness is desired, such as those used for aeronautic work, it
may be made of steel. The use of the more resisting material enables the
engineer to use lighter sections where it is important that the weight
of this member be kept as low as possible consistent with strength.
[Illustration: Fig. 116.--Forms of Pistons Commonly Employed in Gasoline
Engines. A--Dome Head Piston and Three Packing Rings. B--Flat Top Form
Almost Universally Used. C--Concave Piston Utilized in Knight Motors and
Some Having Overhead Valves. D--Two-Cycle Engine Member with Deflector
Plate Cast Integrally. E--Differential of Two-Diameter Piston Used in
Some Engines Operating on Two-Cycle Principle.]
A number of piston types are shown at Fig. 116. That at A has a round
top and is provided with four split packing rings and two oil grooves. A
piston of this type is generally employed in motors where the combustion
chamber is large and where it is desired to obtain a higher degree of
compression than would be possible with a flat top piston. This
construction is also stronger because of the arched piston top. The most
common form of piston is that shown at B, and it differs from that
previously described only in that it has a flat top. The piston outlined
in section at C is a type used on some of the sleeve-valve motors of the
Knight pattern, and has a concave head instead of the convex form shown
at A. The design shown at D in side and plan views is the conventional
form employed in two-cycle engines. The deflector plate on the top of
the cylinder is cast integral and is utilized to prevent the incoming
fresh gases from flowing directly over the piston top and out of the
exhaust port, which is usually opposite the inlet opening. On these
types of two-cycle engines where a two-diameter cylinder is employed,
the piston shown at E is used. This is known as a "differential
piston," and has an enlarged portion at its lower end which fits the
pumping cylinder. The usual form of deflector plate is provided at the
top of the piston and one may consider it as two pistons in one.
[Illustration: Fig. 117.--Typical Methods of Piston Pin Retention
Generally Used in Engines of American Design. A--Single Set Screw and
Lock Nut. B--Set Screw and Check Nut Fitting Groove in Wrist Pin. C,
D--Two Locking Screws Passing Into Interior of Hollow Wrist Pin.
E--Split Ring Holds Pin in Place. F--Use of Taper Expanding Plugs
Outlined. G--Spring Pressed Plunger Type. H--Piston Pin Pinned to
Connecting Rod. I--Wrist Pin Clamped in Connecting Rod Small End by
Bolt.]
[Illustration: Fig. 118.--Typical Piston and Connecting Rod Assembly.]
[Illustration: Fig. 119.--Parts of Sturtevant Aviation Engine.
A--Cylinder Head Showing Valves. B--Connecting Rod. C--Piston and
Rings.]
One of the important conditions in piston design is the method of
securing the wrist pin which is used to connect the piston to the upper
end of the connecting rod. Various methods have been devised to keep the
pin in place, the most common of these being shown at Fig. 117. The
wrist pin should be retained by some positive means which is not liable
to become loose under the vibratory stresses which obtain at this point.
If the wrist pin was free to move it would work out of the bosses
enough so that the end would bear against the cylinder wall. As it is
usually made of steel, which is a harder material than cast iron used in
cylinder construction, the rubbing action would tend to cut a groove in
the cylinder wall which would make for loss of power because it would
permit escape of gas. The wrist pin member is a simple cylindrical
element that fits the bosses closely, and it may be either hollow or
solid stock. A typical piston and connecting rod assembly which shows a
piston in section also is given at Fig. 118. The piston of the
Sturtevant aeronautical motor is shown at Fig. 119, the aluminum piston
of the Thomas airplane motor with piston rings in place is shown at Fig.
120. A good view of the wrist pin and connecting rod are also given. The
iron piston of the Gnome "Monosoupape" airplane engine and the
unconventional connecting rod assembly are clearly depicted at Fig 121.
[Illustration: Fig. 120.--Aluminum Piston and Light But Strong Steel
Connecting Rod and Wrist Pin of Thomas Aviation Engine.]
The method of retention shown at A is the simplest and consists of a set
screw having a projecting portion passing into the wrist pin and
holding it in place. The screw is kept from turning or loosening by
means of a check nut. The method outlined at B is similar to that shown
at A, except that the wrist pin is solid and the point of the set screw
engages an annular groove turned in the pin for its reception. A very
positive method is shown at C. Here the retention screws pass into the
wrist pin and are then locked by a piece of steel wire which passes
through suitable holes in the ends. The method outlined at D is
sometimes employed, and it varies from that shown at C only in that the
locking wire, which is made of spring steel, is passed through the heads
of the locking screws. Some designers machine a large groove around the
piston at such a point that when the wrist pin is put in place a large
packing ring may be sprung in the groove and utilized to hold the wrist
pin in place.
[Illustration: Fig. 121.--Cast Iron Piston of "Monosoupape" Gnome Engine
Installed On One of the Short Connecting Rods.]
The system shown at F is not so widely used as the simpler methods,
because it is more costly and does not offer any greater security when
the parts are new than the simple lock shown at A. In this a hollow
wrist pin is used, having a tapered thread cut at each end. The wrist
pin is slotted at three or four points, for a distance equal to the
length of the boss, and when taper expansion plugs are screwed in place
the ends of the wrist pin are expanded against the bosses. This method
has the advantage of providing a certain degree of adjustment if the
wrist pin should loosen up after it has been in use for some time. The
taper plugs would be screwed in deeper and the ends of the wrist pin
expanded proportionately to take up the loss motion. The method shown at
G is an ingenious one. One of the piston bosses is provided with a
projection which is drilled out to receive a plunger. The wrist pin is
provided with a hole of sufficient size to receive the plunger, which is
kept in place by means of a spring in back of it. This makes a very
positive lock and one that can be easily loosened when it is desired to
remove the wrist pin. To unlock, a piece of fine rod is thrust into the
hole at the bottom of the boss which pushes the plunger back against the
spring until the wrist pin can be pushed out of the piston.
Some engineers think it advisable to oscillate the wrist pin in the
piston bosses, instead of in the connecting rod small end. It is argued
that this construction gives more bearing surface at the wrist pin and
also provides for more strength because of the longer bosses that can be
used. When this system is followed the piston pin is held in place by
locking it to the connecting rod by some means. At H the simplest method
is outlined. This consisted of driving a taper pin through both rod and
wrist pin and then preventing it from backing out by putting a split
cotter through the small end of the tapered locking pin. Another method,
which is depicted at I, consists of clamping the wrist pin by means of a
suitable bolt which brings the slit connecting rod end together as
shown.
ALUMINUM FOR CYLINDERS AND PISTONS
Aluminum pistons outlined at Fig. 122, have replaced cast iron members
in many airplane engines, as these weigh about one-third as much as the
cast iron forms of the same size, while the reduction in the inertia
forces has made it possible to increase the engine speed without
correspondingly stressing the connecting rods, crank-shaft and engine
bearings.
[Illustration: Fig. 122.--Types of Aluminum Pistons Used In Aviation
Engines.]
Aluminum has not only been used for pistons, but a number of motors will
be built for the coming season that will use aluminum cylinder block
castings as well. Of course, the aluminum alloy is too soft to be used
as a bearing for the piston, and it will not withstand the hammering
action of the valve. This makes the use of cast iron or steel imperative
in all motors. When used in connection with an aluminum cylinder block
the cast iron pieces are placed in the mould so that they act as
cylinder liners and valve seats, and the molten metal is poured around
them when the cylinder is cast. It is said that this construction
results in an intimate bond between the cast iron and the surrounding
aluminum metal. Steel liners may also be pressed into the aluminum
cylinders after these are bored out to receive them. Aluminum has for a
number of years been used in many motor car parts. Alloys have been
developed that have greater strength than cast iron and that are not so
brittle. Its use for manifolds and engine crank and gear cases has been
general for a number of years.
At first thought it would seem as though aluminum would be entirely
unsuited for use in those portions of internal combustion engines
exposed to the heat of the explosion, on account of the low melting
point of that metal and its disadvantageous quality of suddenly
"wilting" when a critical point in the temperature is reached. Those who
hesitated to use aluminum on account of this defect lost sight of the
great heat conductivity of that metal, which is considerably more than
that of cast iron. It was found in early experiments with aluminum
pistons that this quality of quick radiation meant that aluminum pistons
remained considerably cooler than cast iron ones in service, which was
attested to by the reduced formation of carbon deposit thereon. The use
of aluminum makes possible a marked reduction in power plant weight. A
small four-cylinder engine which was not particularly heavy even with
cast iron cylinders was found to weigh 100 pounds less when the cylinder
block, pistons, and upper half of the crank-case had been made of
aluminum instead of cast iron. Aluminum motors are no longer an
experiment, as a considerable number of these have been in use on cars
during the past year without the owners of the cars being apprised of
the fact. Absolutely no complaint was made in any case of the aluminum
motor and it was demonstrated, in addition to the saving in weight, that
the motors cost no more to assemble and cooled much more efficiently
than the cast iron form. One of the drawbacks to the use of aluminum is
its growing scarcity, which results in making it a "near precious"
metal.
PISTON RING CONSTRUCTION
As all pistons must be free to move up and down in the cylinder with
minimum friction, they must be less in diameter than the bore of the
cylinder. The amount of freedom or clearance provided varies with the
construction of the engine and the material the piston is made of, as
well as its size, but it is usual to provide from .005 to .010 of an
inch to compensate for the expansion of the piston due to heat and also
to leave sufficient clearance for the introduction of lubricant between
the working surfaces. Obviously, if the piston were not provided with
packing rings, this amount of clearance would enable a portion of the
gases evolved when the charge is exploded to escape by it into the
engine crank-case. The packing members or piston rings, as they are
called, are split rings of cast iron, which are sprung into suitable
grooves machined on the exterior of the piston, three or four of these
being the usual number supplied. These have sufficient elasticity so
that they bear tightly against the cylinder wall and thus make a
gas-tight joint. Owing to the limited amount of surface in contact with
the cylinder wall and the elasticity of the split rings the amount of
friction resulting from the contact of properly fitted rings and the
cylinder is not of enough moment to cause any damage and the piston is
free to slide up and down in the cylinder bore.
[Illustration: Fig. 123.--Types of Piston Rings and Ring Joints.
A--Concentric Ring. B--Eccentrically Machined Form. C--Lap Joint Ring.
D--Butt Joint, Seldom Used. E--Diagonal Cut Member, a Popular Form.]
These rings are made in two forms, as outlined at Fig. 123. The design
shown at A is termed a "concentric ring," because the inner circle is
concentric with the outer one and the ring is of uniform thickness at
all points. The ring shown at B is called an "eccentric ring," and it is
thicker at one part than at others. It has theoretical advantages in
that it will make a tighter joint than the other form, as it is claimed
its expansion due to heat is more uniform. The piston rings must be
split in order that they may be sprung in place in the grooves, and also
to insure that they will have sufficient elasticity to take the form of
the cylinder at the different points in their travel. If the cylinder
bore varies by small amounts the rings will spring out at the points
where the bore is larger than standard, and spring in at those portions
where it is smaller than standard.
It is important that the joint should be as nearly gas-tight as
possible, because if it were not a portion of the gases would escape
through the slots in the piston rings. The joint shown at C is termed a
"lap joint," because the ends of the ring are cut in such a manner that
they overlap. This is the approved joint. The butt joint shown at D is
seldom used and is a very poor form, the only advantage being its
cheapness. The diagonal cut shown at E is a compromise between the very
good form shown at C and the poor joint depicted at D. It is also widely
used, though most constructors prefer the lap joint, because it does not
permit the leakage of gas as much as the other two types.
There seems to be some difference of opinion relative to the best piston
ring type--some favoring the eccentric pattern, others the concentric
form. The concentric ring has advantages from the lubricating engineer's
point of view; as stated by the Platt & Washburn Company in their
text-book on engine lubrication, the smaller clearance behind the ring
possible with the ring of uniform section is advantageous.
Fig. 124, A, shows a concentric piston ring in its groove. Since the
ring itself is concentric with the groove, very small clearance between
the back of the ring and the bottom of its groove may be allowed. Small
clearance leaves less space for the accumulation of oil and carbon
deposits. The gasket effect of this ring is uniform throughout the
entire length of its edges, which is its marked advantage over the
eccentric ring. This type of piston ring rarely burns fast in its
groove. There are a large number of different concentric rings
manufactured of different designs and of different efficiency.
[Illustration: Fig. 124.--Diagrams Showing Advantages of Concentric
Piston Rings.]
Figs. 124, B and 124, C show eccentric rings assembled in the ring
groove. It will be noted that there is a large space between the thin
ends of this ring and the bottom of the groove. This empty space fills
up with oil which in the case of the upper ring frequently is
carbonized, restricting the action of the ring and nullifying its
usefulness. The edges of the thin ends are not sufficiently wide to
prevent rapid escape of gases past them. In a practical way this leakage
means loss of compression and noticeable drop in power. When new and
properly fitted, very little difference can be noted between the
tightness of eccentric and concentric rings. Nevertheless, after several
months' use, a more rapid leakage will always occur past the eccentric
than past the concentric. If continuous trouble with the carbonization
of cylinders, smoking and sooting of spark-plugs is experienced, it is
a sure indication that mechanical defects exist in the engine, assuming
of course, that a suitable oil has been used. Such trouble can be
greatly lessened, if not entirely eliminated, by the application of
concentric rings (lap joint), of any good make, properly fitted into the
grooves of the piston. Too much emphasis cannot be put upon this point.
If the oil used in the engine is of the correct viscosity, and serious
carbon deposit, smoking, etc., still result, the only certain remedy
then is to have the cylinders rebored and fitted with properly designed,
oversized pistons and piston rings.
LEAK-PROOF PISTON RINGS
In order to reduce the compression loss and leakage of gas by the
ordinary simple form of diagonal or lap joint one-piece piston ring a
number of compound rings have been devised and are offered by their
makers to use in making replacements. The leading forms are shown at
Fig. 125. That shown at A is known as the "Statite" and consists of
three rings, one carried inside while the other two are carried on the
outside. The ring shown at B is a double ring and is known as the
McCadden. This is composed of two thin concentric lap joint rings so
disposed relative to each other that the opening in the inner ring comes
opposite to the opening in the outer ring.
The form shown at C is known as the "Leektite," and is a single ring
provided with a peculiar form of lap and dove tail joint. The ring shown
at D is known as the "Dunham" and is of the double concentric type being
composed of two rings with lap joints which are welded together at a
point opposite the joint so that there is no passage by which the gas
can escape. The Burd high compression ring is shown at E. The joints of
these rings are sealed by means of an H-shaped coupler of bronze which
closes the opening. The ring ends are made with tongues which interlock
with the coupling. The ring shown at F is called the "Evertite" and is
a three-piece ring composed of three members as shown in the sectional
view below the ring. The main part or inner ring has a circumferential
channel in which the two outer rings lock, the resulting cross-section
being rectangular just the same as that of a regular pattern ring. All
three rings are diagonally split and the joints are spaced equally and
the distances maintained by small pins. This results in each joint being
sealed by the solid portion of the other rings.
[Illustration: Fig. 125.--Leak-Proof and Other Compound Piston Rings.]
The use of a number of light steel rings instead of one wide ring in the
groove is found on a number of automobile power plants, but as far as
known, this construction is not used in airplane power plants. It is
contended that where a number of light rings is employed a more flexible
packing means is obtained and the possibility of leakage is reduced.
Rings of this design are made of square section steel wire and are given
a spring temper. Owing to the limited width the diagonal cut joint is
generally employed instead of the lap joint which is so popular on wider
rings.
KEEPING OIL OUT OF COMBUSTION CHAMBERS
An examination of the engine design that is economical in oil
consumption discloses the use of tight piston rings, large centrifugal
rings on the crank-shaft where it passes through the case, ample cooling
fins in the pistons, vents between the crank-case chamber and the valve
enclosures, etc. Briefly put, cooling of the oil in this engine has been
properly cared for and leakage reduced to a minimum. To be specific
regarding details of design: Oil surplus can be kept out of the
explosion chambers by leaving the lower edge of the piston skirt sharp
and by the use of a shallow groove (C), Fig. 126, just below the lower
piston ring. Small holes are bored through the piston walls at the base
of this groove and communicate with the crank-case. The similarity of
the sharp edges of piston skirt (D) and piston ring to a carpenter's
plane bit, makes their operation plain.
[Illustration: Fig. 126.--Sectional View of Engine Showing Means of
Preventing Oil Leakage By Piston Rings.]
The cooling of oil in the sump (A) can be accomplished most effectively
by radiating fins on its outer surface. The lower crank-case should be
fully exposed to the outer air. A settling basin for sediment (B) should
be provided having a cubic content not less than one-tenth of the total
oil capacity as outlined at Fig. 126. The depth of this basin should be
at least 2-1/2 inches, and its walls vertical, as shown, to reduce the
mixing of sediment with the oil in circulation. The inlet opening to the
oil pump should be near the top of the sediment basin in order to
prevent the entrance into the pump with the oil of any solid matter or
water condensed from the products of combustion. This sediment basin
should be drained after every five to seven hours air service of an
airplane engine. Concerning filtering screens there is little to be
said, save that their areas should be ample and the mesh coarse enough
(one-sixteenth of an inch) to offer no serious resistance to the free
flow of cold or heavy oil through them; otherwise the oil in the
crank-case may build up above them to an undesirable level. The
necessary frequency of draining and flushing out the oil sump differs
greatly with the age (condition) of the engine and the suitability of
the oil used. In broad terms, the oil sump of a new engine should be
thoroughly drained and flushed with kerosene at the end of the first
200 miles, next at the end of 500 miles and thereafter every 1,000
miles. While these instructions apply specifically to automobile motors,
it is very good practice to change the oil in airplane engines
frequently. In many cases, the best results have been secured when the
oil supply is completely replenished every five hours that the engine is
in operation.
CONNECTING ROD FORMS
The connecting rod is the simple member that joins the piston to the
crank-shaft and which transmits the power imparted to the piston by the
explosion so that it may be usefully applied. It transforms the
reciprocating movement of the piston to a rotary motion at the
crank-shaft. A typical connecting rod and its wrist pin are shown at
Fig. 120. It will be seen that it has two bearings, one at either end.
The small end is bored out to receive the wrist pin which joins it to
the piston, while the large end has a hole of sufficient size to go on
the crank-pin. The airplane and automobile engine connecting rod is
invariably a steel forging, though in marine engines it is sometimes
made a steel or high tensile strength bronze casting. In all cases it is
desirable to have softer metals than the crank-shaft and wrist pin at
the bearing point, and for this reason the connecting rod is usually
provided with bushings of anti-friction or white metal at the lower end,
and bronze at the upper. The upper end of the connecting rod may be one
piece, because the wrist pin can be introduced after it is in place
between the bosses of the piston. The lower bearing must be made in two
parts in most cases, because the crank-shaft cannot be passed through
the bearing owing to its irregular form. The rods of the Gnome engine
are all one piece types, as shown at Fig. 127, owing to the construction
of the "mother" rod which receives the crank-pins. The complete
connecting rod assembly is shown in Fig. 121, also at A, Fig. 127. The
"mother" rod, with one of the other rods in place and one about to be
inserted, is shown at Fig. 127, B. The built-up crank-shaft which makes
this construction feasible is shown at Fig. 127, C.
[Illustration: Fig. 127.--Connecting Rod and Crank-Shaft Construction of
Gnome "Monosoupape" Engine.]
Some of the various designs of connecting rods that have been used are
shown at Fig. 128. That at A is a simple form often employed in
single-cylinder motors, having built-up crank-shafts. Both ends of the
connecting rod are bushed with a one-piece bearing, as it can be
assembled in place before the crank-shaft assembly is built up. A
built-up crank-shaft such as this type of connecting rod would be used
with is shown at Fig. 106. The pattern shown at B is one that has been
used to some extent on heavy work, and is known as the "marine type." It
is made in three pieces, the main portion being a steel forging having a
flanged lower end to which the bronze boxes are secured by bolts. The
modified marine type depicted at C is the form that has received the
widest application in automobile and aviation engine construction. It
consists of two pieces, the main member being a steel drop forging
having the wrist-pin bearing and the upper crank-pin bearing formed
integral, while the lower crank-pin bearing member is a separate forging
secured to the connecting rod by bolts. In this construction bushings of
anti-friction metal are used at the lower end, and a bronze bushing is
forced into the upper- or wrist-pin end. The rod shown at D has also
been widely used. It is similar in construction to the form shown at C,
except that the upper end is split in order to permit of a degree of
adjustment of the wrist-pin bushing, and the lower bearing cap is a
hinged member which is retained by one bolt instead of two. When it is
desired to assemble it on the crank-shaft the lower cap is swung to one
side and brought back into place when the connecting rod has been
properly located. Sometimes the lower bearing member is split diagonally
instead of horizontally, such a construction being outlined at E.
[Illustration: Fig. 128.--Connecting Rod Types Summarized.
A--Single Connecting Rod Made in One Piece, Usually Fitted in Small
Single-Cylinder Engines Having Built-Up Crank-Shafts. B--Marine Type, a
Popular Form on Heavy Engines. C--Conventional Automobile Type, a
Modified Marine Form. D--Type Having Hinged Lower Cap and Split Wrist
Pin Bushing. E--Connecting Rod Having Diagonally Divided Big End.
F--Ball-Bearing Rod. G--Sections Showing Structural Shapes Commonly
Employed in Connecting Rod Construction.]
In a number of instances, instead of plain bushed bearings anti-friction
forms using ball or rollers have been used at the lower end. A
ball-bearing connecting rod is shown at F. The big end may be made in
one piece, because if it is possible to get the ball bearing on the
crank-pins it will be easy to put the connecting rod in place. Ball
bearings are not used very often on connecting rod big ends because of
difficulty of installation, though when applied properly they give
satisfactory service and reduce friction to a minimum. One of the
advantages of the ball bearing is that it requires no adjustment,
whereas the plain bushings depicted in the other connecting rods must be
taken up from time to time to compensate for wear.
This can be done in forms shown at B, C, D, and E by bringing the lower
bearing caps closer to the upper one and scraping out the brasses to fit
the shaft. A number of liners or shims of thin brass or copper stock,
varying from .002 inch to .005 inch, are sometimes interposed between
the halves of the bearings when it is first fitted to the crank-pin. As
the brasses wear the shims may be removed and the portions of the
bearings brought close enough together to take up any lost motion that
may exist, though in some motors no shims are provided and depreciation
can be remedied only by installing new brasses and scraping to fit.
[Illustration: Fig. 129.--Double Connecting Rod Assembly For Use On
Single Crank-Pin of Vee Engine.]
The various structural shapes in which connecting rods are formed are
shown in section at G. Of these the I section is most widely used in
airplane engines, because it is strong and a very easy shape to form by
the drop-forging process or to machine out of the solid bar when extra
good steel is used. Where extreme lightness is desired, as in small
high-speed motors used for cycle propulsion, the section shown at the
extreme left is often used. If the rod is a cast member as in some
marine engines, the cross, hollow cylinder, or U sections are sometimes
used. If the sections shown at the right are employed, advantage is
often taken of the opportunity for passing lubricant through the center
of the hollow round section on vertical motors or at the bottom of the U
section, which would be used on a horizontal cylinder power plant.
[Illustration: Fig. 130.--Another Type of Double Connecting Rod for Vee
Engines.]
Connecting rods of Vee engines are made in two distinct styles. The
forked or "scissors" joint rod assembly is employed when the cylinders
are placed directly opposite each other. The "blade" rod, as shown at
Fig. 129, fits between the lower ends of the forked rod, which oscillate
on the bearing which encircles the crank-pin. The lower end of the
"blade" rod is usually attached to the bearing brasses, the ends of the
"forked" rod move on the outer surfaces of the brasses. Another form of
rod devised for use under these conditions is shown at Fig. 130 and
installed in an aviation engine at Fig. 132. In this construction the
shorter rod is attached to a boss on the master rod by a short pin to
form a hinge and to permit the short rod to oscillate as the conditions
dictate. This form of rod can be easily adjusted when the bearing
depreciates, a procedure that is difficult with the forked type rod. The
best practice, in the writer's opinion, is to stagger the cylinders and
use side-by-side rods as is done in the Curtiss engine. Each rod may be
fitted independently of the other and perfect compensation for wear of
the big ends is possible.
[Illustration: Fig. 131.--Part Sectional View of Wisconsin Aviation
Engine, Showing Four-Bearing Crank-Shaft, Overhead Cam-Shaft, and Method
of Combining Cylinders in Pairs.]
[Illustration: Fig. 132.--Part Sectional View of Renault Twelve-Cylinder
Water-Cooled Engine, Showing Connecting Rod Construction and Other
Important Internal Parts.]
CAM-SHAFT AND CRANK-SHAFT DESIGN
Before going extensively into the subject of crank-shaft construction it
will be well to consider cam-shaft design, which is properly a part of
the valve system and which has been considered in connection with the
other elements which have to do directly with cylinder construction to
some extent. Cam-shafts are usually simple members carried at the base
of the cylinder in the engine case of Vee type motors by suitable
bearings and having the cams employed to lift the valves attached at
intervals. A typical cam-shaft design is shown at Fig. 133. Two main
methods of cam-shaft construction are followed--that in which the cams
are separate members, keyed and pinned to the shaft, and the other where
the cams are formed integral, the latter being the most suitable for
airplane engine requirements.
[Illustration: Fig. 133.--Typical Cam-Shaft, with Valve Lifting Cams and
Gears to Operate Auxiliary Devices Forged Integrally.]
The cam-shafts shown at Figs. 133 and 134, B, are of the latter type, as
the cams are machined integrally. In this case not only the cams but
also the gears used in driving the auxiliary shafts are forged integral.
This is a more expensive construction, because of the high initial cost
of forging dies as well as the greater expense of machining. It has the
advantage over the other form in which the cams are keyed in place in
that it is stronger, and as the cams are a part of the shaft they can
never become loose, as might be possible where they are separately
formed and assembled on a simple shaft.
[Illustration: Fig. 134.--Important Parts of Duesenberg Aviation Engine.
A--Three Main Bearing Crank-Shaft. B--Cam-Shaft with Integral Cams.
C--Piston and Connecting Rod Assembly. D--Valve Rocker Group. E--Piston.
F--Main Bearing Brasses.]
The importance of the crank-shaft has been previously considered, and
some of its forms have been shown in views of the motors presented in
earlier portions of this work. The crank-shaft is one of the parts
subjected to the greatest strain and extreme care is needed in its
construction and design, because practically the entire duty of
transmitting the power generated by the motor to the gearset devolves
upon it. Crank-shafts are usually made of high tensile strength steel of
special composition. They may be made in four ways, the most common
being from a drop or machine forging which is formed approximately to
the shape of the finished shaft and in rare instances (experimental
motors only) they may be steel castings. Sometimes they are made from
machine forgings, where considerably more machine work is necessary than
would be the case where the shaft is formed between dies. Some engineers
favor blocking the shaft out of a solid slab of metal and then machining
this rough blank to form. In some radial-cylinder motors of the Gnome
and Le Rhone type the crank-shafts are built up of two pieces, held
together by taper fastenings or bolts.
[Illustration: Fig. 135.--Showing Method of Making Crank-Shaft. A--The
Rough Steel Forging Before Machining. B--The Finished Six-Throw,
Seven-Bearing Crank-Shaft.]
The form of the shaft depends on the number of cylinders and the form
has material influence on the method of construction. For instance, a
four-cylinder crank-shaft could be made by either of the methods
outlined. On the other hand, a three- or six-cylinder shaft is best made
by the machine forging process, because if drop forged or cut from the
blank it will have to be heated and the crank throws bent around so that
the pins will lie in three planes one hundred and twenty degrees apart,
while the other types described need no further attention, as the
crank-pins lie in planes one hundred and eighty degrees apart. This can
be better understood by referring to Fig. 135, which shows a
six-cylinder shaft in the rough and finished stages. At A the
appearance of the machine forging before any of the material is removed
is shown, while at B the appearance of the finished crank-shaft is
clearly depicted. The built-up crank-shaft is seldom used on
multiple-cylinder motors, except in some cases where the crank-shafts
revolve on ball bearings as in some automobile racing engines.
[Illustration: Fig. 136.--Showing Form of Crank-Shaft for Twin-Cylinder
Opposed Power Plant.]
[Illustration: Fig. 137.--Crank-Shaft of Thomas-Morse Eight-Cylinder Vee
Engine.]
Crank-shaft form will vary with a number of cylinders and it is possible
to use a number of different arrangements of crank-pins and bearings for
the same number of cylinders. The simplest form of crank-shaft is that
used on simple radial cylinder motors as it would consist of but one
crank-pin, two webs, and the crank-shaft. As the number of cylinders
increase in Vee motors as a general rule more crank-pins are used. The
crank-shaft that would be used on a two-cylinder opposed motor is shown
at Fig. 136. This has two throws and the crank-pins are spaced 180
degrees apart. The bearings are exceptionally long. Four-cylinder
crank-shafts may have two, three or five main bearings and three or four
crank-pins. In some forms of two-bearing crank-shafts, such as used when
four-cylinders are cast in a block, or unit casting, two of the pistons
are attached to one common crank-pin, so that in reality the crank-shaft
has but three crank-pins. A typical three bearing, four-cylinder
crank-shaft is shown at Fig. 134, A. The same type can be used for an
eight-cylinder Vee engine, except for the greater length of crank-pins
to permit of side by side rods as shown at Fig. 137. Six cylinder
vertical tandem and twelve-cylinder Vee engine crank-shafts usually have
four or seven main bearings depending upon the disposition of the
crank-pins and arrangement of cylinders. At Fig. 138, A, the bottom
view of a twelve-cylinder engine with bottom half of crank case removed
is given. This illustrates clearly the arrangement of main bearings when
the crank-shaft is supported on four journals. The crank-shaft shown at
Fig. 138, B, is a twelve-cylinder seven-bearing type.
[Illustration: Fig. 138.--Crank-Case and Crank-Shaft Construction for
Twelve-Cylinder Motors. A--Duesenberg. B--Curtiss.]
[Illustration: Fig. 139.--Counterbalanced Crank-Shafts Reduce Engine
Vibration and Permit of Higher Rotative Speeds.]
In some automobile engines, extremely good results have been secured in
obtaining steady running with minimum vibration by counterbalancing the
crank-shafts as outlined at Fig. 139. The shaft at A is a type suitable
for a high speed four-cylinder vertical or an eight-cylinder Vee type.
That at B is for a six-cylinder vertical or a twelve-cylinder V with
scissors joint rods. If counterbalancing crank-shafts helps in an
automobile engine, it should have advantages of some moment in airplane
engines, even though the crank-shaft weight is greater.
BALL-BEARING CRANK-SHAFTS
While crank-shafts are usually supported in plain journals there seems
to be a growing tendency of late to use anti-friction bearings of the
ball type for their support. This is especially noticeable on block
motors where but two main bearings are utilized. When ball bearings are
selected with proper relation to the load which obtains they will give
very satisfactory service. They permit the crank-shaft to turn with
minimum friction, and if properly selected will never need adjustment.
The front end is supported by a bearing which is clamped in such a
manner that it will take a certain amount of load in a direction
parallel to the axis of the shaft, while the rear end is so supported
that the outer race of the bearing has a certain amount of axial freedom
or "float." The inner race or cone of each bearing is firmly clamped
against shoulders on the crank-shaft. At the front end of the
crank-shaft timing gear and a suitable check nut are used, while at the
back end the bearing is clamped by a threaded retention member between
the fly-wheel and a shoulder on the crank-shaft. The fly-wheel is held
in place by a taper and key retention. The ball bearings are carried in
a light housing of bronze or malleable iron, which in turn are held in
the crank-case by bolts. The Renault engine uses ball bearings at front
and rear ends of the crank-shaft, but has plain bearings around
intermediate crank-shaft journals. The rotary engines of the Gnome, Le
Rhone and Clerget forms would not be practical if ball bearings were not
used as the bearing friction and consequent depreciation would be very
high.
ENGINE-BASE CONSTRUCTION
One of the important parts of the power plant is the substantial casing
or bed member, which is employed to support the cylinders and
crank-shaft and which is attached directly to the fuselage engine
supporting members. This will vary widely in form, but as a general
thing it is an approximately cylindrical member which may be divided
either vertically or horizontally in two or more parts. Airplane engine
crank-cases are usually made of aluminum, a material which has about the
same strength as cast iron, but which only weighs a third as much. In
rare cases cast iron is employed, but is not favored by most engineers
because of its brittle nature, great weight and low resistance to
tensile stresses. Where exceptional strength is needed alloys of bronze
may be used, and in some cases where engines are produced in large
quantities a portion of the crank-case may be a sheet steel or aluminum
stamping.
[Illustration: Fig. 140.--View of Thomas 135 Horse-Power Aeromotor,
Model 8, Showing Conventional Method of Crank-Case Construction.]
[Illustration: Fig. 141.--Views of Upper Half of Thomas Aeromotor
Crank-Case.]
Crank-cases are always large enough to permit the crank-shaft and parts
attached to it to turn inside and obviously its length is determined by
the number of cylinders and their disposition. The crank-case of the
radial cylinder or double-opposed cylinder engine would be substantially
the same in length. That of a four-cylinder will vary in length with
the method of casting the cylinder. When the four-cylinders are cast in
one unit and a two-bearing crank-shaft is used, the crank-case is a very
compact and short member. When a three-bearing crank-shaft is utilized
and the cylinders are cast in pairs, the engine base is longer than it
would be to support a block casting, but is shorter than one designed to
sustain individual cylinder castings and a five-bearing crank-shaft. It
is now common construction to cast an oil container integral with the
bottom of the engine base and to draw the lubricating oil from it by
means of a pump, as shown at Fig. 140. The arms by which the motor is
supported in the fuselage are substantial-ribbed members cast
integrally with the upper half.
[Illustration: Fig. 142.--Method of Constructing Eight-Cylinder Vee
Engine, Possible if Aluminum Cylinder and Crank-Case Castings are Used.]
[Illustration: Fig. 143.--Simple and Compact Crank-Case, Possible When
Radial Cylinder Engine Design is Followed.]
The approved method of crank-case construction favored by the majority
of engineers is shown at the top of Fig. 141, bottom side up. The upper
half not only forms a bed for the cylinder but is used to hold the
crank-shaft as well. In the illustration, the three-bearing boxes form
part of the case, while the lower brasses are in the form of separately
cast caps retained by suitable bolts. In the construction outlined the
bottom part of the case serves merely as an oil container and a
protection for the interior mechanism of the motor. The cylinders are
held down by means of studs screwed into the crank-case top, as shown at
Fig. 141, lower view. If the aluminum cylinder motor has any future, the
method of construction outlined at Fig. 142, which has been used in cast
iron for an automobile motor, might be used for an eight-cylinder Vee
engine for airplane use. The simplicity of the crank-case needed for a
revolving cylinder motor and its small weight can be well understood by
examination of the illustration at Fig. 143, which shows the engine
crank-case for the nine-cylinder "Monosoupape" Gnome engine. This
consists of two accurately machined forgings held together by bolts as
clearly indicated.