bearings are used where the bearing is subjected to radial
loads only In tapered roller bearings, the inner and outer-
race bearing surfaces are cone shaped. Such bearings
will withstand both radial and thrust loads. Straight roller
bearings are used in high-power aircraft engines for the
crankshaft main bearings. They are also used in other
applications where radial loads are high.
Maintenance Clean, inspect, and replace
beanngs. in accordance with the applicable maintenance
Connecting Rods. The connecting rod is the link
which transmits forces between the piston and the
crankshaft. Connecting rods must be strong enough to
remain rigid under load and yet be light enough to reduce
the inertia forces which are produced when the rod and
piston stop, change direction, and start again at the end of
Types There are three types of connecting
rod assemblies, as shown in figure 7-9. The plain, fork-
and-blade, and master-and-articulated connecting rod
assemblies are described in the following paragraphs.
connecting rods are used In opposed engines. The end of
the rod attached to the crankpin Is fitted with a cap and a
two-piece beanng. The beanng cap is held on the end of
the rod by bolts or studs. To maintain proper fit and
balance, connecting rods should always be replaced. In
the same cylinder and In the same relative position.
Fork-and-blade connecting rods The
fork-and-blade rod assembly Is used primarily in V-type
engines The forked rod is split at the crankpln end to
allow space for the blade rod to fit between the prongs. A
single two-piece bearing is used on the crankshaft end of
rods. The master-and-articulated rod assembly is only
used in radial engines. In a radial engine the piston in
one cylinder in each row is connected to the crankshaft by
a master rod. All other pistons. In the row are connected
to the master rod by an articulated rod.
connecting rods involves visual Inspection for nicks,
cracks, bending, corrosion, and other damage, magnetic
particle inspection, checking alignment for parallelism and
convergence between the bearing end and the piston end;
rebushing, and replacement of the bearings. Refer to the
applicable maintenance manual before accomplishing any
Pistons. Pins. and Rings. Pistons, pins, and
rings, as shown in figure 7-10, are described and
maintained as explained In the following paragraphs.
Pistons. The piston of a reciprocating
engine is a cylindrical member which moves back and
forth within a steel cylinder. The piston acts as a moving
wall within the combustion chamber. As a piston moves
down the cylinder, it draws In the fuel-air mixture. As it
moves upward, it compresses the charge, ignition occurs,
and the expanding gases force the piston downward. This
force is transmitted to the crankshaft through the
connecting rod. On the return upward stroke, the piston
forces the exhaust gases from the cylinder.
Pins. The piston pin joins the piston to the
connecting rod It Is machined in the form of a tube from a
nickel steel alloy forging, casehardened and ground. The
piston pin is sometimes called a wristpin because of the
similarity between the relative motions of the piston and
the articulated rod and that of the human arm.
Rings. The piston rings prevent leakage of
gas pressure from the combustion chamber and reduce to
a minimum the seepage of oil into the combustion
chamber. The rings fit into the piston grooves but spring
out to press against the cylinder walls, when properly
lubricated, the rings form an effective gas seal. Rings are
always used in sets.
Maintenance. Pistons and pins are
Inspected and checked for cleanliness, wear, scoring,
corrosion, cracks, and any apparent damage New rings
are. Installed each time the engine is overhauled. Refer
accomplishing maintenance on pistons, pins, and rings.
Cylinders. The portion of the engine in which the
power is developed is called the cylinder. The cylinder
provides a combustion chamber where the burning and
expansion of gases take place, and. It houses the piston
and the connecting rod
Description. The cylinder used In the
aircooled engine is the overhead valve type shown in
figure 7-11. Each cylinder assembly consists of the
cylinder head and the cylinder barrel.