Connecting Rod: Parts, Types,
Functions, Uses, and More
Editing Date£º2026-02-23
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Connecting Rod:
Parts, Types,
Functions, Uses, and More
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What is connecting rod?
The connecting rod
is a connection between the piston and a
crankshaft. It joins the piston pin with
the crankpin.
The small end of
the connecting rod is connected to the
piston pin and the big end to the crank
pin.
The purpose of the
connecting rod is to convert the linear
motion of the piston into the rotary
motion of the crankshaft.
The connecting rod
consists of an I-beam cross-section and
is made of forged steel. Aluminum alloy
is also used for connecting rods.
They are precisely
matched in sets of similar weight in
order to maintain engine balance.
The lighter the
connecting rod and piston, the greater
the resulting in power and the lesser
the vibration because the reciprocating
weight is less.
The connecting rod
carries the power thrust from piston to
the crankpin and hence it must be very
strong, rigid and also as light as
possible.
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Parts of Connecting Rod
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Following are the parts
of connecting rod:
•
Small
End
•
Big
End
•
Bushing
•
Bearing inserts
•
Bolt
and Nut
•
Shank
•
Wrist
pin
•
Piston
•
Bearing cap
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1.
Small End
The end at which the
connecting rod is
attached to the face of
the piston pin is known
as the small end of the
connecting rod.
2. Big
End
The end at which the
connecting rod is
attached to the side of
the crank pin is known
as big end of the
connecting rod.
3.
Bush Bearing
Both ends of the
connecting rod are fixed
with a bush bearing. A
phosphor bronze bush is
fitted with the solid
eye is attached to the
small end of the
connecting rod.
The Big end is attached
to the crankpin. The end
is divided into two
parts and is supported
over the crank bearing
shell.
4.
Bearing Insert
In
the big end of the
connecting rod, there is
a bearing insert that is
connected to the bearing
cap, it is known as a
bearing insert. These
are made in two parts
that fit together on the
crankshaft. This is the
position where the
connecting rod travels
along the reverse
direction.
5.
Bolt and Nut
After the connecting rod
is fitted with the crank
at the bottom, both
sides of the big ends
are fastened by some
bolts and nuts. Thus, by
combining these all
components the
connecting rod is ready
to use.
6.
Shank
Furthermore, each of the
bolt and nuts are
employed to connect both
the connecting rod and
bearing cap. And a
section beam is applied
it is known as shank.
The section of the rod
may be rectangular,
tubular, and a circular
section.
The connecting rod
length lies on the ratio
of (I/r)
Where,
I
= is the length of the
shank or beam
r
= is the radius of the
shank.
7.
Wrist Pin
The engine piston is
connected to the
connecting rod with the
help of a hollow
hardened steel tube
called wrist pin. It is
also known as gudgeon
pin. Wrist pin goes
through the short end of
the connecting rod and
pivots on the engaged
piston.
8.
Piston
The piston is connected
to the crankshaft with
the help of a connecting
rod, which is usually
shortened to the rod or
Conrod. The purpose of
the piston is to work as
a movable plug in the
cylinder, which forms
the bottom of the
combustion chamber.
9.
Bearing Cap
Shell bearings have an
adjustment for wear, but
it controls the running
and the side clearance
allows the bearing cap
to be tightened
correctly.
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Construction and
Functions of Connecting
Rod
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There are two types of
ends small end and big
end bearings. The big
end is split at right
angles to its length as
at (a) or at an angle as
at (b), in order that it
may be assembled on the
crankpin. A cap is fixed
to the body of the
connecting rod by two
bolts and nuts.
Modern engines do not
have bearing metal fused
to the bore of a big
end, but it uses
separate low carbon
steel bearing shells.
The shell bearing has
adjustment for wear but
gives the control over
running and side
clearance, providing the
bearing cap to correct
fit. Sometimes, thin
pieces of metal known as
shins are used when spur
bearings are employed.
These can be filled
thinner to compensate
for the wear of the
bearing and also to
secure the correct
bearing clearance
between the connecting
rod and the crankshaft.
The small end is usually
a solid eye fitted with
a phosphor bronze bush
and a screw to close the
eye around the pin.
All the connecting rods
in an engine must be of
equal weight otherwise
noticeable vibration may
occur. In the assembly,
the connecting rods and
caps are individually
matched to each other.
It usually carries
identifying numbers so
that they may not be
mixed if the engine is
dissembled for service.
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Types of Connecting Rod
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Following are the types
of connecting rod, used
in various types of
engines:
•
Plain type rod
•
Fork and blade rod
•
Master and slave rod
•
Billet conrods
•
Cast rods
•
Forged rods
•
Powered metal conrods
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1.
Plain Type Rods
The plain type of
connecting rod is used
in inline and opposed
engines. The big end of
the connecting rod is
attached to the crankpin
and fitted with a
bearing cap.
The bearing cap is
mounted by a bolt or
stud at the end of the
connecting rod. The
connecting rod must be
replaced in the same
cylinder and in the same
relative position to
maintain proper fit and
balance.
2.
Fork and Blade Rods
These types of
connecting rod are used
on V-twin motorcycle
engines and V12 aircraft
engines. In each pair of
engine cylinders, a
¡°fork¡± rod is divided
into two parts at the
big end and a ¡°blade¡±
rod is tapered from the
opposing cylinder to fit
this gap in the fork.
This system eliminates
the rocking couple that
occurs when the cylinder
pairs are balanced along
with the crankshaft.
In
the big-end bearings
type of arrangement, the
fork rod has a single
wide-bearing sleeve that
extends over the entire
width of the rod,
including the central
gap.
The blade rod then runs
directly outside this
sleeve, not on the
crankpin. This causes
the two rods to move
back and forth, this
reducing the force on
the bearing and the
surface speed. But, the
bearing speed also
reciprocates instead of
continuously rotating,
which is a major problem
for lubrication.
3.
Master and Slave Rods
Radial engines typically
use master-and-slave
connecting rods. In this
system, the one piston
consists of a master rod
with a direct attachment
to the crankshaft. Other
pistons connect their
connecting rods to the
rings surrounding the
edge of the master rod.
The disadvantage of
master-slave rods is
that the stroke of the
slave piston is slightly
larger than that of the
master piston, which
increases the vibration
in the V-type engine.
4.
Billet rods
Billet connecting rods
are designed from steel
or aluminum. Compared to
other types of
connecting rod, they are
lighter, stronger, and
longer in lifespan.
It
is commonly used in
high-speed vehicles. It
is sometimes designed to
reduce stress risers and
ease into the natural
grain of the billet
material.
5.
Cast Rods
These types of
connecting rod are
preferred and designed
by manufacturers because
they can capable of
handling the load of a
stock engine.
Cast rods require low
cost to produce and
cannot be used in
applications of high
horsepower. The cast
rods have a noticeable
seam in the middle that
separates them from the
forged type.
6.
Forged Rods
Some of the connecting
rods are manufacture by
forging. These types of
connecting rod made by
forcing a grain of
material to the shape of
the end. Depending on
the required properties
the material may be
steel alloy or aluminum.
Commonly used steel
alloys are chrome and
nickel alloy. The end
product is not designed
to be brittle. Hence,
nickel or chrome alloys
increase the strength of
the connecting rod.
7.
Powered Metal Conrods
Connecting rods are also
designed from power
metal as it is a
suitable choice for
manufacturers. It is
prepared with a metal
powder mixture that is
pressed into the mould
and heated to a high
temperature. This
mixture made into a
solid form.
It
may require light
machining but the
product basically comes
out of a finished
product mould. Conrods
of powder metal are less
costly than steel and
they are stronger than
cast rods.
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Faults
of Connecting Rods
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A
connecting rod is often
subjected to large and
repetitive forces during
each rotation of the
crankshaft. These
created forces are
proportional to the
speed of the engine
(RPM). While the
connecting rod is
continuously working in
the crankshaft, it may
damage or break.
Following are the faults
of a connecting rod:
•
Fatigue
•
Hydrolock
•
Over
revving
•
Pin
failure
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1.
Fatigue
Fatigue often occurs
because the compression
and stretch of the rod
happen most of the time
during the process.
Eventually, this causes
to wear of the rod till
it gets breaks. Lack of
oil and the presence of
dirt in the engine can
exacerbate this problem.
This is the most common
type of defect and often
occurs in older engines
as well. If the engine
is rebuilt, you may also
experience fatigue in
adding a new engine.
Well, this happens when
cheap parts or wrong
parts are used.
2.
Hydrolock
Hydrolock occurs when
water enters the piston
chamber causing
deformation of the
connecting rod. This may
occur when vehicles pass
through a flooded road.
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A
little drop of water in
the cylinder can produce
knocking or tapping in
the engine. That can be
easily corrected. But,
if there is too much
water in the cylinder,
the spark is all over
the place for a period
of time, causing the
cylinder rod to tilt or
break.
3.
Over Revving
Over raving is another
type of fault of the
connecting rod. That
occurs in new and
high-performance
engines. If the
tachometer displays a
red color, it indicates
that the position of the
connecting rod is in
danger. This is because
of forces working on the
con rod rise
dramatically at higher
revolutions.
4. Pin
Failure
Sometimes the piston pin
is also damaged and
results in catastrophic
engine failure. This
occurs when the
connecting rod moves
into the engine block or
when the crankshaft is
bent.
In
some engines, It can
cause heavy power loss.
The engine stops
immediately when the pin
breaks due to this
problem. There is a
possibility that the
engine has survived,
otherwise, a total
breakdown may occur.
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Applications of
Connecting Rods
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It is a
part of a piston engine that
connects the piston to the
crankshaft.
The
connecting rod converts the
reciprocating motion of the
piston to the rotation of the
crankshaft.
Conrods
are applied in various engines
of automobiles.
The
connecting rod is used in all
types of vehicles such as cars,
trucks, and bikes.
In
addition, it is also used in
construction vehicles like
bulldozers, road rollers
(earthmovers).
Therefore,
in the modern era, all types of
machines inevitably depend on
pistons, connecting rods, and
crankshafts.
These
components are needed for the
precise functioning of the
internal combustion engine.
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