1.0 (turbocharger pressure or exhaust gas counterpressure). In the

                  

1.0       Function of Camshaft

 

 

 

 

 

 

Figure 1
Camshaft

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Figure
2 Camshaft in Internal Combustion Engine

The
primary function of the camshaft is to open and close the intake and exhaust
valves so that gases can be exchanged; these actions are synchronized with the
position of the piston and thus with the crankshaft. Normally the valves are
opened by transferring force from the cam to the cam follower, to other
actuation elements where required, and ultimately to the valve, opening(or
lifting) the valve against the force of the valve spring. During the closing
cycle, the valve spring closes the valve. When the follower is in contact with
the cam’s base circle (with the cam exerting no lift), the valve spring keeps
the valve closed against any gas pressure in the port (turbocharger pressure or
exhaust gas counterpressure).

In
the four-cycle engine, the camshaft is driven by the crankshaft and rotates at
half the crankshaft speed. The valve timing for each individual valve is
determined by the geometry and the phase rotation angle of the individual cams,
normally separate for intake and exhaust valves and for the cylinders that are
located along one or more camshafts. In multivalve engines it is possible to
actuate several valves using a single cam with the intervention of linkages or
forked levers. In special designs, the valves of multiple cylinders or the
intake and exhaust valves are activated by the same cam.

In
addition to the movements of the intake and exhaust valves required to control
gas flow, the camshaft can also be used to generate the additional valve
movements required for engine braking systems used in medium- and heavy-duty
utility vehicles. In every application the valve stroke length, velocity, and
acceleration are the products of compromises between the fastest possible
opening and closing for the individual valves and the forces and surface
pressures created thereby. The friction and friction losses at the camshaft and
the valve train as a whole are also important criteria in engineering.

 

 

1.1       Structure of a Camshaft

 

The
main component is the cylindrical shaft (either hollow or solid), upon which
the individual valve actuation cams are located. The
actuation forces are backed at camshaft bearings, most of which are axial bearings
that stabilize the camshaft along the longitudinal direction. The crankshaft is
driven by a drive sprocket that is attached either permanently or detachably to
the drive flange at the end of the camshaft.

Figure
3 Structure of a camshaft

 

1.2       Type
of Material

 

Camshafts
made of cast iron are very widely used and different in terms of the
microstructure and hardness. A
camshaft made of cast iron with nodular or laminar graphite is often the ideal
tribologic match for sliding contact and low-load rolling contact in many
applications. With proper alloying and closely defined hardening of the cams,
tolerable pressure levels of well over 1000 MPa can be attained. In the case of
chilled cast iron the cam area is cooled quickly following casting to create a
wear-resistant carbide structure (ledeburite) with great hardness and good tribologic
compatibility. A gray casting with good machining properties is available for
use in the core area and the camshaft bearing points.

 

Material

Mass production for
passenger car/ utility vehicle

Cast iron with nodular graphite
(GCG), inductance hardened

Passenger cars

Cast iron with laminar graphite
(GG), refluxing hardened(WIG)

Passenger cars

Chilled cast iron, cast iron with
laminar graphite(CCI, GG)

Passenger cars / utility vehicle

Chilled cast iron, cast iron with
nodular graphite (CCI, GGG)

Passenger cars / utility vehicle

Cast steel (GS)

Under development

 

 

 

 

 

 

 

 

 

                                                           

Figure
4 Chilled cast iron in cross section

 

 

 

 

 

 

 

 

 

Figure
5 Physical properties of camshaft casting
material

 

1.3    Operational
Condition

The kinematics of the valve drive is the primary
determinant for camshaft loading. The peripheral geometric conditions such as
the step-down ratio or cam profile (e.g., high acceleration rates) are decisive
here, in particular. Moreover, the camshaft is loaded by the valve train masses
in motion and the total forces exerted by the valve springs and exhaust gas
counterpressure. An integrated engine braking system can impose further and
usually very significant loading on the camshaft (five to ten times the forces
encountered during normal changes of gas charges).  The contact forces created between the cam and
the camshaft induce both torsional and flexural moments in the camshaft which, together
with the drive moment for auxiliary units, give the total torsional and
flexural loads for the camshaft. In addition to the loading, the Young’s modulus
for the cam and the cam follower and the crowning of the components in the
contact area are decisive for pressures and deformations.

 

 

 

 

 

 

 

Figure 6 Factors
influencing
 camshaft
loading
 

 

 

2.0  Failure
Analysis

The
various modes of contact-fatigue failure between a cam and a follower can be classified
according to their appearance and the factor which promote their initiation and
propagation. The main failure
modes of the cam-follower configuration are scuffing and pitting. The
probability of one of these occurring depends on several parameters such as
material properties, lubricants, loads, engine speed, and temperature.

A.   
Scuffing

Scuffing
occurs by a metal-to-metal contact of the surfaces (usually associated with oil
breakdown) leading to welding and tearing. This form of failure depends more on
contact loads than on time, occurring at high contact loads while pitting
occurs at lower loads. The main features of scuffing are:

      
I.  significant
plastic flow occurs on the worn surface

   
II.  the
scuffed surface shows the damage feature in the form of delamination

 III. 
fatigue striation
characteristics can be seen in some places where the delaminated layers

have
just flaked off

 

 

B.    
Pitting

Pitting
on the other hand, is a fatigue process that involves the initiation and
propagation of cracks. Surface layers fail as a result of cyclic stresses due
to the rolling contact nature of the system, with material flaking off
resulting in characteristic pitted surface. This form of failure depends both
on stress and running time. The main characteristics of pitting cracks are:

      
I.           
the majority of cracks
initiate on the very surface or from the bottom of micropits, propagating with
a certain inclination downwards

   
II.           
a smaller percentage of
cracks initiate at a certain depth of sublayer and propagate parallel to the
surface. These cracks can abruptly change direction of propagation upwards
towards the surface, flaking-off a piece of material and leaving behind a pit.

 

C.   
Rolling

Rolling
contact fatigue cracks can be classified into two groups depending on where
they are initiated: cracks may be initiated at the surface and propagate down
into the bulk of the cam at a shallow angle to the surface, or cracks may be
initiated below the surface, in a region of maximum cyclic shear stress.

Surface
cracks can be initiated by the near-surface plastic deformation caused by the
contact stress of the follower, by defects such as dents or scratches, or by
thermal stresses generated during the manufacturing grinding process. Once they
are originated, surface cracks usually propagate at an angle to the surface.
After reaching a critical depth or length, these cracks either branch up toward
the free surface, so that a piece of material is removed thus leaving behind a
pit, or branch down at a steep angle causing catastrophic failure.

Propagation
of surface cracks is dominated by a fatigue mechanism driven by the contact
stress associated with the rolling and sliding of the follower. These contact
stress at the cam-follower interface form a compressive field which by
intuition will prevent crack propagation. To explain the unusual form of
fatigue associated with the propagation of surface cracks, three possible
mechanisms have been proposed:

      
I.           
the cracks are
propagated in a shear mode driven by the cyclic shear stresses caused by
repeated rolling contact

   
II.           
fluid is forced into
the crack by the load, thus prizing apart the faces of the crack

 III.           
fluid is trapped inside
the crack and subsequently pushed towards the crack tip

Subsurface
cracks are initiated in regions of maximum shear stress. Subsurface fatigue
cracks usually propagate parallel to the surface. When a subsurface cracks
propagates upward towards the surface, it forms a pit. Nonmetallic inclusions
act as stress concentrators and are the main cause for subsurface cracking.
Most research done on contact fatigue originated at an inclusion has shown to
be accompanied by changes in microstructure in the region of maximum subsurface
shear stress. The shear mode crack growth rate increases with increasing crack
size and traction force.

 

 

 

 

 

 

Figure
7 Pitted cam
lobes

 

 

(A)                                                                                                                                                                           
(B)

Figure 8  (A) Straight
crack found in the opening ramp of lobe (B) Pitted crack found on the opening
ramp of lobe

D.   
Corrosion

Air
enters the throttle body at the top of the engine, so the top is affected by
ambient air temperatures before the rest of the engine. During the day, the
crankcase is warmed up and filled with warm humid air. In the evening and at
night, the engine cools down and moisture collects in the oil. As more and more
water collects, the air in the crankcase becomes more humid, so in the evenings
the cam cools faster than the rest of the crankcase. Once the cam cools below
the dew point of the air in the crankcase, moisture drops out on the cam. Over
time, this water causes rust to form on the cam and lifter surface. When the
engine is finally started, the rust acts like a lapping compound to start wear
on these surfaces.

 

Figure
9 Corrosion in Camshaft

 

 

 

 

 

2.1       Finite Element Analysis of Camshaft

                                                               

 

 

 

 

 

Figure
10
FEM Camshaft Model

 

 

                                                                                    

 

 

Section

Property name

Value

Tensile property

Ultimate tensile strength (MPa)

720

Tensile property

Tensile yield strength (MPa)

431

Elastic property

Tension elastic modulus (GPa)

206

Hardness

Vickers hardness (HV)

230

 

 

 

 

 

 

 

 

 

 

Figure 11 Contact
Stress Analysis

 

3.0       Fatigue Prevention Methods

 

A.   
Lubrication

A
lubricating oil with the necessary properties and characteristics will provide
a film of proper thickness between the bearing surfaces under all conditions of
operation, remain stable under changing temperature conditions, and not corrode
the metal surfaces. Use only the manufacturer recommended lubricant, which is
generally included with the camshaft. This lubricant must be applied to every
cam lobe surface, and to the bottom of every lifter face of all flat tappet
cams. Roller tappet cams only require engine oil to be applied to the lifters
and cam. Also, apply the lubricant to the distributor drive gears on the cam
and distributor.

In
internal-combustion engines, lubricating oil serves functions:

      
I.           
Protective
Film

Direct
metal-to-metal contact of load-bearing surfaces is similar to the action of a
file as it wears away metal. The filing action is a result of very small
irregularities in the metal surfaces. The severity of the filing action depends
on the finish of the surfaces, the force with which the surfaces are brought
into contact, and the relative hard-ness of the materials. Lubricating oil
fills the tiny cavities in bearing surfaces and forms a film between the
sliding surfaces to prevent high friction losses and rapid wear of engine
parts. The lack of a proper oil film will result in a wear and corrosion of
camshaft.

 

   
II.           
Cooling

Lubricating
oil assists in cooling the engine because the constant flow of oil carries heat
away from localized “hot spots.” The principal parts from which oil absorbs
heat are the bearings, the journal surfaces, camshaft and the pistons. In some
engines, the oil carries the heat to the sump where the heat dissipates in the
mass of oil. However, most modern internal-combustion engines use a centralized
pressure-feed lubrication system. This type of system has an oil cooler (heat
exchanger) where the heat in the oil is transferred to the water circulating in
the jacket-water cooling system.

 

B.    
Correct
Installation of Camshaft

 

      
I.           
Correct
Valve Spring Pressure

Never
install valve springs without verifying the correct assembled height and
pressures. Recommended valve spring pressures are as follows:

Street-type
flat tappet cams: 85-105 pounds

Radical
street flat tappet cams: 105-130 pounds

Street-type
hydraulic roller cams: 105-140 pounds

Mechanical
street roller cams: no more than 150 pounds

Race
roller cams with high valve lift and spring pressure are not recommended for
street use, because of a lack of oil splash onto the cam at low speed running.
Springs must be assembled to the manufacturer’s recommended height. By doing
this, surface cracks can be reduced in camshaft.

 

   
II.           
Spring
coil bind

 This happens when all the coils of a spring
contact each other before the valve fully lifts. Valve springs should be
capable of traveling at least .060 inches more than the valve lift of the cam
from its assembled height. This will increase the service lift of camshaft and
reduce wear.

 

 III.           
Lifter
Rotation

Flat
tappet cams have lobes ground on a slight taper and the lifters appear to sit
offset from the lobe centreline. This will induce a rotating of lifter on the
lobe. This rotation draws oil to mating surface between the lifter and the
lobe. Should view the pushrods during break-in, they should be spinning as an
indication that the lifter is spinning. If do not see a pushrod spinning,
immediately stop the engine and find the cause. This will eliminate camshaft
from scuffing and pitting.

 

4.0       Conclusion

 

A
cam forms a significant part of three-element mechanisms. Its profile,
dimensions of driving and driven elements define a lifting relation taking into
consideration individual deformation ratios and a rigidity of an element for
requested operation. During its movement the cam is exposed to effects of
significant forces at a contact performing a direct influence on its surface that
may result in damaging of contact areas. Such damage becomes evident in form of
pitting that develops from small cracks on a surface of a working surface.
Therefore a correct choice of material of particular elements in a design of a
cam mechanism. The service conditions of the camshaft while in operation and
the factors which affect the service life of the camshaft are explained in this
assignment.

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