The or solid), upon which the individual valve

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.

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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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