0 Function of Camshaft Figure 1Camshaft Figure2 Camshaft in Internal Combustion EngineTheprimary function of the camshaft is to open and close the intake and exhaustvalves so that gases can be exchanged; these actions are synchronized with theposition of the piston and thus with the crankshaft. Normally the valves areopened by transferring force from the cam to the cam follower, to otheractuation elements where required, and ultimately to the valve, opening(orlifting) the valve against the force of the valve spring. During the closingcycle, the valve spring closes the valve. When the follower is in contact withthe cam’s base circle (with the cam exerting no lift), the valve spring keepsthe valve closed against any gas pressure in the port (turbocharger pressure orexhaust gas counterpressure).Inthe four-cycle engine, the camshaft is driven by the crankshaft and rotates athalf the crankshaft speed. The valve timing for each individual valve isdetermined by the geometry and the phase rotation angle of the individual cams,normally separate for intake and exhaust valves and for the cylinders that arelocated along one or more camshafts. In multivalve engines it is possible toactuate several valves using a single cam with the intervention of linkages orforked levers.
In special designs, the valves of multiple cylinders or theintake and exhaust valves are activated by the same cam.Inaddition to the movements of the intake and exhaust valves required to controlgas flow, the camshaft can also be used to generate the additional valvemovements required for engine braking systems used in medium- and heavy-dutyutility vehicles. In every application the valve stroke length, velocity, andacceleration are the products of compromises between the fastest possibleopening and closing for the individual valves and the forces and surfacepressures created thereby. The friction and friction losses at the camshaft andthe valve train as a whole are also important criteria in engineering.
1.1 Structure of a Camshaft Themain component is the cylindrical shaft (either hollow or solid), upon whichthe individual valve actuation cams are located. Theactuation forces are backed at camshaft bearings, most of which are axial bearingsthat stabilize the camshaft along the longitudinal direction. The crankshaft isdriven by a drive sprocket that is attached either permanently or detachably tothe drive flange at the end of the camshaft. Figure3 Structure of a camshaft 1.2 Typeof Material Camshaftsmade of cast iron are very widely used and different in terms of themicrostructure and hardness. Acamshaft made of cast iron with nodular or laminar graphite is often the idealtribologic match for sliding contact and low-load rolling contact in manyapplications. With proper alloying and closely defined hardening of the cams,tolerable pressure levels of well over 1000 MPa can be attained.
In the case ofchilled cast iron the cam area is cooled quickly following casting to create awear-resistant carbide structure (ledeburite) with great hardness and good tribologiccompatibility. A gray casting with good machining properties is available foruse 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 Figure4 Chilled cast iron in cross section Figure5 Physical properties of camshaft castingmaterial 1.3 OperationalCondition The kinematics of the valve drive is the primarydeterminant for camshaft loading. The peripheral geometric conditions such asthe step-down ratio or cam profile (e.g., high acceleration rates) are decisivehere, in particular.
Moreover, the camshaft is loaded by the valve train massesin motion and the total forces exerted by the valve springs and exhaust gascounterpressure. An integrated engine braking system can impose further andusually very significant loading on the camshaft (five to ten times the forcesencountered during normal changes of gas charges). The contact forces created between the cam andthe camshaft induce both torsional and flexural moments in the camshaft which, togetherwith the drive moment for auxiliary units, give the total torsional andflexural loads for the camshaft. In addition to the loading, the Young’s modulusfor the cam and the cam follower and the crowning of the components in thecontact area are decisive for pressures and deformations. Figure 6 Factors influencing camshaft loading 2.0 FailureAnalysisThevarious modes of contact-fatigue failure between a cam and a follower can be classifiedaccording to their appearance and the factor which promote their initiation andpropagation. The main failuremodes of the cam-follower configuration are scuffing and pitting.
Theprobability of one of these occurring depends on several parameters such asmaterial properties, lubricants, loads, engine speed, and temperature.A. ScuffingScuffingoccurs by a metal-to-metal contact of the surfaces (usually associated with oilbreakdown) leading to welding and tearing. This form of failure depends more oncontact loads than on time, occurring at high contact loads while pittingoccurs at lower loads. The main features of scuffing are: I. significantplastic flow occurs on the worn surface II. thescuffed surface shows the damage feature in the form of delamination III.
fatigue striationcharacteristics can be seen in some places where the delaminated layershavejust flaked off B. PittingPittingon the other hand, is a fatigue process that involves the initiation andpropagation of cracks. Surface layers fail as a result of cyclic stresses dueto the rolling contact nature of the system, with material flaking offresulting in characteristic pitted surface. This form of failure depends bothon stress and running time. The main characteristics of pitting cracks are: I. the majority of cracksinitiate on the very surface or from the bottom of micropits, propagating witha certain inclination downwards II. a smaller percentage ofcracks initiate at a certain depth of sublayer and propagate parallel to thesurface. These cracks can abruptly change direction of propagation upwardstowards the surface, flaking-off a piece of material and leaving behind a pit.
C. RollingRollingcontact fatigue cracks can be classified into two groups depending on wherethey are initiated: cracks may be initiated at the surface and propagate downinto the bulk of the cam at a shallow angle to the surface, or cracks may beinitiated below the surface, in a region of maximum cyclic shear stress.Surfacecracks can be initiated by the near-surface plastic deformation caused by thecontact stress of the follower, by defects such as dents or scratches, or bythermal stresses generated during the manufacturing grinding process. Once theyare originated, surface cracks usually propagate at an angle to the surface.After reaching a critical depth or length, these cracks either branch up towardthe free surface, so that a piece of material is removed thus leaving behind apit, or branch down at a steep angle causing catastrophic failure.
Propagationof surface cracks is dominated by a fatigue mechanism driven by the contactstress associated with the rolling and sliding of the follower. These contactstress at the cam-follower interface form a compressive field which byintuition will prevent crack propagation. To explain the unusual form offatigue associated with the propagation of surface cracks, three possiblemechanisms have been proposed: I. the cracks arepropagated in a shear mode driven by the cyclic shear stresses caused byrepeated rolling contact II. fluid is forced intothe crack by the load, thus prizing apart the faces of the crack III. fluid is trapped insidethe crack and subsequently pushed towards the crack tipSubsurfacecracks are initiated in regions of maximum shear stress. Subsurface fatiguecracks usually propagate parallel to the surface.
When a subsurface crackspropagates upward towards the surface, it forms a pit. Nonmetallic inclusionsact as stress concentrators and are the main cause for subsurface cracking.Most research done on contact fatigue originated at an inclusion has shown tobe accompanied by changes in microstructure in the region of maximum subsurfaceshear stress. The shear mode crack growth rate increases with increasing cracksize and traction force. Figure7 Pitted camlobes (A) (B)Figure 8 (A) Straightcrack found in the opening ramp of lobe (B) Pitted crack found on the openingramp of lobeD. CorrosionAirenters the throttle body at the top of the engine, so the top is affected byambient air temperatures before the rest of the engine. During the day, thecrankcase is warmed up and filled with warm humid air.
In the evening and atnight, the engine cools down and moisture collects in the oil. As more and morewater collects, the air in the crankcase becomes more humid, so in the eveningsthe cam cools faster than the rest of the crankcase. Once the cam cools belowthe dew point of the air in the crankcase, moisture drops out on the cam.
Overtime, this water causes rust to form on the cam and lifter surface. When theengine is finally started, the rust acts like a lapping compound to start wearon 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 ContactStress Analysis 3.0 Fatigue Prevention Methods A. LubricationAlubricating oil with the necessary properties and characteristics will providea film of proper thickness between the bearing surfaces under all conditions ofoperation, remain stable under changing temperature conditions, and not corrodethe metal surfaces. Use only the manufacturer recommended lubricant, which isgenerally included with the camshaft.
This lubricant must be applied to everycam lobe surface, and to the bottom of every lifter face of all flat tappetcams. Roller tappet cams only require engine oil to be applied to the liftersand cam. Also, apply the lubricant to the distributor drive gears on the camand distributor.Ininternal-combustion engines, lubricating oil serves functions: I. ProtectiveFilmDirectmetal-to-metal contact of load-bearing surfaces is similar to the action of afile as it wears away metal.
The filing action is a result of very smallirregularities in the metal surfaces. The severity of the filing action dependson the finish of the surfaces, the force with which the surfaces are broughtinto contact, and the relative hard-ness of the materials. Lubricating oilfills the tiny cavities in bearing surfaces and forms a film between thesliding surfaces to prevent high friction losses and rapid wear of engineparts. The lack of a proper oil film will result in a wear and corrosion ofcamshaft. II. CoolingLubricatingoil assists in cooling the engine because the constant flow of oil carries heataway from localized “hot spots.
” The principal parts from which oil absorbsheat are the bearings, the journal surfaces, camshaft and the pistons. In someengines, the oil carries the heat to the sump where the heat dissipates in themass of oil. However, most modern internal-combustion engines use a centralizedpressure-feed lubrication system. This type of system has an oil cooler (heatexchanger) where the heat in the oil is transferred to the water circulating inthe jacket-water cooling system. B. CorrectInstallation of Camshaft I. CorrectValve Spring PressureNeverinstall valve springs without verifying the correct assembled height andpressures. Recommended valve spring pressures are as follows:Street-typeflat tappet cams: 85-105 poundsRadicalstreet flat tappet cams: 105-130 poundsStreet-typehydraulic roller cams: 105-140 poundsMechanicalstreet roller cams: no more than 150 poundsRaceroller cams with high valve lift and spring pressure are not recommended forstreet 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 doingthis, surface cracks can be reduced in camshaft. II. Springcoil bind This happens when all the coils of a springcontact each other before the valve fully lifts. Valve springs should becapable of traveling at least .
060 inches more than the valve lift of the camfrom its assembled height. This will increase the service lift of camshaft andreduce wear. III. LifterRotationFlattappet cams have lobes ground on a slight taper and the lifters appear to sitoffset from the lobe centreline. This will induce a rotating of lifter on thelobe. This rotation draws oil to mating surface between the lifter and thelobe. Should view the pushrods during break-in, they should be spinning as anindication that the lifter is spinning.
If do not see a pushrod spinning,immediately stop the engine and find the cause. This will eliminate camshaftfrom scuffing and pitting. 4.0 Conclusion Acam forms a significant part of three-element mechanisms.
Its profile,dimensions of driving and driven elements define a lifting relation taking intoconsideration individual deformation ratios and a rigidity of an element forrequested operation. During its movement the cam is exposed to effects ofsignificant forces at a contact performing a direct influence on its surface thatmay result in damaging of contact areas. Such damage becomes evident in form ofpitting 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 acam mechanism. The service conditions of the camshaft while in operation andthe factors which affect the service life of the camshaft are explained in thisassignment.