附录附录1Failure Analysis，Dimensional Determination And Analysis，Applications Of CamsINTRODUCTIONIt is absolutely essential that a design engineer know how and why parts fail so that reliable machines that require minimum maintenance can be designedSometimes a failure can be serious，such as when a tire blows out on an automobile traveling at high speedOn the other hand，a failure may be no more than a nuisanceAn example is the loosening of the radiator hose in an automobile cooling systemThe consequence of this latter failure is usually the loss of some radiator coolant，a condition that is readily detected and correctedThe type of load a part absorbs is just as significant as the magnitudeGenerally speaking，dynamic loads with direction reversals cause greater difficulty than static loads，and therefore，fatigue strength must be consideredAnother concern is whether the material is ductile or brittleFor example，brittle materials are considered to be unacceptable where fatigue is involvedMany people mistakingly interpret the word failure to mean the actual breakage of a partHowever，a design engineer must consider a broader understanding of what appreciable deformation occursA ductile material，however will deform a large amount prior to ruptureExcessive deformation，without fracture，may cause a machine to fail because the deformed part interferes with a moving second partTherefore，a part fails(even if it has not physically broken)whenever it no longer fulfills its required functionSometimes failure may be due to abnormal friction or vibration between two mating partsFailure also may be due to a phenomenon called creep，which is the plastic flow of a material under load at elevated temperaturesIn addition，the actual shape of a part may be responsible for failureFor example，stress concentrations due to sudden changes in contour must be taken into accountEvaluation of stress considerations is especially important when there are dynamic loads with direction reversals and the material is not very ductileIn general，the design engineer must consider all possible modes of failure，which include the followingStressDeformationWearCorrosionVibrationEnvironmental damageLoosening of fastening devicesThe part sizes and shapes selected also must take into account many dimensional factors that produce external load effects，such as geometric discontinuities，residual stresses due to forming of desired contours，and the application of interference fit jointsCams are among the most versatile mechanisms availableA cam is a simple two-member deviceThe input member is the cam itself，while the output member is called the followerThrough the use of cams，a simple input motion can be modified into almost any conceivable output motion that is desiredSome of the common applications of cams areCamshaft and distributor shaft of automotive engine Production machine toolsAutomatic record playersPrinting machinesAutomatic washing machinesAutomatic dishwashersThe contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematicallyHowever，the vast majority of cams operate at low speeds(less than 500 rpm) or medium-speed cams can be determined graphically using a large-scale layoutIn general，the greater the cam speed and output load，the greater must be the precision with which the cam contour is machinedDESIGN PROPERTIES OF MATERIALSThe following design properties of materials are defined as they relate to the tensile testFigure 2.7Static Strength The strength of a part is the maximum stress that the part can sustain without losing its ability to perform its required functionThus the static strength may be considered to be approximately equal to the proportional limit，since no plastic deformation takes place and no damage theoretically is done to the materialStiffness Stiffness is the deformation-resisting property of a materialThe slope of the modulus line and，hence，the modulus of elasticity are measures of the stiffness of a materialResilience Resilience is the property of a material that permits it to absorb energy without permanent deformationThe amount of energy absorbed is represented by the area underneath the stress-strain diagram within the elastic regionToughness Resilience and toughness are similar propertiesHowever，toughness is the ability to absorb energy without ruptureThus toughness is represented by the total area underneath the stress-strain diagram， as depicted in Figure 28bObviously，the toughness and resilience of brittle materials are very low and are approximately equalBrittleness A brittle material is one that ruptures before any appreciable plastic deformation takes placeBrittle materials are generally considered undesirable for machine components because they are unable to yield locally at locations of high stress because of geometric stress raisers such as shoulders，hole