1、Failure Analysis ， Dimensional Determination And Analysis ，Applications Of Cams 故障的分析、尺寸的决定以及凸轮的分析和应用 It is absolutely essential that a design engineer know how and why parts fail so that reliable machines that require minimum maintenance can be designed Sometimes a failure can be serious，such as wh
2、en a tire blows out on an automobile traveling at high speed On the other hand， a failure may be no more than a nuisance An example is the loosening of the radiator hose in an automobile cooling system The consequence of this latter failure is usually the loss of some radiator coolant， a condition t
3、hat is readily detected and corrected The type of load a part absorbs is just as significant as the magnitude Generally speaking，dynamic loads with direction reversals cause greater difficulty than static loads， and therefore，fatigue strength must be considered Another concern is whether the materia
4、l is ductile or brittle For example， brittle materials are considered to be unacceptable where fatigue is involved Many people mistakingly interpret the word failure to mean the actual breakage of a part However， a design engineer must consider a broader understanding of what appreciable deformation
5、 occurs A ductile material， however will deform a large amount prior to rupture Excessive deformation， without fracture， may cause a machine to fail because the deformed part interferes with a moving second part Therefore， a part fails(even if it has not physically broken)whenever it no longer fulfi
6、lls its required function Sometimes failure may be due to abnormal friction or vibration between two mating parts Failure also may be due to a phenomenon called creep， which is the plastic flow of a material under load at elevated temperatures In addition， the actual shape of a part may be responsib
7、le for failure For example，stress concentrations due to sudden changes in contour must be taken into account Evaluation of stress considerations is especially important when there are dynamic loads with direction reversals and the material is not very ductile In general， the design engineer must con
8、sider all possible modes of failure， which include the following Stress Deformation Wear Corrosion Vibration Environmental damage Loosening of fastening devices The part sizes and shapes selected also must take into account many dimensional factors that produce external load effects， such as geometr
9、ic discontinuities， residual stresses due to forming of desired contours， and the application of interference fit joints Cams are among the most versatile mechanisms available A cam is a simple two-member device The input member is the cam itself， while the output member is called the follower Throu
10、gh the use of cams， a simple input motion can be modified into almost any conceivable output motion that is desired Some of the common applications of cams are Camshaft and distributor shaft of automotive engine Production machine tools Automatic record players Automatic washing machines Automatic d
11、ishwashers The contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematically However， 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 layout In general， the greater the cam spee
12、d and output load， the greater must be the precision with which the cam contour is machined DESIGN PROPERTIES OF MATERIALS The following design properties of materials are defined as they relate to the tensile test Figure 2.7 Static Strength The strength of a part is the maximum stress that the part
13、 can sustain without losing its ability to perform its required function Thus 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 material Stiffness Stiffness is the deformation
14、-resisting property of a material The slope of the modulus line and， hence， the modulus of elasticity are measures of the stiffness of a material Resilience Resilience is the property of a material that permits it to absorb energy without permanent deformation The amount of energy absorbed is repres
15、ented by the area underneath the stress-strain diagram within the elastic region Toughness Resilience and toughness are similar properties However， toughness is the ability to absorb energy without rupture Thus toughness is represented by the total area underneath the stress-strain diagram， as depic
16、ted in Figure 2 8b Obviously， the toughness and resilience of brittle materials are very low and are approximately equal Brittleness A brittle material is one that ruptures before any appreciable plastic deformation takes place Brittle materials are generally considered undesirable for machine compo
17、nents because they are unable to yield locally at locations of high stress because of geometric stress raisers such as shoulders， holes， notches， or keyways Ductility A ductility material exhibits a large amount of plastic deformation prior to rupture Ductility is measured by the percent of area and percent elongation of a part loaded to rupture A 5%elongation at rupture is considered to be the dividing line between ductile and brittle materials
