凸輪的英文文獻(xiàn)

1、Camsare amongthe 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 thefollower Through the use of cams， a simple input motion can be modified into almost any conceivable output motion that is desired Some of

2、the common applications ofcams are Camshaft and distributor shaft of automotive engine Production machine tools Automatic record players Printing machines Automatic washing machines Automatic dishwashersThe contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematicall

3、y 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 speed and output load ，the greater must be the precision with which the cam contour is machinedDESIGN PROPERTIE

4、S OF MATERIALSThe following design properties of materials are defined as they relate to the tensile test Static Strength The strength of a part is the maximum stress that the part can sustain without losing its ability to perform its required function Thus the static strength may be considered to b

5、e 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-resisting property of a material The slope of the modulus line and，hence ，the modulus of elasticity are measures of the

6、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 represented by the area underneath the stress-strain diagram within the elastic regionToughness Resilience and toughness are sim

7、ilar properties However， toughness is the ability to absorb energy without rupture Thus toughness is represented by thetotal area underneath the stress-strain diagram Obviously ， the toughness andresilience of brittle materials are very low and are approximately equalBrittleness A brittle material i

8、s one that ruptures before any appreciable plastic deformation takes place Brittle 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， holes ，notches ， orkeyw

9、aysDuctility 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 Mall

10、eability Malleability is essentially a measure of the compressive ductility of a material and ， as such ，is an important characteristic of metals that are to be rolled into sheetsHardness The hardness of a material is its ability to resist indentation or scratching Generally speaking ， the harder a

11、material ， the more brittle it is and， hence， the less resilient Also ，the ultimate strength of a material is roughlyproportional to its hardnessMachinability Machinabilityis a measure of the relative ease with which a materialcan be machined In general ，the harder the material，the more difficult it

12、 is tomachine COMPRESSION AND SHEAR STATIC STRENGTHIn addition to the tensile tests ， there are other types of static load testing that provide valuable information Compression Testing Most ductile materials have approximately the same properties in compression as in tension The ultimate strength ，

13、however ， can not be evaluated for compression As a ductile specimen flows plastically in compression ，the material bulges out ， but there is no physical rupture as is the case in tension Therefore ， a ductile material fails in compression as a result of deformation， not stress Shear Testing Shafts

14、，bolts ， rivets ， and welds are located in such a way that shear stresses are produced A plot of the tensile test The ultimate shearing strength is defined as the stress at which failure occurs The ultimate strengthin shear ， however ， does not equal the ultimate strength in tension For example ， in

15、 the case of steel ，the ultimate shear strength is approximately 75%of the ultimate strength in tension This difference must be taken into account when shear stresses are encountered in machine components DYNAMIC LOADSAn applied force that does not vary in any manner is called a static or steady loa

16、d It is also common practice to consider applied forces that seldom vary to be static loads The force that is gradually applied during a tensile test is therefore a static load On the other hand， forces that vary frequently in magnitude and direction are called dynamic loads Dynamic loads can be sub

17、divided to the following three categoriesVarying Load With varying loads ， the magnitude changes ， but the direction does not For example ，the load mayproduce high and low tensile stresses but no compressive stresses Reversing Load In this case， both the magnitude and direction changeThese load reve

18、rsals produce alternately varying tensile and compressive stresses that are commonly referred to as stress reversalsShock Load This type of load is due to impact One example is an elevator dropping on a nest of springs at the bottom of a chute The resulting maximum spring forcecan be many times grea

19、ter than the weight of the elevator， The same type of shockload occurs in automobile springs when a tire hits a bump or hole in the roadFATIGUE FAILURE-THE ENDURANCE LIMIT DIAGRAMThe test specimen in Figure ， after a given number of stress reversals will experience a crack at the outer surface where

20、 the stress is greatest The initial crack starts where the stress exceeds the strength of the grain on which it acts This is usually where there is a small surface defect ，such as a material flaw or a tiny scratch As the number of cycles increases， the initial crack begins to propagate into acontinu

21、ous series of cracks all around the periphery of the shaft The conceptionof the initial crack is itself a stress concentration that accelerates the crack propagation phenomenon Oncethe entire periphery becomes cracked ，the cracks start to move toward the center of the shaft Finally ，when the remaini

22、ng solid inner area becomes small enough，the stress exceeds the ultimate strength and the shaft suddenly breaks Inspection of the break reveals a very interesting pattern The outer annular area is relatively smooth because mating cracked surfaces had rubbed against each other However， the center por

23、tion is rough， indicating a sudden rupture similarto that experienced with the fracture of brittle materialsThis brings out an interestingfact When actual machine parts fail asa result ofstatic loads ， they normally deform appreciably because of the ductility of the material Thus many static failure

24、s can be avoided by making frequent visual observations and replacing all deformed parts However， fatigue failures give to warning Fatiguefail mated that over 90% of broken automobile parts have failed through fatigueThe fatigue strength of a material is its ability to resist the propagation of crac

25、ks under stress reversals Endurance limit is a parameter used to measure the fatigue strength of a material By definition ， the endurance limit is the stress value below which an infinite number of cycles will not cause failureLet us return our attention to the fatigue testing machine in Figurerun a

26、s follows ： A small weight is inserted and the motor is turned on of the test specimen， the counter registers the number of cycles Ncorresponding maximum bending stress is calculated from Equation The test is At failure， and the The brokenweight is insertedto increase the loadA new value of stress i

27、s calculated， and the procedure isspecimen is then replaced by an identicalone， and an additionalrepeated until failure requires only one complete cycle A plot is then made of stress versus number of cycles to failure Figure shows the plot ，which is called the endurance limit or S-N curve Since it w

28、ould take forever to achieve an infinitenumber of cycles ，1 million cycles is used as a reference Hence the endurance limit can be found from Figure by noting that it is the stress level below which the material can sustain 1 million cycles without failureThe relationship depicted in Figure is typic

29、al for steel ， because the curve becomes horizontal as N approaches a very large number Thus the endurance limit equals the stress level where the curve approaches a horizontal tangent Owing to the largenumber of cycles involved， N is usually plotted on a logarithmic scale， as shownin Figure When th

30、is is done，the endurance limit value can be readily detected by the horizontal straight line For steel ， the endurance limit equals approximately 50% of the ultimate strength However， if the surface finish is not of polishedequality ，the value of the endurance limit will be lowerFor example ，for ste

31、elparts with a machined surface finish of 63 microinches (卩 in . ) , the percentagedrops to about 40%. For rough surfaces (300 卩 in . or greater) , the percentage maybe as low as 25%.The most common type of fatigue is that due to bending. The next most frequent istorsion failure ， whereas fatigue du

32、e to axial loads occurs very seldom. Springmaterials are usually tested by applying variable shear stresses that alternate from zero to a maximum value ， simulating the actual stress patterns.In the case of some nonferrous metals ， the fatigue curve does not level off as the number of cycles becomes

33、 very large . This continuing toward zero stress means that a large number of stress reversals will cause failure regardless of how small the value of stress is . Such a material is said to have no endurance limit. For mostnonferrous metals having an endurance limit ， the value is about 25%of the ul

34、timate strength .EFFECTS OF TEMPERATURE ON YIELD STRENGTH AND MODULUS OF ELASTICITYGenerally speaking ， when stating that a material possesses specified values of properties such as modulus of elasticity and yield strength ， it is implied that these values exist at room temperature At low or elevate

35、d temperatures，the propertiesof materials may be drasticallydifferent For example ， manymetals are more brittleat low temperatures In addition ，the modulus of elasticity and yield strength deteriorate as the temperature increases Figure shows that the yield strength for mild steel is reduced by abou

36、t 70% in going from room temperature to 1000oFFigure shows the reduction in the modulus of elasticity E for mild steel as the temperature increases As can be seen from the graph， a 30% reduction in modulusof elasticity occurs in going from room temperature to 1000oF In this figure ， we also can see

37、that a part loaded below the proportional limit at room temperature can be permanently deformed under the same load at elevated temperaturesCREEP: A PLASTIC PHENOMENONTemperature effects bring us to a phenomenon called creep ， which is the increasing plastic deformation of a part under constant load

38、 as a function of time Creep also occurs at room temperature， but the process is so slow that it rarely becomessignificant during the expected life of the temperature is raised to 300oC or more， the increasing plastic deformation can become significant within a relatively short period of time The cr

39、eep strength of a material is its ability to resist creep，and creep strength data can be obtained by conducting long-time creep tests simulating actual part operating conditions During the test ， the plastic strainis monitored for given material at specified temperatures Since creep is a plastic def

40、ormation phenomenon， the dimensions of a partexperiencing creep are permanently altered Thus， if a part operates with tightclearances ， the design engineer must accurately predict the amount of creep that will occur during the life of the machineOtherwise ， problems such binding orinterference can o

41、ccurCreep also can be a problem in the case where bolts are used to clamp tow parts together at elevated temperatures The bolts ，under tension ，will creep as a function of time Since the deformation is plastic，loss of clamping force will result inan undesirable loosening of the bolted joint The exte

42、nt of this particularphenomenon， called relaxation， can be determined by running appropriate creepstrength tests Figure shows typical creep curves for three samples of a mild steel part under a constant tensile load Notice that for the high-temperature case the creep tendsto accelerate until the par

43、t fails The time line in the graph (the x-axis) mayrepresent a period of 10 years， the anticipated life of the productSUMMARYThe machine designer must understand the purpose of the static tensile strength test This test determines a number of mechanical properties of metals that are used in design e

44、quations Such terms as modulus of elasticity ， proportional limit ， yield strength ，ultimate strength ， resilience ，and ductilitydefine properties that canbe determined from the tensile testDynamic loads are those which vary in magnitude and direction and may require an investigation of the machine

45、part' s resistance to failure Stress reversals mayrequire that the allowable design stress be based on the endurance limit of the material rather than on the yield strength or ultimate strengthStress concentration occurs at locations where a machine part changes size， suchas a hole in a flat pla

46、te or a sudden change in width of a flat plate or a groove or fillet on a circular shaft Note that for the case of a hole in a flat or bar，the value of the maximumstress becomes much larger in relation to the average stress as the size of the hole decreases Methods of reducing the effect of stressco

47、ncentration usually involve making the shape change more gradualMachine parts are designed to operate at some allowable stress below the yield strength or ultimate strength This approach is used to take care of such unknownfactors as material property variations and residual stresses produced during

48、 manufacture and the fact that the equations used may be approximate rather that exact The factor of safety is applied to the yield strength or the ultimate strength to determine the allowable stress Temperature can affect the mechanical properties of metals Increases in temperature maycause a metal

49、 to expand and creep and may reduce its yield strength and its modulus of elasticity If most metals are not allowed to expand or contract with a changein temperature ，then stresses are set up that may be added to the stresses from the load This phenomenon is useful in assembling parts by means of in

50、terference fits A hub or ring has an inside diameter slightly smaller than the mating shaft or post The hub is then heated so that it expands enough to slip over the shaft When it cools ， it exerts a pressure on the shaft resulting in a strong frictional force that prevents loosening TYPES OF CAM CO

51、NFIGURATIONSPlate Cams This type of cam is the most popular type because it is easy to design and manufacture Figure shows a plate camNotice that the follower movesperpendicular to the axis of rotation of the camshaft All cams operate on theprinciple that no two objects can occupy the same space at

52、the same time Thus，as the cam rotates ( in this case ， counterclockwise ) ，the follower must either move upward or bind inside the guide We will focus our attention on the prevention ofbinding and attainment of the desired output follower motion The spring is required to maintain contact between the

53、 roller of the follower and the cam contour when the follower is moving downward The roller is used to reduce friction and hence wearat the contact surface For each revolution of the cam，the follower moves through two strokes-bottom dead center to top dead center (BDC to TDC) and TDC to BDCFigure il

54、lustrates a plate cam with a pointed follower Complex motions can be produced with this type of follower because the point can follow precisely any sudden changes in cam contour However， this design is limited to applications in which the loads are very light； otherwise the contact point of both mem

55、bers will wearprematurely ， with subsequent failureTwo additional variations of the plate cam arethe pivoted follower and the offsetsliding follower ， which are illustrated in Figure A pivoted follower is used when rotary output motion is desired Referring to the offset follower ， note that the amou

56、nt of offset used depends on such parameters as pressureangle and cam profileflatness ，which will be covered later A follower that has no offset is called anin-line follower Translation Cams Figure depicts a translation cam The follower slides up and down as the camtranslates motion in the horizonta

57、l direction Note that a pivoted follower can be used as well as a sliding-type follower This type of action is used in certain production machines in which the pattern of the product is used as the camA variation on this design would be a three-dimensional cam that rotates as well as translates For example ，a hand-constructed rifle stock is placed in a special lathe This stock is the pattern ，and it performs the function of a camAs it rotates and translates ， the follower controls a tool bit that machines the production stock from a block of wood 凸輪是被應(yīng)用的最廣泛的機(jī)械結(jié)構(gòu)之一。 凸輪