CH02structure analysis of mechanism

1、Chap.2 structure analysis of mechanismChap.2 structure analysis of mechanism2-1 contents and aim contents and aim2-2 composition of mechanism composition of mechanism2-3 kinematic gram of mechanism kinematic gram of mechanism2-4 condition of exact motion for mechanism condition of exact motion for m

2、echanism2-5 Determining Degree of Freedom Determining Degree of Freedom2-6 matters needing attention for computing DOF matters needing attention for computing DOF2-7 Principle of Mechanism Constitution, Principle of Mechanism Constitution, structure type and structure analysis structure type and str

3、ucture analysis 2-1 contents and aim of mechanism structure analysis Study how to combine mechanism and how to draw the kinematic diagram of mechanism? Know the conditions for exact motion for mechanism Study the principle of mechanism constitution and the structure type.2-2 mechanism constitution1.

4、linkPart is a independent manufacturing unit in machine.Linkis a independent movement unit in machine. From the movement, every machine is composed of several links. Every machine is composed of many parts.氣缸體連桿體連桿頭曲軸齒輪活塞 Link is rigidly composed of many parts and link has an independent movement.真實(shí)

5、連桿連桿體連桿頭螺栓螺母墊圈 is a connection between two or more links (at their nodes), which allows some motion, or potential motion, between the connected links.2.kinematic pairsKinematic pairs例shaft and bearing、 In space a kinematic pair composed of two links, the DOF and constraint should satisfy: f + s = 6s

6、lider and rail 、two gear tooth meshing。2-2 mechanism constitution3）By relative motion typeRotation pair（revolute pair or hinge） sliding pairHelical pair Spherical pairOthers:can be divided into planar kinematic pair and spatial kinematic pair2）By contact type:Higher pairs： line, pointLower pairs： su

7、rface（1）Classification of kinematic pair1）By num of constraint：classpair、classpair、classpair。Mechanism constitution(2/4)3.Kinematic chainAn assemblage of links and joints. If these links are connected in such a way that no motion is possible, a locked chain(structure) result.Closed kinematic chainOp

8、en kinematic chain1234planar closed kinematic chain1234spatial closed kinematic chain1234planar open kinematic chain12345spatial open kinematic chain（2）kinematic pair symbolsKinematic pair can be expressed by simple symbols defined by（GB446084）。Common kinematic pair type Common symbols of kinematic

9、pairs 機(jī)構(gòu)的組成機(jī)構(gòu)的組成(3/4)12344.mechanism Mechanism is a kinematic chain in which at lease one link has been “grounded”.Fixed linkDriving linkDriven link1234planar four-bar mechanism Mechanism can be divided into Planar mechanism and spatial mechanism.spatial four-bar mechanismFixed linkDriving linkDrive

10、d linkFixed linkDriven linkDriving linkMechanism constitution(4/4)planar four-bar mechanismDriving linkDriven linkThere are two forms: a sketch (proportional but not exactly to scale), and the scaled kinematic diagram (usually used for further analysis: position, displacement, velocity, acceleration

11、, force and torque transmission, etc) According to the kinematic dimensions and certain scale, make sure the position of every kinematic pairs by usingthe method of expressing kinematic pairs2-3 kinematic diagram of mechanism1.Whats the kinematic diagram of mechanism？ example: engine kinematic diagr

12、am of mechanism。Kinematic diagram of mechanism: common mechanism kinematic diagram symbols and links expressing method。Kinematic diagram of mechanism(2/2)2. How to draw the kinematic diagram of mechanism?(1)Analyse the construction, motion of the mechanism; (2)Select a visual angle to observe the tr

13、ansmission route from the driving links to output links.(3)Determine the type and number of kinematic pairs； Choose a drawing plane to express the mechanism； (4) Choose a proper scale to draw. Be careful about the relative positions between the pairs. For convenient reference, the links are numbered

14、, while the joints are lettered. The input and output are also labeled. (5)Calculate and check the degree of freedom of the mechanism.kinematic diagram of chaw-crusher mechineFor example：kinematic diagram of engine2-4 conditions for exact motionexample1 four-bar mechanismIf give an independent movem

15、ent,exact motion；If give two independent movement, be broken.motion uncertainty;exact motion. Conclusion：DOF (degree of freedom): F。The num of driving link is equal to the DOF.example2 five-bar mechanismIf give an independent movement,If give two independent movement,The num of driving link F, then

16、be broken;The num of driving linkF, then motion uncertainty.2-5 DOF calculating1.DOF calculating for planar mechanism（1）formula:F3n n- num of active links;pl num of lower pairs；Ph-num of higher pairs.(2)example1）four-bar mechanismF3n(2plph)3324012）five-bar mechanismF3n(2plph)342502123451234(2plph)13

17、478 9101118ABCD,(3)F3n(2plph)362731機(jī)構(gòu)自由度的計(jì)算機(jī)構(gòu)自由度的計(jì)算(2/4)1.4 Points of Attention to Calculating DOF of planar Mechanism A:Multiple joints(compound hinges) B:Passive DOF C:Redundant ConstraintsMultiple joints(compound hinges)A: Multiple joints(compound hinges)7n10lp110273F F = 31 - 22 = -1 F = 31 - 21

18、 = 1計(jì)算平面機(jī)構(gòu)自由度時(shí)應(yīng)注意的事項(xiàng)計(jì)算平面機(jī)構(gòu)自由度時(shí)應(yīng)注意的事項(xiàng)(3/6)B: Passive DOF11F33(231)F3n(2plph) FDOF of cam mechanism=?How many movable links, full joints and half joints does it have?2、要除去局部自由度、要除去局部自由度213233F112223F去掉局部自由度算法去掉局部自由度算法保留局部自由度算法保留局部自由度算法1113233FF3n(2plph) F33(240)01計(jì)算平面機(jī)構(gòu)自由度時(shí)應(yīng)注意的事項(xiàng)計(jì)算平面機(jī)構(gòu)自由度時(shí)應(yīng)注意的事項(xiàng)(3/6)C

19、: Redundant ConstraintsF3n(2plph p) F 34(260-1)01Example 2-8=EDGJFI121311293FF3n(2plph p) F =3*9-（2*11+3-1）-2=1F3n(2plph) =3*6-2*7-3=1links + kinematic pairkinematic chain mechanism機(jī)架機(jī)架原動(dòng)件原動(dòng)件從動(dòng)件組合從動(dòng)件組合F = 02-7 constitution principle, structure type and structure analysis 2-7 constitution principle, structure type and structure analysis 1.Constitution principleExample:1）Assur group： （1）group condition（2）group type1）group： composed of two links and three lower pairs.2）group：ABC12c)ABC12d)ABC12e)ABC12a)ABC12b)ABCDEF1234a)ABCDEF1234b)ABCDEF1234c)平面機(jī)構(gòu)的組成原理、結(jié)構(gòu)分類及