机械原理英文Mechanical principles.docx

十四、Mechanical principlesDegrees of freedomIn mechanics, degrees of freedom are the set of independent displacements and/or rotations that specify completely the displaced or deformed position and orientation of the body or system.Formula Pllower pair , surface contactPhhigher pair , point or line contactnactivity component3every component have 3 degrees of freedom2lower pair have 2 degrees of freedom1high pair have 1 degrees of freedomF1 , we can not know what happen next timeF=1 , we can know what happen next timeF=0 , mechanics will not move三审，笔试，有三个图，分别计算哪个结构是稳定的，第一个是稳定的，第二个是稳定但是有过度约束，第三个不稳定可运动。在过度约束中，审核官问我知道不知道一个公式s=2k-3,我不明白，回来和同学讨论觉得这个应该是计算虚约束的公式，请多注意。Plane mechanisms motion analysisInstantaneous center of velocityRevolute pair join , two mechanism , instantaneous center of velocity in center of revolute pair.Rod 1 and rod 2 are combined by revolute pair, P12 is instantaneous center of velocity.Pure-slide pair join , two mechanism , instantaneous center of velocity infinite distance of vertical motion direction.A move along rod B. P12 in infinite distance of vertical rod B .High pair join , two mechanism , they are pure rolling , instantaneous center of velocity in contact point of two mechanism.P12 is instantaneous center of velocity Higher pair join , two mechanism , they are relative slide , instantaneous center of velocity in base tangent.P12 in t-t.Kennedy-Aronhold theoremP13 in P12 and P23 join line.P13 in P14 and P34 join line.P24 in P12 and P14 join line.P24 in P23 and P34 join line.2 and 4 velocity is same in P24Plane four bar mechanism A and D fix called stander , AB CD called side link.Link can rotation called crank-ABLink can shake in range of degree called rocker-CDCrank-rocker mechanismRadar Double-crank mechanismTrain wheel Double-rocker mechanismCraneCam mechanismAdvantage: structure is simple, respond is quickly.Disadvantage: cam produce is difficult, easily wear between cam and follower.GearA gear is a component within a transmission device that transmits rotational force to another gear or device.Spur gearSpur gear are used for low speed applications and those situations where noise control is not a problem.Helical gearHelical gear operate more smoothly than do spur gear. The use of helical gears is indicated when the application involves high speeds , large power transmission or where noise abatement is important,A disadvantage of helical gear is a resultant thrust along the axis of the gear , which needs to be accommodated by appropriate thrust bearing , and a greater degree of sliding friction between the meshing teeth , often addressed with specific additives in the lubricant.Double helical gearsDouble helical gears , also know as herringbone gears , overcome the problem of axis thrust presented by single helical gears by having teeth that set in a V shape.Bevel gearThe angle between the shafts can be anything except 0 and 180 degree.Worm gear The prime feature of a worm-and-gear set is that it allows the attainment of a high gear ratio with few parts in a sma