MechanicalDesign机械设计

1、Introduction：The basic elements of the machine are Mechanical Parts。In this course, we introduce mainly about the basic knowledge of mechanical design of the whole machine, especially about the design of the common parts in general dimensions(一般尺寸) and under common operating parameters(常用工作参数)，inclu

2、ding Basic design theory and method，technical data and Standard applications. Basic design theoryThe composition of a machineprime mechanism，Transmission mechanism，Executive mechanism，other parts(like: auxiliary system辅助系统、Control System)可以举例说明：a car engine（prime mechanism）；Drive shaft and Gearbox (

3、Transmission part); wheel (Executive part);Fatigue strength of materials1) Definition：Fatigue is the weakening of a material caused by repeatedly applied loads(载荷). The maximum stress values that cause such damage may be much less than the yield stress limit(屈服应力极限).So we use the sN fatigue curve an

4、d equal-life fatigue curve to show the Anti-fatigue properties(抗疲劳特性) of materials.2)Maximum stress max 、Number of stress cycles N 应力循环次数、Stress amplitude a 应力幅值、Mean stress m 平均应力（under the fatigue limit）、static stress静应力Figure Error! Main Document Only. equal-life fatigue curveFigure Error! Main D

5、ocument Only. S-N fatigue curveSymmetry cyclic对称循环variable stress 可变应力:m=0,pulse cyclic脉动循环 variable stress: m=a =0 /2,Known:A(0，-1) ,D (0 /2，0 /2);C ( 0, s ) ,Any point of line C represent the yield limit屈服极限When the parameter of cycle stress（ m，a ）is in OAGC，the materials dont break。When the param

6、eter is out OAGC，the materials must break。When the parameter is on OAGC，the materials reach fatigue limit 。Wear 磨损（Friction摩擦）1) Definition：the material of element surface disappeared or removed because of friction。2) Wear curve:run-in wear period-It results from the roughness of processing surface(

7、加工表面的粗糙度)。stable wear period-The wear speed of element is steady and slow。sharp wear period-the gap of pair increase and cause the vibration（振动） of machine 。The element may be rejected （遗弃，报废）。（可以举例：The running-in period of a new car,···········

8、··，················） Mechanical jointClassification：mechanical Dynamic joint（联系到知识点：kinematic pair运动副）mechanical static joint（Relative movement相对运动 is not allowed）Screw joints and screw drive：1) Classification

9、of Screw thread(螺纹)：According to the type of tooth牙型 of screw threadRectangle矩形 screw thread；Triangle三角形 screw thread；Trapezium梯形 screw thread；Hackle锯齿形 screw threadexternal thread外螺纹；internal thread内螺纹2) Basic types of screw thread joint:Bolted joint螺栓连接；Screw joint螺钉连接；Stud joint双头螺柱连接；holding scr

10、ew joint紧定螺钉连接3) pre-tightening force 预紧力：the majority of screw thread joint need to be tightened while fitting装配 before 8supporting service load负载. This load is called pre-tightening force.To enhance the reliability and tightness of joint, to prevent appearance of gap or relative move after bearing

11、 load.4) Screw drive：Function: to change rotating motion回转运动 to rectilinear movement直线运动 and transferring power in the same time.Classification：According to purposeTransferring power screw（car jack汽车千斤顶）；Transferring movement screw (in machine tool机床)；Adjusting screwThe coupling between the axle（轴）

12、and the hub（轮毂）1) The key joint键连接Function：hoop location(固定) and transfer torque（传递力矩） between the axle and the hubType（多画图解释）：straight key平键（主要记）, woodruff key半圆键, taper key楔键, tangent key切向键.2) The spline coupling花键连接Component（一对零件）：external spline, internal splineAdvantage：uniform pressure均匀受力；hi

13、gh load carrying ability；improve working accuracyDisadvantage：Working cost highlyWelding焊接（联系“工程材料”相关）Mechanical drivesSummaryAim：To transmit power and change the state of motion between prime mechanism and Executive mechanismClassification：by friction，mesh（啮合），······&#

14、183;······；By electric driveBelt drives1. Type（多画图，图见书P144）: Friction belt driveFlat belts平带；Round belts圆带； V belts（It is now the basic belt for power transmission. because Greater contact area接触面积，bigger friction）；Multi-groove belts多楔带Mesh belt drive2. Features：Belt dr

15、ive is simple, inexpensive, They need no lubrication润滑 and minimal maintenance维护.Slipping(打滑)the Friction of belt drive reached its maximumFriction, so wear of belt is a problem3. Calculate：机械设计课本P146P147三条公式Chain driveChain drive is Mesh drive(Sprocket链轮 and chain链条)Advantage：Chain drive has no sli

16、pping and is Mesh drive, so it has precise transmission ratio and high efficiency.Chain is made of metal, it can work in extreme environment.(machice of Coal transportation)Disadvantage: NoiseType: *Roller chain; Tooth Chain (Silent Chain)Calculate：机械设计课本p172公式1*Gear Drives机械原理+机械设计相关章节1. Advantage：

17、high efficiency( can reach 99)；compact structure结构紧凑；high reliability and long life；2. Disadvantage：high precision request of installing高安装精度；higher price。3. Type：spur gear；helical gear斜齿轮；（double- helical gear人字齿；）bevel gear锥齿轮；worm(蜗杆)and worm wheel；rack齿条 and pinion小齿轮4. 基础知识（of Spur gear）：1) 见机械

18、设计.pdf2) how can a pair of involute gears mesh?写公式3) The pressure angles（）and the module（m） are equal ；the center distance between the gears is equal to the sum of Pitch radius4) Helix angle of helical gear(螺旋角)注意斜齿轮，锥齿轮，蜗轮蜗杆旋向（The direction of rotation）和力的方向的判断。5. Produce gears:Gear milling铣齿；Gear

19、hobbing滚齿；Gear shaping插齿1) Gear millinguse the Milling cutter; machine the tooth one by one2) Gear hobbingGear hobbing is a multipoint machining process多点加工 in which gear teeth are machined by a series of cuts with a helical(螺旋的) cutter (hob). the hob and the workpiece Both revolve旋转 constantly3) Ge

20、ar shaping(画运动方向图)Both the gear blank轮坯 and cutter are set in a vertical plane and rotated such as the two gears in meshing.; The cutter have Cutting movement切削运动, Feed motion进给运动 ;the gear blank also have small movment.6. Types of gear failures：Breakage（崩齿）; Surface pitting点蚀Small piece off pitting

21、(麻点) ，increase the hardness of materialgear scoring胶合scoring（划痕），increase the hardness of the tooth surfacewear磨损（联系上面磨损相关知识）; plastic deformation塑性变形7. Gear design:（相关见机械设计课程设计.doc）A. Choose gear type, precision grade精度等级and material Type: spur gear drive order bevel gear drivePrecision grade: 13 g

22、rade (0 to 12, 0 is the best)Material: 40 chrome (Cr铬) to small gear and45 steel( 45号钢)B.Design according to tooth contact strength（齿面接触疲劳强度）C.Design according to tooth root bending strength（齿根弯曲疲劳强度）D.Compare the results from different method, choose the module, get the numbers of teeth（齿数z1, z2）E.

23、Calculate the size (pitch diameters节圆直径, the distance centers between two ,width of the gear)Splash lubrication飞溅润滑（浸油润滑）BearingPlain bearing1. Definition：Bearings working under sliding friction；no rolling elements；The simplest example of a plain bearing is a shaft rotating in a hole.2. Feature：use

24、in High speed, harsh working environment, large impact and vibration, small space3. Structure：(画剖面图展示)the journal轴颈 (i.e., the part of the shaft in contact with the bearing)bushing轴瓦；（is inserted into a housing to provide a bearing surface for rotary applications）Bearing lining轴承衬（Friction-reducing

25、material layer）4. Material：Babbitt巴氏合金(a kind of alloy，make as bearing lining,expensive)Rolling Element bearing1. Definition：Bearings working under Rolling friction; Is standardized被标准化 and widely used；（画图表示）2. Structure: Inner ring, outer ring, rolling element, cage保持架（to Separating the rolling elements）3. Type：According to the shape of the rolling elementballs, cylind