1、Part 2: Machine Elements and Mechanisms第1頁(yè)2.1 Introduction Machine elementsMachine elements refer to an elementary component of a machine. They include: structural components such as bearings, axles, keys, fasteners, seals, and lubricants.Mechanisms such as gear trains, belt or chain drives, linkage

2、s, cam and follower systems, including brakes and clutches第2頁(yè)Control components such as buttons, switches, indicators, sensors, actuators and computer controllers. Machine elements are basic mechanical parts and features used as the building blocks of most machines.第3頁(yè)Figure 2.1 Machine elements are

3、 the building blocks of most machines第4頁(yè)2.2 Fastenings2.2.1 Introduction A fastener is a device used to connect or join two or more components. Traditional forms of fastening include nuts, bolts, screws and rivets. They are frequently used and most of them have been standardized with their shapes an

4、d sizes. The importance of fasteners in a machine第5頁(yè)2.2.2 Thread TerminologyFigure 2.2 Screw threads(b) Left-hand thread.(a) Right-hand thread. 第6頁(yè)Figure 2.3 Specialist terminology used for describing threads.第7頁(yè)2.2.3 Thread Forms3. Square thread1. Metric threads2. Acme threads第8頁(yè)4. Buttress threads

5、Basic pipe thread with 55 angle.5. Pipe thread第9頁(yè)2.2.4 Power ScrewFigure. 2.9 A worm-gear screw jack . A power screw, also known as a leadscrew (or lead screw) or translation screw, is a device used in machinery to change angular motion into linear motion, and, usually, to transmit power.第10頁(yè)The adv

6、antages of a leadscrew are： Large load carrying capability Compact Simple to design Easy to manufacture; no specialized machinery is required Large mechanical advantage Precise and accurate linear motion Smooth, quiet, and low maintenance Minimal number of partsMost are self-locking The disadvantage

7、s are that most are not very efficient. Leadscrew efficiency is typically between 25 and 70%第11頁(yè)A higher performing but more expensive alternative is the ball screw.Figure 2.10 Ball screws.第12頁(yè)2.2.4 Bolts, Studs, Nuts and Plain WasherFigure 2.11 Bolts and screws. 1. Bolts2. StudsFigure 2.12 Studs.第1

8、3頁(yè)Figure 2.13 Standard nuts. 3. Nuts4. WashersFigure 2.14 Washers.第14頁(yè)2.2.5 Alternative Screw Heads and End PointsFigure 2.15 Alternative screw heads.90。 SlottedcountersunkSlottedround headSlottedcheese headSquareheadSlottedfillister headSlottedinstrumentheadSlotted headHexagonsocketheadPhillips hea

9、d第15頁(yè)Figure 2.16 Alternative screw points.FlatDogConical RoundCup2.2.6 Selection of Screwed FastenersSelection criteria for the screw materials ： temperature； required strength； resistance to corrosion； joint material and cost ； assembly considerations.第16頁(yè)2.2.7 Systems for Specifying the Dimensions

10、 of Screws1. ISO metric screw thread第17頁(yè)2. British Standard Whitworth第18頁(yè)3. British Association screw thread第19頁(yè)4. Unified Thread Standard第20頁(yè)2.2.8 Applications of Screwed FastenersFigure 2.17 Applications of screwed fasteners.Name all parts and fasteners in the drawing第21頁(yè) 2.3 Keys and Pins (a) Par

11、allel keys 2.3.1 Keys (b) Semicircular keys Figure 2.19 Parallel key joints. （1-Shaft. 2-Hub, 3-Key, 4-Nut, 5-Washer） (c) Tapered keys Figure 2.18 Keys.第22頁(yè) Figure 2.20 Semicircular keys.Figure 2.21 Tapered keys. 2.3.2 Pins (a) Taper pins(b) Parallel pins(C) split pinsFigure 2.22 Pin joints.第23頁(yè) 2.4

12、 Riveted Joints Figure 2.23 Conventional rivet before and after setting 2.5 Welded Joints Welding can be described as the process of joining material together by raising the temperature of the surfaces to be joined so that they become plastic or molten.第24頁(yè) Types of Welding : ungsten inert gas weldi

13、ng (TIG)submerged arc welding (SAW)metal inert gas welding (MIG)manual metal arc welding (MMA)resistance weldinggas welding第25頁(yè) 2.6 Spring ClampStore elastic energy Reduce shock. Functions of springs:Helical compression springs. (b) Helical extension springs. (c) Torsion springs.(d) Spiral torsion s

14、prings.(f) Leaf springs.第26頁(yè)2.7 Bearings2.7.1 Bearing classificationJournal bearingDeep groove ball bearings第27頁(yè)Types:Figure2.26 Bearing classification.第28頁(yè)Factors for consideration when selecting bearings：Load speed location size coststarting torque noiselubrication supply 2.7.2 Bearing type select

15、ion第29頁(yè)2.27 Bearing type selection by load capacity and speed.第30頁(yè)Table 2.9 Comparison of bearing performance for continuous rotation 第31頁(yè)Table2.10 Merits of different rolling contact bearings 第32頁(yè)2.8 Clutches and Brakes Clutches and brakes provide frictional, magnetic, hydraulic or mechanical conne

16、ction between two machine elements, usually shafts. In reality, the same device can function as a brake or clutch by fixing its output element to a shaft that can rotate or to ground, respectively.第33頁(yè)The similarities between clutches and brakes?Figure 2.28 Idealized friction disc clutch or brake. D

17、rivingdiscActuatorPusher discstogetherDrivendiscFrictionmaterialOutputInputFigure 2.29 Typical applications of clutches and brakes.第34頁(yè)Method ofengagementMagneticPneumaticand hydraulicMechanicalElectricalOverrunningPositivecontactFrictionMagneticFluidcouplingSquare jawSpiral jawToothedDiscDrumConeRo

18、llerSpragSpring woundMagnetic particleHysteresisEddy currentDry fluidHydraulicClutch classification:第35頁(yè)2.9 Shafts and CouplingsCommonly used shafts： (b) Stepped shafts. （a）Plain shafts. (c) Crankshafts.第36頁(yè)Shaft design considerations include:1. size and spacing of components (as on a general assemb

19、ly drawing), tolerances;2. material selection, material treatments;3. deflection and rigidity: bending deflection; torsional deflection; slope at bearings; shear deflection;4. stress and strength：static strength；fatigue；reliability;5. frequency response;6. manufacturing constraints.第37頁(yè)Figure 2.32 A

20、 typical shaft arrangementName parts in the drawing 2.9.2 ShaftHub ConnectionFigure 2.30 Alternative methods of shaft-hub connection.第38頁(yè)（a）Pin jointsPinShaftHub（b）Key joints（c)Set screw joints第39頁(yè)Functions of couplings 2.9.3 ShaftShaft Connection Couplingstransmit power from one shaft to another.Tw

21、o general types of coupling: Rigid couplingsUniversal jointsFlexible couplings第40頁(yè)2.10.1 Introduction 2.10 Belts and Chains（a）Belt driveBelt and chain drives consist of flexible elements running on either pulleys or sprockets.(b) Chain drive第41頁(yè)The advantages and disadvantages of a belt over gears a

22、nd chains:1. Consequence of Failure.2.Versatility in Shaft Connection.3.Effect on Shaft Bearing Life.4.Speed Ratio.5.Cost. 6.Noise and Vibration. 7.Speed and Power.8.Maintenance. 第42頁(yè)Types: 2.10.2 Belt Drives（a）Flat belt drives. (b) Round belt drives.(c) V belt drives(d) Synchronous belt drives.第43頁(yè)

23、Types:Figure 2.39 Various belt cross-sections.Figure 2.40 Pulley configuration.第44頁(yè)A chain is a power transmission device consisting of a series of pin-connected links 2.10.3 Chain Drives(a)Simple chainFigure 2.41 Roller chain. (b) Duplex chain （c）Triplex chainFigure 2.42 Simple chain drive.第45頁(yè) Cha

24、in types:Figure 2.43 Chain types.ChainsSilentConveyorLeafRollerBritish standardrangeStandardseriesDoublepitchBushchainANSIrangeStandardseriesHeavyseries第46頁(yè)Figure 2.44 Roller chain components.第47頁(yè)Figure 2.45 Conveyor chainFigure 2.47 Silent chain Figure2.46 Leaf chain第48頁(yè)Figure 2.48 Sperber gear box, silent chains 第49頁(yè)2.11.1 Introduction 2.11 GearsFigure 2.49 Spur gear schematic showing principle terminology. Gears are toothed cylindri