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1、Part 2: Machine Elements and Mechanisms,2.1 Introduction,Machine elements,Machine 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,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.,Figure 2.1 Machine elements are the
3、 building blocks of most machines,2.2 Fastenings,2.2.1 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 and siz
4、es.,The importance of fasteners in a machine,2.2.2 Thread Terminology,Figure 2.2 Screw threads,(b) Left-hand thread.,(a) Right-hand thread.,Figure 2.3 Specialist terminology used for describing threads.,2.2.3 Thread Forms,3. Square thread,1. Metric threads,2. Acme threads,4. Buttress threads,Basic p
5、ipe thread with 55 angle.,5. Pipe thread,2.2.4 Power Screw,Figure. 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.,The advantages of
6、 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 parts Most are self-locking,The disadvantages are tha
7、t most are not very efficient. Leadscrew efficiency is typically between 25 and 70%,A higher performing but more expensive alternative is the ball screw.,Figure 2.10 Ball screws.,2.2.4 Bolts, Studs, Nuts and Plain Washer,Figure 2.11 Bolts and screws.,1. Bolts,2. Studs,Figure 2.12 Studs.,Figure 2.13
8、Standard nuts.,3. Nuts,4. Washers,Figure 2.14 Washers.,2.2.5 Alternative Screw Heads and End Points,Figure 2.15 Alternative screw heads.,90。 Slotted countersunk,Slotted round head,Slotted cheese head,Square head,Slotted fillister head,Slotted instrument head,Slotted head,Hexagon socket head,Phillips
9、 head,Figure 2.16 Alternative screw points.,Flat,Dog,Conical,Round,Cup,2.2.6 Selection of Screwed Fasteners,Selection criteria for the screw materials ：,temperature； required strength； resistance to corrosion； joint material and cost ； assembly considerations.,2.2.7 Systems for Specifying the Dimens
10、ions of Screws,1. ISO metric screw thread,2. British Standard Whitworth,3. British Association screw thread,4. Unified Thread Standard,2.2.8 Applications of Screwed Fasteners,Figure 2.17 Applications of screwed fasteners.,Name all parts and fasteners in the drawing,2.3 Keys and Pins,(a) Parallel key
11、s,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.,Figure 2.20 Semicircular keys.,Figure 2.21 Tapered keys.,2.3.2 Pins,(a) Taper pins,(b) Parallel pins,(C) split pins,Figure 2.22 Pin joints.,2.4 Riveted Join
12、ts,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.,Types of Welding :,ungsten inert gas welding (TIG),submerge
13、d arc welding (SAW),metal inert gas welding (MIG),manual metal arc welding (MMA),resistance welding,gas welding,2.6 Spring,Clamp,Store elastic energy,Reduce shock.,Functions of springs:,Helical compression springs.,(b) Helical extension springs.,(c) Torsion springs.,(d) Spiral torsion springs.,(f) L
14、eaf springs.,2.7 Bearings,2.7.1 Bearing classification,Journal bearing,Deep groove ball bearings,Types:,Figure2.26 Bearing classification. .,Factors for consideration when selecting bearings：,Load speed location size cost starting torque noise lubrication supply,2.7.2 Bearing type selection,2.27 Bea
15、ring type selection by load capacity and speed.,Table 2.9 Comparison of bearing performance for continuous rotation,Table2.10 Merits of different rolling contact bearings,2.8 Clutches and Brakes,Clutches and brakes provide frictional, magnetic, hydraulic or mechanical connection between two machine
16、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.,The similarities between clutches and brakes?,Figure 2.28 Idealized friction disc clutch or brake.,Figure 2.29 Typical applications of clutches and brakes.,Clutch classification:,2.9 Shafts and Couplings,Commonly used shafts：,(b) Stepped shafts.,（a）Plain shafts,(c) Crankshafts.,Shaft design considerations include:,1. size an
