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This chapter introduces some of the more common mechanical components, used in technical design, as well as their graphic representations. Gears, cams, pulleys and belts, chains and sprockets, and mechanical linkages are used to transmit power and motion. Some are described in this chapter, along with the engineering drawing standards for each type of mechanism.
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BASIC DEFINITIONS |
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| 22.1 An important emphasis of this chapter is that manufactured products — more often than not — are made up of numerous components, not a single component. Quite often a student will spend a majority of an introductory course drawing single parts without having an appreciation for how these parts are integral to larger mechanisms. Another important point to make is that most assemblies contain components which are not manufactured on site, but are supplied by an outside manufacturer. Proper specification of these standard components is critical if the supplier is going to properly manufacture the components for your company. Part of the specification process is the technical drawing. In the process of engineering a mechanism, drawings of the components as part of an assembly can be a valuable tool during the analysis phase. All of the mechanisms described in this chapter are used to transfer energy from one part of the mechanism to another. For that reason, the forces being applied to the mechanism and how they transfer it is important to analyze. The types of analysis which might be performed on the mechanism includes: Statics Kinematics Dynamics Emphasize that with both kinematic and dynamic analysis, the mechanism must be evaluated with time as one of the variables. This can be done both by graphing techniques and by creating drawings and/or models of the mechanism changing over time. Three dimensional modeling is a powerful technique for analyzing mechanisms, as is appropriate use of graphing and animation techniques. |
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GEARS |
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| 22.2 The primary purpose of gears is to transmit rotational power and motion between portions of a mechanism. Typically a shaft is used to both apply and receive power from the gears. In the process of passing through two or more gears, the rotational motion may change direction, orientation, speed, or location in space. |
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| 22.2.1 In order to acheive the proper change in the rotational energy, you must be familar with the types of gears available and how they work together. The shafts which the gears attach to can be oriented with respect to each other in three ways: parallel intersecting nonintersecting Gears themselves are classified by the shape of the gear tooth: spur helical bevel face crossed helical hypoid worm |
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| 22.2.2 Parallel shafting Spur gears Probably the simpliest type of gear to manufacture or to draw. Note that the smaller of the two gears is called the pinion. The shape of a spur gear tooth is an involute. Helical and herringbone (double-helical) gears |
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| 22.2.3 Intersecting shafting Bevel gears These gears are used to change the orientation of the shafts to each other, usually 90 degrees. This is acheived by having the teeth oriented other than parallel to the shaft. The gear shape is based on a right circular cone. |
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| 22.2.4 Nonintersecting shafting Crossed helical gears Used to change orientation of the shaft in low load situations. Hypoid gears Worm gears Rack and pinion |
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| 22.2.5 As mentioned earlier, the geometry of spur gears is based on the involute (see Section 6.7.3). The important elements for specifying or drawing a spur gear are: line of centers base circle pitch circle line of action pressure angle |
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| 22.2.6 The pressure angle helps determine the load carrying capacity of a spur gear. The standard angles are 14 1/2 °, 20°, and 25°. |
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| 22.2.7 The ratio of the diameters of the gear and pinon determine the degree of rotational speed reduction or increase. |
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| 22.2.8-13 These sections give an introduction to how gears are designed and drawn. They include a more detailed look at the elements which define and specify a gear. This information is used to both design a safe load carrying capacity for the gears and provide information for the machinist cutting the gears. |
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CAMS |
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| 22.3 Cams are used to translate rotational motion into linear motion. Though cams often operate at high speeds, they usually aren't used to transfer large amounts of power. Instead, cams are typically used as mechanical acuators, such as opening and closing valves in an internal combustion engine. The cam is a non-circular shaped surface on a shaft which controls the linear motion of the follower. Cams are classified as one of three types: Face Groove Cylindrical Followers are classified as one of three types: Knife edge Flat face Roller This last type is the most common since it can operate at high speeds with a minimum of wear or heat buildup. |
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| 22.3.3 Displacement diagrams are 2-D line graphs which typically map the rotational motion of the cam on the horizontal axis and the linear displacement of the follower on the vertical axis. The shape of the line graph describes the geometry of the cam profile. See Chapter 20 for more information on creating line graphs. The line graph describes three types of motion of the follower: Rise Fall Dwell |
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| 22.3.4-8 The displacement diagrams describe four basic types of motion which the follower can make. They differ in terms of the rate of speed change (acceleration) the follower undergoes. They are: Uniform Harmonic Uniformly accelerated Combination |
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| 22.3.9 In addition to a displacement diagram, the cam profile can also be drawn directly. The view of face or groove cam profile is parallel to the primary axis of the shaft. |
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| 22.4 Linkages are the most common type of mechanism in use to transmit force. A linkage consists of a (usually) linear element, called the bar, and one or two joints. The length and shape of the bar and the location of the joint(s) determine the movement of the linkage. Quite often more than one bar/joint combination are used, with joints being shared between the bars. Sliders are used to direct the motion of a bar via a joint. Some of the most common types of links are: Crank Lever or rocker Rocker arm Bell crank Four-bar |
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| 22.4.3 Linkages can be analyzed graphically, mathematically, or both. As with cams, the graphical analysis can be either a graph representing the displacement of elements of the linkage or the actual geometry of the linkage. 3-D modeling and animation techniques can be powerful tools for analyzing the motion of the linkages (see Chapters 7 and 20). |
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BEARINGS |
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| 22.5 Bearings are used in conjunction with other mechanisms — such as gears and linkages — to minimize friction and wear between the parts. Bearings are usually used with some type of friction reducing compound (lubricant) such as grease or oil. Bearings are commonly used as an interface between rotating shafts and their stationary supports. |
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| 22.5.1 Plain bearings provide a sliding contact on which a lubricant can be applied. Note that these bearings are a single element and not multiple spherical or cylindrical elements as is commonly assumed. |
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| 22.5.2 Rolling contact bearings are what most people think of when they think of bearings. The bearings come in a number of shapes, determining their application. The geometries of the bearings include: spherical (ball) clindrical (roller or needle) truncated cone (taper) Figure 22.59 shows the different classifications of rolling contact bearings. Notice how the classifcation is based both on the geometry of the bearing and the type of contact between the moving part(s) and the stationary element. |
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SUMMARY |
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Gears, cams, bearings, and linkages are represented on technical drawings using standard practices. There are many types of gears that are represented on drawings for the machinist, through details and a cutting data table. Cams are represented on technical drawings by creating a displacement diagram, which is a graph representing the travel of the follower along the face of the cam. Cam profile drawings are used to detail the contour of the cam. Linkages are represented on the drawings using a standard symbol system. Graphical analysis can be used to determine the extreme positions of the links. Bearings are not shown on detail drawings because they are standard parts, but they must be represented on assembly drawings and listed in the parts list. |