University of Waterloo Department of Mechanical Engineering ME MACHINE DESIGN 1. Fall 2005

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1 University of Waterloo Department of Mechanical Engineering ME MACHINE DESIGN 1 Fall 2005 Mechanical Design - "the process of formulating workable plans for the construction of machinery, devices or equipment to satisfy some human needs" 1. General Information Instructor: G. Glinka Office: E Phone extension: 3339 Teaching Assistants Mr. Amir Noroozi: E2-2354F, Phone ext Mr. Ramtin Kavahie: E2-2354B, Phone ext Lecture Schedule: Monday (10:30 AM), Wednesday (10:30 PM), Thursday (3:30 AM) + 3 makeup lectures Marking Scheme ME 322 (Mini-project: Shaft Design) 10-15% Mid-term test (2 hrs - closed book) 30-35% Final Exam (2.5 hrs - course textbook allowed) 50-60% Course Text: Mechanical Engineering Design, McGraw Hill, 7th Edition, 2004 Joseph E. Shigley, Charles R. Mischke, and Richard G. Budynas Course format Review and consolidation of analysis techniques, Mid-term test (closed book) Machine Component design Mini-project Final Exam (Course textbook only) Where we are: ME219, ME220- Mechanics of Deformable bodies (Statics) ME321 Kinematics and Dynamics of Machines (Dynamics) ME322 Mechanical Design 1 ME423 Mechanical Design 2 1

2 ME 322 Course Summary 1. Basic stress analysis Stresses and strains (stress tensor) Hooke s law Plane stress vs. plane strain state Transformation of the stress tensor, principal stresses and Mohr s circle 2. Material properties Material stress-strain curve The yield strength and ultimate strength Hardness Strength criteria (Mohr, Tresca and H-M-H strength hypothesis) 3. Stress calculation Equations of static equilibrium Free body diagram External and internal forces (normal forces, shear forces, bending moments) Bending and shear stresses in beams Stress concentration factors 4. Deflection of beams and buckling of columns Deflection of beams Elastic buckling Design applications 5. Fatigue failure Definitions The S-N curve Estimation of the endurance limit Correcting factors Stress concentration effect Mean stress effect Design applications 6. Shaft design Internal forces Stresses in shafts Static strength analysis of shafts under combined loads Deflection of shafts Fatigue strength of shafts 2

3 7. Static strength analysis of welds Weld classification Basic conventions concerning static stress analysis in welds Welds in tension Welds in bending Welds in shear 8. Bolted joints Bolted joints in tension The pre-stressing effect Force analysis Static strength of bolted joints Fatigue analysis of bolted joints 3

4 Assignments and Tutorials, ME322 - Fall 2005 The assignments and tutorial problems can be found in the texbook: Mechanical Engineering Design, McGraw Hill, 7 th Edition, 2004, authors: Joseph E. Shigley, Charles R. Mischke, and Richard G. Budynas. The solutions are available on the website. Assignments: Chapter4: 4-3(f), 4-12(c), 4-25(c), 4-27, 4-47 Chapter5: 5-35, 5-36, 5-40, 5-75, 5-76 Chapter6: 6-25, 6-27 Chapter7: 7-17, 7-20, 7-23, 7-26 Chapter8: 8-15, 8-20, 8-28, 8-32, 8-34, 8-38, 8-39, 8-43 Chapter9: 9-7, 9-11, 9-12, 9-13, 9-14, 9-23 Tutorial problems: Chapter4: To be handed out in the class Chapter5: 5-73 Chapter6: 6-14, 6-23, 6-24 Chapter7: To be handed out in the class Chapter8: 8-22, 8-35, 8-48 Chapter9: 9-5 4

5 ME322 - MECHANICAL DESIGN 1 Topic 1 & 2 - Review of Stress Analysis and Material Properties This part of the course is a quick review of the stress analysis concepts which you studied in the ME219 and ME220 courses. Stress analysis is important in machine design, particularly when strength, wear or reliability is critical. The ability to calculate stresses and to combine various load cases is a basic requirement for a design engineer. Stress is defined as internal force per unit area on a specified surface, in a specified direction and may be "shear" or "normal". Basic Stress Cases: In my opinion, Chapter 4 is not logically organized, since the most difficult topics (principal stresses, combined stresses, Mohr's Circle and Tri-axial Stress) are discussed first, before the review of simple stress cases. In the lectures we will reverse the order, and review the simpler stress cases first: tension, compression, bending, torsion, beam shear, pure (scissors-like) shear and thin-shell pressure vessels (see Sections 2.4 to 2.16). These are the most common cases (although there are many other less-common situations, such as curved beams, thick shell pressure vessels, etc.) and equations for these cases are in the text or in more specialized reference books. Free-body diagrams (FBDs) are basic and essential for proper stress analysis, so if you have problems with FBDs, please review the discussion of load calculations in Section Equilibrium, and try the static problems suggested previously. Beams are probably the most common structural elements and are therefore important. The "singularity function" approach (Section 2.9) for calculating beam shear and bending moment is useful for computer programs (Math CAD), but is very difficult to interpret and to check. Always sketch the shear and bending moment diagrams by hand whenever you solve beam problems. Combined Stresses: We will review basic cases before considering combined stress, Mohr's Circle and Triaxial Stresses (Sections 2.1 to 2.3). Combined stresses occur when machine parts are subject to several loads simultaneously. For example, a motor shaft may be twisted and bent at the same time, and the stresses due to torsion and bending must be combined. In problems such as this, Mohr's Circle (p. 26) is a useful tool for visualization, since it is a graphic form of the equations for combined stress, and is an effective way to remember the equations (and an easy way to check your answers). Sometimes Mohr's Circle is not covered in the earlier courses, so students should review the construction procedure. The case of triaxial (or 3-D) stresses is important, and may also be a new concept. The tri-axial Mohr's Circle can help you visualize stress directions. Stress Concentrations: If a machine part has "discontinuities" such as holes or sharp corners, the stress will not "flow" smoothly through the part and "stress concentrations" result (Section 2.15, p. 57). Stress concentrations do not affect the strength of ductile materials under static loads, since a little local yielding or "work-hardening" occurs, and relieves the concentration. However, stress concentrations are dangerous sources of fatigue cracks. 5

6 Material properties: Knowledge of basic material properties is essential for machine design. Standard tests and standard specifications for materials give the designer confidence that a specified material will have the properties needed for strength and safety. This topic is covered in Chapter 5 - Materials, of your course text (Shigley & Mischke). The static tensile test is very important because many other properties (even fatigue strength) can be related to static test data, which is widely available. The static test, and related topics, are described in Sections 5.1 to 5.3 of the course textbook. Review the tensile, compression and torsion tests, as well as the definitions of Stress, Strain, Modulus of Elasticity, Proportional Limit, Elastic Limit, Yield Strength, Ultimate Strength, Ductility, Brittleness, Modulus of Rigidity, Fatigue Strength, Endurance Limit, Resilience, and Toughness. The relationship between hardness and strength, for steel and cast iron, is very useful, and is given as an equation (Eq. 5.20, p. 197) in Section Hardness, in the textbook. Section 5.14 explains the hardening process and the various methods of hardening, such as through-hardening (quenching), and casehardening (carburizing, nitriding, induction hardening), and methods of reducing hardness (annealing and tempering). 6

MECHANICS OF SOLIDS IM LECTURE HOURS PER WEEK STATICS IM0232 DIFERENTIAL EQUAQTIONS

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