Introduction to Design.pptx
- 1. 1 Course Outline 1. Fundamentals of Design • Definitions and Basic Concepts • The Mechanical Design Process • Allowable Stresses and Factor of Safety • Stress Concentration Factors 2. Design for D/t Types of Loadings • Types of Loads • Endurance Strength • Theories of Failure 3. Strength Calculation of Joints • Threaded Fasteners • Power Screws 4. Torque Transmitting Joints • Design of Keys • Spline Joints and Pin Joints 5. Design of Springs • Spring Materials • Design of Helical Compression Springs 6. Ergonomic and Aesthetic Considerations
- 2. 2 Chapter 1 Fundamentals of Design Chapter Contents Fundamentals of Design – Introduction: Definitions and Basic Concepts – The Mechanical Design Process • Steps in the Design Process • General Design Procedures – Mechanical Properties of Materials – Design Factor and Allowable Stress – Stress Concentrations – Notch Sensitivity and Strength Reduction Factor
- 3. 3 Introduction: Definitions and Basic Concepts • Machine Design: process of formulating a plan to solve a specific problem (by applying various scientific principles, laws, techniques, etc) • Machine Design Process involves: – Selecting design criteria for d/t machine components – Use design data books & IS codes for design – Select standard components with specifications as per design • Machine Component Design involves: – Analyze & evaluate loads, forces, stresses involved in machine components and decide their forms/shapes & sizes/dimensions – Select proper materials for machine components
- 4. 4 The Mechanical Design Process • It is the designer's responsibility to ensure that a machine part is safe for operation under reasonably foreseeable conditions. • The designer should evaluate carefully: – the application in which the component is to be used, – the environment in which it will operate – the nature of applied loads (static, repeated and reversed, fluctuating, shock, or impact) • Will high mean loads be applied for extended periods of time, particularly at high temperatures, for which creep must be considered
- 5. 5 The Mechanical Design Process – the types of stresses to which the component will be exposed, • direct tension/compression, direct shear, bending, or torsional shear? Will two or more kinds of stresses be applied simultaneously? Are stresses developed in 1D (uniaxially), 2D (biaxially), 3D (triaxially)? Is buckling likely to occur? – the type of material to be used, • Consider the required material properties of Sy, Sut, Suc, endurance strength, stiffness, ductility, toughness, creep resistance, corrosion resistance, and others in relation to the application, loads, stresses, and the environment; Is ductile/brittle material appropriate? – the degree of confidence he/she has about the application, • How reliable are the data for loads, material properties, & stress calculations? Are controls for manufacturing processes adequate to ensure that the component will be produced as designed with regard to dimensional accuracy, surface finish, etc.? These considerations will affect your decision for the design factor, N.
- 6. 6 The Mechanical Design Process • All design approaches must define the relationship between the applied stresses on a component and the strength of the material from which it is to be made, considering the conditions of service. • The strength basis for design can be Sy in tension, compression, or shear; Su in tension, compression, or shear; endurance strength; or some combination. • The goal of the design process is to achieve a suitable design factor. N, (sometimes called a factor of safety) that ensures the component is safe. • The strength of the material must be greater than the applied stresses.
- 7. 7 The Mechanical Design Process • The sequence of design analysis will be d/t depending on what is already specified and what is left to be determined. – Geometry of component & load are known: apply the desired design factor, N, to the actual expected stress to determine the required strength of the material. Then a suitable material can be specified. – Load known & material for component specified: compute a design stress by applying the desired design factor, N, to the appropriate strength of the material (this is the max. allowable stress to which the component can be exposed); then complete the stress analysis to determine what shape and size of the component will ensure that stresses are safe. – Load known, material and complete geometry of component specified: compute both the expected max. applied stress & the design stress; compare these stresses & determine the resulting design factor, N, & judge its acceptability. A redesign may be called for if the design factor is either too low (unsafe) or too high (over designed).
- 8. 8 Steps in General Design Process • Steps:
- 9. 9 Practical Considerations in Design Process • Each design decision should be tested against cost • Material availability must be checked. • Manufacturing considerations may affect final specifications for overall geometry, dimensions, tolerances, or surface finish. • Components should be as small as practical unless operating conditions call for larger size or weight. • After computing the min. acceptable dimension for a component, standard sizes should be specified using tables of preferred sizes • Before a design is committed to production, tolerances on all dimensions & acceptable surface finishes must be specified so as to specify suitable manufacturing processes • Surface finishes must be applied for a particular area of a component, considering appearance, effects on fatigue strength, and whether or not the area mates with another component (producing smoother surfaces increases cost dramatically).
- 10. 10 Mechanical Properties of Materials • Machine elements are very often made from metals/metal alloys such as steel, aluminum, cast iron, zinc, titanium, or bronze. • Some strength, elastic, and ductility properties of metals: – Tensile Strength, Su – Yield Strength, Sy – Modulus of Elasticity in Tension, E, – Ductility and Percent Elongation – Shear Strength, Sys and Sus – Modulus of Elasticity in Shear, G – Poisson's Ratio, v – Hardness – Machinability – Fatigue Strength or Endurance Strength – Creep
- 11. 11 Mechanical Properties of Materials • Typical Stress-Strain Diagram for Steel
- 12. 12 Mechanical Properties of Materials • Typical Stress-Strain Diagram for aluminum and other metals with no yield point
- 13. 13 Mechanical Properties of Materials • Hardness Conversion
- 14. 14 Mechanical Properties of Materials • Typical creep behavior
- 15. 15 Design Factor and Allowable Stress • Design Factor (N): a measure of the relative safety of a load- carrying component. 1. Mostly, the strength of the material is divided by the design factor to determine a design stress (σd) or an allowable stress; then the actual stress to which the component is subjected should be less than the design stress. 2. For some kinds of loading, the design factor, N, is computed from the actual applied stresses and the strength of the material. 3. Still in other cases, particularly for the case of the buckling of columns, the design factor is applied to the load on the column rather than the strength of the material. • Often the value of the design factor or the design stress is governed by codes established by standard-setting organizations (ASME, AGMA, AISI, AISC, U.S. Department of Defense, the Aluminum Association, etc.) • In the absence of codes or standards, the designer must use judgment to specify the desired design factor; The following guidelines can be used:
- 16. 16 Design Factor and Allowable Stress Ductile Materials – N = 1.25 to 2.0: Design of bodies under static loads for which there is a high level of confidence in all design data. – N = 2.0 to 2.5: Design of machine elements under dynamic loading with average confidence in all design data. – N = 2.5 to 4.0: Design of static machine elements under dynamic loading with uncertainty about loads, material properties, stress analysis, or the environment. – N = 4.0 or higher: Design of static machine elements under dynamic loading with uncertainty about some combination of loads, material properties, stress analysis, or the environment. The desire to provide extra safety to critical components may also justify these values. Brittle Materials • N = 3.0 to 4.0: Design of bodies under static loads for which there is a high level of confidence in all design data. • N = 4.0 to 8.0: Design of static machine elements under dynamic loading with uncertainty about loads, material properties, stress analysis, the environment.
- 17. 17 Design Factor and Allowable Stress Example 1
- 18. 18 Design Factor and Allowable Stress Example 2
- 19. 19 Design Factor and Allowable Stress Example 3
- 20. 20 Stress Concentration Origins of Stress Concentrations Machine members often have regions in which the state of stress is significantly greater than theoretical predictions as a result of: 1. Geometric discontinuities or stress raisers such as holes, notches, and fillets; 2. Internal microscopic irregularities (non-homogeneities) of the material created by manufacturing processes (casting & molding) 3. Surface irregularities such as cracks and marks created by machining operations. These stress concentrations are highly localized effects which are functions of geometry and loading.
- 27. 27 Stress Concentration Example 3 Contd ...
- 28. 28 Stress Concentration Tables SC Table A-15-1-3
- 29. 29 Stress Concentration Tables SC Table A-15-4-6
- 30. 30 Stress Concentration Tables SC Table A-15-7-9
- 31. 31 Stress Concentration Tables SC Table A-15-10-12
- 32. 32 Stress Concentration Tables SC Table A-15-13-15