Chapter 1: Introduction

WHAT IS A MACHINE MACHINE : A device for transforming or transfering energy An apparatus consisting of interrelated units (machine elements) A device that modifies force and motion

A machine receives energy in some available form and uses it to do some particular kind of work A petrol engine is a machine, which may use the heat energy derived from the combustion of the fuel to propel a vehicle along the road

A lathe is a machine which receives mechanical energy from the line shaft through the belt or gears and uses that energy to remove metal from a bar or other piece of work LINK OR ELEMENT : Each part of a machine which has motion relative to some other part STRUCTURES : Made up of series of members of regular shape that have a particular function for load carrying

SYNTHESIS : Concerned with the problem of selecting the size of the mechanism to perform a given function STRESS : Internal reacting force per unit area due to the effects of external applied forces

DESIGN Formulate a plan for the satisfaction of a human need The need for the problem has to be identified Design problem have no unique answer

A good answer today may well turn out to be a poor answer tomorrow, if there is a growth of knowledge during the period A design is always subject to certain problem-solving constraints A design problem is not a hypothetical problem

Design has an authentic purpose the creation of an end result by taking definite action, or the creation of something having physical reality

ENGINEERING DESIGN The process in which scientific principles and the tools of engineering mathematics, computers, graphics and English are used to produce a plan which, when carried out, will satisfy a human need

MECHANICAL ENGINEERING DESIGN Design of things and systems of mechanical nature, machines, products, structures, devices, and instruments For the most part, mechanical design utilizes mathematics, the materials sciences, and the engineering mechanics sciences

The ultimate goal in machine design is to size and shape the parts ME-307 Machine Design I Week 1 9-13, Jun-12 The ultimate goal in machine design is to size and shape the parts choose appropriate material and choose manufacturing process So that resulting machine can be expected to perform its intended function without failure Mohiuddin Mohammed Taher

An engineer should be able to calculate and predict the mode and conditions of failure for each element and then design it to prevent that failure This requires stress and deflection analysis for each part

Stresses are functions of applied and inertial loads An analysis of the forces, moments, torques and dynamics of system must be done before stresses and deflections can be completely calculated

Design A design must be: Functional- fill a need or customer expectation Safe- not hazardous to users or bystanders Reliable- conditional probability that product will perform its intended function without failure to a certain age. Competitive- contender in the market Usable- accommodates human size and strength Manufacturable- minimal number of parts and suitable for production Marketable- product can be sold and serviced

Design Process Actions Conceive alternative solutions Analyze, test, simulate, or predict performance of alternatives Choose the “best” solution Implement design

Design is… An innovative and iterative process A communication intensive activity Subject to constraints

Steps to Design

Design Considerations ME-307 Machine Design I Week 1 9-13, Jun-12 Design Considerations Strength Stiffness Wear Corrosion Safety Reliability Friction Usability Utility Cost Processing Weight Life Noise Styling Shape Size Control Thermal Properties Surface Lubrication Marketability Maintenance Volume Liability Recovery Mohiuddin Mohammed Taher

Codes and Standards Code- a set of specifications for the analysis, design, manufacture, and construction of something Standard- a set of specifications for parts, materials, or processes intended to achieve uniformity, efficiency, and a specified quality

Organizations Aluminum Association (AA) American Gear Manufacturers Association (AGMA) American Institute of Steel Construction (AISC) American Iron and Steel Institute (AISI) American National Standards Institute (ANSI) American Society for Metals (ASM) American Society of Mechanical Engineers (ASME) American Society of Testing Materials (ASTM) American Welding Society (AWS) American Bearing Manufacturers Association (ABMA) British Standards Institute (BSI) Industrial Fasteners Institute (IFI) Institution of Mechanical Engineers (I. Mech. E.) International Bureau of Weights and Measures (BIPM) International Standards Organization (ISO) National Institute for Standards and Technology (NIST) Society of Automotive Engineers (SAE) American Society of Agricultural and Biological Engineers (ASABE)

Economics Cost plays an important role in design decision process No matter how great the idea may be, if it’s not profitable it may never be seen The use of standard sizes and large manufacturing tolerances reduce costs Evaluating design alternatives with regard to cost Breakeven Points Cost Estimates

Product Liability “Strict liability” concept prevails in the U.S. Manufacturers are liable for any damage or harm that results from a defect.

Uncertainty Roman Method- repeat designs that are proven Factor of Safety Method of Philon- separate the loss-of-function load and the impressed load using a ratio Permissible Stress- fraction of significant material property (i.e., strength)

Uncertainty Design Factor Method- factor of safety is increased with rounding error to achieve nominal size (5.3 mm designed bolt size is increased to 6.0 mm) Stochastic Design Factor Method- uncertainty in stress and strength is quantified for linearly proportional loads

Measures of Strength S – Strength Ss – Shear Strength Sy – Yield Strength Su – Ultimate Strength - Mean Strength

Measures of Stress t – Shear Stress s – Normal Stress s1 – Principal Stress sy – Stress in y-direction sr – Radial Stress st – Tangential Stress

Stress Allowable (AISC) Tension: 0.45 Sy ≤ sall ≤ 0.60 Sy Shear: tall = 0.40 Sy Bending: 0.60 Sy ≤ sall ≤ 0.75 Sy Bearing: sall = 0.90 Sy

Loads Used to Obtain Stresses Where: Wd- dead loads Wl- live loads k- service factor Fw- wind load Fmisc- locality effects (earthquakes)

Service Factors Applications Elevators Traveling Crane Supports Light Machinery Supports Reciprocating Machinery Supports Floor and Balcony Supports k 2 1.25 1.20 1.50 1.33

Factor of Safety Design factors (nd) are defined as: and where ns-accounts for uncertainty of strength nd-accounts for uncertainty of loads

Realized Factor of Safety

Reliability Probability that a mechanical element will not fail in use Reliability approach to design: judicious selection of material, processes, and geometry to achieve reliability goal Factor of Safety Method- time proven, widely accepted Reliability Approach- new, requires data