EML 3004 C CHAPTER 4 What is Design

EML 3004 C CHAPTER 4 What is Design? Namas Chandra Introduction to Mechanical engineering Chapter 4 -1

EML 3004 C Objectives The meaning of the word “design” Difference between design, analysis and reproduction Difference between good and bad design Engineering Design Process Design Cycle A design competition example Sections 4. 1 Concept of Design 4. 2 Difference between design and analysis 4. 3 Good design versus bad design 4. 4 Design Cycle Namas Chandra Introduction to Mechanical engineering Chapter 4 -2

EML 3004 C 4. 1 Concept of Design § Design is the development of a product or device to meet a specific need § Design may not necessarily be a physical product ü Development of Microsoft word software is considered a software design of a word processing program. ü In mechanical engineering world, by device we typically mean a product. It may range from an automobile, engine or even a spark plug. Namas Chandra Introduction to Mechanical engineering Chapter 4 -3

EML 3004 C 4. 2 Difference Between Design and Analysis § Analysis is different from design or reproduction § Analysis is the application of engineering science and mathematics to a well-defined, well-posed process with a unique solution. § Design is a solution to a problem with ill-posed constraints. There are many possible solutions all valid to the problem. § Analysis……. . If a 6' man reaches the top of the building at 32° standing 40' away from base, what is the height of the building? § Design……. . Given a building, device a method to measure the height of the building. . Namas Chandra Introduction to Mechanical engineering Chapter 4 -4

EML 3004 C Design example of a Catapult § Rewind back to middle ages! We need to design a device to haul objects from a distance accurately on to a target. § Suppose we need to determine the trajectory given all geometric variables, we can use basic dynamic equation: § Given specific x, angle and distance we have a unique velocity to get to the target- Analysis § But there are many possible ways to choose combination of these variables- Design. Namas Chandra Introduction to Mechanical engineering Chapter 4 -5

EML 3004 C Analysis of the Catapult Trajectory Namas Chandra Introduction to Mechanical engineering Chapter 4 -6

EML 3004 C 4. 3 Good vs. Bad Design 1. In general a design that just barely meets the expectation is a bad design. 2. If the design fails to meet the requirement it is no design. 3. If a design exceeds expectation in terms of cost, performance, maintenance, rugged, eventual disposal and safety then it is a good design. Namas Chandra Introduction to Mechanical engineering Chapter 4 -7

EML 3004 C 4. 3 Good vs. Bad Design 1. Some characteristics of Good Design 2. 1. Does the product meet technical requirements? 1. Should work under all conditions (favorable and unfavorable) 2. Different ranges of stones in (weight, size, shape), climactic conditions (wind, rain, hot/cold) 3. 2. Does the product work? ü Product should work for a long time ü Cracked timber or faulty launch mechanism ü Should exceed “period of expected performance” ü Satisfied customer-key to success Namas Chandra Introduction to Mechanical engineering Chapter 4 -8

EML 3004 C Good vs. Bad Design… 2 Does the product meet cost requirements? ü Cost is the predominant factor in many engineering application 4. Will the product require the least maintenance? ü Durability ü Maintenance-free product 5. Is the product safe? ü Safety is an important issue ü Cannot eliminate failure but reduce it as much as possible 6. Does the product create an ethical dilemma? ü A larger question of defense vs. offense, security vs. threat ü More practical aspect is cost and time in relation to potential dangers to the users and by-standers. 3. Namas Chandra Introduction to Mechanical engineering Chapter 4 -9

EML 3004 C 4. 4 Design Cycle § § § Namas Chandra Introduction to Mechanical engineering Design is a creative process and from concept to completion undergoes a “design cycle. ” There are basically seven steps in the process, including an iterative loop. While full scale testing is one possible way, computer simulations and mock-up testing are frequently very useful. Chapter 4 -10

EML 3004 C Step 1: Define overall objective 1. Define the overall objective. ü This is an important step ü A well-defined and thought-out problem leads to a much better solution Inquire carefully, v v v Why do you need the product? Who needs it and how they will use it? What are the critical features? What are the desirable features? Can this be produced easily? Is there a similar product already available. If so why there is a new need? Is it due to lack of performance, cost, safety? v How much will the new product cost? v What are the essential safety features? v Is the product governed by federal, state and local laws? At this stage, you should have an open-ended problem statement with all constraints (essential and desirable) Namas Chandra Introduction to Mechanical engineering Chapter 4 -11

EML 3004 C Step 2: Gather Information 2. Gather information. ü This process is made simpler in the internet age. ü Do not reinvent the wheel ü Search for old case histories (modifications) ü Look at the failure case histories on similar designs ü Discuss with other designers, analysts, fabricators, maintenance workers, architects ü Consult the latest technical journals ü Search for databases/trade magazines ü Make sure of all the relevant patents on the product At this stage, you should have a comprehensive specification of the design Namas Chandra Introduction to Mechanical engineering Chapter 4 -12

EML 3004 C Step 3: Choose a Design Strategy 3. Select a design strategy that satisfies objectives. 1. Design solution should satisfy all objectives and meet constraint 2. For complex systems break the overall goals into smaller goals 3. Take the design of an automobile as an example. Break into 1. 2. 3. 4. 5. 6. Engine Cooling Systems Suspension Brakes Chassis Drive Train Namas Chandra Introduction to Mechanical engineering Exploded view of a solar array panel showing component level design Chapter 4 -13

EML 3004 C Step 3: Choose a Design Strategy… 2 Breaking into components is useful since v v 1. Testing/Validation/redesign is easy Use of existing concepts/technology faster Newer technology can be easily ported Modular approach is useful from maintenance and training May be less innovative due to 1. 2. Individual modules may work but not in unison (Hubble) Overall goal of the project may not be fully met at component level Namas Chandra Introduction to Mechanical engineering Chapter 4 -14

EML 3004 C Step 4: Make a Preliminary Design Make a preliminary design of the product building up from component level. 1. This is the embodiment stage 2. You should have an engineering drawing at this stage 3. The drawing should have 1. 2. 3. 4. Approximate dimensions Materials lists, supplier number if off-shelf item Manufacturing/Assembly procedure Cost estimates 4. Check if the design is feasible based on all the objectives, constraints (irrevocable) and desires (flexible) Namas Chandra Introduction to Mechanical engineering Chapter 4 -15

EML 3004 C Step 5: Build, Document and Test This is the actual (preceded by virtual) building. 1. Build a computer model 2. Use simulation tools (AUTOCAD, Pro-E, Working model) to identify major glitches or bugs 3. Build a ‘physical model’ or ‘proto-type’ 4. Document all the details and design changes 5. Design change documents are very useful 1. 2. 3. For recording changes and reasons Safety related issues Potential litigations Namas Chandra Introduction to Mechanical engineering Chapter 4 -16

EML 3004 C Step 7: Thorough testing Once the final design is reached the product should be thoroughly tested. Assess the performance from various aspects 1. For physical entity: temperature, humidity, loading, prolonged use, initial run-in 2. Human response (ergonomics) 3. Beta testing for software products 4. Functionality after the testing process 5. Accelerated test methods for wear and tear Namas Chandra Introduction to Mechanical engineering Chapter 4 -17

EML 3004 C Design competition Using a design competition of a vehicle we can demonstrate the seven stages of design. ü ü ü ü Define the objectives Gather information Choose a strategy Make a preliminary design Build, document and test Revise, revise and revise again Testing Namas Chandra Introduction to Mechanical engineering Chapter 4 -18

EML 3004 C 4. 5 Peak-Performance Design Competition Objective ü Design a vehicle that can climb the ramp under its own power, stop at the top of the ramp, sustain its position against the opposing vehicle Constraints: v Vehicle should be autonomous; no remote control v Should fit within a 30 cm cube v Should start with a switch located on the vehicle v Vehicle’s power is fixed using • A single 9 Volt battery • Rubber bands (4 mm by 10 mm unstretched) • Mouse trap springs Namas Chandra Introduction to Mechanical engineering v Weight less than 2 Kg v No toxic substance v At the end of the run, vehicle should be at the top v No guide rails v There are six runs with 15 sec. Each. Winner moves next round. Chapter 4 -19

EML 3004 C 4. 5. 2: Define the Overall Objective Never start the design without careful planning. Though building directly may be more fun, the end result may not be. ü Design for speed v Need to reach the top before 15 seconds v Speed is not the main concern ü Design for defensive and offensive strategies v Need to defend the position at the top ü Design for easy modification v Since changes are permitted between runs allow for swaps ü Design for durability v Should sustain at least six runs v Rugged but flexible for modification ü Design for simplicity v Make the design simple for ease of repairs Namas Chandra Introduction to Mechanical engineering Chapter 4 -20

EML 3004 C 4. 5. 3: Choose a Design Strategy Break the problem into basic components. üEnergy source üPropulsion systems üStopping device üStarting device Namas Chandra Introduction to Mechanical engineering Chapter 4 -21

EML 3004 C 4. 5. 3: Some components Propulsion device. ü Rubber band gets energy by twisting or stretching. ü Here we see a mouse trap. When bale retracts, energy is released. Stopping mechanism. ü Should stop at the precise point and depends on the energy source chosen. ü Here is a design when the vehicle is horizontal the contact is made to stop. Contact switch to stop. ü Using a timer and by trial and error you can shut off the power if we have battery source. Namas Chandra Introduction to Mechanical engineering Chapter 4 -22

EML 3004 C 4. 5. 4: Preliminary design Front wheel drive ü Assume that the opposing vehicle is a box. ü Goal is to resist being pushed down. ü Bottom figure shows what happens if the power is supplied to the front wheels. Rear wheel drive ü Since energy and torque is applied at the rear wheels, tilting action is avoided. Namas Chandra Introduction to Mechanical engineering Chapter 4 -23

EML 3004 C 4. 5. 4: Preliminary design Rough diagram of the vehicle ü Should have design calculations. ü Parts list. ü Sketches of various parts. Preliminary design shows ü Wedge shaped design. ü Single drive shaft with motor, pulley and belt. ü Switch to turn off at the top. Mock ups and models ü Before building, you check the wedge shape concept using a wooden model ü Result is …. . . Namas Chandra Introduction to Mechanical engineering Chapter 4 -24

EML 3004 C 4. 5. 4: Design alternatives Initial design with small wheels ü Cannot negotiate the corner. Increase in wheel size leads to ü Clearing the ramp ü More unstable ü May have a problem in pushing the other vehicle Decrease in length ü Clears the corner and is more stable ü So length, wheel size can be altered suitably-matter of choice-design Namas Chandra Introduction to Mechanical engineering Chapter 4 -25

EML 3004 C 4. 5. 6: Revisions, revisions and more revisions Moving the real wheel forward ü Can negotiate the corner. But due to extra weight at the back or vehicle at the back ü Vehicle completely tilts ü Becomes very unstable ü May have a problem in pushing the other vehicle Namas Chandra Introduction to Mechanical engineering Chapter 4 -26

EML 3004 C 4. 5. 8: Reality Check and More revisions Many decisions are taken as we go along in the design process ü We observe different situations and keep modifying ü Always recall why we changed our design in the first place ü This is a process of learning Motor power determination ü ü ü Motors come as ac or dc Motors are available with different voltages Different ampere rating Drive shaft sizes Methods to cut off the power supply All under the constraint of time and funding Namas Chandra Introduction to Mechanical engineering Chapter 4 -27

EML 3004 C CHAPTER 4…. concludes What is Design? Namas Chandra Introduction to Mechanical engineering Chapter 4 -28