Orientation · Module 2 of 10
The Mechanical Design Process
Design is not a flash of genius; it is a repeatable process. It moves from a need to a product through requirements, concepts, and detail, looping back whenever the evidence says to.
Readiness check
This module is about how design works. Tick what you can do comfortably.
- Multiply a number by a factor.
- Take a weighted average of two scores.
- Tell a goal apart from a way of reaching it.
- Recall that testing can send you back a step.
- Read a simple flow diagram.
The core idea
Design moves from a need to a product through stages: develop requirements, generate concepts, choose one, work out the details, and produce it, iterating whenever tests demand. Requirements turn a vague need into measurable targets, and a factor of safety turns a load requirement into a design number.
design load = working load × factor of safetyweighted score = Σ(weight × score)weights sum to 1Good design follows a process, so the outcome does not depend on luck. It begins with a need, then requirements development, the crucial step of turning that need into specific, measurable targets: not "make it strong" but "carry 500 newtons with a safety factor of 2.5." Next comes conceptual design, deliberately generating several different concepts rather than committing to the first idea, because the best concept is rarely the first. Those concepts are compared, often with a weighted decision matrix that scores each against criteria whose weights add to one, so the choice is explicit and defensible rather than a matter of taste. The chosen concept then goes through detailed design, where dimensions, materials, and tolerances are fixed and a factor of safety converts each load requirement into a design load the part must actually withstand. Finally the design is produced, and throughout, testing and review feed back: a failed test sends you to an earlier stage. This loop, requirements, concepts, selection, detail, production, and iteration, is the backbone of every course that follows.
The skills, taught in order
Five skills describe how a design is actually done.
2.1 The stages of design
A design moves through recognizing a need, developing requirements, conceptual design, detailed design, and production. Naming the stage you are in keeps effort focused: you do not fix tolerances before you have chosen a concept.
2.2 Requirements development
Requirements turn a need into measurable, testable targets, with numbers and units. "Light" becomes "under 2 kilograms"; "safe" becomes "factor of safety at least 2.5." Requirements state what must be true, not how to achieve it.
2.3 Generating concepts
Conceptual design produces several distinct ways to meet the requirements. Generating options widely, before judging them, avoids anchoring on the first idea and usually surfaces a better one.
2.4 Choosing a concept
A weighted decision matrix scores each concept against criteria whose weights sum to one, giving a single comparable number per concept. It makes the choice explicit and reviewable.
| Concept | Cost (weight 0.6) | Performance (weight 0.4) | Score |
|---|---|---|---|
| A | 8 | 6 | 7.2 |
| B | 5 | 9 | 6.6 |
| C | 7 | 7 | 7.0 |
Each score is the weighted sum. Concept A wins here because cost carries the greater weight.
2.5 Detail, production, and iteration
Detailed design fixes dimensions, materials, and a factor of safety, turning requirements into a buildable part. Production makes it, and testing feeds back: a shortfall returns you to an earlier stage. Design is a loop, not a line.
Engineering connection: a mousetrap-powered car starts with the requirement to travel a set distance, spawns several drivetrain concepts, and is chosen and refined exactly through this process, as in the course text.
Worked example 1: a design load
A bracket must support a working load of 500 N. Company policy requires a factor of safety of 2.5. What load must the bracket be designed to withstand?
- ProblemFind the design load for the bracket in Figure 1.
- Given / findWorking load 500 N, factor of safety 2.5. Find the design load.
- AssumptionsThe factor of safety multiplies the expected working load.
- Modeldesign load = working load × factor of safety.
- EquationsFdesign = 500 × 2.5
- SolveFdesign = 1250 N.
- Check1250 N is 2.5 times the expected load, giving margin for overloads, wear, and unknowns.
- ConclusionThe bracket is sized for 1250 N even though it normally sees 500 N, which is what "safe" means as a number.
Worked example 2: choosing a concept
Three concepts are scored 0 to 10 on cost (weight 0.6) and performance (weight 0.4). A scores 8 and 6, B scores 5 and 9, C scores 7 and 7. Which concept wins?
- ProblemFind the winning concept in Figure 2.
- Given / findWeights 0.6 cost and 0.4 performance; scores A(8,6), B(5,9), C(7,7). Find each weighted score.
- AssumptionsWeights sum to 1 and scores are on one comparable scale.
- Modelweighted score = 0.6 × cost + 0.4 × performance.
- EquationsA = 0.6(8) + 0.4(6)B = 0.6(5) + 0.4(9), C = 0.6(7) + 0.4(7)
- SolveA = 7.2, B = 6.6, C = 7.0. Concept A wins.
- CheckB has the best performance but its low cost score, carrying more weight, pulls it below A.
- ConclusionThe matrix makes the choice explicit: A is best given these weights, and changing the weights could change the winner.
Misconceptions and diagnostics
| Mistake | Symptom | Diagnostic question | Correction |
|---|---|---|---|
| Treating design as linear | No plan for redoing a step | "What if the test fails?" | Design iterates; testing loops you back. |
| Detailing the first idea | Only one concept ever considered | "What are the alternatives?" | Generate several concepts before choosing. |
| Requirements as solutions | Specs that name a part, not a target | "Is this a goal or a way to reach it?" | State measurable targets, not solutions. |
| Skipping the factor of safety | Part sized only for the expected load | "What if it is overloaded?" | Multiply by a factor of safety for the design load. |
Practice ladder
A hook carries a working load of 800 N with a required factor of safety of 3. Find the design load.
Show answer
Design load = 800 × 3 = 2400 N.
Two concepts are scored on cost and performance, each weighted 0.5. A scores 6 and 9, B scores 8 and 6. Which wins?
Show answer
A = 0.5(6) + 0.5(9) = 7.5; B = 0.5(8) + 0.5(6) = 7.0. A wins.
A phone must survive a 1.5 m drop onto concrete. Write two measurable requirements for it.
Show answer
For example: survive 10 drops from 1.5 m onto concrete with no functional damage; screen glass withstands the resulting impact stress with a factor of safety of at least 1.5. Both are testable with numbers.
For a reusable water bottle, write the need, three measurable requirements, and two distinct concepts.
What good work looks like
Need: carry and dispense water on the go. Requirements: hold at least 0.75 L, weigh under 200 g empty, survive a 1 m drop full without leaking. Concepts: a single-wall aluminium bottle with a screw cap, and a double-wall insulated steel bottle with a flip spout. Requirements are measurable; concepts are genuinely different.
Working with AI, and proving it yourself
Use AI as a guide, not an oracle
Portfolio task
Run one small design through the process: a need, three requirements, two concepts, a weighted choice, and a factor-of-safety design load.
Retrieval and spaced review
Closed notes. Answer out loud, then reveal.
1. Name the design stages in order.
Need, requirements, conceptual design, detailed design, production, with iteration.
2. What makes a good requirement?
It is measurable and testable, a target with a number, not a solution.
3. Write the design load.
Design load = working load × factor of safety.
4. What does a decision matrix give?
One comparable weighted score per concept, making the choice explicit.
5. Why is design a loop?
Testing feeds back, sending you to fix an earlier stage.
Textbook mapping
This module follows Wickert and Lewis, An Introduction to Mechanical Engineering, 3rd edition. Use these references to read further.
| Topic in this module | Where to read more |
|---|---|
| The design process and requirements | Wickert and Lewis, Section 2.2, The Design Process |
| Conceptual design case study | Wickert and Lewis, Section 2.4, Mousetrap-Powered Vehicles |
| Manufacturing and production | Wickert and Lewis, Section 2.3, Manufacturing Processes |
Section numbers refer to Wickert and Lewis, 3rd edition. Any edition with the same chapter titles is equivalent for study.