---
title: "Role and types of models in design"
subject: "Machine Design II"
chapter: 1
key: "semester_7/machine_design_ii:models-role-engineering-design"
source: https://www.bctengineer.com/q/semester_7/machine_design_ii/models-role-engineering-design
site: BCT Engineering
---

# Role and types of models in design

Machine Design II · Chapter 1: Modeling and Simulation

- Asked in: year not given, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/machine_design_ii/models-role-engineering-design
- Whole chapter: https://www.bctengineer.com/md/semester_7/machine_design_ii/chapter-1

## Question

Explain the role of models in engineering design. Describe iconic, analog and symbolic models with one example of each, and explain how computer simulation with parameter variation helps a designer.

## Short answer

Models simplify reality to predict performance, reduce cost and risk. Iconic (clay casing) looks like the object; analog (electrical circuit for a spring-mass) shares equations; symbolic (σ=Mc/I) uses maths. Computer simulation with parameter variation shows sensitivity and quickly finds feasible designs.

## Answer

A **model** is a simplified representation of a real part, machine or process that keeps the features important to the design question and ignores the rest. Models let the designer predict performance, compare alternatives and find faults before any metal is cut.

#### Role of models in design
- Predict stress, deflection, temperature, life or cost before manufacture.
- Reduce cost and risk: a failed model is cheaper than a failed prototype.
- Allow comparison of many concepts quickly and support optimisation.
- Help communication between designer, manufacturer and customer.

#### Types of models
| Type | Idea | Example |
|---|---|---|
| Iconic | Looks like the real object (scaled or full size) | Clay or 3D-printed model of a casing; scale model of a bridge |
| Analog | A different physical system with the same governing equation | Electrical circuit used for a spring-mass-damper; electrical analog of heat flow |
| Symbolic (mathematical) | Equations or computer code | $\sigma = Mc/I$, $\delta = FL^3/3EI$, a finite element model |

#### Mathematical modelling
Steps: define the problem and variables, make assumptions, write governing equations (equilibrium, material law, geometry), solve, then **validate** against experiment or handbook data. Always state the assumptions, because the model is valid only inside them.

#### Computer simulation and parameter variation
The mathematical model is programmed and run many times while one parameter (diameter, load, material, speed) is varied at a time. The results give a **sensitivity** of the performance to each parameter, show which parameters are critical, and give a safe operating range. Example: varying wire diameter and number of coils of a spring on a spreadsheet shows how stiffness and stress change, so the designer chooses a feasible design quickly.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which description fits an analog model?
   A. A scaled clay or 3D-printed copy of the part
   B. A different physical system that obeys the same governing equation, such as an electrical circuit for a spring-mass-damper
   C. A set of equations such as sigma = Mc/I
   D. A full-size prototype tested to destruction
   Answer: B. An analog model uses another physical system with the same governing equation. A look-alike copy is iconic and equations are symbolic. (Key point: Analog model)

2. Why is a model used before any metal is cut?
   A. It removes the need for any assumptions
   B. It guarantees the final part will never fail
   C. It predicts performance and finds faults cheaply, since a failed model costs less than a failed prototype
   D. It replaces the customer's requirements
   Answer: C. Models let the designer predict stress, life or cost and compare alternatives early, reducing cost and risk. (Key point: Why models are used)

3. After the governing equations of a mathematical model are solved, what must be done and why?
   A. Nothing more, the solution is exact
   B. Increase every load by 10 percent
   C. Validate it against experiment or handbook data and state the assumptions, because the model is valid only inside them
   D. Replace the variables by their average values
   Answer: C. The steps end with validation, and the stated assumptions limit where the model can be trusted. (Key point: Validating a mathematical model)

4. What does running a simulation many times while varying one parameter at a time give the designer?
   A. The sensitivity of performance to each parameter, the critical parameters and a safe operating range
   B. A proof that the material never yields
   C. The exact manufacturing cost of the part
   D. A physical scale model of the part
   Answer: A. Parameter variation shows how performance changes with each parameter, which identifies critical ones and safe ranges. (Key point: Parameter variation)

## Related in Machine Design II

- [Risk, society and design standards](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Arisk-society-standards-regulation) — Machine Design II, chapter 4
- [FEM steps and geometric modelling](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Afem-steps-geometric-modelling) — Machine Design II, chapter 1

## Related in other subjects

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- [Levels of abstraction in computer simulation](https://www.bctengineer.com/question.md?key=semester_8%2Fsimulation_and_modeling%3Acomputer-system-abstraction-levels) — Simulation and Modeling, chapter 10