ME321 - Kinematics and Dynamics of Machines Design Process Notes

Steve Lambert Mechanical Engineering, U of Waterloo 4/29/2020

Design Definitions

“Engineering design is an iterative, decision-making activity, whereby scientific and technological information is used to produce a system, device or process, which is different in some degree from what the designer knows to have been done before, and is meant to meet human needs.” [Middendorf, 1986] 4/29/2020

Design Definitions

“… the selection of materials and geometry which satisfies specified and implied functional requirements while remaining within the confines of inherently unavoidable limitations.” [Johnson, 1980] An

adequate

design “… represents a synthesis which merely satisfies the functional requirements within the confines of the existing limitations”, while an

optimum

design “… is the best possible [design] from the standpoint of the most significant effect.” 4/29/2020

Design Objectives

 Evolving the product in the mind rather than in the marketplace,   Developing insights during the design process from an in-depth understanding of the need rather than customer dissatisfaction and feedback, and Practicing “break-through” (as opposed to evolutionary) design by identifying and breaking development barriers.

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Design Process

 Preliminary Design – Product planning and clarifying the task (needs analysis),    Conceptual design, Embodiment (layout) design, and Detail (systems) design.

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Preliminary Design

     Identification of the real need, Generation of the function structure, Identification of criteria and constraints, Order of magnitude calculations, and Development of design requirements (specifications).

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Need Statement

 Expressed in terms of the most significant criteria along with the most important constraint  Avoid the use of a particular configuration in describing the need  Examples:  Brake system  transform kinetic energy at maximum  rate (or over shortest distance) Canal  move ships across land mass.

  Think conceptually rather than in terms of configurations This fosters divergent thinking and innovation.

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Function Structure

 Represents a statement of the primary functions, secondary functions and functional alternatives that our design must perform to satisfy the need.

 The primary functions should be independent to facilitate breaking the design team into coherent, independent groups.

 They should be solution-independent to facilitate innovation.

 Often, they will deal with different ‘transmissions’.

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Function Structure Example

 Electrical fuse to mount on a circuit board.

 Need:  prevent the flow of excessive current while maintaining the integrity of the surroundings.

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Initial Function Structure:

1. Allow flow of current under normal operating conditions 2. Create a discontinuity in the current flow 2.1.

2.2.

Under short-circuit conditions Under overload conditions 3. Maintain integrity of the surroundings.

3.1.

Prevent escape of hot gases 3.2.

3.3.

Reduce heat transfer rate Prevent explosion 4/29/2020

Modified Function Structure:

1. Allow/disallow flow of current 1.1 Allow flow of current under normal conditions 1.2 Create a discontinuity in the current flow path 1.2.1 Under short circuit conditions 1.2.2 Under overload conditions 2. Maintain integrity of the surroundings 2.1 Prevent escape of hot gases 2.2 Reduce heat transfer rate 2.3 Prevent explosion 4/29/2020

Criteria and Constraints



Design Criteria

are what you will use to choose the

best

design  performance standards to be met by the design 

Design Constraints

must be met by all possible designs  limitations placed on the designer, the final design or the manufacturing process 4/29/2020

Constraints

 Since each constraint may eliminate possible solutions, it is important to identify only real constraints to foster innovation.  It is also necessary to quantify these constraints.  Order of magnitude calculations can be used to help with this.

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Sources of Constraints

  Interface Size  Time (on your design process as well as on your design)  Safety  Market  Financial  Regulations 4/29/2020

Order of Magnitude Calculations

 Used to further refine your design problem  to help identify and quantify important parameters and constraints  Example order of magnitude calculations include energy and/or mass balance calculations.

 Use only as much detail as is required to provide insight into the problem.  Often used to verify the feasibility of a design  Identify critical parameters and hence focus your design effort on the most important aspects of a design 4/29/2020

Design Requirements

 Summarize your results of the needs analysis in the form of a set of design requirements or specifications list  should include:  a clear statement of the design problem  the function structure, and   the design criteria and constraints Design constraints should be presented in a quantitative and organized manner. 4/29/2020

Design Requirements

 Example constraint specification (not complete) Type Size Weight Cost Design Requirement Must be less than Minimize Reduce manufacturing cost Magnitude 1.2 m <2 kg <$5.00

Source Packaging limits Shipping limits Market analysis 4/29/2020

Design Requirements

 The design requirements (specifications) must be  Complete, i.e., one should be able to complete the design based only on your design requirements (specifications) list, and it should eliminate the need for assumptions by the team members.

 Be solution independent to allow maximum design freedom  Be consistent, i.e., no conflicts.

 Be entirely quantifiable.

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