Ch 2 Intro to Mech Design Problems and Process

8/13/2019 Ch 2 Intro to Mech Design Problems and Process

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Mechanical Design Problems & Process


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OutlineDefinition of DesignNeed for Mechanical design

Product function, behavior and performanceTypes of Mechanical Design ProblemsLanguages of Mechanical DesignSummary


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Design definition

Set of decision making processes and activitiesto determine the form of an object,

given the customers desired function .


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Need for Mechanical DesignBefore 20 th century discipline of mechanical design requiredknowledge of only mechanical parts and assembliesPresently, now most devices are mechatronic systems have

mechanical, electronic and software. E.g, Cameras, office copiers,cars and just about everythingNo matter, how electronic or computer-centric devices become, nearlyall products require mechanical functions and a mechanical interfacewith humans

Additionally, all products require mechanical machinery formanufacture and assembly and mechanical components for housingThus, no matter how smart products become, there will always bethe need for mechanical design


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Decomposition of a system A system is generally considered a grouping of objects that perform aspecific functionDuring the design process: the function of the system and its

decomposition are considered first After the function has been decomposed into the finest subsystemspossible, assemblies and components are developed to provide thesefunctionsFor mechanical devices, the general decomposition is: system subsystem assembly componentDuring the early phases of the design process, when developingsystems is the focus of the effort, it is often unclear whether theactual function will be met by mechanical assemblies, electricalcircuits, software programs or a mix of these elements


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A Kodak Zoom camera

Example of a Mechatronic system Function: Record images Can be decomposed into: photographic system, exposure system and the shutter

system (system view)

Many different physical forms can accomplish the function of each system variety ofcameras


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The shutter assembly

Subsystem whose function is to control light coming through thelens

Part of the exposure system that includes the light meter, thecontroller and the auto focus system

Assembly of components in terms of the physical components orform of the camera (Assembly view)


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Decomposition of design disciplines

Steps in the design process:1. The function of the system and its decomposition are considered first2. After the function has been decomposed into the finest subsystems

possible, assemblies and components are developed to provide thesefunctions

Feature refers to specific attributes of systems, assemblies orcomponents that are important, such as dimensions, material properties,shapes or functional details


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Product function, behavior and performance

In mechanical Engineering, the terms function, operation and purpose are used to describe what a device doesMechanical devices are classified by their function. E.g., Screw driver

whose function is to insert or remove a screwIn telling the function of the screw driver, no information is given onhow this function is achieved for this we need some information onthe form of the deviceForm relates to any aspect of physical shape, geometry, construction,material or sizeBehavior and performance are also related to the functionPerformance is the measure of function and behavior how well thedevice does what it is designed to do Eg., of a cycle handlebar

support 50 kg is a measureable desired performance (specification)


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Function and behavior subtle difference

Behavior can be simulated or measured, whereas function is only a desire


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Types of Mechanical Design Problems

For example, we might be designing a new type of consumer product that will accept awhole raw egg, break it, fry it, and deliver it on a plate.Since this is a new product, there will be a lot of original design work to be done.

As the design process proceeds, we will configure the various parts.

To determine the thickness of the frying surface we will analyze the heat conduction ofthe frying component, which is parametric design .

And we will select a heating element and various fasteners to hold the componentstogether.Further, if we are clever, we may be able to redesign an existing product to meet someor all of the requirements.

It is rare to find a problem that is purely one type. So the following are the differenttypes of design problems:

1. Selection design2. Configuration design3. Parametric design4. Original design5. Redesign


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Selection Design selecting a bearing to support a shaft

Involves choosing one item or more from a list of similar items choosing anitem form a catalog; E.g. of choosing a cutting tool from the supplier catalog

To solve a selection problem, we must start with a clear need The catalog then effectively generates potential solutions for the problem

The potential solutions are evaluated with respect to specific requirements tomake the right choice


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Table 02_03

f l h l l f d k


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Configuration Design a slightly more complex type of design packagingdesign

All the components have been designed and the problem is how to assemble them into thecompleted product playing with an Erector set or construction toy or arranging living-roomfurniture

Configuration design answers the questions How do we fit all the assemblies in anenvelop? Or Where do we put what?

Randomly select one component from the list and position it so that all the constraints on

that assembly are met; then second component is selected and place

if a conflict arises,we back up and try again


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Parametric Design

Finding values for the features that characterize the objectbeing studied Example of designing a cylindrical storage tank that must

hold 4 m 3 of liquid. This tank is described by the parameters r, its radius, and

l, its length and its volume is determined by


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Original Design

Any time the design problem requires the development ofa process, assembly, or component not previously inexistence it calls for an original design.

It can be said that if we have never seen a wheel and wedesign one, then we have an original design

Though most selection, configuration, and parametricproblems are represented by equations, rules, or someother logical scheme, original design problems usuallycannot be reduced to any algorithm.

Each one represents something new and unique.


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Redesign

Most design problems solved in industry are for theredesign of an existing product Modification of an existing product to meet new

requirements. Example of hydraulic cylinder from 0.25m to 0.3m long Many redesign problems are routine and the design

domain is so well understood that the method used can beput in a handbook as a series of formulas or rules.


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1890 Humber bicycle


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Recumbent bike (FantasE)


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The Martin Mount Vision mountain bike


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Mature design and its evolution

Mature design - remained virtually unchanged over many years.There are many examples of mature designs in our everyday lives: pencil sharpeners, holepunches, and staplers are a few found on the average desk.For these products, knowledge about the design problem is high. There is little more to learn.

Why did a mature design like a bicycle begin evolving again?1. Customers are always looking for improved performance. Mountain bicycles of the style shown

in Figure are better able to handle rough terrain than traditional bikes.2. There is improved understanding of human comfort, ergonomics, and suspensions.3. Customers are always looking for something new and exciting even if performance is not

greatly improved.4. Materials and components have improved.

So, even mature designs change to meet new needs, to attract new customers, or to takeadvantage of new materials.Part of the design of a new bicycle like the Recumbent bike (FantasE) is routine , and part isoriginal Many subproblems were parametric problems, selection problems, and configuration problems.Thus, the redesign of a product, even a mature one, may require a wide range of designactivit .


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Variant Design

Sometimes companies will produce a large number ofvariants of their products.

A variant is a customized product designed to meet theneeds of the customer.

For example, when you order a new computer fromcompanies such as Dell, you can specify one of threegraphics cards, two battery configurations, threecommunication options, and two levels of memory.

Any combination of these is a variant that is specifically

tuned to your needs.


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Conceptual Design and Product Design

Two other terms that will be used throughout this courseare conceptual design and product design.

These are general terms for two parts of the productdevelopment process.

First, you must develop a concept and then refine theconcept into a product.

The activities during the conceptual and productdevelopment phases may make use of original,parametric, configuration, selection design and redesign

as needed.


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Languages of Mechanical Design

Many languages or representations can be used to describe amechanical objectDifference between detailed drawing of a component and the actual

hardware (component) both drawing and the hardware representthe same object however, they each represent it in a differentlanguageExample of a bolt

1. Word bolt is a textual (semantic or word) description of the

component2. Can also be represented through equations that describe its

functionality and possibly its form3. Ability of a bolt to carry shear stress (function) is described by the

equation = F / A


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Different representations (languages) todescribe the bolt

1. Semantic : verbal or textual representation of the object bolt2. Graphical : drawing of the object orthogonal drawings, sketches

or artistic renderings3. Analytical : Equations, rules or procedures representing the form or

function of the object - = F / A4. Physical : the hardware or a physical model of the object

In most mechanical design problems, initial need is expressed in asemantic language written specification or a verbal request by acustomer or supervisorFinal result of the design process is a physical productIn between, the Designer produces a graphical representation of theproduct

All the languages will be used as the product is refined from its

initial, abstract semantic representation to its final form


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Abstract sketch and final drawing of a component

Table 02 01


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Table 02_02


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_


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Summary A product can be divided into functionally oriented operating systems. These aremade-up of mechanical assemblies, electronic circuits, and computer programs.The important form and function aspects of mechanical devices are calledfeatures.

Function and behavior tell what a device does; form describes how it isaccomplished.Mechanical design moves from function to form.One component may play a role in many functions, and a single function mayrequire many different components.

There are many different types of mechanical design problems: selection,configuration, parametric, original, redesign, routine, and mature.Mechanical objects can be described semantically, graphically, analytically, orphysically.Mechanical design is the refinement from abstract representations to a finalphysical artifact.


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Ch 2 Intro to Mech Design Problems and Process

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