Mechanical Design

By Dr. Yangzhi Chen

School of Mechanical Engineering,

South China University of Technology

• References:1. Robert L.Mott. Machine Elements in Mechanical Design.

Third edition.1999 by Prentice-Hall,Inc.

2. Weng-jian Zhu, Huang-ping. Mechanical Design.2005 by High education press.(Chinese)

3. Liang-gui Pu,Ming-gang Ji.Mechanical Design.Seventh edition.2001 by High education press.(Chinese)

PART 1 Principles of Design• Objectives and content PART 1 of this book• As you complete chapter 1 of this book,you

will gain an understanding of design philosophies.

• You should have built on earlier-learned principles of strength of materials, materials science, and manufacturing processes ,from other courses including MATERIALS in MECHANICAL ENGINEERING, MECHANICS of MATERIALS, and MECHANICAL MANUFACTURING PROCESSES

1 The Nature of Mechanical Design

• What are features of machine?

There are three features of machine:

(1) Machine is composed of mechanical parts;

(2) There are relationships of assured relative motion

between different parts;

(3) it involves moving parts that transmit power and accomplish specific patterns of motion.

What is comprised of machine?

Normally machine is composed of three Devices:

Motor Drive Devices Working Devices

Appended: Control System, Accessorial devices

To design mechanical components and devices, you must be competent in the the design of individual elements that comprises the system.

• You must also be able to integrate several components and devices into a coordinated, robust system that meets your customer’s needs.

Mechanical products in different fields• Consumer products: Household appliances, lawn mowers, c

hain saws, power tools, garage door openers, air conditioning systems, and so on. Fig. 1-1,1-2

• Manufacturing systems: Material handling devices, conveyors, cranes, transfer devices, industrial robots, machine tools, automated assembly systems, special-purpose processing systems, forklift trucks,and packaging equipment.1-3,4,5

• Construction equipment: Tractors with front-end loaders or backhoes, mobile cranes, power shovels, earthmovers,graders, dump trucks, road pavers, concrete mixers, powered nailers and staplers, compressors .Fig. 1-5,6

• Agricultural equipments: Tractors, harvesters, rakes, hay balers, plows, disc harrows, cultivators, and conveyor.Fig1-6,,7,8

Mechanical products in different fields

• Transportation equipment:(a) Automobiles, trucks,and buses;(b)Aircraft;

• Ships:Winches to haul up the anchor,cargo-handling cranes,rotating radar antennas,rudder steering gear,drive gearing and drive shafts;

• Space systems:Satellite systems,the space shuttle,and launch systems;

1-2 The Mechanical design process• Product refers to any device or system that is designed and produced

for use by a customer. Such as production machinery, product performance-evaluation system;

• Many methods are used to determine what the customer wants,one popular method is called quality function deployment or QFD that seeks (1) to identify all of the features and performance factors what customer desire and (2) to assess the relative importance of these factors. The result of the QFD process is a detailed set of functions and design requirements for the product.

• it is important to consider how the design process fits with all of the functions that must happen to deliver a satisfactory product to the customer and to service the product throughout its life cycle.;

• The total of all such functions that affect the product is sometimes called the product realization process or PRP.

Some of the factors included in PRP

• Marketing functions to assess customer requirements;

• Research to determine the available technology that can reasonably be used in the product;

• Availability of materials and components that can be incorporated into the product;

• Product design and development;

• Performance testing;

• Documentation of the design;

• Vendor relationships and purchasing functions;

• Consideration of global sourcing of materials and global marketing;

• Work-force skills;

• Physical plant and facilities available;

• Capability of manufacturing systems;

• Production planning and control of production systems;

• Production support systems and personnel;

• Quality systems requirements;

• Operation and maintenance of the physical plant;

• Distribution systems to get products to the customer;

• Sales operations and time schedules;

• Cost targets and other competitive issues;

• Customer service requirements;

• Environmental concerns;

• Legal requirements;

• Availability of financial capital;

1-3 Skills needed in mechanical design

• Technical drawing and computer-aided design(CAD);• Properties of materials;• Manufacturing processes;• Statics, dynamics, and strength of materials;• Kinematics and mechanisms;

1-4 Functions and design requirements• The ultimate objective of mechanical design is to produce a

useful device that is safe, efficient, economical, and practical to manufacture.

• Statements of functions tell what the device is supposed to do; they are often somewhat general, but they should always employ action phrases such as to transmit, to support, or to lift;

• After the functions are defined, a set of design requirements is prepared;

• Show an example of statements of function and design requirements in P.9-10;

1-5 Criteria for evaluating machine design decisions

• Safety;• Performance: the degree to which the design meets or exceeds the design

objectives;• Reliability: a high probability that the device will reach or exceed its

design life;• Ease of manufacture;• Ease of service or replacement of components;• Ease of operation;• Low initial cost;• Low operating and maintenance costs;• Small size and low weight;• Low noise and vibration;smooth operation • Use of readily available materials and purchased components;• Prudent use of both uniquely designed parts and commercially available

components • Appearance that is attractive and appropriate to the application;

1-6 example of the integration of machine elements into a mechanical design

• Mechanical design is the process of designing and/or selecting mechanical components and putting them together to accomplish a desired function.

• Machine elements must be compatible,must fit well together, and must perform safely and efficiently.

1-7 Computational aids in this book

• Many of the design procedures require long, complex calculations , spreadsheets, mathematical analysis software, computer programs, or programmable calculators are often useful in performing the design analysis.

1-8 Design calculation • Identify the machine element being designed and the

nature of the design calculation;• Draw a sketch of the element,showing all features that

affect performance or stress analysis;• Show in a sketch the forces acting on the element (the

free-body diagram), and provide other drawings to clarify the actual physical situation;

• Identify the kind of analysis to be performed, such as stress due to bending, deflection of a beam,buckling of a column,and so on;

• List all given data and assumptions;• Write the formulas to be used in symbol form,and clearly

the values and units of the variables involved.• Solve each formula for the desired variable;• Insert data, check units, and perform computation;

• Judge the reasonable, change the design decisions and recompute. Perhaps a different geometry or material would be more appropriate;

• If the result is not reasonable,satisfactory result has been achieved,specify the final values for all important design parameters,using standard sizes,convenient dimensions,readily available materials,and so on;

• Fig.1-12 shows a sample design calculation.

1-9 preferred basic sizes, screw threads and standard shapes

• Preferred basic sizes;• American standard screw threads;

1-10 Unit systems

• Computations are performed by using either the U.S. Customary Unit System (inch-pound-second) or the International System (SI). Table 1-2 lists the typical units used in the study of machine design.

1-11 Distinction among weight,force,and mass