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2014-09-17

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BASICS

MECHANICAL ENGINEERING DEPARTMENT

PALESTINE POLYTECHNIC UNIVERSITY

DR. MOMEN SUGHAYYER

ME 351: Machine Design I 2014-09-17

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What is Engineering?

Engineering is the art of applying scientific and

mathematical principles, experience, judgment, and

common sense to make things that benefit people.

In other words, engineering is the process of

producing a technical product or system to meet a

specific human need.

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ME 351: Machine Design I - Dr. Momen Sughayyer

2014-09-17

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Who are Engineers?

Engineers are people who use their training in

mathematics, physics, and chemistry to understand

the physical world and develop creative solutions to

societies complex needs.

They are

designers, planners, managers,

analysts, researchers, consultants,

sales specialists, and more

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What is Design?

Design is an interplay between what we want to achieve and how we want to achieve it.

The designers (mechanical engineer, electrical engineer, etc) must do the following.

Know or understand their customers’ needs.

Define the problem they must solve to satisfy the needs.

Conceptualize the solution through synthesis.

Perform analysis to optimize the proposed solution (Adequacy assessment).

Check the resulting design solution to see if it meets the original customer needs.

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Mechanical Engineering Design

Mechanical engineering design involves all the disciplines of

mechanical engineering;

It involves fluid flow, heat transfer, friction, energy transport,

material selection, thermomechanical treatments, statistical

descriptions, and so on.

Mechanical design concentrates mostly on loading, stress

analysis, and material mechanical properties.

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Design process

The phases of the

design process

acknowledge the

many feedbacks and

iterations.

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Adequacy of Design

Design product should be

Functional: satisfy the intended need and customer expectation.

Safe: not hazardous to the user, bystanders, or surrounding

property with appropriate directions or warnings provided.

Reliable: perform its intended function satisfactorily or without

failure at a given age.

Competitive: product survival.

Usable: user friendly product.

Manufacturable: suited to mass production with a minimum

number of parts (minimum information).

Marketable: purchasable with repair available.

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Design Considerations

Functionality

Strength/stress

Distortion/deflection/stiffness.

Wear

Corrosion

Safety

Reliability

Manufacturability

Utility (electricity, gas. etc)

Cost

Friction

Weight

Life

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Noise

Styling

Shape

Size

Control

Thermal Properties

Surface

Lubrication

Marketability

Maintenance

Volume

Liability

Remanufacturing/resource recovery

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Design Tools and Resources

Computational Tools

CAD (Computer-aided design) software:

Aries, AutoCAD, CadKey, I-deas/Unigraphics, ProEngineer, etc.

CAE (Computer-aided engineering):

Finite element analysis/method (FEA or FEM):

Algor, ANSYS, MSC/NASTRAN, ABAQUS, etc.

Computational fluid dynamics:

CFD++, FIDAP, Fluent, etc.

Dynamic force and motion in mechanics:

ADAMS, DADS, Working Model, etc.

Acquiring Technical Information

Libraries, Government sources, Professional societies, commercial vendors, internet.

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Engineer’s Professional Responsibilities

The design engineer is required to satisfy the needs of customers (management, clients, consumers, etc.) and is expected to do so in a competent, responsible, ethical, and professional manner.

Success in engineering (achievements, promotions, raises, etc.) may in large part be due to competence but if you cannot communicate your ideas clearly and concisely, your technical proficiency may be compromised.

The design engineer’s professional obligations include conducting activities in an ethical manner.

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Codes and Standards

Standard: a set of specifications for parts, materials, or processes

intended to achieve uniformity, efficiency, and a specified quality.

Code: a set of specifications for the analysis, design, manufacture,

and construction of something.

All of the organizations and societies have established

specifications for standards and safety or design codes.

AA, AGMA, AISC, AISI, ANSI, ASM, ASME, ASTM, AWS, ABMA,

BSI, IFI, I. Mech. E., BIPM, ISO , NIST, SAE, JIS, DIN

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Economics

Standard sizes (Table A-17)

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Economics

Large Tolerances

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Economics

Breakeven points

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Economics

Cost estimates:

Cost per weight

Number of parts

Area

Volume

Horsepower

Torque

Capacity

Speed

Various performance ratios

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Safety and Product Liability

The strict liability concept of product liability generally

prevails in the United States (laws exist).

The manufacturer of an article is liable for any damage or

harm that results because of a defect. It does not matter

whether the manufacturer knew about the defect, or even

could have known about it.

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The Adequacy Assessment

An adequacy assessment consists of the cerebral, empirical,

and related mathematical modeling steps that the designer

takes to ensure that a given specification set is satisfactory

(suitable, feasible, and acceptable).

The adequacy assessment draws from the analysis portions of

prior course work.

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Uncertainty in Mechanical Design

Composition of material and the effect of variation on properties.

Variations in properties from place to place within a bar of stock.

Effect of processing locally, or nearby, on properties.

Effect of nearby assemblies such as weldments and shrink fits on stress

conditions.

Effect of thermomechanical treatment on properties.

Intensity and distribution of loading.

Validity of mathematical models used to represent reality.

Intensity of stress concentrations.

Influence of time on strength and geometry.

Effect of corrosion.

Effect of wear. 2014-09-17

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Uncertainty in Mechanical Design

Methods to address uncertainties:

(1) Deterministic

Design factor or safety factor

(2) Stochastic

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Load Allowable Maximum

Load) (Failure LoadFunction of Lossdn

Example

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Stress and Strength Notations

The designer must allow the maximum stress to be less than the strength by

a sufficient margin so that despite the uncertainties, failure is rare.

Strength is an inherent property a part, a property built into the part

because of the use of a particular material and process.

S: Strength

Ss: shear strength

Sy: yield strength

Su: ultimate strength

s: normal stress

t: shear stress

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Allowable Stress and Strength

The American Institute of Steel Construction has published the Manual of Steel Construction Allowable Stress Design (ASD).

The relationship between allowable stresses and specified minimum strengths:

Tension 0.45 Sy<sall<0.60 Sy

Shear tall=0.40 Sy

Bending 0.60 Sy<sall<0.75 Sy

Bearing sall=0.90 Sy

The minimum strength is that at least 99% of the population of values obtained from all standard material in size range meets. (ANSI-ASTM) ANSI-ASTM: American National Standard Institute- American Society for Testing and Materials.

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Loads and Forces

The loads or forces are:

F = SWd + SWl + SKFl + Fw + SFmisc

SWd: Sum of the dead loads

SWl: Sum of the stationary or static live loads

Fl: Forces that may cause impact or dynamic loading

K: Service factors in Table 1.2.

Fw: Wind load on the structure

Sfmisc: The effects of earthquakes, hurricanes, or other

extraordinary conditions

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Design Factor and Factor of Safety

The AISC method for relating stress and strength is also used in some

other specialized design areas. However, it is not general approach,

since it addresses only specific materials and loadings.

Three Categories of Design:

The product is made in large quantities justifying elaborate testing

of materials, components, and prototypes in the field.

The product is made in sufficient quantities to justify a modest

material test program, perhaps as small as ultimate tensile tests.

The product is made in such small quantities that no testing of

materials is performed at all.

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Deterministic Design Factor of Safety

The general approach to the allowable load-loss of function load problem is the deterministic design factor method.

Allowable load =loss of function load/nd

nd=S(loss of function)/s(allowable)=strength/stress

When the stresses are linearly proportional to the loads. For contact stress problems where stresses are not linearly proportional to loads, the form changes to

nd=(strength/stress)3 for spheres in contact

nd=(strength/stress)2 for cylinders in contact

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Reliability

The reliability method of design is one in which we obtain the

distributions of stresses and the distribution of strengths and

then relate these two in order to achieve an acceptable

success rate.

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Units and Preferred Units

Newton’s second law, F = ma

(1) U.S. Customary foot-pound-second system (fps) and inch-

pound-second system (ips)

In fps system, the unit of mass is kip = 1000 lbf or 1000 lb

The weight of 1 slug is W= mg = 1 slug ∙32.2 ft/s2=32.2 lbf

The unit of pressure and stress is lbf/in2 = psi (6890 Pa)

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Newton’s second law, F = ma

(2) The International System of Units (SI: Systeme Internaional

d’Unites) with the base units of kg, m, s. The force is

expressed as

The weight of 1 kg is W= mg = 1 kg ∙9.81 m/s2=9.81 N

The unit of pressure and stress is N/m2 = Pa

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192 423.618 50 : correct notation, but should be expressed

as 1.924 ∙105.

Use of prefixes G, M, k, m, micro (m), n, p

Prefixes should not be used in the denominators of derived unit

such as N/mm2 →MN/m2.

Double prefixes should not be used such as mmm → mm.

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Calculations and Significant Figures

Usually three or four significant figures are necessary for engineering accuracy.

Make all calculations to the greatest accuracy possible and reports the results within the accuracy of the given input.

To display 706 to four significant figures:

706.0, 7.060ⅹ102, 0.7060ⅹ103

To display 91600 to four significant figures: 91.60ⅹ103

When d=0.40 in

pd=3.1(0.40)=1.24in=1.2 in

pd=3.141592(0.40)=1.256in=1.3 in

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Power Transmission Case Study

Assume that a company wishes to provide off-the-shelf speed reducers in various capacities and speed ratios to sell to a wide variety of target applications. The marketing team has determined a need for one of these speed reducers to satisfy the following customer requirements.

Notice that the list of customer requirements includes some numerical specifics, but also includes some generalized requirements, e.g., low maintenance and competitive cost.

These general requirements give some guidance on what needs to be considered in the design process, but are difficult to achieve with any certainty. In order to pin down these nebulous requirements, it is best to further develop the customer requirements into a set of product specifications that are measurable.

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Design Requirements

Power to be delivered: 20 hp

Input speed: 1750 rev/min

Output speed: 85 rev/min

Targeted for uniformly loaded applications, such as conveyor belts, blowers, and generators

Output shaft and input shaft in-line

Base mounted with 4 bolts

Continuous operation

6-year life, with 8 hours/day, 5 days/wk

Low maintenance

Competitive cost

Nominal operating conditions of industrialized locations

Input and output shafts standard size for typical couplings

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Design Specifications

Power to be delivered: 20 hp

Power efficiency: >95%

Steady state input speed: 1750 rev/min

Maximum input speed: 2400 rev/min

Steady-state output speed: 82–88 rev/min

Usually low shock levels, occasional moderate shock

Input and output shaft diameter tolerance: ±0.001 in

Output shaft and input shaft in-line: concentricity ±0.005 in, alignment

±0.001 rad

Maximum allowable loads on input shaft: axial, 50 lbf; transverse, 100 lbf

Maximum allowable loads on output shaft: axial, 50 lbf; transverse, 500 lbf

Base mounted with 4 bolts

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Design Specifications

Mounting orientation only with base on bottom

100% duty cycle

Maintenance schedule: lubrication check every 2000 hours; change of

lubrication every 8000 hours of operation; gears and bearing life

>12,000 hours; infinite shaft life; gears, bearings, and shafts replaceable

Access to check, drain, and refill lubrication without disassembly or opening

of gasketed joints.

Manufacturing cost per unit: <$300

Production: 10,000 units per year

Operating temperature range: −10◦ to 120◦F

Sealed against water and dust from typical weather

Noise: <85 dB from 1 meter

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