Introduction to Engineering Design - Concordia Universityusers.encs.concordia.ca/~nrskumar/Index_files... · the shaft will be held in position axially and how power transmission

Concordia University

Department of Mechanical, Industrial and

Aerospace Engineering

MECH 390 – Mechanical

Engineering Design Project

Lecture 02

Introduction to Engineering Design

Design Process

1. Recognizing the need

2. Defining the problem

3. Planning the project

4. Gathering information

5. Conceptualizing

alternative approaches

6. Evaluating the alternatives

7. Selecting the preferred alternative

8. Communicating the design

9. Implementing the preferred design

Step 2: Definition of the Problem

The recognizing need and problem definition are “critical steps”

in the design.

Failure to define the need statement & goal statement

will almost certainly lead to a design failure.

Tacoma Narrow

Bridge Collapse

The fact that it

was narrow,

caused the bridge

to collapse

Case Study 1: Formulation of Need/Goal

A customer asks you to design a new surveillance system that has to

be deployed at 5-10m high when the vehicle is on the move.

Describe a plausible "need" that might have triggered that request.

Then identify two fundamentally different goals for designs that can

satisfy that need

Case Study 2: Formulation of Need/Goal

A customer asks you to design car tires that have a better grip on the

road. Describe a plausible "need" that might have triggered that

request.

Then identify two fundamentally different goals for designs that can

satisfy that need.

Case Study 3: Defining objectives

A customer asks you to design a screen door from house to backyard.

Describe reasonable set of objectives and constraints for this project

Case Study 3: Defining objectives

Objectives

Light weight (lbs)

Secure latch (lb. required to break)

Easily replaceable (minimum effort)

Cost ($)

Reasonable opening force (lb)

Easy removal for winter storage (min effort)

Constraints

Made from rust resistant materials

To fit standard frame sizes

• Design system to transmit power between parallel shafts

• Three possibilities (from many possibilities)

• Belt, Chain or Gear.

• How do you choose.

• List advantages of each

• Belt

1. Electrical insulation

2. Less noisy

3. Used for long center distance

4. High speeds

5. No lub

6. CD variation better than gear, chain

7. Misalingment – no problem

Developing Design Criteria for Power Transmission Between

Parallel Shafts






• Chain

1. CD variation better than gear

2. Easy to install

3. No tension on slack side – less bearing load

4. No slip or creek unlike belts

5. Compact (smaller sprockets, narrow chain)

6. No static charge

7. No deterioration under oil, grease, or age

8. Higher operating °C than belts


Parallel Shafts






• Gears

1. CD samllest of 3, so compact

2. High speed capability

3. Large speed ratios (better than chain, belt)

4. Higher power transfer at higher speeds

5. No deterioration under oil, grease, or age

6. Do not develop static charge


Parallel Shafts

• From the individual advantage, we can develop Consolidated

List of Criteria, for the drive

1. Shock Protection (b1, c6, g6)

2. Noise (b2)

3. Large separation distance (b3)

4. Speed capability (b4, g2)

5. Lubrication requirement (b5)

6. Misalignment (b6)

7. Flexible CD (c1)

8. Installation/replacement ease (c2)

9. Bearing loads (c3)

10.Slippage/creep (c4)

11.Size (cf, g1)

12.Life expectancy (c7, g5)

13.Operating temperature (c8)

14.Speed flexibility (g3)

15.High torque capability (g4)


Parallel Shafts

Criteria Tree for Power Transmission Between Parallel Shafts

Power

Transmission

Between

Parallel Shafts

Geometry

Health &

Safety

Maintenance

Shaft

Configuration

Noise

(b2)

Shock

Protection

(b1,c6,g6)

Size

(c5,g1)

Install and

Replace Easily

(c2)

Lubrication

Requirement

(b5)

Life

Expectancy

(c7,g5)

Separation

Distance Flexible

(c1)

Misalignment

(b6)

Large Separation

Distance

(b3)

Operating

Conditions

Operating

Speed

Operating

Temperature

(c8)

Speed

Flexibility

(g3)

High Speed

Capability

(b4, g2)

Power/Load

Ratings

Slippage/

Creep

(c4)

Bearing

Loads

(c3)

High Torque

Capability

(g4)

Level 1 Level 2 Level 3 Level 4

Design Criteria for an Automobile Horn*

1. Ease of achieving 105-125 DbA

2. Ease of achieving 2000-5000 Hz

3. Resistance to corrosion, erosion, and water

4. Resistance to vibration, shock, and acceleration

5. Resistance to temperature

6. Response time

7. Complexity: number of stages

8. Power consumption

9. Ease of maintenance

10.Weight

11.Size

12.Number of parts

13.Life in service

14.Manufacturing cost

15.Ease of installation

16.Shelf life

Design Criteria for an Automobile Horn*

service

life

maintenance

shelf

life

installation

power req'ts

DbA

response

time

Hz

performance

durability

temperature

corrosion

shock

manufacturing

cost

# of parts

complexity

size

weight

marketing

automobile

horn

Functional Analysis

systeminputs outputs

system

energy inputs energy outputs

information inputs information outputs

material outputsmaterial inputs

Fig. 2.9

Fig. 2.8

Simplest functional analysis diagram to formulate a problem

Slightly modifying it, we have multiple inputs and multiple outputs

That is dividing the inputs and output into categories

Functional Analysis

generate

electricity

non-electric

energy

electricity

conversion

losses

(a) conventional

power plant

generate

electricity and

useful thermal

energy

non-electric

energy

electricity

conversion

losses

thermal

energy

(b) cogeneration

power plantFig. 2.10

Designing an electric power generation plant

General level

Convert non electric energy to electric energy

Not considering how the electricity is generated

Now if we give some consideration how electricity is generated, we may also have thermal energy as output

Functional Analysis

4. condense

steam

1. generate

steam

non-electric

energy

3. drive

generator

electricity

conversion

losses

2. spin

turbine

system

boundary

Fig. 2.11

We can now divide the primary function of generating electricity into subfunctions

Generate steam, rotate turbine, drive generator

While spin turbine is complete is the excess steam is condensed and feedback into step 1

If we go deeper into this, we can still divide the sub function 1 as 1.1, 1.2 etc

Where it will be pulverize coal, deliver coal to boiler, burn coal etc…

Functional Analysis

Functional analysis diagram for 3 speed desk fan

The fan has the ability to oscillate

Identify input output

Describle key function that transforms input to output

Problem Formulation Terminology

• Need

• Goal

• Objectives

• Constraints

• Criteria

• Attributes

• Characteristics

• Functions

• Specifications

• Performance Specifications

• Design Specifications

• Customer Requirements

• Engineering Requirements

• Design Parameters

• Performance Parameters

Design Process - Step 3: Project Planning

Design Process - Step 3: Project Planning

Design Process - Step 4: Information Gathering

Step 4: Information Gathering

A good engineer is a good researcher


• Customer:

The person/group that has commissioned the work or design.

• Stakeholders:

Other parties that are affected or impacted by the work or

design.

• Experts/Professionals:

Persons with specific expertise/experience related to work or

design.


• Never underestimate the amount of information available on

any subject

• It is always better to base your design on existing information

rather than relying exclusively on your own ideas.


Type if Information

• Technical

• Stimulation

• Economic

• Acquisition


• Professional Association Magazines and Articles:

o ASME ( http://www.asme.org/kb/news-articles )

o SAE ( Society of Automotive Engineers,

http://www.sae.org/pubs/ )

o IEEE ( https://www.ieee.org/publications_standards/index.html )

o SPIE ( http://spiedigitallibrary.org )

• Technology Magazines and Articles:

o IEEE Spectrum ( http://spectrum.ieee.org )

o New Scientist ( http://www.newscientist.com/section/tech )

o MIT Technology Review ( http://www.technologyreview.com )

o Popular Mechanics ( http://www.popularmechanics.com )

• Codes & Standards:

o Federal and State Standards

o Engineering Standards (ISO, AGMA, ASTM, SAE)

o Company Standards


• Textbooks:

o Library

o Local Community Library

• Journal or Conference Papers:

o Compendex ( http://www.engineeringvillage.com )

o PubMed ( http://www.ncbi.nlm.nih.gov/pubmed )

o Google Scholar ( http://scholar.google.com )

• Patents:

o ( http://www.cipo.ic.gc.ca )

o ( http://www.uspto.gov )

• Internet:

o News-based websites:

o Professional/Association Community and Forum websites:

o Search Engine Sites (Google, Yahoo, Bing, etc...)


• Stages of Information Acquisition

o Identify the kind of information required.

o Physically or electronically gather the information.

o Determine how reliable and credible the information is.

o Decide when to stop looking.


• Referencing

o Reference all your materials/information obtained in your

reports.

o http://www.ieee.org/documents/ieeecitationref.pdf

o Many other standards also exist. An overview of all reference

standards can be found at: http://en.wikipedia.org/wiki/Citation


Statistical Data

• Federal and state government can collect, analyze, and disseminate lots of statistical data that no other entity can.

• Gateway to statistics from over 100 federal agencies (www.fedstats.gov)

o Economic Census

o Manufacturing Energy Consumption Survey

• Similar role played by trade associations for specific industries.

• State and local governments

o National Conference of State Legislators (NCSL)

o National Governors Association (NGA).

• International Organizations

o Index to International Statistics.


U.S. Energy Flow 2000

Shaft

• A shaft is the component of a mechanical device that

transmits rotational motion and power.

© 2017 Ayhan Ince. All rights reserved. 34

Shaft Design• Material Selection

• Geometric Layout

• Stress and strength

– Static strength

– Fatigue strength

• Deflection and rigidity

– Bending deflection

– Torsional deflection

– Slope at bearings and shaft-supported elements

– Shear deflection due to transverse loading of short shafts

• Vibration due to natural frequency

Shaft Design

• Shaft Materials

Deflection primarily controlled by geometry, not material

Stress controlled by geometry, not material

Strength controlled by material property

• Shaft Materials

Shafts are commonly made from low carbon, CD or HR steel, such

as ANSI 1020–1050 steels.

Fatigue properties don’t usually benefit much from high alloy

content and heat treatment.

Surface hardening usually only used when the shaft is being used

as a bearing surface.

Shaft Design

• Cold drawn steel typical for d < 3 in.

• HR steel common for larger sizes. Should be machined

all over.

• Low production quantities

– Lathe machining is typical

– Minimum material removal may be design goal

• High production quantities

– Forming or casting is common

– Minimum material may be design goal

Shaft Design

• Issues to consider for shaft layout

– Axial layout of components

– Supporting axial loads

– Providing for torque transmission

– Assembly and Disassembly

Axial Layout of Components

Supporting Axial Loads Axial loads must be supported through a bearing to the frame.

Generally best for only one bearing to carry axial load to shoulder

Allows greater tolerances and prevents binding

Providing for Torque Transmission

• Common means of transferring torque to shaft

– Keys

– Splines

– Setscrews

– Pins

– Press or shrink fits

– Tapered fits

• Keys are one of the most effective

– Slip fit of component onto shaft for easy assembly

– Positive angular orientation of component

– Can design key to be weakest link to fail in case of overload

Assembly and Disassembly

Shaft Design for Stress

• Stresses are only evaluated at critical locations

• Critical locations are usually

– On the outer surface

– Where the bending moment is large

– Where the torque is present

– Where stress concentrations exist

Shaft Design Procedure

1. Determine the rotational speed of the shaft.

2. Determine the power or the torque to be transmitted by the shaft.

3. Determine the design of the power-transmitting components or other devices that will be

mounted on the shaft, and specify the required location of each device.

4. Specify the location of bearings to support the shaft. Normally two and only two bearings

are used to support a shaft. The reactions on bearings supporting radial loads are

assumed to act at the midpoint of the bearings.

5. Propose the general form of the geometry for the shaft, considering how each element on

the shaft will be held in position axially and how power transmission from each element to

the shaft is to take place.

6. Determine the magnitude of torque that the shaft sees at all points.

7. Determine the forces that are exerted on the shaft, both radially and axially.

8. Resolve the radial forces into components in perpendicular directions, usually vertically

and horizontally.

9. Solve for the reactions on all support bearings in each plane.

10. Produce the complete shearing force and bending and torque moment diagrams to

determine the distribution of torque bending moments in the shaft.


11. Select the material from which the shaft will be made, and specify its condition: cold-

drawn, heat-treated, and so on. As indicated in Table 2–9, suggested steel materials for

shafts are plain carbon or alloy steels

12. Determine an appropriate design stress, considering the manner of loading (smooth,

shock, repeated and reversed, or other)..

13. Analyze each critical point of the shaft to determine the minimum acceptable diameter of

the shaft at that point in order to ensure safety under the loading at that point. In general,

the critical points are several and include those where a change of diameter takes place,

where the higher values of torque and bending moment occur, and where stress

concentrations occur.

14. Specify the final dimensions, surface finishes, tolerances, geometric dimensioning details,

fillet radii, shoulder heights, keyseat dimensions, retaining ring groove geometry, and other

details for each part of the shaft, ensuring that the minimum diameter dimensions from

Step 13 are satisfied.

Because of simultaneous occurrence of torsional shear stresses and normal

stresses due to bending, stress analysis of shaft involves use of combined stress

approach.



• Typically the torque comes into the shaft at one gear and leaves the shaft at another gear.

A free body diagram of the shaft will allow the torque at any section to be determined. The

torque is often relatively constant at steady state operation. The shear stress due to the

torsion will be greatest on outer surfaces.

• The bending moments on a shaft can be determined by shear and bending moment

diagrams. Since most shaft problems incorporate gears or pulleys that introduce forces in

two planes, the shear and bending moment diagrams will generally be needed in two

planes. Resultant moments are obtained by summing moments as vectors at points of

interest along the shaft.

• Axial stresses on shafts due to the axial components transmitted through helical gears or

tapered roller bearings will almost always be negligibly small compared to the bending

moment stress. They are often also constant, so they contribute little to fatigue.

Consequently, it is usually acceptable to neglect the axial stresses induced by the gears

and bearings when bending is present in a shaft

Forces Exerted on Shafts by Machine Elements


Spur Gears

Torque:

T = 63 000 (P)/n

Tangential Force:

Wt = T/(D/2)

Radial Forces:

Wr = Wt tan ϕ

where P = power being transmitted in

hp

n = rotational speed in rpm

T = torque on the gear in lb.in

D = pitch diameter of the gear in inches


Spur Gears


Helical Gears

Torque: T = 63 000(P) / n [lb . In]

Tangential force:

Wt = (33 000)(P) / vt [lbf]

Axial force:

Wx = Wt tan ψ

Radial force:

Wr = Wt tan Φt


Chain Sprockets

Force in Chain

Fc = T/(D/2)

Fcx = Fc cos θ

Fcy = Fc sin θ


V-Belt SheavesNet Driving Force

FN = F1 - F2

Net Driving Force

FN = T/(D/2)

Bending Force on the shaft carrying the

sheave is dependent on the sum,

FB= F1 + F2

For V-belt drives, the ratio is normally

taken to be

F1/F2 = 5

FB = C.FN

Bending Force on Shaft for V-Belt

Drive

FB = 1.5 FN = 1.5T/(D/2)

Bending Force on Shaft for Flat-Belt

Drive

FB = 2.0 FN = 2.0T/(D/2)

Stress Concentrations in Shafts

• Shafts typically contain:

Several diameters

Keyseats

Ring grooves

Other geometric discontinuities

• Stress concentration factors typically based on

diameter which is the objective of the design


• Stress analysis for shafts is highly dependent on stress

concentrations.

• Stress concentrations depend on size specifications, which

are not known the first time through a design process.

• Standard shaft elements such as shoulders and keys have

standard proportions, making it possible to estimate stress

concentrations factors before determining actual sizes.


Keyseats

Kt = 2.0 (profile)

Kt = 1.6 (sled runner)


Fillets

Kt = 2.5 (sharp fillet)

Kt = 1.5 (well-rounded fillet)


Forces Exerted on Shafts

Forces and Moments Exerted on Shafts

Resulting Bending Moments in Shafts

Torque Diagram in Shafts

Torque Diagram in Shafts

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Introduction to Engineering Design - Concordia Universityusers.encs.concordia.ca/~nrskumar/Index_files... · the shaft will be held in position axially and how power transmission