Introduction to Engineering Design - Concordia...
Transcript of Introduction to Engineering Design - Concordia...
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
• 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
• 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
Developing Design Criteria for Power Transmission Between
Parallel Shafts
• 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
• 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
Developing Design Criteria for Power Transmission Between
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)
Developing Design Criteria for Power Transmission Between
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
Step 4: Information Gathering
• 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.
Step 4: Information Gathering
• 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.
Step 4: Information Gathering
Type if Information
• Technical
• Stimulation
• Economic
• Acquisition
Step 4: Information Gathering
• 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
Step 4: Information Gathering
• 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...)
Step 4: Information Gathering
• 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.
Step 4: Information Gathering
• 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
Step 4: Information Gathering
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.
Step 4: Information Gathering
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.
Shaft Design Procedure
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.
Shaft Design Procedure
Shaft Design Procedure
• 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
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
Forces Exerted on Shafts by Machine Elements
Spur Gears
Forces Exerted on Shafts by Machine Elements
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
Forces Exerted on Shafts by Machine Elements
Chain Sprockets
Force in Chain
Fc = T/(D/2)
Fcx = Fc cos θ
Fcy = Fc sin θ
Forces Exerted on Shafts by Machine Elements
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 Concentrations in Shafts
• 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.
Stress Concentrations in Shafts
Keyseats
Kt = 2.0 (profile)
Kt = 1.6 (sled runner)
Stress Concentrations in Shafts
Fillets
Kt = 2.5 (sharp fillet)
Kt = 1.5 (well-rounded fillet)
Stress Concentrations in Shafts
Forces Exerted on Shafts
Forces and Moments Exerted on Shafts
Resulting Bending Moments in Shafts
Torque Diagram in Shafts
Torque Diagram in Shafts