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University of Toronto Faculty of Applied Science and Engineering
Department of Mechanical & Industrial Engineering
MIE243H1F - Mechanical Engineering Design I
Final Examination
December 2018
Duration: 2 Y2 hours Calculator Type: 2
Instructor: Peter Series
Rules and Instructions: - The examination is 150 minutes and is worth a total of 100 marks. - You should attempt ANY TWO OF THE FOUR design problems. Clearly
indicate your choice of options in the answer booklet. - This exam is closed book. The only materials allowed at your desk are
writing materials, a type 2 calculator, your exam paper, and the provided reference booklet. No outside material is allowed.
- The exam contains five pages, including this one. You must provide your answers on the separate answer booklet. Answers written on this exam booklet will not be marked.
- Designs will be evaluated based on your entire response, relative to your classmates. Clearly state all assumptions, design considerations, and explain the logic behind your design choices wherever ambiguous.
- Sketching and drafting protocols will not be enforced, however sketches should be clear and labeled in order to best elaborate your design choices.
Design 1 - You've been asked to design the lifting mechanism for a Fork Lift, which is a construction tool designed to lift heavy loads using two forks up to a designated height.
Electi Motc
Motor Output
These devices are typically used by contractors, plant workers, or other professionals and so are commonly higher cost and high reliability. The lift is required to lift its own weight of 100 kg as well as a payload of 700 kg with a factor of safety of 1.5 over a distance of 2m. The lift must be able to raise and lower in a reasonable amount of time, and hold the payload at a given height. The forks for the lift as well as the driving system are already designed. The lifting mechanism can be coupled to the fork lift's driving motor with a shaft located right above, and parallel to the rear axle, and which is an electric motor operating at (v240 rpm and r80 Nm. Assume no power losses. If a gearbox is required, general design considerations will suffice.
Write an engineering specification for the problem at hand. (5 marks) Sketch, briefly describe, and briefly compare/evaluate two candidate designs for your main mechanism. (10 marks) Choose one design as the final candidate and explain its operation, including relevant sketches and labels. Include a brief explanation of how your design works, plus any analysis/justification of major design decisions. (20 marks) Where would you expect the highest amount of wear to occur in your design? How might you design around this? (8 marks) Describe how your design addresses the potential duty cycle requirements placed upon it in terms of lubrication, cooling etc. (7 marks)
Design 2 - You've been asked to design a small Wind Turbine for use on local farms which transmits rotational wind power to a generator located at the base of the turbine.
The blades of the turbine have been developed by a fluid engineer to maximize efficiency and during normal operation are expected to spin up to 180 rpm with 40 Nm of torque. The generator is too heavy to be mounted in the turbine so is located at the base of the structure and generates maximum power when operating at speeds exceeding 3000 rpm. Depending on wind direction, the turbine housing should be able to pivot on top of the structure to maximize efficiency. The structure of the turbine ranges from 6m to 8m in height depending on the wind profile of the region. Assume no power losses. If a gearbox is required, general design considerations will suffice.
Write an engineering specification for the problem at hand. (5 marks) Sketch, briefly describe, and briefly compare/evaluate two candidate designs for your main mechanism. (10 marks) Choose one design as the final candidate and explain its operation, including relevant sketches and labels. Include a brief explanation of how your design works, plus any analysis/justification of major design decisions. (20 marks) What would happen to your design during a particularly windy day? How could you prevent damage to the turbine given high wind speeds? (8 marks) What environmental considerations would you expect for this device and how does your design accommodate this? (7 marks)
Design 3 - You've been asked to design a diesel-powered elevator drivetrain and gearbox for use in emergency situations for apartment buildings.
Output Drum
The driving mechanism is located on the top floor of the apartment building and must lift a collective mass of 2400 kg with a factor of safety of 1.25 by winding the supporting cable around a drum with a 50 cm radius. The clutch and brake mechanisms have already been selected to meet occupational health & safety requirements for lift design and are mounted next to the drum. As the elevator must go both up and down, there should be minimal backlash in the system. As this system is critical to emergency situations of a building, reliability and performance are paramount. One of the three following diesel engine can be selected as the power source and coupled to depending on design parameters:
P = 120 hp, t= 200 Nm, w= 4250 rpm, shaft output parallel to drum P = 150 hp, 'r= 300 Nm, co= 3500 rpm, shaft output parallel to drum P = 100 hp, t= 150 Nm, (= 4750 rpm, shaft output 90° to drum
Assume no power losses. If a gearbox is required, full design analysis is recommended.
Write an engineering specification for the problem at hand. (5 marks) Sketch, briefly describe, and briefly compare/evaluate two candidate designs for your main mechanism. (10 marks) Choose one design as the final candidate and explain its operation, including relevant sketches and labels. Include a brief explanation of how your design works, plus any analysis/justification of major design decisions. (20 marks) Describe how your design meets the requirements of a high-reliability and high-performance system. What major design choices are critical to this requirement? (8 marks) What would you expect to be this design's first point of failure? Why? (7 marks)
Design 4 - You've been tasked to design an Electric Sewing Machine that is able to sew thin fabrics at a series of speeds and is a low-cost alternative to your competitors.
As many sewing machines currently exist on the market, your client has requested that this be designed as a low-cost alternative. The active output sewing motion requires a reciprocating linear motion in order to pass the needle in and out of the fabric being fed through the machine. The location of the motor & output shaft as well as needle output are determined by the shape of the sewing machine as shown in the above figure. The client has requested the ability to alter the speed of reciprocating output based on a user input with a minimum of three speed settings. The electric motor operates at a single speed which provides more than sufficient force to penetrate fabric at high speeds without the requirement of a gear ratio. Assume no power losses.
Write an engineering specification for the problem at hand. (5 marks) Sketch, briefly describe, and briefly compare/evaluate two candidate designs for your main mechanism. (10 marks) Choose one design as the final candidate and explain its operation, including relevant sketches and labels. Include a brief explanation of how your design works, plus any analysis/justification of major design decisions. (20 marks) What happens to your design if the output needle hits something hard that it isn't able to penetrate? How might you prevent damage to your other components? (8 marks) Describe how you would redesign this product in order to increase the number of minimum speed settings to a completely variable system. (7 marks)
Four-Bar Drag-Link
Input Motion: Rotational (Continuous)
Output Motion: Rotational (Continuous)
Type of System: Motion Transmission & Motion Modification
Characteristics: Can be used to transmit rotational motic distance/move center of rotation Can modify radius of rotation
hi) Can create complex closed motion paths
Four-Bar Crank-Rocker
Input Motion: Rotational (Continuous)
Output Motion: Rotational (Oscillating)
Type of System: Motion Conversion
Characteristics: I) Converts Incoming continuous rotation motion into oscillating rotational motion
illLength of arc is controllable iii) Not reversible
I
across
adding rigid follower
MIE 243 Final Exam Quick Reference Guide Fall 2018
The Engineering Design Process
Conceptual Design Detailed Design
Identify Conceptualize Analyze Select & Model Finalize Prototype Evaluate Deliver
Engineering Candidate - Feasibility & - Preliminary Selected 3D Print or Ability to Final
Specifications Designs Performance
t Designs
-1 Design Machine meet criteria Product
II
Planar Linkages & Mechanisms
Scotch-Yoke
Input Motion: Rotational (Continuous)
Output Motion: Linear (Oscillating)
Type of System: Motion Conversion
Characteristics: I) Output position Is the sine/cosine function of Input angle h) Good conversion of torque to up/down force uI) Peak of force at middle of cycle
Uses a rolling pair to generate up-down motion (high wear) Reversible motion Input/output
Slider-Crank --
Input Motion: [
Rotational (Continuous)
Output Motion: Linear (Oscillating)
Type of System: Motion Conversion
Characteristics: i) Same conversion type as Scotch-Yoke
Better torque conversion in some applications
Uses only turning pairs - less wear More parts than Scotch-Yoke - higher cost
Quick-Return Mechanism
Input Motion: Rotational (Continuous)
Output Motion: Linear (Oscillating)
Type of System: Motion Conversion, Motion Modification
Characteristics: ii Same conversion type as Scotch-Yoke + pivot point for yoke Asymmetrical motion -faster in one direction Driven by rolling pair (more wear than slider-crank) More parts than Scotch-Yoke
Geneva Mechanism
Rotational (Continuous) Input Motion:
Output Motion: Rotational (Stepped) IV
Type of System: Motion Conversion, Motion Modification
Characteristics: ifWroduces a Stepped Motion
Discrete Increment, discrete amount of time In between
Can be constructed using gears too (later lectures) All contact sliding -high wear rates
Four-Bar Double Rocker
Input Motion: Rotational (Oscillating)
Output Motion: Rotational (Oscillating)
Type of System: Motion Transmission & Motion Modification
Characteristics: I) Links one oscillating motion with another
ii) Can modify radius, arc length, and power output
ill) Mostly used for creating complex paths
Double Hinge
Input Motion: Rotational or Linear
Output Motion: Linear (Extension)
Type of System: Motion Conversion, Motion Transmission
Characteristics: Driven by linear force or rotation of first pivot - creates
extending linear motion
Three links, two kinematic pairs- extended length up to 3x
link length
Four-Bar Parallelogram F_7 Input Motion: Rotational (Continuous)
Output Motion: Rotational (Continuous)
Type of System: Motion Transmission
Characteristics: Used primarily to transmit identical motion with no change In
angle, radius etc.
Stiff transmission of forces (can push)
Toggle Clamp area!x' Input Motion: Rotational (Constrained)
Output Motion: Linear (Pushing or Pulling), Rotational (Pressing)
Type of System: Motion Conversion, Motion Modification
Characteristics: ( Uses rotational change-point to lock the output against
counter-movement
ii) Minor force required to latch, amplified by lever
JJ) Four links, three kinematic pairs
3. Motion Support & Transmission
Bushing/Sleeve Bearing
Supported Motions: Rotational or Linear
Supported Loading: Radial: Low/Moderate Thrust: Low
Shaft Speed: Low
Manufacturing Cost: Cheapest bearing type - Single piece of material
Tribology: High - Surface-surface slip, can have oil impregnated
Characteristics: Provides reduced friction by: a( Separating shaft from
counterfpce, b) Elastically deforming under stresses
So should be made of a softer material than the shaft (plastic, PTFE, brass, bronze)
Roller Bearing j(Ji Supported Motions: J Rotational
Supported Loading: Radial: High (Greater area than ball bearing)
Thrust: Low to High (Angle of rollers)
Shaft Speed: High
Manufacturing Cost: High
Tribology: Moderate - No-slip pair, but wider surface area more heat
Characteristics: Ranges from straight rollers to 30 contact angle
Many different iterations for varying applications
Larger area compared to ball bearing accommodates more radial loading but creates more heat
Ball Bearing
0
Supported Motions: Rotational
Supported Loading: Radial: Moderate/High Thrust: Moderate
Shaft Speed: High
Manufacturing Cost: Moderate- standardized and mass produced but many parts
Tribology: Low friction and wear - Balls provide (relatively) no-slip pair
Characteristics: Most commonly used type of bearing Many different iterations for varying applications
Limited contact area means deformation of the balls is possible thus ruining the bearing
Thrust Bearings
Supported Motions: Rotational
Supported Loading: Radial: Low
Thrust: Extremely High
Shaft Speed: Low
Manufacturing Cost: Low to High-depends on configuration -
Tribology: Moderate• High normal force = higher friction and wear
Characteristics: Designed to be loaded parallel to the shaft
Retaining ring/rigid connection can limit radial forces
Common in items such as lazy Susan or bar stool
Ball Joint/Swivel Bearing
Supported Motions: Multi-DOF Rotational
Supported Loading: Radial: Moderate
Thrust: Moderate
Shaft Speed: Very Low
Manufacturing Cost: High - very specialized to application
Tribology: Surface-surface slip - High wear, can have oil Impregnated
Characteristics: I. Designed for motion In two or more DOF
ii. Not suited for high speed or loading
lii. Often used In non-planar assemblies (car
suspension/steering)
Bearing lopLinear
Supported Motions: Unear, Rotational
Supported Loading: Radial: Moderate
Thrust: N/A
Shaft Speed: Moderate to high linear, moderate rotational
Manufacturing Cost: Moderate - standardized but lens common
Tribology: - Balls and rollers both low friction and wear
Characteristics: I. Supports motion parallel to the shaft
IL Can be roller or ball, similar characteristics to radial
versions
Iii. Balls in direct contact with shaft - easier to collect debris
Radial Thrust
Motions Loading Loading Speed Cost Tribology Best Applications
Bushing! Rotational ,JJ.. .JJ.. $ Bad
Inexpensive, low speed, low
Sleeve or Linear loading
$$ Good
Generic, high speed, loading moderate Ball Bearing Rotational - 'fi"
Roller Expensive, high speed, high
Rotational JJ. - fl' ir $$$ loading in both coordinates
Bearing
Thrust Loading parallel to the shaft
but rotational movement Rotational .JJ, $ - $$$ Bearing
Ball/Swivel Multi-DOF JJ.Jj.. $$$ Bad
Require more than one DOF
Bearing Rotational
Linear Linear, N/A fr $$ Good-
Shaft movement parallel to
Bearing Rotational - the shaft
Shaft Design Summary: Driveshaft - Transmits torque from power source to machine element Gearshaft - Primary purpose is supporting motion transmission between elements Passive Axle - Goes through wheel, does not transmit torque Drive Axle - Goes through wheel, transmits torque
Material Considerations: Low grade - Plain Carbon Steel Medium grade - Alloy Steel High grade - High Strength Steel *Corrosion resistant - Stainless
4 - -
Rigid Coupling
Supported Misalignment: None/Forced Alignment --- -
Motion Type: Transmission
Characteristics: i. Forces pre-alignment during assembly
ii. Can cause deformation/vibration if misalignment is
large ill. Rigid connection means torque and speed matched
perfectly between shafts
Beam Coupling
Supported Misalignment: Angular, Small Parallel
Motion Type: Transmission
Characteristics: Low cost method for angular misalignment
Low torque due to thin strips of material
Prone to fatigue when misalignment is large
Universal Joint 4 74
Supported Misalignment: Angular -
Motion Type: Transmission, Modification
Characteristics: i. Allows for high-torque transmission, even across
significant angular misalignment
ii. Simple construction with bearings - low cost/wear
iii. Not a constant velocity coupling (proportional to angle)
Constant Velocity Coupling
Supported Misalignment: Angular
Motion Type: Transmission
Characteristics: Transmits input-output with no change in velocity
Generally expensive/high wear due to many parts
required
ill. Range of solutions exist - low to high torque
Oldham Coupling
Supported Misalignment: Parallel
Motion Type: Transmission
Characteristics: Compensates significant parallel alignment
Three sliding disks- high wear
ill. No angular misalignment is allowed
iv. Low torque - middle disk is commonly plastic
4. Gearbox Design
Important Equations:
a2 = —a.1(' /N2)
'r2 = _T1(N2/N1)
N DP=
Standard Gear Sizes
Diametral Pitch (DP): 64, 48, 32, 24, 20,
16, 12, 10, 8, 6
Pitch Diameter (D): 0.25", 0.375", 0.5",
2'', Y, 411, 651, 811, 12"
Number of Teeth (N): 10-100 (Even
numbers)
'V Minimum Number of Teeth to avoid Undercut
Noise Torque Capability
14.50 32 Quieter Medi
20° 17
250 13 Louder Hight
Planetary Gearbox Equations: Gear Ratio for Planetary Config. 1: Ring Fixed (Large N, forward) =
R+S Number of teeth: R = 2P + S-
S R+S Evenly Spaced Planets:
Config. 2: Sun Fixed (Near N1, forward) = - R - integer,
S = integer Config. 3: Planets Fixed (Large N, reverse) = -
R G- S where G = # planetary gears
Shaft Torque Speed Contact
Alignment Trans. Trans. Ratio Noise Tribology Cost Notes Applications
Straight Spur Parallel 44 (ti,) it 1.3-1.8 it tj) Good $ Very cheap and standardized
Non-specialized applications
Helical Spur Parallel -(f3) 4(3) 2-3 4 CW & CON, High High speed, large thrust forces power transmission
Herringbone Parallel tft Th 2.5-3 CW&CCW, Specialized tooting
Highspeed,large power transmission
Rotational it 4 4 Good $
61-directional Steering mechanisms, Rack & Pinion
to Linear 1.S-3 motion convert Rack Railway
Crossed Helical 90° 4 4 2 4 0 = 45°, both Orientation and CW/CCW positioning
Straight Bevel 90° 4-it (ti,) it 1.3-1.6 t() Good Cheapest 90° gear Lathe, Mill pair
Spiral Bevel 90° it it >3 4 Bad CW & CCW Hand Drill amplify & transmit
Hypoid Bevel 90°, offset ¶it >3 4 Very Bad $$$$ Non-intersecting axis of rotation
Driveshaft to differential
Worm Gear 90° U 2-40 4 Very Bad Cut axially down I Big speed reductions, the shaft self-locking
Gear-to-shaft Attachment Methods Lower t
j• Press-Fit
Shrink-Fit Pins Set Screws Keys & Keyways
Higher t
Housing Cooling/Lube Design: Lowest cooling/lube
I. Open Air
Forced Air Grease Filled
. Oil Filled Forced Oil
Highest cooling/lube
5. Belt and Chain Drives
Belt Drives
Input Motion: Rotational (Continuous)
Output Motion: Rotational (Continuous)
Type of System: Motion Transmission, Modification
Characteristics: I) Can drive multiple output shafts over long distances
Typically planar, but flexible so misalignment acceptable
Can modify direction and speed ratio of input and output shafts
Chain Drives
Input Motion: Rotational (Continuous)
Output Motion: Rotational (Continuous)
Type of System: Motion Transmission, Modification
Characteristics: Can drive multiple output shafts over long distances
Very little out of plane misalignment allowed
Individual teeth with no slipping - almost perfect efficiency
Can modify direction of rotation and change the speed ratio
Types of Belts: Flat Belt Timing Belt VBe1t
. Ribbed Belt
Types of Chains: Roller Chain Inverted Tooth/Silent Chain Offset Sidebar Chain
Power Sources
W F'P
100W 500W 1 h 50 hp 500 hp 1000 hp 2000+ hp
Size :..:"''.;.4
Cost Very Low Low Medium High Very High
-;
0% 20% 40% 60% 80% 100%
Brakes & Clutches
Drum Brakes Input Motion: Rotational (Continuous or otherwise) Output Motion: Slower Rotation + Heat/Waste Energy SON
Type of System: Motion Modification (decrease magnitude)
Characteristics: Function: Brake pads are forced outwards to engage with rotating
drum which transmits braking motion to shah
Characteristics: High actuating force, compact, moderate-high
stopping force -
Disc Brakes
Input Motion: Rotational (Continuous or otherwise)
Output Motion: Slower Rotation + Heat/Waste Energy
Type of System: Motion Modification (decrease magnitude)
Characteristics: Function: Braking created by clamping stationary high friction
surfaces Onto another rotating surface (disc)
Characteristics: Less complex and cheaper than drum, low-moderate
input force, very high stopping force, compact design
Band Brakes
Input Motion: Rotational (Continuous or otherwise)
('•",, Output Motion: Slower Rotation + Heat/Waste Energy
Type of System: Motion Modification (decrease magnitude)
Characteristics: Function: Braking action created by a band that constricts (tightens)
around spinning hub or cylinder attached to shaft
Characteristics: Simple and cheap, Very high stopping force with low
input/actuating force, ON-OFF use profile (either applied or not),
Rugged design with moderate wear
Cone Brakes ,
Input Motion: Rotational (Continuous or otherwise)
Output Motion: Slower Rotation + Heat/Waste Energy
Type of System: Motion Modification (decrease magnitude)
Characteristics: Function: Braking created by pushing a cone-shaped fixed drum into
an angled rotating drum attached to a spinning shaft
Characteristics: Compact and very high stopping force (higher
surface area), moderate application force, standard wear profile,
more complex than a drum brake
Disc Clutches .-
Input Motion: Rotational (Continuous or otherwise)
Output Motion: Rotational (Continuous or otherwise)
Type of System: Motion Transmission
Characteristics: Function: Coupling is achieved by pressing one or more shin friction discs attached to one shaft, between steel discs connected to the
other shaft Characteristics: Torque limit Is determined by material and number of discs, commonly med-high t, high actuation force Similar wear to other clutch types, capable of high vi
Cone Clutches ft Rotational (Continuous or otherwise) Input Motion:
Output Motion: Rotational (Continuous or otherwise)
Type of System: Motion Transmission
Characteristics: Function: Uses a cone with a friction surface pressed into a matching cone to couple/decouple input and output shafts Characteristics: High transmission oft, very compact configuration, starts engaging before fully in contact - faster/smoother engagement, generally lower ii
Centrifugal Clutches
Input fnitiOfl Rotational (Continuous or otherwise)
Output Motion: Rotational (Continuous or otherwise)
Type of System: Motion Transmission, Motion Modification (idle, slip, engaged)
Characteristics: Function: Centrifugal force created by spinning shaft automatically
engages the friction pads when the input reaches a high enough us
Characteristics: Low transmitted c, self-limiting torque transfer
(engaging too much slows us causing retractior, and slip), no external
engagement mechanism needed, slipping clutch at high r
8. Cams & Followers
Disc Cams Face Cams Input Motion: Rotational (Continuous) -
Input Motion: Rotational (Continuous)
4.
Output Motion: Linear or rotational (constrained) Output Motion: Linear (constrained)
Type of System: Motion conversion Type of System:
i) Cam is made of a fiat disc and follower traces path along outer Motion Conversion
Characteristics:
Characteristics:
I
I) Axis of input rotation and line of output motion are parallel
ill Not as suitable for some follower types (plate, needle etc.)
perimeter called cam profile ill Motion conversion type depends on follower Ili) Generally only useful for moderate F or r
Wedge Cams
Input Motion: Linear (Oscillating)
Output Motion: Linear (constrained) or Rotational
Type of System: Motion Conversion
Characteristics: I) Linear input cam
ii) Axes of input and output motion are commonly (but not always) perpendicular
Ili) Suitable for high forces
Barrel Cams
Input Motion: Rotational (Continuous)
Output Motion: Linear (constrained) or Rotational (oscillating)
Type of System: Motion Conversion
Characteristics: I) Follower motion constrained in both directions
ill Linear follower motion parallel to axis of rotation
ill) Output force is bidirectional (push and pull)
Knife Edge Roller Flat Face Spherical
4, AbilIty Motion Coherence F/r Capability
Knife Edge Bad Exact tracing of cam
Low Must be <10 profile
Good Slight averaging of cam
Roller Usable up to 3O-4O
profile based on size of Moderate-High roller
Flat Plate N/A Extremely averaged cam
High -Very High Will always be O profile
Spherical Moderate Moderately averaged
High Usable <20 cam profile
9. Useful Formulae and Conversions
6.3RadJs=60rpm 745W= 1 hp
= * 'r (units: W orNm!s) wv/r Radls T F*r Nm
C=27rr
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