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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


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


Output Motion: Linear (Oscillating)


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)



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



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


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)


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)


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


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


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


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



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



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


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


('•",, Output Motion: Slower Rotation + Heat/Waste Energy


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 ,


Output Motion: Slower Rotation + Heat/Waste Energy


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 .-


Output Motion: Rotational (Continuous or otherwise)


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:



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)


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) -


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


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


Output Motion: Linear (constrained) or Rotational (oscillating)


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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