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Department of Mechanical & Aerospace Engineering
Coursework Assignment Cover Sheet
Class No: 16429 – Computer Aided Engineering Design
Coursework Title: Creo CAD Coursework, 2015
Submission to: Dr T Comlekci
Date Stamp
Surname: Simpson
First Name: Campbell
Degree Course: Mechanical Engineering
Year: 4th
I confirm that this work is my own and is the final version
Signed Campbell Simpson
Submission Details
DEADLINE: This assignment must be submitted both online and in hardcopy to MAE
Central Services before 3pm on Monday 23rd November 2015.
Assignments handed in after this date may incur a penalty or may not be accepted.
Students with a medical certificate should contact the appropriate academic staff asap.
An announcement will be made when assignments are ready for collection (where applicable).
You must download a submission receipt from the class page on Myplace when submitting coursework
via Central Services. This will be date stamped by a member of CS staff and will act as proof of submission.
Please note: this is a ‘Cover Sheet’; you must also bring a ‘Submission Receipt’.
NB: Submit assignments to MAE Central Services, Reception (James Weir Building level8)
OPENING HOURS: 10am – 4pm
Responsibility lies with the student to complete a receipt to be date stamped at time of submission.
IMPORTANT INFORMATION
2
16429 Computer Aided Engineering Design
Semester 1 Course Work: Human Powered Generator
Campbell Simpson 4th Year MEng Mechanical Engineering
2012 11 629
3
Abstract This report covers the processes undertaken to design a Human Powered
Generator; my thought processes from initial design criteria and conceptual ideas to
the finished and analysed model. It includes the reasons for my geometry, gearing
system and component selection as well as calculations to justify the forces required
to power it. An analyses section is also included showing what parts of the design
were weak and unfit for purpose and what parts were redundant. As well as
optimisations to show how these areas were improved to increase strength while
reducing mass.
Nomenclature Equations
𝜔 = Rotational Velocity (revolutions per minute / rpm)
𝑇 = Torque (Newton metres / Nm)
𝑃 = Power (Watts / W)
𝐼 = Current (Amperes / A)
𝑉 = Voltage (Volts / V)
DC = Direct Current
𝑟 = radius = ½ pitch diameter (metres / m)
𝐹 = Force (Newton’s / N)
deg = degrees
Gearbox Diagrams Red = Motor
Blue = Large Gears
Green = Small Gears
Grey / Silver = Bearings
Contents Abstract ...................................................................................................................... 3
Nomenclature ............................................................................................................. 3
Equations ................................................................................................................ 3
Gearbox Diagrams .................................................................................................. 3
4
Introduction ................................................................................................................ 5
Aims ........................................................................................................................ 5
Scope ..................................................................................................................... 5
The Design ................................................................................................................. 6
Ergonomics of a Hand Cranked Device .................................................................. 6
Generator Selection Process .................................................................................. 6
Gearing Selection Process ..................................................................................... 7
Bearing Selection Process ...................................................................................... 8
Modelling .................................................................................................................... 9
Assembly .............................................................................................................. 10
Arrangement & Connections ................................................................................. 11
Materials ................................................................................................................... 13
Parts List ............................................................................................................... 13
Material Properties ................................................................................................ 13
Justification for Homogeneous Linear Elastic Constitutive Approach ................... 14
Analysis & Optimisation ............................................................................................ 15
Applied Force ....................................................................................................... 15
Handle .................................................................................................................. 16
Crank Assembly.................................................................................................... 17
Main Body ............................................................................................................. 18
Internal Gearbox ................................................................................................... 19
Gears .................................................................................................................... 20
Conclusion ............................................................................................................... 21
Appendices .............................................................................................................. 22
Appendix 1: Component Drawings ....................................................................... 22
Appendix 2: Sourced Part Specifications .............................................................. 29
Small Gear – ..................................................................................................... 29
Large Gear – ..................................................................................................... 30
Ball Bearings – .................................................................................................. 30
Radial Bearing – ................................................................................................ 30
5
Introduction The aim of this report is to detail the processes I went through, designing a Human
Powered Generator which will be able to power a USB charging socket. In this
report, the mechanical design process including the reasons for geometry and
connections, material selection and parts selections will all be highlighted. As well as
this, I will show how I revised the design and why I believe it is fit for purpose.
Aims The Aim of this project was to design a hand cranked generator capable of
producing a 10W, 5V, 2A output, to be used as a USB charger. The brief was to
create a product which could be attached to a table or similar object to be used while
stationary that was also light enough to be transported by hand. I plan to create a
design which will work well for some of the suggested uses such as natural disaster
areas and on boats, where a robust and well clamped device could be used by
multiple people. I will also design my device to be capable of folding its handle away
so to not get damaged unnecessarily by passers-by.
Scope
Design a product that will produce the required electrical output based upon a
‘human hand crank’ input
Ensure the product will be fit for purpose with appropriate ergonomic features
and USB socket
Ensure the product is versatile, with the ability to be securely fastened to a
wide variety of tables and surfaces
Ensure the product is robust enough to stand up to communal use / abuse
e.g. acting as a ‘charging station’ in natural disaster refugee accommodation
The design should be optimised to be as lightweight as possible while
remaining robust and strong
The product should be carefully analysed to ensure it will not fail mechanically
under reasonable working stresses
Suitable gears, bearings and any other externally sourced components should
be selected based upon manufacturers specifications
All fastenings and connections should be detailed in the report so the product
could feasibly be assembled
6
The Design
Ergonomics of a Hand Cranked Device I began my design process by trying to understand the
one area of the system which I would not be able to
alter, how humans will interact with the device. I used a
piece of string tied to my table at home and
experimented with tensioning and ‘cranking’ it around
its anchor point, trying different lengths and rotational
velocities. I timed what felt comfortable with a
metronome and concluded that, as suggested in class,
around 100 to 120 rpm at a relatively small radius
would feel comfortable for most people for a good
length of time.
Generator Selection Process The next step was to select the appropriate motor which I could turn into a generator.
The requirements were that it had to
Be a Permanent Magnet DC Motor
Be small and light enough to not restrict the design specifications
Have the potential to meet the requirements of a 5V, 2A and 10W output
Have suitable torque and speed to allow a simple gearing system to transfer
the expected (hand cranked) input into the required output from the system
The motor that was eventually selected was the Maxon Brushless DC Motor, 30W,
12V DC, 54.3 mNm and 4360 rpm. For the desired voltage output, the shaft would
have to be rotated at 5
12 of its maximum rotational velocity thanks to the
proportionality of open circuit voltage to rotational velocity in a DC motor.
7
Gearing Selection Process In order to get a 5V generator output from a motor which normally runs at 4360 rpm
at 12V, the shaft would have to rotate at:
5
12 × 4360 = 1816
2
3 𝑟𝑝𝑚
This would result in the ideal output for a USB charger as,
𝑃 = 𝐼 × 𝑉
10𝑊 = 2𝐴 × 5𝑉
In order to achieve the desired input to the generator from the expected human
input, the gearbox would have to convert 100 − 120 rpm into 18162
3 rpm.
The required gearing to turn a 120 rpm input to this 18162
3 input is 15:1. However the
solution that was decided upon was a 16:1, meaning the desired input would be
reached at a slightly slower 113.5 rpm by the hand crank. This does not mean the
function of the generator is limited at that velocity, if the input exceeds 113.5 rpm the
electronics will ensure that the maximum charging output is maintained.
In order to achieve this 16:1 ratio, two 4:1 steps were used, this allowed a smaller diameter of gear to be used than doing the 16:1 in one step. The selected gears were HPC Skive Hobbed Spur Gears – Module 0.5, of pitch diameter 10mm and 40mm respectively. This would allow a small and lightweight gearbox while remaining durable thanks to the material and high torque rating of these gears. Specifications for these gears are included in Appendix 2.
The gearbox arrangement
and corresponding rotational
velocities are shown here:
1. 1131
2 𝑟𝑝𝑚
2. 454 𝑟𝑝𝑚
3. 454 𝑟𝑝𝑚
4. 18162
3 𝑟𝑝𝑚
5. Hand Crank (Input)
6. Motor (Output)
8
Bearing Selection Process Free rotation is key to the function of this design. The motor shaft, gears and crank
must all rotate with as little friction as possible to maximise the efficiency of the
device. For this reason, it was deemed appropriate to use a combination of radial
bearings and roller bearings.
One roller bearing would be used for the middle gear while the other would sit at the
end of the gearbox where the crank powers the slowest moving gear. An additional
radial bearing was decided upon to stabilise the hand cranked gear to avoid any
excess torsion or bending on the shaft from rough treatment.
Here we can see how the forces will act
upon the slowest gear from the hand
crank. It is clear that bending will occur
along the shaft in normal use. Considering
that the handle will go through
unnecessary excess force at points during
its working life, it was decided to fix the
shaft at two points, with the two bearings
shown in this diagram.
The radial bearing that was chosen for the
job is the grey component, shown fixed in
this image. This is the AST bearings,
IR5x8x12 Inner Rings, Metric Series. This would suit the job in hand as it is
recommended to function at a load of over 100N, far in excess of the working loads it
will be dealing with.
The roller bearings that were decided upon were the SKF Deep Groove Ball
Bearings 6000-2Z as they met the design specifications. They have a maximum rpm
of way above the speeds which we will be operating at, they have high load ratings
and are smaller than the gears we will be using, under 3cm diameter, so will not
affect the size of the device at all.
Bearing Specifications can be found in Appendix 2
9
Modelling The general sizing and shape, pre optimisation, was designed to be ergonomic, to
comfortably and securely house the necessary mechanical and electronic systems
and to be versatile and robust in use. This resulted in a large C-clamp extrusion at
the bottom of the housing, allowing it to securely fasten to a wide variety of table tops
thanks to its extremely long travel.
Initial geometry was specified with toughness in mind, opting for thick walls and a
robust clamp. The handle was also designed to be sturdy though its shape was
dictated by the specification of it having to fold away.
10
Assembly
Above shows the full assembly of the device with its handle tucked away. Here we
can clearly see how all of the components fit together with the hidden detail of the
interior shown in grayscale. Below shows exploded pictorial views of the entire
assembly from two different angles. The layout and connections of components are
broken down into individual part assemblies in the following sections.
11
Arrangement & Connections The arrangement of the internal
components is shown here.
This layout allows the motor, gears
and bearings to fit together in a neat
and space saving way.
1. Here we can see where the crank arm enters the
housing. It goes through the first ball bearing and
attaches to the first large gear. We can also see the
extension of the shaft which rotates on the radial
bearing, all bolted together through the crank and into
the gear itself.
2. Here the middle gears are shown, sitting in the
gearbox on the second ball bearing. This
bearing sits on a rib in the centre of the gearbox
and the assembly is again bolted together
through the centre of the gears.
3. This part assembly shows the motor connecting to
the fastest rotating gear by a grub screw. It also
shows a plate which screws into the lower half of
the gearbox, keeping the motor axially in position
while the two halves of the gearbox clamp down to
keep it in its radial position.
12
4. This assembly shows the relatively simple assembly
of the handle core and its lightweight rubber grip that
slides onto it.
5. This view depicts the two halves of the crank shaft. The
smaller part having already been detailed in part 1, this
half rotates only around the same axis as the slowest
gear. The larger part however connects to the smaller
part by a pin so it is able to sit in a tucked away
position then swing around and rotate about the same
axis as the rest of the system when being used. Once
in this ‘active position’, the two parts of the crank shaft
interlock in the grooves that are visible here and the
handle sticks out perpendicularly to receive power.
Below we can see the external and more simply connected components. The Clamp
section of the body is comprised of a threaded lower cylinder extrusion, a long
threaded rod, a rubber grip (to aid with friction on the under side of the table) and a
wingnut to allow easy attachment and detachment of the device from the table.
We can also see the main body of the device in explosion with the pins on the lower
part clearly showing. These slot into holes on the upper part and we can also see the
USB port and “electronics” components which, like the two halves of the body, will be
glues in place.
13
Materials
Parts List All Part Drawings are in Appendix 1
Part Name Part Number Material Source
Gearbox Bottom 1 UPVC Original Design
C-Clamp Screw 2 Steel Original Design
C-Clamp Grip 3 Rubber Original Design
C-Clamp Wingnut 4 Steel Original Design
USB Port 5 Plastic & Metal International Standard
“Electronics” 6 Copper & Plastic Representative
Motor 7 Steel Manufacturer’s Dimensions
Motor Plate 8 Steel Original Design
Gear 4 9 Steel Manufacturer Website
Gear 3 10 Steel Manufacturer Website
Gear 2 11 Steel Manufacturer Website
Roller Bearing 12 Steel Manufacturer Website
Radial Bearing 13 Bronze Manufacturer Website
Gear 1 14 Steel Manufacturer Website
Crankshaft prt1 15 UPVC Original Design
Crankshaft prt2 16 UPVC Original Design
Handle Core 17 UPVC Original Design
Handle Grip 18 Closed Cell Foam Original Design
Gearbox Top 19 UPVC Original Design
Pin 20 Steel Original Design
Small Bolt 21 Steel International Standard
Large Bolt 22 Steel International Standard
Grub Screw 23 Steel International Standard
The majority of components in this design are totally original; they have been
designed for purpose and modelled on CAD. The dimensions of the gearbox are
based upon the selected gears and bearings and the design of the rest of the device
followed. Some modelled components are dimensioned from manufacturer’s
websites and international standards though gears and bearings were downloaded
directly from the manufacturer’s websites.
Material Properties Properties of materials chosen for load bearing and critical applications.
Material Density (kg/m^3) Yield Strength (MPa)
PVC (minimum grade) 1400 31
UPVC 1420 51
Steel 7600 320
Rubber 1100 N/A
Bronze 8000 140
Closed Cell Foam 50 N/A
Copper 8960 70
14
Justification for Homogeneous Linear Elastic Constitutive Approach Using a homogeneous linear elastic constitutive approach suits the analysis for this
model as we are assuming the model does not deform to any damaging extent when
we apply our required loads and hence does not Yield. This is because we are
studying the expected deformations and internal stresses and strains within the
model due to the loading conditions of the device under normal use. We know that
no critical areas of the model approach yield thanks to computer based analysis in
the “Analysis” section later in the report.
We can therefore consider our components to be acting in the Linear-Elastic region
and we can assume “small” deformations and a linear relationship between stress
and strain at all points.
We assume the model is homogeneous, so when we apply a material choice to one
of the components, the material is consistent throughout. It will, for example, have
the same value of Young’s Modulus at all points, hence no areas will deform under
any greater or lesser stresses than any other. There will also be no defects in the
material to affect density or any other mechanical properties.
This type of simple, linear analyses is ideal for static analysis of individual parts and
small, simple assemblies with just a few forces acting upon them. For a larger and
more complex system however, such as the entire assembly, nonlinear FEA should
be considered. The areas of higher load, moving and force translating parts, non-
metallic components which do not yield and bolted connections would all benefit
from this type of analyses.
15
Analysis & Optimisation
Applied Force In order to find our minimum necessary forces to be applied by the user, we
calculate the necessary torques upon the motor shaft, and hence the necessary
torques on the handle.
𝐺𝑒𝑎𝑟 𝑅𝑎𝑡𝑖𝑜 =𝐼𝑛𝑝𝑢𝑡 𝑆𝑝𝑒𝑒𝑑
𝑂𝑢𝑡𝑝𝑢𝑡 𝑆𝑝𝑒𝑒𝑑=
𝜔1 (𝑟𝑝𝑚)
𝜔2(𝑟𝑝𝑚)=
𝑇2(𝑚𝑁𝑚)
𝑇1(𝑚𝑁𝑚)
The power transmitted by the torque to the shaft is given by,
𝑃 =2𝜋𝜔𝑇
60
Assuming an ideal gearbox,
𝑖𝑛𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟 = 𝑜𝑢𝑡𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟
So,
2𝜋𝜔1𝑇1
60=
2𝜋𝜔2𝑇2
60
From our “Gearing Selection Process” section, we know,
𝜔1 = 113.5 𝑟𝑝𝑚 & 𝜔2 = 18162
3 𝑟𝑝𝑚
And from our motor specifications (“Generator Selection Process” section),
𝑇2 = 54.3 𝑚𝑁𝑚
Therefore, we can calculate our necessary input torque,
2𝜋 ∗ 113.5 ∗ 𝑇1
60=
2𝜋 ∗ 181623 ∗ 54.3
60
𝑇1 = 862.12 𝑚𝑁𝑚
𝑇1 = 0.87 𝑁𝑚
We can now calculate the necessary force from the human hand by using this torque
and the distance from the centre of the handle to the axis of rotation of the crank,
𝑇 = 𝐹𝑟
0.87𝑁𝑚 = 𝐹 ∗ 0.05662𝑚
𝐹 = 15.4𝑁
16
The following analyses consider the stresses and strains on components due to this
necessary force acting upon the handle. It was however rounded up to 20N acting
along the handle to account for 75 – 80% efficiency as well as excess force
inadvertently provided by the user.
Handle
The first design iteration of the handle proved to be easily strong enough though
large sections of it seemed redundant. There was almost no stress on the main
cylinder (the part which would be gripped) and this component was 2.5e-5 cubic
metres in volume.
This revision increased the maximum stress slightly but remained well within the
elastic region while reducing the volume to 0.92e-5 cubic metres. This new design
would be combined with a ‘sheath’ of 4.5e-5 cubic metres of a far lower density
foam, reducing weight and improving grip, comfort and usability.
17
Crank Assembly
Iterations of Crank prt2, 2.65e-5 cubic metres being reduced to 1.57e-5 cubic metres
by shelling the thicker end and boring weight saving holes. The resulting stresses
were past yield and unacceptable so were amended by reducing the hole width with
use of an optimisation study and enclosing the shelled end to become a hollow
object.
Optimisation of Crank prt1, 1.24e-5 cubic metres being reduced to 1.03e-5 cubic
metres while remaining under a reasonable working stress.
Once combined, the assembly is stiffer and the max VM stress reduces to 30 MPa,
located at a small stress concentrator around the connection with the gearbox.
18
This concentrator is amended by rounding this extrusion and the max stress is
reduced to an acceptably low 14 MPa. The max displacement is around 1mm at the
end of the handle.
Main Body The following body analyses are carried out assuming 50N Force from the clamp in
order to fasten the device securely to a rigid table.
Initial analyses, clamp is strong but has redundant areas and a stress concentrator
where the clamp meets gearbox.
Redundant areas on clamp are bored away to reduce mass. Connection to gearbox
has also been rounded however max stress has still been exaggerated.
19
Connection to top is rounded further, reducing max stress to below un-optimised
shape. Displacement is around 1.3 mm at the clamp end, perfectly acceptable.
When this load is applied again but with an extra load of 50N on the top surface
(suggesting a human hand pushing down with the same force) the stress is reduced
further.
Internal Gearbox
This represents an absolute worst case load on the gearbox from the crank. This is
assuming all the required force is applied downwards (not rotationally) from the
crank to the housing. Even in this unlikely situation, the body does not yield.
20
Under these same “worst case” loading conditions we can see here that the gearbox
roof does not endure any severe stresses. This allows us to reduce the thickness of
the ribs in the non-critical areas as well as shaping weight saving holes into its
internal fittings, reducing volume from 6.32e-5 to 5.44e-5 cubic metres.
Gears
Taking our “𝑇1 = 0.87 𝑁𝑚” from the “Applied Force” section, we can find our 𝑇2 by
dividing by 4 from our first gear ratio.
𝑇2 =𝑇1
4=
0.87
4= 0.2175 𝑁𝑚
Hence the transmitted forces through each gear connection, from gear 1 to gear 2
and from gear 3 to gear 4 is:
𝐹 =𝑇
𝑟
𝐹1 =𝑇1
𝑟1=
0.87 𝑁𝑚
0.02 𝑚 = 43.5 𝑁 @ 20 𝑑𝑒𝑔
𝐹2 =𝑇2
𝑟2=
0.2175 𝑁𝑚
0.02 𝑚= 10.9 𝑁 @ 20 𝑑𝑒𝑔
To apply this transmission to an analysis
𝐹𝑥 = 43.5 cos 20 = 41𝑁
𝐹𝑦 = sin 20 = 15𝑁
21
These analyses show the results (in KPa) of the stresses in both sizes of gear where
the largest force is applied. This proves that the steel gears are totally unnecessary
and Nylon or other, less dense materials would have been a better option both for
reducing overall weight of the system and reducing inertia of the gearbox.
Conclusion Once completing the analyses and optimisations, it was clear that the device could
be improved. The Handle and Crank assembly were both optimised to reduce weight
while remaining easily strong enough for purpose with use of rounding and boring
weight saving holes. The Main body and clamp were also optimised with use of the
same modelling features. The top half of the gearbox was optimised by reducing the
size of the unnecessarily thick ribs while hollowing out other internal sections to
further reduce mass.
Additionally, the gears were shown to be over engineered being of steel and able to
take loads far in excess of the working stresses of the model. Improvements on the
design could include Nylon gears instead of steel, reducing weight without
compromising function as well as increased optimisation on other parts of the
design. A lighter body may have been possible with reduced thickness at less critical
areas but reinforced areas where the clamp is acting.
This device ended up being a reasonably light 769 grams, making it possible to be
carried by hand. It was however designed with stationary use in mind and is perfect
for that. It is perfectly fit for purpose as all components are robust and strong,
standing up to greater loads than required as well as being made of correct
materials. The design was also ergonomic with a good required input rpm and
reasonably low required force while being capable of producing the desired electrical
output. The design is also extremely versatile, with a wide range of table thicknesses
to which it could attach.
22
Appendices
Appendix 1: Component Drawings Part # Part Name Drawing
#1 Gearbox Bottom
#2 C-Clamp
Screw
#3 C-Clamp Grip
23
#4 C-Clamp Wingnut
#5 USB Port
#6 “Electronics”
24
#7 Motor
#8 Motor Plate
#9 Gear 4
25
#10 Gear 3
#11 Gear 2
#12 Roller
Bearing
26
#13 Radial Bearing
#14 Gear 1
#15 Crankshaft
prt1
27
#16 Crankshaft prt2
#17 Handle Core
#18 Handle Grip
28
#19 Gearbox Top
#20 Pin
#21 Small Bolt
29
#22 Large Bolt
#23 Grub Screw
Appendix 2: Sourced Part Specifications
Small Gear – Skive hobbed spur gears - Case hardened steel - Module 0.5
REF (Part number) PSG0.5-20HC
DET (Detail) Full
Z (N° of teeth) 20
P (ØP Pitch / mm) 10
D (ØD External / mm) 11
A (ØA Bore / mm) 4
M (ØM Boss / mm) 8
CL (Keyway) -
J (Backlash / mm) 0.029 - 0.061
C (Torque / Nm) 1.11
MAS (Weight / kg) 0.006
INFO (INFO) INFO
30
Large Gear – Skive hobbed spur gears - Case hardened steel - Module 0.5
REF (Part number) PSG0.5-80HCK DET (Detail) Full
Z (N° of teeth) 80
P (ØP Pitch / mm) 40
D (ØD External / mm) 41
A (ØA Bore / mm) 8
M (ØM Boss / mm) 25
CL (Keyway) 2
J (Backlash / mm) 0.033 - 0.079
C (Torque / Nm) 5.8
MAS (Weight / kg) 0.082
INFO (INFO) INFO
Ball Bearings –
SKF Deep Groove Ball Bearing 6000-2Z 10mm I.D, 26mm O.D Ball Bearing Type Deep Groove
Bore Type Parallel
Cage Material Steel
Dynamic Load Rating 4.75kN
End Type Shielded
Inside Diameter 10mm
Limiting Speed 34000rpm
Number of Rows 1
Outside Diameter 26mm
Race Type Plain
Race Width 8mm
Reference Speed 67000rpm
Static Load Rating 1.96kN
Radial Bearing –
AST Bearings, IR5x8x12, Metric Series
Attributes Values
Units mm
Bearing Type inner ring
Bore Dia (d) 5.0000
Outer Dia (D) 8.0000
Width (B) 12.0000
Weight (g) 2.79
Material 52100 Chrome steel, or equivalent