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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.

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An announcement will be made when assignments are ready for collection (where applicable).

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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

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16429 Computer Aided Engineering Design

Semester 1 Course Work: Human Powered Generator

Campbell Simpson 4th Year MEng Mechanical Engineering

2012 11 629

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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

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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

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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.

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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)

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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

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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.

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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.

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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.

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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.

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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

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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.

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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

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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.

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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.

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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.

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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.

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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

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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.

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Appendices

Appendix 1: Component Drawings Part # Part Name Drawing

#1 Gearbox Bottom

#2 C-Clamp

Screw

#3 C-Clamp Grip

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#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

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#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

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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