gear box designing

8/13/2019 Gear Box Designing

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Overview

A machine consists of a power source and a power

transmission system, which provides controlled application of

the power.

Transmissionis an assembly of parts including the speed-changing gears and the propeller shaft by which the power is

transmitted from an engine to a live axle.

Often transmission refers simply to the gearboxthat usesgears and gear trains to provide speed and torque conversions

from a rotating power source to another device.


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Objective

To build a gearbox for a electric run motor car:

Minimize travel time required for a run on a straight route

Minimize the cost of the gearbox

Minimize the weight of the gearbox

Maximize the performance and durability

Maintain aesthetic design


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Design Steps

Geometry of gears

Type of gear, module, no. of teeth, etc.

Using C++ code and ANSYS

Design of shafts and bearings

Using force calculations

Design of Casing

As per final gear design and ANSYS


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CAD

Dimensions of gears:

First Pinion = 20* mm and Module = 2

First Gear = 18 mm and Module = 2

Second Pinion = 24* mm and Module 2.5 Second Gear = 22 mm and Module 2.5

Dimensions of Shafts:

(for relaxed condition of only max torque=60Nm)

OD25mm shafts

(for a conservative design: FOS=3 and max torque=60Nm)

OD 36mm shafts


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CAD

Dimensions of bearings:

Outer Shafttapered roller

(single row)

32007 X/Q

(From SKF)

ID=35 OD=62

T=18

wt=224 gm

Intermedia

te Shaft

tapered roller

(single row)

32205 BJ2/Q

(From SKF)

ID=25 OD=52

T=19.25

wt=187 gm

Input shafttapered roller

(single row)

32007X

(From SKF)

ID=35 OD=62

T=21

wt=263 gm


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CAD


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CAD


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CAD


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CAD


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CAD


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CAD


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CAD


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Code

For design of gears, ShigleysMechanical Engineering Design

was referred to

Design analysis was carried out for both spur gears and helical

gears


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Allowable Bending Stress


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Bending Stress Number


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Stress Cycle Factor


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Reliability Factor

For a reliability of 95%, the following equation was used:


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Other Factors

Temperature factor = 1 for temperatures less than 120oC

Factor of safety is taken to be 1


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Output


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Shaft Design

3 types of shaft

Connected to motor on which pinion is attached

Intermediate

Connected to wheel drive

Steps:

Calculated radial, axial and tangential forces from helical gear

design provided for both interfaces of gear and pinion.

Used maximum torque of 60Nm that motor can provide forcalculation (though in steady state the torque will be less)

Accordingly calculated axial and radial forces on shaft


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Shaft design (continued)

Plotted moment diagram and torque diagram and found critical

point on shaft

Material used was SAE 4340, got maxhere. Used factor of 0.75 as

the location was gear location.

Used FOS of 3 to get new maxas max/FOS Now used this value to get lower bound on OD of shaft

Repeated the procedure for all the 3 shafts


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Design of Bearings

To account for axial forces, used Tapper Roller Bearing

Because, they can take large axial forces (i.e., they are

good thrust bearings) as well as being able to sustain

large radial forces.

Commonly used for moderate speed, heavy duty

applications where durability is required.

We can also use Deep Groove Ball Bearings(calculations

for this are provided, if we want to use ball bearings), But

Its life is limited and does not fulfill our requirement .


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Taper Bearing Design Steps

Given: Fa, Fr, Shaft diameter(OD) (for all three shafts)

L= millions of revolutions (50,000 Km)

L10>L (~170 millions of revolution) (for D=22 in for tyre)

Also Our bearing should be of light weight

Used a catalog available online (reference is given) Choose bearing(initially of minimum weight available)

Calculate L10 for that bearing (SAE 50-oil is used for calculation)

Check for L10>L.

If L10


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Ball Bearing calculations

For the Shaft Connected to motor on which pinion is attached

SKF bearing no. 6307

Fa=4112N, Fr=4825.25N, Shaft Speed= 5500rpm

C0=17600N, C=26000N, Limiting Speed (SL)=8000rpmFa/C0= 0.2336

From Page 4.4, DDB: closest to 0.2336 Fa/C0 -> 0.25

As Fa/Fr >e -> X=0.56, Y=1.2

P=XFr+YFa= 7636.54N

L10= (C/P)^3 = 40 million revolutions < L (~ 170 mr) , hence not ok

Also its weight is higher(0.46kg) than taper roller bearing (0.22kg) for

the shaft connected to the motor.

Hence working with Taper Roller Bearing is much better.


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References

http://www.skf.com/skf/productcatalogue/jsp/calculation/calc

ulationIndex.jsp

http://www.irusa.com.br/catalogos/timken/usa_chap_1.pdf
http://www.skf.com/skf/productcatalogue/jsp/calculation/calculationIndex.jsphttp://www.skf.com/skf/productcatalogue/jsp/calculation/calculationIndex.jsphttp://www.irusa.com.br/catalogos/timken/usa_chap_1.pdfhttp://www.irusa.com.br/catalogos/timken/usa_chap_1.pdfhttp://www.skf.com/skf/productcatalogue/jsp/calculation/calculationIndex.jsphttp://www.skf.com/skf/productcatalogue/jsp/calculation/calculationIndex.jsp


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ANSYS

Purpose:

ANSYS was used to

confirm the output of C++ code decrease the weight of gears

design casing

decrease weight of casing

confirm shaft calculations

Ansys Analysis


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Optimization of weight of

Gears Initially used conventional arms for both gears as taught in the

course

But these arms were heavy

So, thought of using other profiles than arms

Used circular patterns to remove the material Did static structural analysis in Ansys

Iterative process of removing material and simulating it

Maximum material was removed keeping FOS > 3


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Gear 1 (FOS)


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Gear 1 (Total Deformation)


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Gear 1 (Von-Mises Equivalent Stress)


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Gear 2 (FOS)


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Gear 1 (Total Deformation)


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Gear 1 (Von-Mises Equivalent Stress)


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Casing

Initially used MS casing

but too heavy (from solidworks CAD model)

used Al for casing

Initially made casing with 15 mm thickness (starting guess)

Did Ansys analysis for it and found out it is overdesigned

Then designed the casing with 10 mm thickness

This design was also overdesigned at some places

Then designed a casing with thickness 5 mm at some places and

10 mm at other, using Ansys analysis to minimize the weight


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FOS

(min value = 3.9)


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Total Deformation

(max value = 12 microns)


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References

Motor Specification:

http://agnimotors.com/95_Series_Performance_Graphs.pdf

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