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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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