Transcript SAE 1: Four Wheel Steering

SAE 1: Mini-Baja Four
Wheel Steering
Team:
Clyde Baker
Ken Brown
Alex Cherukara
Kevin Eady
Sponsor:
Dr. Patrick Hollis
Problem Statement
• Reduce Turning Radius
• Applicable to Formula SAE Car
• Budget of $500
Needs Assessment
• As defined by group:
– Turning radius of 7’ or less
– Strong enough to withstand forces experienced
during competition
– Must not interfere with other components
– Must remain stable
– Least weight possible
Concepts
•
–
–
–
Steering Configuration
Opposite Wheel Steering
Parallel Wheel Steering
Combination
Concepts
• Front Wheel Angle
– 35 degrees: Toggling becomes a concern
– 20 degrees: Must have high front to rear wheel angle ratio
to satisfy turning radius
– 30 degrees: Standard for most production cars. Good
compromise
Concepts
• Wheel Angle Ratio
– 1:1 : Greatly reduces turning radius, unstable
during high speed turns.
– 2:1 : Good compromise. Will meet turning
radius requirement, more stable than 1:1.
– 3:1 : Cannot satisfy turning radius requirement
Steering Ratio
Concepts
• How to achieve front to rear ratio?
– Geometry: Easier to construct, more reliable, less
components.
– Gearing: Easier to design, identical geometry and
components for subsystems.
Final Concept
• The complete concept is a combination of all
other concepts.
– Opposite wheel steering
– 30 degree front wheel angle
– 2:1 front to rear wheel angle ratio
– Gearing to achieve ratio
System Configuration
• Layout/Operation of the system:
– Two subsystems: Rack and pinion for front
and rear, identical geometry and components.
– Steering column fitted with 2” bevel gear.
Meshes with 2” bevel gear on shaft to front
system and 4” gear on shaft to rear system.
– As steering wheel is turned the shafts turn.
Front rack will travel 5” total, the rear 2.5”.
Front Steering Assembly
Center Gearbox
Rear Steering Assembly
Force Analysis
• Performed by hand initially, vector analysis of
statically loaded system.
• Equations written in Mathcad equation solver
software, angle of wheels and impact changed.
• Given stresses through components and design
strengths of components based on factor of
safety.
Vector Analysis
• Vector analysis performed in following way:
• Performed for each individual component: Steering
arm, tie rod, tie rod ends, bolts and rack
Initial Force
• Initially force was taken as the force with the largest
expected driver and a 30 – 0 mph velocity with an
elapsed time of .7 seconds.
• Following more research the initial velocity was
lowered to 20 mph and the E.T to .55 seconds.
Force Breakdown
• Forces in steering arm as wheel travels from 0-30 degrees
TOTAL STRESS (MPa)
STEERING ARM
TOTAL STRESS VS. WHEEL ANGLE
500
400
300
200
100
0
0
10
20
WHEEL ANGLE
30
Force Breakdown
• Forces on bolt as wheel travels from 0-30 degrees
TOTAL STRESS (MPa)
TIE ROD BOLT
TOTAL STRESS VS. WHEEL ANGLE
80
70
60
50
40
30
20
10
0
0
10
20
WHEEL ANGLE
30
Force Breakdown
• Forces on tie rod as wheel travels from 0-30 degrees
TOTAL STRESS (MPa)
TIE ROD
TOTAL STRESS VS. WHEEL ANGLE
250
200
150
100
50
0
0
10
20
WHEEL ANGLE
30
Force Breakdown
• Forces in tie rod end as wheel travels from 0-30
degrees
TIE ROD END
TOTAL STRESS (MPa)
TOTAL STRESS VS. WHEEL ANGLE
500
400
300
200
100
0
0
10
20
WHEEL ANGLE
30
Force Breakdown
• Forces on rack as wheel travels from 0-30 degrees
TOTAL STRESS (MPa)
RACK
TOTAL STRESS VS. WHEEL ANGLE
500
400
300
200
100
0
0
10
20
WHEEL ANGLE
30
Stress Reduction
• Initially the design strengths of the components were too high.
The stress and therefore design strength of the components
were lowered by increasing their sizes.
Maximum Stress (MPa)
Comparison of Stresses Pre/Post Reduction
1000
900
800
700
600
500
400
300
200
100
0
Pre Stress Reduction
Post Stress Reduction
Steering
Arm
Tie Rod
Bolt
Tie Rod
Component
Tie
RodEnd
Rack
Design Strength
• The final design strengths of components
COMPONENT STRENGTH VS. SELECTED MATERIAL STRENGTH
1200
STRENGTH (MPa)
1000
800
COMPONENT
DESIGN STRENGTH
600
400
SELECTED
MATERIAL
STRENGTH
200
0
STEERING
ARM
TIE RO D
BO LTS
TIE RO D
CO MPO NENT
TIE RO D END
RACK
Modeling
• All parts and assemblies were modeled in ProEngineer
• ADAMS was inoperable during the first half of
the semester, therefore work is still being done
to confirm the hand calculated force analysis
numbers.
• Predict our force analysis values are 50% high.
Modeling
Modeling
Modeling
Component Selection
• Components selected based on force analysis
results. Picked to match the design strength of
the components.
– Tie Rod Ends: ¾” Chrome-moly ends
– Rack: 11” steel rack with 5” total travel
– Gears: 2” Steel bevel pinion(2), 4” Steel bevel
gear
– All other components will be fabricated by
team
Final Car Specs
Turning Radius
6.5’
Front Wheel Angle
30 degrees
Rear Wheel Angle
15 degrees
Cost
$475 (est.)
Car Weight
800 lbs. (estimated)
Current Work
• Complete ADAMS modeling.
• Reselect components if needed
• Order Components
Spring Agenda
•
•
•
•
•
Test current steering system.
Construct system.
Test new steering system.
Analyze test results.
Rework system if necessary.
Downhill From Here