Steering System DFMEA and Design Review for sports car and racing vehicle

Please read previous blog for vehicle parameters, steering components, steering mechanism, child parts and related system components mechanism.

Here we will make summary table of Design Review which was explained in previous blogs.

Child Part	Requirement	Material	Manufacturing	Cost	Weight	Life	Design Review
Rack Housing	High Surface Finish, High Strength, Fine Dimension Tolerance, Wear Resistance, Corrosion Resistance, High Toughness	Steel	Machining	A	C	B	Al Alloy is light weight and has good life. Steel and Aluminum are lesser costly but have high weight. Also Rusting is more for Steel. So Al Alloy will be used for Rack Housing Components.
		Aluminum ADC12	PDC Pressure Die Casting and Machining	B	B	B
		Al Alloy		C	A	A
Support Tube		ADC12	PDC Pressure Die Casting and Machining	B	C	B	Al Alloy will be used for Support Tube as it has less weight.
		Al Alloy		C	A	A
Pinion Housing		Steel	Machining	A	C	B	Al Alloy will be used for Support Tube as it has less weight and lesser wear.
		ADC12	PDC Pressure Die Casting and Machining	B	B	B
		Al Alloy		C	A	A
Housing Cap + Bush		Steel + Bronze	Casting/Machining	A	B	B	Bronze Bush with ADC12 housing Cap will be used due to light Weight. (HPDC High Pressure Die Casting)
		Bronze	Casting/Machining	C	B	A
		ADC12+ Bronze	HPDC, Machining	A	A	B
Pinion Housing Clamp		Alloy Steel	Casting/Machining	C	B	A	Driver applied torque is transmitted though the lower pinion gear to the rack gear, So very high Strength is required here.
		Al Alloy	HPDC, Machining	C	A	B
Pinion  Helical Gears	Number of Spiral Tooth Anti Backlash ID/ OD	Alloy Steel	Forged, Milling, Hobbing Lapping	C	B	A	Alloy Steel Helical Gears will be used as it has less weight and long life.
		Cast Iron		B	C	C
Rack with variable Teeth  Helical  Gear	Rack Speed=60mm / Revolution Travel Max = 75mm	Carbon Fiber	Molding	C	A	B	Carbon Fiber is very costly.  Both Pinion and Rack should have same material for same wear rate. Alloy Steel will be used for rack.
		Alloy Steel	Casting, Machining.  Gear=Milling, Hobbing Lapping	A	C	A
		Al Alloy		B	B	B
Pinion Shaft	High Surface Finish, High Strength, Fine Dimension Tolerance,  Low Wear Rate, Corrosion Resistance, High Toughness	Steel	Casting, M/c	A	B	B	Aluminum will be used for Pinion Shaft.
		ADC 12	HPDC, M/c	B	A	B
Rack Ends		Steel	Casting, M/c	A	C	B	ADC12 will be used for Rack Ends.
		ADC12	HPDC, M/c	B	B	B
		Steel Alloy	HPDC, M/c	C	A	A
Housing End Supports		ADC 12	HPDC, M/c	A	B	B	Al Alloy will be used for Housing End Support as it has less weight.
		Al Alloy	HPDC, M/c	B	A	A

DFMEA:

I have explained in detail severity, occurrence and detection in blogs of Throttle DFMEA, Intake System DFMEA, Exhaust System DFMEA and Brake DFMEA in previsions blogs.   

NO	ITEM/ FUNCTION	POTENTIAL FAILURE MODE	POTENTIAL EFFECT OF FAILURE	SEVERITY	POTENTIAL CAUSES	OCCURANCE	CURRENT DESIGN CONTROLS		DETECTION	RPN
							PREVENTION	DETECTION
1	RACK AND PINION	Mechanical Failure, Lubrication not Uniform, Loosing of Nut and Bolt	Steering Failure	10	Obstruction in movement of pinion over rack, damage to components	2	Simulation for Burst Pressure, Stress and Strain. Design Calculation for Flared Thread  Dimension	Lubrication Check, Vibration Measurement on Nut/Bolts, Material and Strength Test, Weld Strength Test.	3	10*2*3= 60
			Steering Wheel Hard	8		5			3	8*5*3= 120
2	STEERING COLUMN AND WHEEL	Breakage, Mechanical Failure, High Vibration	Steering Failure, High Debris	10	Excess Load Applied by Driver, Bump Steer	2	Strength and Vibration Simulation of each component	Physical Durability Testing of each component	3	10*2*3= 60
		Driver Tired, Ergonomic Issue	Drivability Issue	5	Wrong Position of Steering Wheel	4	Calculate Ergonomic Position	Steering Wheel Check	5	5*4*5= 100
3	DRIVABILITY	Improper Cornering, Difference in Left and Right Turn	Safety Issue, Vehicle Skid, Slip or topple	10	Wrong Length and Position of Steering Rack, Wrong Gear Ratio.	4	Calculation, Layout and Simulation of Rack Location , and Gear Ratio	Minimum Corner Radios Measurement at highest speed	4	10*4*4= 160
		Vehicle Not moving in Straight Line ( Waving)	Safety Issue	8	Rack Pinion Mismatch, Gear Tooth Wear or Broken	2	Gear Mess Simulation, Gear Tooth Strength Test	Gear Wear Test, Tooth Breakage Force Measurement	2	8*2*2 =32
4	STEERING SENSOR	Electric Failure	Safety Issue	9	Water Entry, Electromagnetic Noise, High Vibration,	2	Select Sensor Specification After checking humidity, electromagnetic noise and vibration on vehicle	Water Spray Test, Electromagnetic Antenna Test , Vibration Measurement on Sensor	2	9*2*2= 36

Recomendation based on above DFMEA

NO	ITEM/ FUNCTION	RECOMMENDED ACTION
1	RACK AND PINION	1) Use Good Lubrication 2) Use High Surface Finish Gears 3) Reduce Welding and Joints in system
2	STEERING COLUMN AND WHEEL	1) Use Aluminum Alloy or Steel Alloy for Manufacturing
		1)Take many expert drivers opinion
3	DRIVABILITY	1)Do many iteration for optimization of Rack Length and Gear Ratio 2) Benchmarking of Race Vehicle 3) Do only small modification in previous designs
		Use Alloy Steel Helical Gears with high surface finish and high toughness
4	STEERING SENSOR	1)Use sensor cover 2)Use twisted shielded wire 3)Use high Vibration Resistance  Sensor