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Mounting System
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Vibration Isolation- Mounting Systems Design of Mounts for better NVH of vehicles
Dr. M N Ambardekar
ERC- NVH Centre
Tata Motors Ltd.,Pune
August’23, 2011
Automotive NVH: major transfer paths of vibrations
Engine Mounts, Body Mounts, Suspension Bushes, Radiator
Isolators, A/C compressor mounts, exhaust hangers
Vibration source
ReceiverTransmission path
Low frequency models of vibration isolation
• Two problems
j t
tF e ω
Isolator
Receiver
Source
j teF e ω
j tXe ω
k c
m
(1)
Isolator
Host structure
equipment
k c
m
j t
tX e ω
j t
eX e ω
(2)
Vibration Isolators in a Car or a Truck
Supporting engine on a Car-Frame
Uniform load sharing
Cost / weight
Drivability of a car [koko /TITO jerks]
Durability
Engine-motion control + vehicle ride-comfort
Vibration Isolation + structure-borne noise control
Requirements of Engine-mounting
systems
Isolator stiffness calculations
Compression
l
A
stiffness, EA
kl
=
Shear
A
h
stiffness, GA
kh
=
( )2 1E Gν= +
Poisson's ratioν =
Young's modulusE =
shear modulusG =
Stiffness calculations
Inertia Forces due to Reciprocating Masses
In-line 4-cylinder
Engine
In-line 2-cylinder 180 deg phase shift
In-Line 2-cylinder 360 deg phase shift
In-line 3-cylinder
0 0 Unbalanced 0
0 Unbalanced 0 Unbalanced
Unbalanced 0 Unbalanced 0
1st order 2nd order
0 Unbalanced Unbalanced 0
Force Couple Force Couple
Combustion Torque Input on PT-mounting
Low frequency Mathematical model
j t
tF e ω
Isolator
Receiver
Source
j teF e ω
j tXe ω
k c
m The transmitted force, Fr is given by:
The equation of motion is:
( )mx cx kx f t+ + =&& &
For harmonic excitation:
( )2- em j c k X Fω ω+ + =
( )tF k j c Xω= +
(1)
(2)
force transmissibility
2
tF
e
F k j cT
F k m j c
ω
ω ω
+= =
− +
Force transmissibility from SDOF model (viscous damping)
• Transmissibility
2
1 2
1 2
nF
n n
j
T
j
ωζ
ω
ω ωζ
ω ω
+
=
− +
,n
k
mω =
2 n
c
mζ
ω=
ζ=0.01 ζ=0.03 ζ=0.1 ζ=0.3
1 10 10
-3
10 -2
10 -1
10 0
10 1
10 2
Non-dimensional frequencyn
ω
ω
Fo
rce
tra
nsm
issib
ility
, T
F
2n
ω
ω=
amplification isolation
mobility analysis
= =( ) (2)
( ) (
without isolator
with isolator 1)r
r
VE
VIsolator effectiveness
So = ++
1 i
s r
YE
Y Y
Source Isolator Receiver
sYiY rY
fVr
VFree velocity
. rr f
r s i
YV V
Y Y Y=
+ +
• If no isolator is fitted, i.e., the receiver is rigidly connected directlyto the source, then Yi=0, and
. rr f
r s
YV V
Y Y=
+(2)
Effect of frame flexibility
Rubber Mount-
stiffness N/mm
Engine-structure stiffness kN/mm
Mtg.Brkt stiffness kN/mm
Body-stiffness kN/mm
Isolation Effectiveness [dB] at freq. 100 Hz 20 dB = 90 % Isolation
500 50 10 2 12.5
500 50 1 2 7.3
200 50 10 2 19.1
500 50 10 1 8.9
Rubber mounts focused on Principal MI Axis of engine -
Decoupling
---- 6 DOFs
Torque Roll Axis of PT
KEF for pure modes
Decoupling Requirements: Focused Mounts
• a/m v/s
α
Ride comfort aspects
• C entre o f Percussion type m ounting
• D e-coupling requ ired or not for P itch and Bounce ?
• E n g in e -m o u n t in g m o d e s a n d V e h ic le
S u s p e n s io n m o d e s … .. K e y -o n /k e y -o f f je rk s ; T o r tu re - tra c k
p e r fo rm a n c e ?
Location of PT as a part of vehicle Assly.
• E n g in e - a s a D y n a m ic a b s o r b e r t o f l e x i b le f r a m e s
Quality problem
• Combustion variations --- cylinder to cylinder
variations --- OR--starving of engine -----freq. 8 to 10
Hz .. A critical band --------
• Production variation …
Modal Map of vehicle
Dynamic characteristics of Rubber Mounts
� Dynamic Stiffness of Rubber Mounts till 200 Hz to be < 1.4 times Static Stiffness
� Loss Tangent = tan [δ] = damping force in rubber / spring force in rubber = C * 2 * pi * freq. / K
where K and C are respectively stiffness [N/m] and C damping coeff. [N sec/m] of the rubber-mount
� Natural Rubber damping increases as its shore hardness
� Dynamic stiffness of the rubber-mounts of higher δ increases
Damping versus Isolation
Dynamic characteristics of Rubber Mounts
� Damping is desirable only at Resonance
� Damping increases vibration transmissibility at off-resonant conditions; so
δ < 6 deg or tan [δ] < 0.1
Thermal Fatigue of Rubber-mounts ?
Notching effect ----- reduce dynamic stiffness of the mounts only at a particular freq. of in-cab boom ??
Standing Waves in Rubber Isolators
Frequency [Hz] * 100
10-1
100
101
102
-40
-20
0
20
40
60
80
100
Isola
tor
effectiveness (
dB
)
Effectiveness of isolator with rigid
source and receiver and massless
isolator
Isolator resonance
frequencies
Fundamental resonance
frequency
Source and receiver
resonance frequencies
Advanced Materials
Conflicting requirements
• Low stiffness and damping for excellent Vibration isolation against
Inertia Forces & couples
Combustion-torque
• Restricted
movement of PT:
Vehicle key-on
Key-.off jerks
Tip-in-tip-out
Launch-shake
Costly
delicate
Less reliable
Cost –effective and Robust
•Progressively non-linear stiffness curve
[stoppers to Rubber]
•Multi-directional Rubber-bushes
Excellent
OA performance
But large static and dynamic displacements
of Engine
Bad Drivability of Vehicle
Poor fatigue life of Rubber-part
Stiff isolators can be used with
frequency dependent damping
&.or stiffness
Soft Rubber Isolators
for Vibration Isolation
Active Mounts Passive Rubber Mounts
Solutions
Hydra-mount
• Rubber mount with a fluid
Specialty
• Hydra-mounts -----
Dynamic stiffness
increases as a function
of frequency
• Semi-active mounts ----
Electro-Rheological
fluids usage
Active Mount
Engine Motion Control
• Side-Stoppers
in Rubber-mounts
Torque-control
arm
• shock absorbers
Transfer Paths of Vibrations in a car
Suspension System Bushes : Jounce Bumpers, Stabilizer Bar-
bushes and Link-pivots
Driveline-Dynamics
Steering Wheel system dynamics
Damping Mass 0.5 kg Tuned freq. ~ 30 Hz
Exhaust system: vibration Isolation
Body / cab mounts
Various Tuned Dampers
Gear-box dynamic absorber [ freq. 150-200 Hz] Power-train
bending mode
Various Isolators
• 1. Door Lock De-coupler
• 2. Radiator Isolators
• 3. ECU de-couplers
Some other Mass Dampers found in premium cars
• 1. Brake Dampers
• 2. Battery Dampers
• 3. Sub-frame dampers
• 4. Floor Panel Dampers
• 5. Gear-shift Lever Damper / Mass
Engine parts with vibration isolators
• 1. Valve Tappet covers
• 2. Oil-sump isolators
• 3. Rubberized pulleys
• 4. Timing Pulley cover isolation
• 5. fuel-Injection-pump-cover isolation
Crankshaft Damper
Full vehicle NVH and Vibration Isolation elements
On engines that have a normal second order vibration, the following components, if defective or misadjusted, can allow the vibrations to be transferred into the passenger compartment.– Engine mounts
– Transmission mounts
– Exhaust mounts
– Body mounts
– Propshaft Slip Yoke
– A/C or power steering hoses
– Aftermarket accessories
– Other components
pipework
• Reference: Noise Control Engg. 2007
A need of reduction of Engine-exciation level
Critical evaluation
• Idle shake is heart of NVH [ a first impression of car-customers]
• This is a real test of Power-train mounting system since excitation freq. is close to natural freq. of Mounting system
• Idle In-cab noise is also dominated by structure-borne noise of engine and hence here, too, the rubber-mounts below the power-train play a majoprrole ..
Transfer Path Analysis and PT-mounts
NVH performance of vehicle:
Subjective Rating [0-10 scale]
0
2
4
6
8
10Idle-shake
key-on /off jerk
Tip-in-tip-out
secondary ride Running
vibrations
Boom
Average score
Vehicle drivability ---- low torque driving – TITO / KOKO jerks
Dynamics
Overall Design of Engine Mounting
Rubber Mounts
Vibration Isolation Engine Rocking Fatigue Life
Low and High Frequency Excitations On Rough Roads
Heat
Ozone
oil
Fatigue Life of Rubber Mounts
• Tensile Strength is not a useful indicator of Fatigue
Performance
• Laboratory Tests of uni-axial dynamic loading rarely
correlate with service Performance of rubber mounts
Sinusoidal Testing for bond-strength
• Laboroatry Test
• Freq. 2 Hz Temp.
• - 0.5 to + 1.5 times static service load
• 1 million Cycles
• To test minimum 10 samples to arrive at B10 life
Rubber Material Properties
Lower mechanical strength, dynamic properties change
high damping & good weather
resistance Butyl
poor resistance to grease
Good resistance to
Heat, aging, Ozone EPDM
poor resistance to oil & Ozone
Excellent
mechanical strength Natural Rubber
Engine Mount Force Measurements
……. Deformation of Rubber mounts as predicted by FEA
•
Allowable Stress & Strain in Rubber-mounts
• Dynamic Strain < 40 %
• Static strain < 20 %
[compression]
• Maximum stress < 0.50 N/
mm2
Reference: (1) Engg. with Rubber – ed. by A N Gent
(2) Theory and Practice of Engg. with Rubber – by Freakley and Payne
(3) Rubber Spring Design –by Goebel
Crash safety needs from Engine-mounts
• Inertia of power-train during frontal-impact
• Damping
• Nonlinear force-deflection response
• ==================================================
• Foundation Flexibility effects ??
Balancing conflicting requirements
For both Vibration Isolation and longer Fatigue Life]
Mount Cross-section area --- to be higher
(lower stresses)
Thickness or Height of Rubber– to be higher
(lower stiffness in compression]
Shore hardness --- to be higher
{ good mechanical properties}
Optimization output --- Constraints!!
• X-axis – mount inclination ; Y-axis –vibrations at Driver’s Seat ; Z-axis –
mount stiffness
Multi-disciplinary optimization– Volvo Approach [Ref. SAE 2011-01-1674]
DFMEA for Rubber-mounts
Item A: Adhesive bond failure
Item B: Fouling with stoppers
THANK YOU !