7. Vibration Isolation- Mounting Systems

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 !