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Analysis of Dynamic Pressure Puild-up

in Twin-tube Vehicle Shock Absorbers

with Respect to Vehicle Acoustics

Vehicle Dynamics Expo 2008

Dr.-Ing. Alexander Kruse ZF Sachs AG

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Overview

� Damping principle

� Description of processes in shock absorber valves

� Vibration analysis of shock absorber hydraulics

� Optimization of the dynamic vibration behavior

� Summary

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Damping principle of a twin-tube shock absorber

upper working chamber

lower working chamber

compensating chamber

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Damper Measuring at the Test Rig

F/v diagram F/s diagram Force, displacement, piston rod acceleration

diagram

Frequency spectrum ofpiston rod acceleration

Force

Piston rod acceleration

Displacement

Sound pressure

Servohydraulic Test RigData acquisition and analyse

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Comparison of F/v diagrams of shock absorber statically “– „dynamically“

Rebound

Compression

Excitation 2 Hz, 41mm, 0.5m/s Excitation 25 Hz, 3,3mm, 0.5m/s

Shock absorber velocity

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Shock absorber displacementDamping forcePiston rod acceleration

Damper Measuring at the Test Rig

Excitation 0.5m/s (2 Hz, 41mm) Excitation 0.5m/s (25 Hz, 3,3mm)

Highly dynamic excitation brings significant differences to the described "quasi-static" behavior of the shock absorber

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Load cell

Test arrangement on rig

1

Shock absorber

Acceleration sensor

Test rig frame

Sensors

Test rig actuator

2

4

5

6

3

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“Quasi-static" behavior of the shock absorber

Time, s

Comp Reb

BRV

p_lower p_upper

D_F

PR_aD_W

D_WBCV

Measuring signals at excitation of 2 Hz and velocity of 1,0 m/s

D_W - shock absorber displacement

D_F - damping force

PR_a - piston rod acceleration

p_upper - oil pressure in upper working chamber

p_lower - oil pressure in lower workingchamber

p_ausgl - oil pressure in compensatingchamber

BRV - lift of base replenishing valve

BCV - lift of base compression valve

The processes occurring in the shock absorber correspond to classic concepts. Piston and base valves open at the defined shock absorber design speed (0.2 – 0.3 m/s).

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Measuring of damper characteristics

Time [s]

0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026

0.006 0.00

8

0.01 0.012 0.014 0.01

6

0.018 0.02 0.022 0.024 0.026Time [s]

D_W

D_W

BCV

p_upper

p_lower

D_F

PR_a

0 1 2 3

Drop D_W - shock absorber displacement

D_F - damping force

PR_a - piston rod acceleration,

p_upper - oil pressure in upper working chamber

p_lower - oil pressure in lower working chamber

p_ausgl - oil pressure in compensating chamber

BRV - lift of base replenishing valve

BCV - lift of base compression valve

With high-frequency excitation, both working chambers are under far greater hydraulic pressure at the reversal point than with slow excitation. The maximum damping force is reached shortly after the start of the compression stage.

high-frequency excitation of 25 Hz and a shock absorber velocity of 1,0 m/s

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Valve opening phases

BCV

BCV

KDV

KDV

BRV

BRV

KZV

KZV

0 180 360

Excita

tion

fre

qu

en

cy

BDV

BDV

KDV

KDV

BRV

BRV

KZV

KZV

0 180 360

BDV

BDV

KDV

KDV

BRV

BRV

KZV

KZV

0 180 360

BDV

BDV

KDV

KDV

BRV

BRV

KZV

KZ

BCV

BCV

PCV

PCV

BRV

BRV

PRV

PRV

Phase φ°0 180 360

2 Hz

25 Hz

High-frequency excitation causes a length reduction of the opening time in thecase of check valves (delay on opening and premature closing) and extension of the opening time (delay on closing and premature opening) of the main valves

Excitation

frequency, Hz

Ratio

p upper chamber/p lower chamber

Phase φ at maximum

damping force

φ= 0° φ= 180° Compressi

on

Rebound

2 1.5 1.0 90° 270°

25 8 3 20° 290°

These phase shift cause problems in harmonic pressure and force build-up

"quasi-static" and highly dynamic shock absorber excitation

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Measuring characteristics of modified shock absorber setting

high-frequency excitation of 25Hz and a shock absorber velocity of 1 m/s

Significant improvement of vibration behaviour, through elimination of pressure retention at modified setting

0 0 .0 0 2 0 . 0 0 4 0 .0 0 6 0 .0 0 8 0 . 0 1 0 .0 1 2 0 . 0 1 4 0 .0 1 6 0 .0 1 8 0 . 0 2 0 .0 2 2 0 . 0 2 4 0 .0 2 6 0 .0 2 8 0 .0 3 0 .0 3 2 0 . 0 3 4 0 .0 3 6 0 .0 3 8 0 .0 4 0 .0 4 2 0 . 0 4 4 0 .0 4 6 0 .0 4 8 0 .0 5 0 .0 5 2

Z e i t [s ]

- 3 5 0 0

- 3 0 0 0

- 2 5 0 0

- 2 0 0 0

- 1 5 0 0

- 1 0 0 0

- 5 0 0

0

5 0 0

1 0 0 0

0 0 .0 0 2 0 .0 0 4 0 .0 0 6 0 . 0 0 8 0 . 0 1 0 .0 1 2 0 .0 1 4 0 . 0 1 6 0 . 0 1 8 0 . 0 2 0 .0 2 2 0 .0 2 4 0 . 0 2 6 0 .0 2 8 0 .0 3 0 .0 3 2 0 . 0 3 4 0 . 0 3 6 0 .0 3 8 0 . 0 4 0 .0 4 2 0 . 0 4 4 0 .0 4 6 0 .0 4 8 0 .0 5 0 .0 5 2

Z e it [ s ]

CompReb

BRV

p_lower

p_upper

D_F

PR_a

D_W

D_W

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Summary

� Analysis of the dynamic vibration processes going on in the working chambers showed that the damping force irregularities and noise occur at the start of valve opening

� Improvement of dynamic pressure build-up through valve optimization brought distinct noise reduction

� With knowledge of the valve operating characteristics in conjunction with the internal pressure characteristics, effective optimization strategies can be pursued

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Thank you very much for your attention!