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Microcellular polyurethane as
steering coupling element
Material specific properties of solutions based onCellasto®-Elastolit-X® composite structures
28.05.2010 [email protected] 2
Contents
Cellasto® material specificproperties:
Classification, materialstructure and production
Volume compressibility
Compression set,dynamical characteristics
Steering couplings:
Function in steeringsystems
Requirements and differentdesigns
Composite technologies –combining Cellasto® with hardmaterials:
Motivation
Production technology
Adhesion characteristics
Cellasto®-Elastolit-X prototypes
Experimental prototypedata
Optimisation potential andcomparable applications
References
Summary
28.05.2010 [email protected] 3
Material specific propertiesClassification, material structure, production
Cellasto® = microcellular polyurethane thatis applicable as technical elastomer
Cellular material structure with
Volume compressible air component
and a rubber-like matrix
high density (350-650kg/m³)
Very closed cell structure
Molecular level: hard & soft segments
Cellasto® necessitates a specialized andchallenging production process.
SEM-photo
Flexible foam –open cell structure
Cellasto® special elastomer– closed cell structure
SEM-photo
Mixing the main components for Cellasto®
► the foam rises and fills the mold
28.05.2010 [email protected] 4
Average Compression Stress
Standard Test Cylinder
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
0 10 20 30 40 50 60 70 80 90
Compression in %
Co
mp
res
sio
nS
tre
ss
inM
Pa
MH24-35
MH24-40
MH24-45
MH24-50
MH24-55
MH24-60
MH24-65
Material specific propertiesCompressive stress and effect of volume compressibility
Compression test with acylindrical specimen:
Compression anddisplacement of the aircomponent
At the beginning the lateralextension is negligible andraises at high compressionlevels
The stiffness and themaximal deformability ishighly dependent on thedensity
Compressing rubber andCellasto® samples in a glastube demonstrates the effect ofvolume compressibility Compression test with
rubber and Cellasto®
28.05.2010 [email protected] 5
Material specific propertiesSetting, dynamical characteristics and others
Statical compression set:
Taking into account when dimensioningparts
Extent of creeping very small comparedto the reversible deflection
Dynamical properties:
Favourable stiffening properties for manyapplications
Dependent on state of stress andmaterial density
Amplitude dependent damping
Outstanding fatigue limit for compressionstress
Characteristics such as elongation break, tearresistance, heat aging and so on are fullycompetitive for many automotive applications.
Density 600 kg/m³
time [h]
co
mp
ressio
n[%
]
f = 15 Hz, test objects: top mounts
Amplitude [mm]
Lo
ssan
gle
[°]
4
5
6
7
8
9
10
11
12
13
14
0 0,5 1 1,5 2 2,5 3
Cellasto®
Rubbercompoud
Highdamping
Lowdamping
f = 15 Hz, test objects: top mounts
Amplitude [mm]
Lo
ssan
gle
[°]
4
5
6
7
8
9
10
11
12
13
14
0 0,5 1 1,5 2 2,5 34
5
6
7
8
9
10
11
12
13
14
0 0,5 1 1,5 2 2,5 3
Cellasto®
Rubbercompoud
Highdamping
Lowdamping
28.05.2010 [email protected] 6
Material specific propertiesExamples of automotive applications
Top mount „dual-path“,Jounce bumpers
Top mount„one-path“
Steel springsupport
Air spring mount
Roll restrictor
Gear mountChassis mounts
Cellasto®
is automotivesuccess
28.05.2010 [email protected] 7
Steering couplingsFunction in steering systems
Tube-in-tubedamper
Steeringcoupling
Steering systems need to
Transmit and reinforce input from thesteering wheel and give feedback aboutthe road.
Decouple acoustical excitations andvibrations excited by uneven roads.
Decoupling is realized by damping elementslike
Tube-in-tube dampers
Steering couplings
Bushings for the steering gear
There exist different solutions depending on
The engine‘s orientation
The car‘s market segment
The manufacturer‘s philosophy
The steering system technology (hydraulic,electrical, point of torque amplification, ...)
www.sgf.de
Steering gearbushing
28.05.2010 [email protected] 8
Steering couplingsRequirement specification
Torsional stiffnesses betweensome Nm/° and about 30Nm/° arerequired.
The torsional load spectrumdepends on the position relative tothe torque amplifier.
position before the torqueamplifier ► max. torquesseldomly higher than 1-6Nm.
Otherwise the torques canreach much higher values.
Steering couplings have towithstand extreme axial loads (10-15kN) due to crash or mountingprocedures.
Low axial stiffness is desirable forimproving the vibrational comfort.
They have to come up with aredundance in case the elastomerpart is destroyed.
As far as we know there is noradial function.
The damping elements also needto decouple acoustical excitation:
from the wheels
from sources like electrical orhydraulical actuators
the steering column as noisetransferring path couldbecome more relevant withelectrical powertrains.
Load peaks have to be reduced.
Temperature requirements varyvery much with the couplingsposition.
► Solution in Cellasto feasible?
28.05.2010 [email protected] 9
Composite technologiesMotivation for combining Cellasto® with harder materials
Multi-axial loads are common withmany automotive applications forelastomers.
Tensile and shear forces betweenCellasto and metal surfaces can betransferred via friction or chemicalbonding.
Use of friction is limited andimplicates the danger of sliding.
Steering couplings such as tube-in-tube-dampers need to
withstand high axial forces dueto light crash, mounting andrepair processes.
Prevent rotational Sliding forsome designs.
28.05.2010 [email protected] 10
Composite technologiesProduction of composite structures
Isoc
yana
te
1. Mold charging:
Semifinished products e.g. water-cutted Cellasto-profiles
Metal parts treated with bonding agent
2. Component dosing and pressure-free mold filling by static mixer
Dosing of polyol and isocyanate ► very liquid resin „Elastolit X“
„Elastolit X“ fills out fine and detailed cavities, can be reinforcedby fiber glass (e.g. for replacing metal parts)
3. A composite structure with strong adhesion between layers ofCellasto®, Elastolit X and a hard material is realized.
Polyol
1 1 2
3
28.05.2010 [email protected] 11
Composite technologiesValidating the adhesion characteristics
Cellasto-Elastolit-X material samples have been used for
finding appropriate bonding agents and
testing the system‘s adhesive strength.
The composite structure‘s adhesive strength exceeds Cellasto‘s tensilestrength which is absolutely competitive compared to rubber compounds.
These results have been further verified by destructive testing ofprototypes.
A steering coupling prototype with about 40mm in Ø and 45mm in lengthsupports up to 14kN in axial direction whereby the requirement was 13kN.
28.05.2010 [email protected] 12
Experimental prototype dataPhotos, global geometry, overview of experimental data
Different Cellasto prototypes
and rubber bushings
Comparable use of design
space:
Lengths: 25mm - 45mm
Outer-Ø: 33mm - 38mm
Thickness elastomer: 3.5–
4.5 mm
Statical and dynamical testing:
torsional
axial
radial
Axial and Radial also in
frequency range of 3kHz - 5kHz
Setting as consequence of the
first torsional load
Endurance tests
28.05.2010 [email protected] 13
Experimental prototype dataStatical torsion, preconditioning: +/-4°, 30Nm/°
Preconditioning of the specimen
Statical torque-angle-of-rotation-curve
Torsional stiffness for Cellasto prototypes higher (+less design space!)
► softer and more progressive characteristics are easily realizable
Less friction in Cellasto‘s hysteresis
The stiffening as relation of the dynamical torsional stiffness and the statical
torsional stiffness shows much lower values for Cellasto
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
torsional angle [°]
-30
-20
-10
0
10
20
30
torq
ue
[Nm
]
0 10 20 30 40 50 60
Frequenz [Hz]
0
0.5
1
1.5
2
2.5
3
stiff
enin
g(d
yn.to
rs.
/sta
t.to
rs.)
left y-axis specimen
rubber #1
Cellasto #1
Cellasto #4
rubber #2
rubber #3
Cellasto #3
Cellasto #16zone rubber
zone Cellasto®
28.05.2010 [email protected] 14
Experimental prototype dataReaction on compression load
and bushing properties for a rubber bushing
the rubber‘s spot face in the radial direction of a typical bushing is quite large
Radial loads will force the rubber to evade through a narrow channel in theaxial and circumferential direction, since the rubber‘s volume has to bemaintained
The high resistance versus this deformation is equivalent to high radialstiffnesses
the shear stresses that are used for tranferring axial and torsional loads willcause lower stiffness values
A Cellasto bushing will show a different behaviour because it can react onthe radial load with its ability to shrink
Necking
Expansion
Spot face
28.05.2010 [email protected] 15
Experimental prototype dataRadial characteristics, statical hysteresis and stiffness
The test bench‘s stiffness has to be taken into account because of a high stiffnesslevel and very small maximal deflections
Preconditioning by radial loads and torsional testing
Measurement cycles force controlled ► different deflection amplitudes
Therefore the comparability of stiffness values is constrained
The radial stiffness is much higher for the rubber mounts because of the volume-incompressiblity
► higher potential for acoustical isolation
-1 -0.5 0 0.5 1
deflection [mm]
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
forc
e[k
N]
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
deflection [mm]
0
10
20
30
40
50
stiff
ness
[kN
/mm
]
left y-axis
zone rubber
zone Cellasto®
specimen
rubber #1
Cellasto #1
Cellasto #4
rubber #2
rubber #3
Cellasto #3
Cellasto #16
28.05.2010 [email protected] 16
-2 -1 0 1 2 3
deflection [mm]
0
0.5
1
1.5
2
2.5
3
stiff
ness
[kN
/mm
]
Preconditioning of the specimen
Statical force-deflection-curve and stiffness-deflection-curve
Force-controlled ► different deflection amplitudes
Comparability is limited because of the amplitude dependency
Bushings using the same design space are statically softer
Stiffening as dynamical axial stiffness at amplitude 0.5mm divided bystatical torsional stiffness @+/-1.5Nm after setting cycles
Experimental prototype dataAxial characteristics, statical hysteresis and stiffness
left y-axis specimen
rubber #1
Cellasto #1
Cellasto #4
rubber #2
rubber #3
Cellasto #3
Cellasto #16
c(0N) [kN/mm]
0.690
0.764
1.311
0.828
0.825
0.778
0.958
0 2 4 6 8 10 12 14 16 18 20
Frequenz (Hz)
0.15
0.16
0.17
0.18
0.19
0.2
0.21
0.22
0.23
0.24
0.25
stiff
enin
g(c
_dyn
_ax
/c_s
tat_
tor
@0N
)
zone Cellasto®
zone rubber
28.05.2010 [email protected] 17
Experimental prototype dataamplitude dependency, driving situations and angle of torsion
Slow statical steering at high speeds ►acoustical isolation & stiffness around 0°
Maximal torque during parking situation
Vibrations as consequence of anuneven road surface ► axialstiffnesses / torsional stiffening?
Dynamical steering at average speeds ►statical torsional stiffness
28.05.2010 [email protected] 18
Optimisation potentialOverview of design parameters
The Cellasto bushing offers differentdesign parameters (see the picture).
The parameters directly influence theparts properties and offer optimisationpotential.
An important variation of the shownconcept are designs that do not only makeuse of tensile stress but also ofcompression.
This can be realized by non-circularprofiles.
length
Inner-Ø
density
RadialPre-compression Thickness of
precompressedCellasto
„Pure“ tensile sress Little tensile stress,Compressive stress
Extreme design,almost only compression
28.05.2010 [email protected] 19
Optimisation potentialcosts for different design concepts
Metall-Elastolit-X- normal
Metall-Elastolit-X- low cost
Elastolit-X- normal
Elastolit-X- low cost
name of the concept
outer metal sleeve treating the steel tube Cellasto sleeve
Elastolit-X moulding narrowing
Ca
.E
uro
X/S
tck
.
Ca
.–
20
%zu
mB
as
isp
reis
Ca
.–25
%zu
mB
as
isp
reis
Ca
.–
30
%zu
mB
as
isp
reis
Substitution of metal parts
reduces cost and weight
28.05.2010 [email protected] 20
ReferencesTorque transfering and decoupling parts
Torsional load change damperin cardan shaft for motorcycles
Damping element in pulleysfor start-stopp-systems
www.bmw.de
28.05.2010 [email protected] 21
Summary
Cellasto® is microcellularpolyurethane that can be usedas elastomer for sophisticatedtechnical applications.
A composite technology basedon the resin system Elastolit Xallows bonding of Cellasto tohard structures.
Part designs are realizable thatcan fulfill all requirements forsteering couplings and othertorsional busings:
tests with Tube-in-tubebushings are running.
coupling-like concepts canstill be worked out.
A steering coupling based onCellasto® offers someadvantageous characteristics:
Saving design space,substitution of metal parts(price, weight)
More freedom forrealization of L-D-curves(e.g. higher torsionalstiffness, more progression)
Very different axial andradial characteristics
The chances of these conceptsin a rapidly changing steeringsystem market are currentlyelaborated.