Microcellular polyurethane as steering coupling element Material specific properties of solutions based on Cellasto -Elastolit-X composite structures
Contents Cellasto material specific properties: Classification, material structure and production Volume compressibility Compression set, dynamical characteristics Steering couplings: Function in steering systems Requirements and different designs Composite technologies combining Cellasto with hard materials: Motivation Production technology Adhesion characteristics Cellasto -Elastolit-X prototypes Experimental prototype data Optimisation potential and comparable applications References Summary 2
Material specific properties Classification, material structure, production Cellasto = microcellular polyurethane that is applicable as technical elastomer Cellular material structure with Volume compressible air component and a rubber-like matrix high density (35-65kg/m³) Very closed cell structure Molecular level: hard & soft segments Cellasto necessitates a specialized and challenging production process. SEM-photo SEM-photo Flexible foam open cell structure Cellasto special elastomer closed cell structure Mixing the main components for Cellasto the foam rises and fills the mold 3
Material specific properties Compressive stress and effect of volume compressibility 4,5 Compression test with a cylindrical specimen: Compression and displacement of the air component At the beginning the lateral extension is negligible and raises at high compression levels The stiffness and the maximal deformability is highly dependent on the density Compressing rubber and Cellasto samples in a glas tube demonstrates the effect of volume compressibility Com pression Stress in M Pa 4, 3,5 3, 2,5 2, 1,5 1,,5, Average Compression Stress Standard Test Cylinder 1 2 3 4 5 6 7 8 9 Compression in % Compression test with rubber and Cellasto MH24-35 MH24-4 MH24-45 MH24-5 MH24-55 MH24-6 MH24-65 4
Material specific properties Setting, dynamical characteristics and others Statical compression set: Taking into account when dimensioning parts Extent of creeping very small compared to the reversible deflection Dynamical properties: Favourable stiffening properties for many applications Dependent on state of stress and material density Amplitude dependent damping Outstanding fatigue limit for compression stress Characteristics such as elongation break, tear resistance, heat aging and so on are fully competitive for many automotive applications. time [h] Density 6 kg/m³ 5 compression [%] Loss angle [ ] 14 13 12 11 1 9 8 7 6 5 f = 15 Hz, test objects: top mounts 4,5 1 1,5 2 2,5 3 Amplitude [mm] Cellasto Rubber compoud High damping Low damping
Material specific properties Examples of automotive applications Steel spring support Top mount one-path Cellasto Top mount dual-path, Jounce bumpers Air spring mount is automotive success Roll restrictor Chassis mounts Gear mount 6
Steering couplings Function in steering systems Tube-in-tube damper Steering systems need to Transmit and reinforce input from the steering wheel and give feedback about the road. Decouple acoustical excitations and vibrations excited by uneven roads. Decoupling is realized by damping elements like www.sgf.de Steering coupling 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,...) Steering gear bushing 7
Steering couplings Requirement specification Torsional stiffnesses between some Nm/ and about 3Nm/ are required. They have to come up with a redundance in case the elastomer part is destroyed. The torsional load spectrum depends on the position relative to the torque amplifier. As far as we know there is no radial function. position before the torque amplifier max. torques seldomly higher than 1-6Nm. Otherwise the torques can reach much higher values. Steering couplings have to withstand extreme axial loads (115kN) due to crash or mounting procedures. Low axial stiffness is desirable for improving the vibrational comfort. The damping elements also need to decouple acoustical excitation: from the wheels from sources like electrical or hydraulical actuators the steering column as noise transferring path could become more relevant with electrical powertrains. Load peaks have to be reduced. Temperature requirements vary very much with the couplings position. Solution in Cellasto feasible? 8
Composite technologies Motivation for combining Cellasto with harder materials Multi-axial loads are common with many automotive applications for elastomers. Tensile and shear forces between Cellasto and metal surfaces can be transferred via friction or chemical bonding. Use of friction is limited and implicates the danger of sliding. Steering couplings such as tube-intube-dampers need to withstand high axial forces due to light crash, mounting and repair processes. Prevent rotational Sliding for some designs. 9
e cy an at Is o Production of composite structures l lyo Po Composite technologies 3 1 1 2 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 reinforced by fiber glass (e.g. for replacing metal parts) 3. A composite structure with strong adhesion between layers of Cellasto, Elastolit X and a hard material is realized. 1
Composite technologies Validating 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 tensile strength which is absolutely competitive compared to rubber compounds. These results have been further verified by destructive testing of prototypes. A steering coupling prototype with about 4mm in Ø and 45mm in length supports up to 14kN in axial direction whereby the requirement was 13kN. 11
Experimental prototype data Photos, 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 12
Experimental prototype data left y-axis rubber #1 Cellasto #1 Statical torsion, preconditioning: +/-4, 3Nm/ stiffening (dyn. tors. / stat. tors.) 3 torque [Nm] 2 1-1 -2-3 -2-1.5-1 -.5.5 1 1.5 2 torsional angle [ ] specimen Cellasto #4 rubber #2 3 rubber #3 Cellasto #3 2.5 zone rubber Cellasto #16 2 1.5 zone Cellasto 1.5 1 2 3 4 Frequenz [Hz] 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 13 5 6
Experimental prototype data Reaction on compression load and bushing properties for a rubber bushing Necking Spot face the rubber s spot face in the radial direction of a typical bushing is quite large Expansion Radial loads will force the rubber to evade through a narrow channel in the axial and circumferential direction, since the rubber s volume has to be maintained The high resistance versus this deformation is equivalent to high radial stiffnesses the shear stresses that are used for tranferring axial and torsional loads will cause lower stiffness values A Cellasto bushing will show a different behaviour because it can react on the radial load with its ability to shrink 14
Experimental prototype data left y-axis specimen rubber #1 Radial characteristics, statical hysteresis and stiffness Cellasto #1 Cellasto #4 5 6 5 rubber #3 Cellasto #3 stiffness [kn/mm] 4 force [kn] rubber #2 zone rubber 3 2 1-1 -2-3 4 Cellasto #16 3 2 sto a l l e ec n zo 1-4 -5-6 -1 -.5.5 1 -.8 -.6 -.4 deflection [mm] -.2.2 deflection [mm] The test bench s stiffness has to be taken into account because of a high stiffness level 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 volumeincompressiblity higher potential for acoustical isolation.4 15.6.8 1
Experimental prototype data left y-axis.25 3.24 2.5.23 stiffness [kn/mm] stiffening (c_dyn_ax / c_stat_tor @N) Axial characteristics, statical hysteresis and stiffness.22.21zone rubber.2.19.18 sto a l l e ec n zo.17.16.15 2 4 6 specimen c(n) [kn/mm] rubber #1.69 Cellasto #1.764 Cellasto #4 1.311 rubber #2.828 rubber #3.825 Cellasto #3.778 Cellasto #16.958 2 1.5 1.5 8 1 12 14 16 18 2-2 Frequenz (Hz) -1 1 deflection [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.5mm divided by statical torsional stiffness @+/-1.5Nm after setting cycles 16 2 3
Experimental prototype data amplitude dependency, driving situations and angle of torsion Slow statical steering at high speeds acoustical isolation & stiffness around Vibrations as consequence of an uneven road surface axial stiffnesses / torsional stiffening? Maximal torque during parking situation Dynamical steering at average speeds statical torsional stiffness 17
Optimisation potential Overview of design parameters The Cellasto bushing offers different design parameters (see the picture). Radial Pre-compression density The parameters directly influence the parts properties and offer optimisation potential. Thickness of precompressed Cellasto Inner-Ø An important variation of the shown concept are designs that do not only make use of tensile stress but also of compression. This can be realized by non-circular profiles. Pure tensile sress Little tensile stress, Compressive stress length Extreme design, almost only compression 18
Optimisation potential Ca. Euro X/Stck. Sub stitu redu tion o f ces cost metal p ar and weig ts ht Metall-Elastolit-X - normal Metall-Elastolit-X - low cost Elastolit-X - normal Ca. 3% zum Basispreis Ca. 25% zum Basispreis Ca. 2% zum Basispreis costs for different design concepts Elastolit-X - low cost name of the concept outer metal sleeve Elastolit-X moulding treating the steel tube narrowing Cellasto sleeve 19
References Torque transfering and decoupling parts www.bmw.de Torsional load change damper in cardan shaft for motorcycles Damping element in pulleys for start-stopp-systems 2
Summary Cellasto is microcellular polyurethane that can be used as elastomer for sophisticated technical applications. A composite technology based on the resin system Elastolit X allows bonding of Cellasto to hard structures. Part designs are realizable that can fulfill all requirements for steering couplings and other torsional busings: tests with Tube-in-tube bushings are running. coupling-like concepts can still be worked out. A steering coupling based on Cellasto offers some advantageous characteristics: Saving design space, substitution of metal parts (price, weight) More freedom for realization of L-D-curves (e.g. higher torsional stiffness, more progression) Very different axial and radial characteristics The chances of these concepts in a rapidly changing steering system market are currently elaborated. 21