ENERGY DISSIPATION OF AN ELASTOMERIC FRICTION DAMPER IN SUB-ZERO TEMPERATURES

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1 3-04 PROBLEMY EKSPLOATACJI MAINTENANCE PROBLEMS 83 Aam JUNGOWSKI, Jerzy OSIŃSKI Warsaw University of Technology, Institute of Machine Design Funamentals Warsaw ENERGY DISSIPATION OF AN ELASTOMERIC FRICTION DAMPER IN SUB-ZERO TEMPERATURES Key wor Damper, elastomer, friction, research, temperature. Abstract The impact of sub-zero temperatures on energy issipation levels is very essential in Polish climatic conitions. Experimental research results (mae on shimmy self-oscillation amper use in a laning gear of a small plane) show that energy issipation level of an elastomeric friction amper (in opposition to hyraulic) is ecreasing with ropping temperature. This situation is a result of significant ifferences in thermal expansion of materials, especially the specific elastomer feature large thermal expansion (elastomer have thermal expansion about 50 times larger than steel). Simulation of amper work, using Finite Element Metho, confirme those epenencies. An estimation of the friction coefficient has been mae between the elastomer an aluminium surface. It has been ascertaine that those values are relatively small, from 0.3 initial moment eclining to then rising to Introuction Significant changing of energy issipation levels is a very important problem uring the operation of a vibration amper (shock absorber) in the

2 84 PROBLEMY EKSPLOATACJI MAINTENANCE PROBLEMS 3-04 highly changing temperature conition of Polish climate. Hyraulic ampers have a high gain of energy issipation levels in temperatures below 0 egrees Centigrae, an it is cause by the increase of the viscosity of flui cause by ropping temperature. Elastomeric friction ampers may be the alternative, because they have a smaller sensitivity for changing working temperature. This article represents an assessment of temperature impact on elastomeric friction ampers use to amping shimmy oscillation that occurs in laning the gear of a small plane.. Energy issipation level research of amper The tests were mae on amper applie on a Cessna areoplane shimmy type, serial number P/N-SE-069-, an patent number 6,90,038, prouce by LORD. The amper consists of a tubular aluminium case, an elastomeric friction part, an piston ro. The elastomer is rigily fixe to the piston ro. elastomer iameter case iameter 3 piston ro iameter Fig.. Section scheme of amper The research was one in the Institute of Machine Design Funamentals (Warsaw University of Technology) in the Department of Mechanics Laboratory an also in the Institute of Funamental Technological Research (Polish Acaemy of Sciences) in the Division of Experimental Mechanics. Figure represents a recor of the amper work cycle at 0 Hz frequency (typical range of amper work) at a temperature of -5 C. Figure 3 shows, for comparison, the same cycle at 5 C.

3 3-04 PROBLEMY EKSPLOATACJI MAINTENANCE PROBLEMS 85 Fig.. Damper work cycle in -5 C temperature Fig. 3. Damper work cycle in 5 C temperature The measure of issipation energy is the area insie one cycle, an to assess this, a level amene program from work [] was use. Dissipation energy amounte to Nm in the plus temperature cycle, an below zero it was Nm, which is 60% lower. It is noticeable that the elastomeric friction amper has a lower issipation energy level below 0 C in opposition to the hyraulic amper. A lot of research conucte by the authors confirms that it is

4 86 PROBLEMY EKSPLOATACJI MAINTENANCE PROBLEMS 3-04 a very powerful epenence (the isparity of issipation levels compoun with increasing frequency). It was notice that one cycle issipation energy insignificantly grows with escalating frequency, an the issipation energy in relation to time quickly increases with increasing frequency.. Temperature sensitivity analysis of elastomeric friction amper The main cause of energy issipation is friction between the elastomeric element an the insie surface of the casing of the elastomeric friction amper mae of aluminium. Energy issipation levels are epenent on contact pressure between those elements. Changes of imensions, which have an impact on pressure, are cause by thermal expansion of materials. Differences in coefficients of thermal expansion are very large. There are some average values, which are use in textbooks for example: [] steel 0-6, aluminium 0-6 K, elastomer K If we take the simple moel casing, the axisymmetric elastomer an steel elements will be subject to temperature changes; therefore, we can calculate longituinal strain using this formula: ε x = α T an ε x = α T, an for transverse strain: ε x = α T an ε x = α T. Changes of transverse imension can be calculate using the following formulas: W = ε α T an W = ε α T. The ifference of the above values is a measure of interference, an it can be compute with the following formula: ( W W ) = T ( α α ) U. If U takes the zero value, the whole system will be unaffecte by changing temperature as follows: α = α. Then calculation of case iameter is α =, α. K

5 3-04 PROBLEMY EKSPLOATACJI MAINTENANCE PROBLEMS 87 If numerical values are use = 6 40 () It means that the amper casing must be 40 times wier than the elastomer element, an this is impossible to achieve because amper will be too heavy. In the case when we take all amper elements (Fig. ), we can use the following formulas: Ε x = α T ; Ε x = α T ; Ε x3 = α3 T. The very small strain, cause by rigi fix of elements an 3, in transverse imension evaluation was omitte. So the following formulas will be use: W = α T W = α T, W3 = 3α 3 T 3, Interference U is calculate from U = ( W + W3) W = T ( α + 3α 3 3 α ) () Taking a conition of zero value α + α α = 0, α + α α α =, = + α α α = , so = The conition of zero value can be achieve by changing of imensions increasing 3 iameter (insie steel shaft piston ro) or leaving 3 iameter (elastomer element) without moification. It is possible to fulfil these conitions with a change of casing material from aluminium or steel to plastic with a larger coefficient of thermal expansion. The analysis has a qualitative nature, an

6 88 PROBLEMY EKSPLOATACJI MAINTENANCE PROBLEMS 3-04 precise calculations require consiering many nonlinear phenomena. It can be compute by numerical analysis. 3. Simulation using Finite Element Metho The Finite Element Metho was use to analyse the elastomeric friction amper work cycle using the ABAQUS program. All nonlinear phenomena were taken into account, i.e. the contact problem with friction, the large eformation of the elastomer, an they were escribe by a reuce sixth orer polynomial moel. The moel was mae from cubic elements, an the calculation was mae with retention of all Gauss points, without reuce integration. The amper was mae as an interference fit. The external elastomer iameter is larger than the internal case iameter at first. The first stage is the simulation of making an interference fix (thermal or mechanical). Then stages of temperature changes an work cycles can be repeately mae. This task requires the elineation of the thermo-mechanical problem with the possibility of changing temperature an tension. The elastomer was escribe by a reuce polynomial moel, accoring to test result shown above, with the coefficient changing as a kg function of temperature function as in [3], a ensity of 00 m 3, an amping proportional to the inert matrix with coefficient near 0 at 5 o C an increasing to a value of 000 at -30 o C. Aluminium an steel was escribe as elastic materials. A simulation of pressing the roller fixe to the elastomer element was mae, with an insert to zero point (work start). It is possible to assemble the amper by han, an we presuppose that the amper was mae that way, so we can acknowlege the obtaine results as real values. The coefficient of friction was assesse by a comparison of obtaine pressure values (average values on the surface) with figures from experimental research. The coefficient of friction was 0.3 at the start (spee 0), an ecrease at low spees. The minimum occurre at m/s an ha a value of Next, the coefficient of friction increase, an at maximum spee (in the analysis was 0. m/s), it ha a value of These coefficient of friction values will be use in further calculations. Execute examples confirme the nature of the phenomenon notice uring experimental research, i.e. issipation energy significantly recees with lower working temperatures. This phenomenon can be consierably reuce by changing elements imensions, accoring to analysis from Section, an it was valiate by subsequent calculate examples. In conclusion, it can be sai that an elastomeric friction amper is a possible alternative to the hyraulic amper in appliances working in an environment with changing temperatures. It is possible to achieve more

7 3-04 PROBLEMY EKSPLOATACJI MAINTENANCE PROBLEMS 89 regularity of the energy issipation level in an environment with a high fluctuation of temperature. Using of this type amper can be especially avantageous in winter work conitions (sub-zero temperature). References. Żach P.: Strukturalna ientyfikacja właściwości sprężysto-tłumiących materiałów hiperokształcalnych, Biblioteka Problemów Eksploatacji Wyawnictwo Instytutu Technologii Eksploatacji w Raomiu, 03.. Ashby Michael F., Jones Davi R.H.: Materiały inżynierskie, Tom kształtowanie struktury i właściwości, obór materiałów WNT, Warszawa Anna Boczkowska, Kamil Babski, Jerzy Osiński, Piotr Żach - Moelowanie charakterystyki przy ściskaniu oraz właściwości użytkowe materiałów poliuretanowych stosowanych w buowie maszyn POLIMERY 7/8 tom LIII, Dyk J., Jungowski A. Osiński J.: Irregular motion in the vibration systems with an elastomeric friction element, Monografia Wyawnictwa Politechniki Łózkiej DYNAMICAL SYSTEMS - Applications, Eitors: Awrejcewicz, J., Kaźmierczak, M., Olejnik, P. an Mrozowski, J.), Łóź, December 5, Polan, Rozpraszanie energii w tłumiku elastomerowo-ciernym w warunkach ujemnych temperatur Słowa kluczowe Tłumik, elastomer, tarcie, baania, temperatura. Streszczenie Problem wpływu ujemnych temperatur na zmienność poziomu rozpraszania energii w polskich warunkach klimatycznych ma ważne znaczenie. Przestawione w pracy wyniki baań oświaczalnych (wykonane na przykłazie tłumika rgań samowzbunych typu shimmy w powoziu małego samolotu) pokazują, że tłumiki elastomerowo-cierne (w przeciwieństwie o hyraulicznych) charakteryzują się obniżaniem poziomu rozpraszania energii z spakiem temperatury. Przyczyny takiej sytuacji wynikają ze znacznych różnic rozszerzalności cieplnej materiałów, w szczególności specyficznych cech elastomerów, istotna jest uża (około pięćziesięciokrotnie większa o stali) rozszerzalność cieplna elastomerów. Symulacja ziałania tłumika wykonana Metoą Elementów Skoń-

8 90 PROBLEMY EKSPLOATACJI MAINTENANCE PROBLEMS 3-04 czonych potwierziła te zależności. Wykonano również oszacowanie wartości współczynnika tarcia pomięzy elastomerem a ścianą wykonaną z aluminium stwierzono, że są to wzglęnie małe wartości o 0,3 w chwili początkowej spaające o wartości 0,085 i następnie rosnące z prękością o 0,56.