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Study of an experimental prototype simulating a mechanical vibration damper with rotational friction pairs

https://doi.org/10.38013/2542-0542-2020-4-77-86

Abstract

This paper presents the results of a computational and experimental study of an experimental prototype simulating an aircraft engine vibration damper installed in an engine nacelle on the wing pylon of a mainline aircraft.

For citation:


Vermel V.D., Zichenkov M.Ch., Koryakin A.N., Paryshev S.E. Study of an experimental prototype simulating a mechanical vibration damper with rotational friction pairs. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2020;(4):77-86. https://doi.org/10.38013/2542-0542-2020-4-77-86

Introduction

Typically, aircraft structures are exposed to vibrations caused by a variety of factors. One of the most influencing factors is the impact of aircraft engines throughout the flight. Along with an adverse effect on on-board equipment, crew and passengers, vibrations result in cumulative structural fatigue damage. The vibration level can be reduced by stiffening the aircraft structure, but it requires a heavier weight. A more efficient method that can help suppress unwanted vibrations is the application of special-purpose dampers, which are successfully implemented in various technical applications.

The requirements to aircraft structure vibration dampers include small weight, low operational costs, and high reliability. Efficient vibration damping of individual objects (for instance, the engine installed on the aircraft wing pylon) is often hampered, because there is no option to install a damper at best suitable points where displacements reach maximum values and loads on a damper are minimal. The only possible way is to install a damper in close proximity to the mounting supports of the object to be damped or install it instead of one of the supports. In this case, conditions for damper placement leave much to be desired as vibrations are to be damped together with intense dynamic loads at extremely low amplitudes. In order to meet such requirements, a dry friction damper was developed where low reciprocal vibratory displacements are converted into considerable angular displacements of shafts in rotational pairs with friction surfaces [1][2][3]. Specifications of patents [1] and [2] contain a broad list of similar inventions which efficiently operate if securely attached to the objected to be protected, but they squander many advantages when operated in the above-mentioned specific conditions. A specific feature of the damper under study is the dependence of friction factors in rotational pairs due to loads on friction surfaces and velocities of their relative motion. For calculations, we considered the impact of structural strength and inertia masses on the performance of the damper.

1. Description of damper design

The schematic diagram of the damper is shown in Figure 1. The damper comprises base 1 with attached semiannular spring 2. Ring 3 is attached to the end of the semiannular spring. Ring 4 is inserted in the hole in base 1. Eccentric shaft 5 with the bracket and shaft attached to its end is mounted inside ring 4. Ring 6 with vertical tie rod shaft 7 is mounted in the hole in the eccentric shaft. The shaft attached to the end the of the bracket and ring 3 form a rotational pair with rotation axis А. The vertical tie rod shaft and ring 6 form a rotational pair with rotation axis В. The eccentric shaft and ring 4 form a rotational pair with rotation axis С. Axes А, В, and С are located in the same plane. Internal cylindrical surfaces of all the rings have tribological coatings.


Fig. 1
. Diagram of special-purpose friction damper: a) general view, b) sectional view in plane of symmetry in loaded condition

When an external force acts along vertical rod 7, shaft 5 rotates around rotation axis С together with the end of annular spring 2 attached to the bracket, straining the latter and loading up the friction surfaces of all the rotational pairs. The work of friction torques in rotational pairs results in the dissipation of vibration motion energy of the object being damped into dissipated heat energy. The relation (Fig. 1b) between force Fon the damper tie rod, the spring reaction, and the parameters of shaft displacement is represented below (for details, see [4]):

where:

FB – force acting on vertical tie rod,
RA – spring force acting on turning joint А,
ρB, ρC, ρA – friction circle radii,
ρB = fB·rB, ρC = fC·rC, ρA = fA·rA,
rB, rC, rA – radii of shafts relative to axes В, С, and А, respectively,
fB, fC, fA – friction factors of shafts contacting the internal surfaces of the rings in rotational pairs with rotation axes В, С, and А, respectively, ωk – rate of change of angle αk

At that:

sB = ΔB sin αk,
ΔB cos αk > ρC + ρB. (2)

Expression ΔB cos αk > ρC + ρB is the basic condition for implementation of damper motion. Failure to meet this condition results in jamming of the damper.

The theoretical dependence of load FB on the damper tie rod on its displacement is shown in Figure 2. Friction factors are assumed to be constant values, and the tie rod is considered absolutely rigid. The damper structure is considered weightless.


Fig. 2
. Theoretical dependence of load applied to damper tie rod on its displacement at constant friction factors in rotational pairs

The hysteresis loop area shown in Figure 2 is equal to the operation of the damper per vibration cycle and is calculated using the following formula:

where AB and NA – amplitudes of parameters sB and RA respectively.

According to formula (3), it is possible to make a damper with the required amount of work executed per vibration cycle by selecting the eccentricity value ΔB at relatively small friction circle radii. The advantage of the damper design allows to use it in rotational pairs with the coating of friction surfaces with low friction factors, and therefore with high wear resistance.

2. Results of experimental and analytical studies

The damper under study (Fig. 3) is one of intermediate prototype versions of the damper [2]. A camshaft with adjustable eccentricity value ΔbB was used as the eccentric shaft. Its principle of operation is similar to the principle described
above. Antifriction self-lubricating organoplastic (ASO) used for coating internal surfaces of bearing rings is well-known and widely used material in mechanical engineering and aerospace industry (for details, see [3] and [4]). High wear resistance, a wide range of operating temperatures and good load bearing characteristics allowed to use this material as a reliable coating in friction bearings, in particular in the series of the ShLT bearings produced by Saratov Bearing Plant. The damper design allows to adjust backlash sizes in rotational pairs in accordance with the specification of patent [2].


Fig. 3
. Installation of prototype (demonstration mock-up) of damper on test bench

During the experiment, we set sinusoidal motion of the damper tie rod with certain frequencies (from 4 to 5.5 Hz), amplitudes (from 0.15 to 1.0 m) and a number of cycles for the individual stage of continuous motion (at least 150 vibration cycles). Time dependencies of the damper’s tie rod displacements and the acting force were recorded. Interruption duration was not longer than the time period required to change the amplitude value or cycle frequency in the computer.

Figure 4 shows experiment results as an example. The displacement of the vertical tie rod end is designated by symbol y as it differs from the vertical displacement sB of axis B of the camshaft due to deformation of the vertical tie rod. The action of internal inertia forces in the damper taken into account for calculations also requires to assume that there is some difference between the forces acting on axis B and on the vertical tie rod end. That is why the force at the vertical tie rod end is designated by symbol FuB. Experiment and calculation results agree satisfactorily. Blue lines in diagrams depict the experimental characteristics, while red lines – the design characteristics. Non-linearity of characteristics is determined by the dependence of friction factors in rotational pairs on pressures on friction surfaces, velocities of relative motion of the latter, elasticity and delayed action of the damper structure. The peculiarities of application of the ASO coating onto internal surfaces of bearing rings, which feature certain undulation, entail asymmetry of local pressures on friction surfaces at variable loads, and therefore the asymmetry of the resulting characteristics. The design featuring adjusted eccentricity ΔB along with the method of attachment of the annual ring also contributed to the asymmetry. The asymmetry indicates certain drawbacks of a particular design, which shall be eliminated in further products.


Fig. 4
. Dependence of load applied to damper tie rod on its vertical vibrations

This damper was manufactured in order to determine dependencies of friction factors of the selected tribological coatings on pressures on working surfaces of rotational pairs and velocities of their relative motion, using a test bench. The damper design, in particular adjustable eccentricity ΔB of the camshaft and mounting attachments on a test bench, was selected in accordance with the experiment objective. That is why the damper’s overall dimensions and weight increased. This is acceptable for a device developed to achieve a particular goal of the experiment, but unacceptable for operation. The damper parts, including the spring, are made of steel 30KhGSA. Based on the conducted studies, we determined the parameters of of the damper’s working specimen [2] for its installation on the engine pylon of a mainline aircraft, the parameters of which are near to those of the MS-21 aircraft with the engine weight of 4000 kg. Comparison of overall dimensions of the working specimen and the prototype of the damper is shown in Figure 5. It is worth mentioning that with loads on the working specimen of the damper increased by 5 times in comparison to the prototype, its weight was reduced by 1.63 times.


Fig. 5
. Comparison of working specimen and prototype in unified scale

Patent specification [1] describes the damping of free vibrations of the beam (Fig. 6 shows the original image) of length L1 = 8.4 m with three concentrated masses. The dampers (demonstration mock-ups) are attached to the beam at distances L2 = 0.2 m from supports with the beam and dampers equipoised (without initial forces). The beam stiffness is equal to the stiffness of a box structure made of two channel sections No. 14 as per GOST 8240-56 (moment of inertia J = 2×491 cm4). Weight values are as follows: middle weight – m1 = 97.5 kg, two extreme positions – m2 = 440 kg. For calculations, we assumed that the beam weight is negligibly small in comparison to the weight mass. Free vibration frequencies without damping are as follows: ω1 = 6.4 Hz, ω2 = 47.9 Hz, ω3 = 57.5 Hz. Friction factors in rotational pairs are assumed to be constant values without account of damper deformation.


Fig. 6
. Diagram of the beam with concentrated masses and dampers from patent specification [1]

This work represents the similar calculation of the same object (Fig. 6) with the real characteristic of the damper under study. Free vibrations of the beam were investigated upon application of a rectangular impulse of force of 2.9 kN with duration of action of 0.05 s. Figures 7–9 show calculation results. We obtained almost similar dependencies for the left-hand and right-hand dampers as expected. The first mode frequency reached 8.3 Hz instead of 6.4 Hz specific for free vibrations without dampers. The exponent enveloping local minimum values of the dependence shown in Figure 7 has the value of –2.5 t. This figure corresponds to logarithmic vibration decrement of 0.3. This example illustrates high performance of dampers in close proximity to the supports of the beam to be protected.


Fig. 7
. Free vibrations in the middle of the beam

Fig. 8
. Free vibrations of the connection point between damper and beam (LH and RH dampers)

Fig. 9
. Dependence of load applied to damper on displacement of the connection point between damper and beam (LH and RH dampers)

Based of the study results, damper versions with rotational friction pairs using the ShLT series spherical bearing were developed. The methods of damper installation in the engine pylon of a mainline aircraft, observing basic proportions, are shown in Figures 10 and 11. The weight data are specified in Figure 11. These dampers feature the improved method of backlash adjustment in rotational pairs and higher structural strength. Besides, in collaboration with Rostov State Transport University, the method of inservice diagnostics of damper dissipative characteristics was proposed. Backlash adjustment in rotational pairs along with in-service diagnostics of their dissipative characteristics, plus operational experience in application of the ASO coatings in aviation allows to expect the acceptable service life of dampers in the course of operation.


Fig. 10
. Installation of damper in front of front supports of the engine pylon of mainline aircraft

Fig. 11
. Installation of damper as the rear shock-absorbing support of the engine pylon of mainline aircraft

Mechanical engineering involves a variety of scenarios that require the damper either to be installed near supports or to be used as a shockabsorbing support in order to suppress vibrations. The specified examples of such application can be useful as study cases for designers of various objects exposed to vibrations. We should emphasize that the shown dampers were developed regarding particular situations. For any individual operating conditions a customized damper configuration may apply. As only rotational pairs are the key structural elements of the damper, jointing efforts of designers of a particular object and the damper is more reasonable solution than the development of a multi-purpose damping device. Problems of vibration damping in development of aerospace defence equipment and facilities are no less urgent than in aviation, but they are familiar to and can be understood only by particular specialists. In comparison to other fields of design and development, joint efforts of designers are definitely required for vibration damping.

Conclusion

Calculation gives satisfactory results indicating the behaviour of variable loads on the damper within the investigated range of combination of friction velocity (up to 0.1 m/s) and pressures on friction surfaces (up to 12 MPa) in temperature conditions of the completed experiment. We should note that the ASO coating application technique failed to ensure total absence of undulation during the experiment. Fitting of shafts in rotational pairs was observed at some local points and changed in the course of fitting-in. Therefore, the designed pressure and real local pressures on contact surfaces between shafts and coatings may differ from one another.

Studies prove that efficient working specimens of dampers with rotational friction pairs can be designed. The above examples of vibration damping of the beam on hinged supports and the engine installed in the aircraft wing pylon show broad options for successful implementation of such a damper in various technical applications, including aerospace defence articles.

References

1. Механический демпфер низкоамплитудных колебаний с вращательными парами трения: пат. 2544046 РФ: МПК F16F 7/06 / О.Е. Барышников [и др.]; заявитель и патентообладатель Федеральное государственное унитарное предприятие «Центральный аэрогидродинамический институт им. проф. Н.Е. Жуковского. № 2013134428/11; заявл. 24.07.2013; опубл. 27.01.2015. Бюл. № 3. 14 с.

2. Демпфер низкоамплитудных колебаний: пат. 181778 РФ: МПК F16F 7/06 / О.Е. Барышников [и др.]; заявитель и патентообладатель Федеральное государственное унитарное предприятие «Центральный аэрогидродинамический институт им. проф. Н.Е. Жуковского. № 2017136536; заявл. 17.10.2017; опубл. 26.07.2018. Бюл. № 21.

3. Дроздов Ю.Н. и др. Трибологические свойства и применение антифрикционных покрытий в шарнирах и подшипниках скольжения - демпферах колебаний авиационных конструкций // Результаты фундаментальных исследований в прикладных задачах авиастроения: сб. ст. / Российская академия наук, Центральный аэрогидродинамический институт имени проф. Н.Е. Жуковского (ЦАГИ). М.: Наука, 2016. С. 461-472.

4. Вермель В.Д. и др. Разработка и исследование механического демпфера с вращательными парами трения скольжения // Результаты фундаментальных исследований в прикладных задачах авиастроения: сб. ст. / Российская академия наук, Центральный аэрогидродинамический институт имени проф. Н.Е. Жуковского (ЦАГИ). М.: Наука, 2016. С. 445-460.


About the Authors

V. D. Vermel
Central Aerohydrodynamic Institute (TsAGI)
Russian Federation

Vermel Vladimir Dmitrieviich - Dr. Sci. (Engineering), Sectoral Head. Research interests: mathematics, mechanics, digital models of motion trajectories, mechanics of technological processes, aviation, control theory, management and automation of production technologies.

Zhukovsky, Moscow region


M. Ch. Zichenkov
Central Aerohydrodynamic Institute (TsAGI)
Russian Federation

Zichenkov Mikhail Cheslavovich - Cand. Sci. (Engineering), Deputy General Director, Head of the Aircraft Strength Department, Research interests: aviation, aeroelasticity, aerodynamics, mathematics, mechanics, oscillation theory, static and dynamic strength, aircraft, strength and aeroelasticity.

Zhukovsky, Moscow region


A. N. Koryakin
Central Aerohydrodynamic Institute (TsAGI)
Russian Federation

Koryakin Alexander Niikolaevich - Cand. Sci. (Engineering), Leading Researcher, Research interests: aviation, aeroelasticity, aerodynamic experiment, mathematics, mechanics, vibration theory, static and dynamic strength, aircraft, strength and aeroelasticity.

Zhukovsky, Moscow region


S. E. Paryshev
Central Aerohydrodynamic Institute (TsAGI)
Russian Federation

Paryshev Sergey Emiilievich - Cand. Sci. (Engineering), Head of the Department for Strength, Load and Aeroelasticity Aircraft Standards, Research interests: aviation, aeroelasticity, aerodynamics, mathematics, mechanics, vibration theory, static and dynamic strength, aircraft, strength and aeroelasticity.

Zhukovsky, Moscow region


For citation:


Vermel V.D., Zichenkov M.Ch., Koryakin A.N., Paryshev S.E. Study of an experimental prototype simulating a mechanical vibration damper with rotational friction pairs. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2020;(4):77-86. https://doi.org/10.38013/2542-0542-2020-4-77-86

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