Foil bearings have been attracting considerable attention for their applications to micro turbomachinery, such as blowers and compressors, because of their excellent stability at high speeds and durability in high-temperature environments. This paper investigates experimentally and numerically the high-speed instability of a rotor supported by small aerodynamic foil journal bearings. Two types of foil journal bearings were prepared: a first-generation bump-type foil bearing and a dimple-type foil bearing; these consist of a top foil and a support foil with bumps or dimples, respectively. The dynamic characteristics of a support foil using the frequency response and the threshold speed of instability at high speeds were measured experimentally. Furthermore, the numerical threshold speed of instability was obtained using the nonlinear orbit method. It was confirmed experimentally and numerically that a 6 mm diameter rotor with a mass of 4.7 g supported by either of the two types of foil journal bearing treated in this paper could rotate stably at speeds of more than 760,000 rpm.

Due to thermodynamically adverse effects of liquid-type lubricants on the operation of air conditioning and refrigeration compressors, oil-less compressors are of current research interest. Under such conditions, advanced tribologically coatings are needed for critical interacting surfaces to ensure reliable operation under oil-less conditions. In this paper, seven commercially available polymeric coatings (PTFE-, PEEK-, resin- and fluorocarbon-based coatings) coated on gray cast iron were tribologically evaluated through unlubricated oscillatory experiments simulating a realistic wrist pin interface in a piston-type compressor. The experiments were performed in the presence of the environmentally friendly (as far as minimal global warming potential is concerned) carbon dioxide refrigerant, simulating oil-less piston-type compressors. It was found that polymer-based coatings exhibited excellent tribological performance, and in general PTFE-based coatings performed better than PEEK-based coatings. Also, their friction and wear behavior is affected by additives, which polymeric coatings were blended with. Specifically, PTFE coatings blended with pyrrolidone showed relatively low friction coefficient, and coatings blended with MoS 2 had higher wear resistance.

The aim of this study is to develop an interferometric technique to measure micron level clearances occurring in small piston-cylinder arrangements; such as hermetically sealed compressors. A Cartesian model of the piston cylinder arrangement is manufactured and lateral and vertical motions are generated via a step motor, and micrometers, respectively, to simulate the piston motion and the change of clearance during the motion. Experiments are conducted in air and a lubricant. Measurements show the important effect of surface waviness of the mirror and the effect of the motion of the carriage at micron level measurements.

The aim of this study is to develop an interferometric technique to measure micron level clearances occurring in small piston-cylinder arrangements; such as hermetically sealed compressors. A Cartesian model of the piston cylinder arrangement is manufactured and lateral and vertical motions are generated via a step motor, and micrometers, respectively, to simulate the piston motion and the change of clearance during the motion. Experiments are conducted in air and a lubricant. Measurements show the important effect of surface waviness of the mirror and the effect of the motion of the carriage at micron level measurements.

Natural refrigerants have gained significant interest due to environmental reasons. Among different natural refrigerants such as water, air, and ammonia, Carbon dioxide (CO 2 ) is the main candidate to replace commonly used hydrofluorocarbon (HFC) refrigerants in air-conditioning compressors due to its environmental benefits. One of the main advantages of implementing CO 2 , from an environmental point of view, lies in its relative low global warming potential (GWP) which is three orders of magnitude lower than commonly used HFC refrigerants. Coupled with the new refrigerants is also the need to use some form of protective coatings on the tribopairs. In this study unlubricated (presence of CO 2 refrigerant only) experiments were performed to evaluate the tribological performance of three different polymeric-based coatings deposited on Durabar G1 gray cast iron disks. Specifically, Fluorolon 325, Impreglon 1704 PEEK, and PEEK/Ceramic/ PTFE blends were tested against 52100 hardened steel pins. Results showed lower friction coefficient and wear in the Fluorolon 325 case compared to the Impreglon 1704 PEEK and PEEK/Ceramic/PTFE coatings. These results are comparable with earlier studies using PTFE-based coatings [1].

As conventional refrigerants phased out, Hydrocarbons such as Isobutane emerged as the main alternative in refrigeration industry. However, compatibility of these new ozone friendly refrigerants with compressor oils and bearing materials need to be evaluated. This study investigates wear performance of Titanium and Niobium added MoS 2 coatings running against 100Cr6 steel under isobutane exposure. Wear performance is established under starved lubrication condition by comparing the maximum achieved sliding distance before the sudden increase and fluctuation of the friction curve. Sinter steel samples were coated with MoS 2 , MoS 2 -Ti, MoS 2 -Nb thin films by magnetron sputtering. The concept of durability distance has been defined as the distance until a sudden fluctuation of the friction curve is observed. It was found that addition of Titanium and Niobium adversely affects wear performance of the MoS 2 coatings under isobutane exposure. This can be attributed to the possible degradation of MoS2 lamellar structure that acts as solid lubricant under such exposure.

This work describes in details the steps involved within the mathematical modelling of multibody systems (rigid and flexible) interconnected via controllable thin fluid films. The dynamics of the mechanical components are described with help of multibody dynamics and finite element method. In this paper, the methodology is applied to reciprocating machines such as hermetic reciprocating compressors and internal combustion engines. In previous studies [1], it has been shown that for a light duty vehicle, the friction losses may reach until 48% of the total energy consumption of an engine and from that, almost 30% are coming from bearings and crankshaft. Therefore, considering that the dynamics of the fluid films in the journal bearings can be actively controlled by means of different types of actuators, allowing significant reduction of wear and vibrations, one of the aims of this paper is to study the feasibility of applying active lubrication to the main journal bearings of reciprocating machines. In this framework the paper gives a theoretical contribution to the combined fields of fluid-structure interaction and active vibration control. The hydrodynamic pressure distribution for an active lubricated finite journal bearing dynamically loaded can be calculated by numerically solving the modified Reynold’s equation [2], by means of finite-difference method and integrated over the pressure area in order to obtain the dynamic reaction forces among components. These forces are strongly nonlinear and dependent on the relative kinematics of the system. From the point of view of active lubrication and specifically considered the case of a dynamically loaded journal bearing, the injection pressure should be controlled in the time domain. However, taking into account that the pressures and reaction forces in a reciprocating machine have a cyclic behaviour, the fluid film thickness of the main bearings may be modified by controlling the oil pressure injection, depending on the crank angle and the load bearing condition. It can be mentioned that the pressure and flow may be controlled by mechanical cam systems, piezoelectric nozzles [3] [4] or servovalves [5] [6], therefore, an adequate control strategy has to be defined. The fluid film forces are coupled to the set of nonlinear equations that describes the dynamics of the mechanical system. Such a set of equations is numerically solved giving some insights into the following parameters: a) maximum fluid film pressure, b) minimum fluid film thickness, c) maximum vibration levels and d) viscous frictional forces. The behaviour of such parameters is investigated when the system operate with conventional hydro-dynamic lubrication, passive hybrid lubrication and controlled hybrid lubrication.

Foil gas bearings are self-acting hydrodynamic bearings made from sheet metal foils and are used in selected lightly loaded, highspeed turbo-machines such as compressors and small microturbines. The general lack of familiarity of the foil bearing design and manufacturing process has hindered their widespread dissemination. Using information from publicly available literature, this paper demonstrates design, fabrication and performance testing of first and second generation bump style foil bearings and serves as an effective starting point for new Oil-Free turbomachinery development activities.

Squeeze Film Dampers (SFDs) are routinely employed to reduce vibration amplitudes and isolate structural components in gas jet engines, high performance compressors and, occasionally, water pumps. Most open-ended squeeze film dampers in practice present the phenomenon of air entrapment. It is generally accepted that the presence of air reduces the damping capability of the SFD, especially at large amplitudes and high frequencies of vibrations. Thus, there is a need for a reliable model of practical use in the analysis of high performance turbomachinery SDFs operating with air entrainment. Di´az and San Andre´s advance a model for estimation of air entrapped into the film lands. This model is based on a dimensionless number that relates geometric and operational parameters, but is strictly valid only for infinitesimal length bearings. The present research has by objective extending the previous work of Di´az and San Andre´s for prediction of air entrainment and entrapment on finite length bearings. The Reynolds lubrication equation for a homogeneous mixture is solved using the finite volume method. The results are shown in a map that allows determining air volume content in the film as a function of the dimensionless parameter created by Di´az and San Andre´s (Squeeze-Feed Flow Number, γ) and the length-diameter ratio (L/D). This work represents a significant step towards a better understanding of air entrainment in finite length Squeeze Film Dampers.

The objective of the 2006–07 Virginia Tech Turbocharger Project was to obtain and analyze vibrations data from different bearing designs. The supporting tasks were divided amongst various sub-teams; a turbocharger team, an engine test operating team, a dynamometer team, and an ADRE software team. The turbocharger team has successfully developed a procedure for disassembly and reassembly of the turbocharger. A consistent and well-documented assembly procedure has ensured turbocharger operation free from human error. Consistency in the rebuild process is vital to properly compare vibration data with the different shaft bearings. The most important aspect of this reassembly is the torque applied to the tie-bolt nut. This torque causes the tie-bolt to stretch, and this stretch must be accounted for because it is a measure of preload force on the compressor wheel. The team designed and made a stand to carefully apply this torque on the tie-bolt nut without creating a bending moment. The original stock bearings were used for the first turbocharger rebuild to see if the rebuild procedure would lead to similar data. Two other bearings have been tested and analyzed. This paper will give a brief overview of the results to date.

Journal bearings are widely used for supporting high speed rotating machinery such as turbine, compressor and HDD spindle etc, because of their high load carrying capacity, stability and durability. Recently, the engineering trend is oriented to higher speed and smaller sizes of rotating machinery, and then it is important to enhance stabilities for the journal bearings. Tilting pad journal bearing is generally used for high-speed rotating machineries because of their advantages of stabilization. However, there are some problems using for compact size rotating machineries for their high manufacturing cost and complicated structure. Therefore, it is necessary to enhance the stability of more simple bearings compared with tilting pad bearing in order to use for small size rotating machinery. This paper describes the stabilization method of journal bearing based on the combination mechanism with starved lubrication and orientation angle change. From the results of stability experimental study of journal bearings, it is clarified that the stabilization of bearings is improved drastically by using above techniques.

Tribological investigations of air-conditioning compressors have been a topic of great interest in recent years and gray cast iron has been a commonly used material by various compressor manufacturers. The scope of this paper is to determine the role of oxygen and in particular carbon dioxide refrigerant (R744) in cast iron samples tribologically tested using an Ultra High Pressure Tribometer that is suitable for tribological testing of compressor contact interfaces that operate with carbon dioxide refrigerant. A series of experiments was performed in environments of air, nitrogen (N 2 ), oxygen (O 2 ) and carbon dioxide (CO 2 ). While it was found that the presence of oxygen is beneficial, CO 2 has a more positive effect on the surfaces than in the case of pure O 2 suggesting that the use of CO 2 promotes a different wear mechanism. Also, it was found that CO 2 has better tribological performance over a range of pressures between 100 psi (0.69 MPa) to 600 psi (4.1 MPa), compared to lower pressures. Furthermore, CO 2 was compared with tetrafluorethane (R134a), a common hydrofluorocarbons refrigerant and found to have superior tribological performance. Two surface chemical analysis techniques were utilized to examine the surfaces after tribological testing. Auger electron spectroscopy (AES) was used to track changes in the elemental composition while X-ray photoelectron spectroscopy (XPS) was utilized to detect the different chemical states resulting from compound formation on the tribologically tested surfaces. It was found that CO 2 leads to better tribological performance of the interface due to the formation of carbonates on the surface, which reduce friction and prevent wear.

Test results are presented for a smooth seal and three centrally-grooved seals that are representative of buffered-flow oil seals in centrifugal compressors. The seals are short (L/D ≅ .21), with a diameter of 117 mm and a nominal radial clearance of 0.085 mm, netting the clearance-to-radius ratio .0015. The grooves have groove depth to clearance ratios ( D g / C r ) of 5, 10, and 15. Test conditions include three shaft speeds from 4000 to 10000 rpm, three inlet oil pressures from 24 to 70 bars, and seal eccentricity ratios from 0 (centered) to 0.7. Dynamic results include stiffness, damping, and added-mass coefficients; static results include stator position, attitude angles, and seal leakage. Stiffness, damping, and mass coefficients plus leakage are compared for the seal geometries. Results show that all rotordynamic coefficients consistently decrease with increasing seal groove depths, and seal leakage is largely unchanged. Comparisons are also made between experimental results and predictions from a computer program based on a Reynolds + energy equation model. The model includes the assumption that a groove is large enough to create separate lands within the seal, creating a zero or negligible pressure perturbation within the groove. Test results show that even the deepest groove depth tested is not deep enough to satisfy this assumption.

Increasing in rotational speed in turbo machineries such as turbochargers or turbo-compressors, several types of fluid film bearings are operated in turbulent flow regime. The transition from laminar to turbulent flow regime causes a remarkable change in bearing characteristics. To predict the transition condition, therefore, is of importance for the design of high speed bearings. The purpose of this paper is to investigate the critical Reynolds number in thin lubricating film of thrust bearings and the fluid film characteristics in transient to turbulent flow regime. An experimental study for a sector-shaped parallel plane thrust bearing shows a decrease in bearing temperature due to the transition, and results in that the critical Reynolds number is 400∼600. The critical Reynolds number obtained is smaller than the result of linear stability analysis, in which an infinitesimal perturbation has been assumed. The reason of this difference is considered to be an effect of finite disturbance and local temperature rise in the bearing. THL analysis in laminar to turbulent flow regime results in reasonable agreement with the experimental results with respect to bearing temperature.