Ceramic Rolling Elements Research Articles

Experience in using ceramics (Si3N4) as a material for rolling bearings of high-speed machines

To analyse the influence of the material of the rolling elements (steel and ceramics) of a high-speed ball bearing on the stresses in the contact zone of the rolling elements and rings and on friction losses in the contact zone.The study considers the outer ring of an angular contact ball bearing, which is loaded with additional centrifugal force. A nonstationary nonlinear volumetric interaction problem between the ball and the raceway was solved to determine the contact angle using the “DYNA” application package. The values of contact stresses for ceramic and steel rolling elements of a hybrid ball bearing are determined, as well as the value of deformations of balls and raceways based on Hertz's theory. The specific friction power deals with the differential slipping of the rolling elements relative to the raceway was estimated.Analysis of the behaviour of the bearings at different rotation speeds showed that at rotational speeds of more than 7000 rad/s, bearings with ceramic rolling elements of the considered standard size have advantages in contact strength. In addition, it was found out that at high rotational speeds, the friction power of the ceramic rolling elements is almost two times less than the friction power of the steel rolling elements.The research results relate to rolling ball bearings operating at a speed parameter dn of up to 4∙106 mm∙rpm and the analysis of friction losses is limited to losses from differential slip, which suggests further research of other components of losses.The study can justify the choice of bearing type (steel or ceramic rolling elements) depending on the operating speed and the lubrication and cooling system, which depends on the level of heat generation in the bearing.The work obtained original results of contact stresses and friction loss components for a certain type of bearing in a wide range of rotation speeds.

The investigation of the contact and wear of railway axle bearings under various race conformities

The bearing race conformity is a design parameter that determines the degree of contact between rollers and races. This paper investigates the effect of race conformity on wear distribution in a railway spherical roller bearing under boundary lubricated condition in which contact behaves like a dry contact. The results showed that the traction/micro-slip distributions were substantially dependent on the level of conformity. The wear is found to be highly sensitive to the race conformity, with lower conformity resulting in lower wear. For example, 5.2% increase of conformity factor, a parameter that represents race conformity mathematically, reduces worn area by 52% in inner contact that is 60% in outer contact where conformity factor increase is 5.8%. This is mainly due to high value micro-slip being truncated at the edges as the traction distribution is compressed into the smaller contact patch. The bearings with ceramic rolling elements show lower wear than for steel rolling elements due to lower wear coefficient. The higher elastic modulus of ceramic causes the traction magnitude to be higher in the ceramic-steel case over the steel-steel case. Therefore, race conformity can be taken into consideration to optimize roller bearing wear life.

Ultra-high-speed hybrid ceramic triboelectric bearing with real-time dynamic instability monitoring

The reliability and stability of bearing operation are crucial factors limiting the performance of high-speed rotating machinery. In this study, an ultra-high-speed hybrid ceramic rolling element triboelectric bearing (US-HCTEB) was developed to provide real-time dynamic behavior and stability monitoring. Herein, we adopted a floating rolling–sliding combination freestanding mode that ensures bearing structural integrity. Furthermore, a crown-shaped cage was employed with an opening, allowing the ceramic rolling element to be used as a dielectric material that was coupled with a sector-shaped interdigital electrode set on the bearing end cover to form a floating freestanding triboelectric nanogenerator (TENG). The rolling element self-rotated and revolved owing to the traction of the raceways, sweeping the electrodes to generate an alternating current. Additionally, the superiority and reliability of the US-HCTEB were demonstrated using a 30 million cycle durability test and an ultra-high rotating speed test at 16000 rpm, which far exceeds the capabilities of previously reported triboelectric bearings. The US-HCTEB output was then evaluated considering variable working conditions, structural optimization, and the surrounding environment. Dry contact lubrication and low humidity were found to be beneficial for the output performance. The spectra of the output current and the statistics describing the rolling element instantaneous speed suggest that the high-speed heavy radial load of the US-HCTEB could reduce overall skidding at the cost of increased instability. Subsequently, an extremely fast acceleration and deceleration test program was conducted on the US-HCTEB to evaluate its performance under the complex and nonstationary conditions in which high-speed bearings operate. The results revealed that the US-HCTEB not only achieved real-time and high-resolution dynamic behavior identification but also exhibited high reliability as it functioned appropriately until bearing failure. Thus, the proposed high-precision US-HCTEB can serve as an essential basis for the development of smart rolling bearings.

A Study of Possible Reasons of the Damage by Electric Current of Traction Induction Motors Bearings of Electric Trains Lastochka and the Ways to Prevent This

There is a problem of damage of bearings of traction induction motors of electric trains Lastochka by electric current. Based on the fact that the bearings have a ceramic insulation coating, and the cases of its electrical breakdown is not fixed, the authors put forward a version that a high-frequency current flows through the bearing as through an electric capacitance. The analysis show that the cause of the current can be a radio standard GSM-R. The proposed technical solution against the damage of bearings: the use of grounding shaft rings, replace the bearings on the other with ceramic rolling elements, arrangement of separate high-frequency grounding for the GSM-R antenna.

The Crack Propagation Trend Analysis in Ceramic Rolling Element Bearing considering Initial Crack Angle and Contact Load Effect

Silicon nitride ceramic bearings are widely used for their excellent performance. However, due to their special manufacturing method, cracks will occur on ceramic ball surface, and this initial surface crack will propagate under the action of cyclic stress, which will lead to material spalling. This will greatly limit its service life in practical applications, especially under heavy load at high speed. Therefore, it is necessary to study the surface crack propagation of silicon nitride ceramic bearings. In this paper, the effect of initial crack angle and contact load on crack growth is analysed by the finite element method (FEM). A three-dimensional finite element model of a silicon nitride bearing ball containing an initial crack is created by the FEM. The cracks are initially classified based on the angle between the crack and the bearing ball surface, and the location of the most dangerous load for each type of crack is known by theoretical analysis. The stress intensity factors (SIFs) are calculated for the crack front to investigate the effect of load position on crack growth. Subsequently, the SIFs are calculated for each type of crack angle subdivided again to investigate the effect of crack angle on crack propagation.

Empirical correlation of heat generation in hybrid ball bearings, depending on the operational conditions in the aero-engine rotor supports

Modern gas turbine bearings operate at high rotational speeds. Their speed index reaches dmn=3…3, 5-106 mmrpm, where dm = bearing mean diameter, and n = rotational speed. At high speeds, significant heat is generated in conventional steel bearings, and substantial cooling oil consumption may be required. A promising solution to this problem is using hybrid bearings with steel rings and ceramic rolling elements. Due to ceramic’s relatively small coefficient of thermal expansion, hybrid bearings retain radial clearance in a wide temperature range. Hybrid ball bearings, with an inner diameter of 130mm and 150mm, were tested in the bearings test rigs at the Central Institute of Aviation Motors (CIAM). From these tests, an empirical method for determining the thermal state of the bearings was developed. The calculated values correspond well with the experimental data presented in the extant literature.

Modelling the criticality of silicon nitride surface imperfections under rolling and sliding contact

Ceramic rolling elements of hybrid bearings may initially include surface imperfections. In order to provide reliable operation of a bearing, the criticality of such imperfections under rolling contact fatigue is examined by defining them as Star features: intersecting semi-elliptical surface cracks. Parametric study is conducted using Finite Element Method and discussed with help of previously published experimental observations. The effects of the Star feature morphology and configuration, contact pressure and crack face friction are investigated in terms of stress intensity factors. Possible crack propagation scenarios are explained in the present study.

Silicon nitride-based ceramic composite materials for corrosion-resistant rolling bearings

The technological process of obtaining material and manufacturing ceramic rolling elements of the bearing is considered. The composition of a silicon nitride-based ceramic composite material was investigated. The chemical inertness of ceramic materials determines the low adhesive wear in the mating parts of the supporting elements and makes it possible to use the rolling elements of the bearings in conditions of poor lubrication. Bearings with ceramic rolling elements show excellent running performance in emergency conditions and do not lead to sudden failures. The excellent corrosion resistance of ceramic materials allows them to be used in aggressive environments. The combination of such properties is accompanied by a decrease in temperature in the working area and allows the use of higher rotational speeds. The feasibility of using ceramics for the production of balls for bearings and the main advantages of using a ceramic material compared to metal products are shown. The process mode of hot pressing of blanks of ceramic rolling elements has been developed.

Experimental and Numerical Investigation of the Outer Ring Cooling Concept in a Hybrid and in an All-Steel Ball Bearing Used in Aero-Engines by the Introduction of a Helical Duct

Rolling element bearings for aero engine applications have to withstand very challenging operating conditions because of the high thermal impact due to elevated rotational speeds and loads. The high rate of heat generation in the bearing has to be sustained by the materials, and in the absence of lubrication these will fail within seconds. For this reason, aero engine bearings have to be lubricated and cooled by a continuous oil stream. When the oil has reached the outer ring it has already been heated up, thus its capability to remove extra heat from the outer ring is considerably reduced. Increasing the mass flow of oil to the bearing is not a solution since excess oil quantity would cause high parasitic losses (churning) in the bearing chamber and also increase the demands in the oil system for oil storage, scavenging, cooling, hardware weight, etc. A method has been developed for actively cooling the outer ring of the bearing. The idea behind the outer ring cooling concept was adopted from fins that are used for cooling electronic devices. A spiral groove engraved in the outer ring material of the bearing would function as a fin body with oil instead of air as the cooling medium. The method was first evaluated in an all steel ball bearing and the results were a 50% reduction in the lubricating oil flow with an additional reduction in heat generation by more than 25%. It was then applied on a Hybrid ball bearing of the same size and the former results were reconfirmed. Hybrid bearings are a combination of steel made parts, like the outer ring, the inner ring, and the cage and of ceramic rolling elements. This paper describes the work done to-date as a follow up of the work described in, and demonstrates the potential of the outer ring cooling for a bearing. Friction loss coefficient, Nusselt number, and efficiency correlations have been developed on the basis of the test results and have been compared to correlations from other authors. Computational Fluid Dynamics (CFD) analysis with ANSYS CFX has been used to verify test results and also for parametric studies.

Characterization and modelling of the degradation of silicon nitride balls with surface missing-material defects under lubricated rolling contact conditions

Hybrid bearings in which ceramic rolling elements are used in combination with hardened steel rings outperform the conventional full-steel bearings in some demanding applications where high speed, high temperature, electric current, and thin-film or medium lubrication are involved. Despite the continued effort and progress made to improve the toughness, ceramics are regarded as brittle solids that can suffer in general from lack of damage tolerance in the form of sensitivity to pre-existing features that are seen as being potentially harmful. Under specific conditions, these imperfections may become critical for the reliability of hybrid bearings.This paper reports on a study of the degradation of silicon nitride ceramic balls, caused by surface missing-material (MM) defects under lubricated rolling contact conditions. Rolling contact fatigue test is conducted on the ceramic balls containing artificial defects of different configuration and sizes. Numerical simulation by means of finite element analysis indicates that the lubricant entrapped inside the MM cavity will be pressurized under rolling contact. The hydraulic pressure of the entrapped lubricant can result in significant tensile stress at the corner of the cavity that may cause crack initiation, initial crack propagation, and eventual spalling of the ball surface. Such a degradation mechanism is confirmed by the characteristics of the fatigue damage observed from the tested balls. Based on the understanding of the failure mechanism, a model based on the nonlocal approach to crack initiation at a V-notch is developed to describe the damage tolerance of the silicon nitride balls with the MM defects of specific geometry with respect to the rolling contact load. The prediction of the tolerance limits agrees well with the endurance limits of the silicon nitride balls tested under lubricated rolling contact conditions.This research has contributed to a fundamental understanding on what type and size of the MM defects can be tolerated in hybrid bearings in relation to used application running conditions.

Ceramic Bearings for Electric Motors: Eliminating Damage with New Materials

Since the advent of variable frequency drive motors, standard rolling element bearings have been failing from electrical arcing because of stray electrical currents in the motors. This article will explore the damage mechanism in the bearings and introduce the use of ceramic rolling element bearings to eliminate this issue. Case histories will be provided.

Testing Methods on Ceramic Rolling Elements for Hybrid Bearings

Abstract Different manufacturing routes for ceramic balls and rollers used in hybrid bearings can lead to different microstructures and mechanical properties of a silicon nitride grade, even being delivered by one supplier. This fact, experienced by the authors, makes it necessary to do testing on the different balls or rollers themselves. This will make sure that the minimum properties on strength, fracture toughness, and microstructure requirements according to ASTM F2094/F2094M-14 and ISO 26602 are fulfilled on the bearing component and not only on special bar samples prepared for bending tests. A first approach done in the bearing industry was a simple crush test of two or three balls on top of each other. This gave a very rough estimate of a “static load rating.” The material resistance to cracks and Hertzian contact damage was tested by indentations with tungsten carbide spheres or doing impact strength testing. A first approach for “real” strength measurement of ceramic spheres was introduced by using a diametrally compressed “C-Sphere” specimen. This was further developed to be more easily machinable and was simply called “Notched ball test.” A similar test was also developed in recent times for ceramic cylindrical roller rollers, called the “Notched roller test.” The goal of this paper was to discuss and summarize test methods for strength and combinations of strength with other properties or effects like toughness and/or residual stresses for ceramic rolling elements that have been established so far for hybrid bearings. This supported the understanding for the need to establish the notched ball test, which was already established as a National Standard ÖNORM M6341:2013 in Austria, and also in ISO and ASTM.

An Experimental and Theoretical Study of Hybrid Bearing Micropitting Performance under Reduced Lubrication

Hybrid bearings—that is, bearings with ceramic rolling elements and steel rings—are often used in applications with reduced (i.e., boundary or mixed) lubrication conditions. The mechanisms by which hybrid bearings perform significantly better than full-steel ones in these cases are so far unclear, although a number of published works have shown experimental results in which appreciable performance benefits were obtained by the use of hybrid bearings under boundary or mixed lubrication. In this article, the reduced lubrication performance of hybrid rolling contacts, versus full-steel ones, is studied in detail by means of rolling bearing fatigue experiments and a theoretical micropitting model. It is found that the large improvement in surface fatigue resistance of hybrid contacts cannot be explained solely on the basis of the unavoidable differences in some of the roughness parameters existing between the full-steel and hybrid contacts. It is also necessary to take into account a considerable reduction in the effective boundary friction coefficient of the hybrid contact. In the numerical micropitting simulations it was found that the boundary friction coefficient of a hybrid contact must be about two times lower than that for the corresponding full-steel contact, in order to be able to predict the experimental observations reasonably well. A similar ratio of the boundary friction coefficients was obtained in a number of dedicated tests, thus confirming the results of the micropitting model. The mechanisms of the strong micropitting resistance of hybrid bearings under reduced lubrication conditions are discussed in detail, shedding new light on the operational tribology and performance capabilities of bearings with rolling elements made of silicon nitride ceramics.

Process Compensated Resonance Testing: Whole Body Surface NDT for Ceramic Balls

Process Compensated Resonance Testing (PCRT) is a non-destructive evaluation method for ceramic balls using measured resonance frequencies of parts to detect surface and internal flaws. It offers fast, accurate and affordable defect detection for ceramic rolling elements for hybrid bearings. PCRT has demonstrated capabilities for bulk property/internal defect detection, process monitoring, material property characterization, and surface defect detection. In co-inspection comparison, PCRT demonstrated superior defect detection and/or throughput versus visual, FPI (Fluorescent Penetrant Inspection) and UT (Ultrasonic Testing). Vibrant’s automated ball testing system tests more than 75 balls per hour with no ball-to-ball contact. This research is funded in part by the US Navy and AFRL (Airforce Research Laboratory) with support of Vibrant’s Industrial partners at Honeywell, Rolls-Royce Corporation, GE Aviation, Winsted Precision Ball and Barden Precision Bearing.

CONCEPT, THEORY, DESIGN AND EXPERIMENTAL TESTS OF SELFPOWERED LUBRICATION IN CERAMIC COMPOSITE AND ROTORS

ABSTRACTDue to a modulus of elasticity higher than that of high carbon chrome bearing steel,ceramic rolling elements exhibit less deformation at contact points and therefore agreater stress under the same load. Combined with a lower thermal conductivity,their working temperature increases and their load capacity decreases more rapidlyin operation. In Nature, we have a ceramic composite system, Bone, known for itssuperior load-bearing capacity, and its self-protection at the high-stress movingcontactpoints (joints) with built-in lubrication. Less known but no less important isthe fact that it has a built-in capillary networks for self-powered supply of lubricantsand coolants, as well as nutrient and growth factor, from within. It has served as aninspiration to an effort and a model system for the study we report here that aim toIncorporate some of these functionalities into man-made composite structures. Inthis report, we first highlight our attempt to develop a method for generating networksof micro- and nano-capillaries within a ceramic composite structure during thesintering process. We then present test results of self-powered supply of fluids tothe contact (load-bearing) surface via the capillary networks from the fluid reservoir.As a further extension, a self-regulation mechanism is added into the design toenable temperature-controlled self-powered lubrication, and tested in a modelsystem. The method is adaptable to various structural shapes, and scalable in size,and to both biophysiologic and mechanic composite systems.

Experimental Study of the Smoothing Effect of a Ceramic Rolling Element on a Bearing Raceway in Contaminated Lubrication

The effects of steel and ceramic rolling elements on protrusions from the raceway of a bearing were experimentally investigated. Such protrusions, which are normally caused by solid contaminants in the lubricating oil, create stress concentrations and lead to a reduction in the rolling contact fatigue life of the bearing. To compare the over-rolling effects of steel and ceramic rollers, experiments with steel discs with artificial dents on the surfaces were performed using a modified twin-disc machine. The results show that ceramic rollers can reduce the height of the protruded edge of an artificial dent more than steel rollers, which means that they are more effective in smoothing a damaged surface. The stresses at the contact were calculated by finite element analysis based on the deformed profile of the dented surface. The reduction in the stress level due to the smoothing effect of ceramic rollers is greater than that of steel rollers. According to the Lundberg–Palmgren bearing fatigue model, that smoothing ensures a significantly longer rolling contact fatigue life for a bearing. To put the idea into practice, a rolling ball bearing with two of its nine steel balls replaced with silicon nitride balls (referred to as a “partial hybrid bearing”) was run, together with a full steel bearing of the same model, on a bearing tester in a highly contaminated lubrication condition. The results show that the partial hybrid bearing suffers from less damage in terms of wear. The post-experiment examination of the damaged surface of the bearing raceway found that the surface of the partial hybrid bearing was smoother than that of the full steel bearing. This reveals the smoothing effect of the rolling ceramic element on the contaminant-damaged bearing surface.

Rolling contact fatigue studies on spray formed ceramics composite rolling elements

Spray formed ceramic rolling elements can enhance the scope of high speed anti-friction bearings. Spray forming offers distinct advantages over other manufacturing processes, in that it is possible to tailor make structural properties of the rolling element. Selection of ceramic composite material, hollowness ratio ( D/ d) and also post-heat treatment after spray forming will ensure the desired rolling contact performance. Alumina–titania (AT40) composite rings of different wall thicknesses (1, 2 and 3 mm) were spray formed and post-heat treated at 1200, 1400, 1500 and 1600 °C. observations clearly illustrate best possible performance of thinner rings with post-heat treatment at 1200 °C. Performance was evaluated in terms of structure–property relationship and also rolling contact wear.

RESEARCH OF PRESENT STATE AND PROSPECTIVE ON GYRO-SPIN BEARINGS

The summarization and comment are presented according to the accumulation of research work and the investigation of related literatures on gyro-spin bearing, included the technology in optimum design,material, lubricating mechanism of PEHL,instability of cage,lifetime and reliability experiment,how to prolong service period of gyro-spin bearing,different kinds of aspect on measurement techniques and experimental approaches to the bearings,et al. Several promising research fields are suggested accordingly, such as the mechanism of thin film lubrication,nano-particles lubricant additives with anti-friction and prolonging lifetime capability,estimation in performances of lubricants,optimal design after using ceramic rolling elements and experimental technology of the accelerated life tests.The current situation and the severe challenge facing to us in technical area in China are analyzed,which are about research of basic theory and application in the engineering fields.The corresponding countermeasures and some programming suggestions in this field have also been proposed.

Ceramic rolling elements with ring crack defects—A residual stress approach

Experimental results of rolling contact fatigue on ceramic bearing elements with refrigerant lubrication are presented. Residual stress measurements located on the contact path and other locations on the surface are described. An X-ray method was employed. Residual stress measurements are helpful in predicting rolling contact fatigue life. In addition, analysing the relationship of residual stress with rolling contact fatigue is an important study, which will provide guidelines on the design, process and manufacturing of these elements. During this research, ring crack defects were induced in ceramic rolling contact bearing elements. A compressive residual stress value of −73 MPa near the ring crack and a comparatively lower value of −12 MPa on the contact indicate sub-surface crack initiation and propagation. The average fatigue spall ranges from 100 to 148 μm in depth. Within the spall area residual stress measurements suggest that compressive residual stress is relieved much faster in the region of sub-surface damage.

Current Status of Hybrid Bearing Damage Detection

Advances in material development and processing have led to the introduction of ceramic hybrid bearings for many applications. The introduction of silicon nitride hybrid bearings into the high-pressure oxidizer turbopump on the space shuttle main engine led NASA to solve a highly persistent and troublesome bearing problem. Hybrid bearings consist of ceramic balls and steel races. The majority of hybrid bearings utilize Si3N4 balls. The aerospace industry is currently studying the use of hybrid bearings and, naturally, the failure modes of these bearings become an issue in light of the limited data available. In today's turbine engines and helicopter transmissions, the health of the bearings is detected by the properties of the debris found in the lubrication line when damage begins to occur. Current oil debris sensor technology relies on the magnetic properties of the debris to detect damage. Because the ceramic rolling elements of hybrid bearings have no metallic properties, a new sensing system must be developed to indicate the system health if ceramic components are to be safely implemented in aerospace applications. The ceramic oil debris sensor must be capable of detecting ceramic and metallic component damage with sufficient reliability and forewarning to prevent a catastrophic failure. The objective of this research is to provide a background summary on what is currently known about hybrid bearing failure modes and to report preliminary results on the detection of silicon nitride debris in oil using a commercial particle counter.