Self-lubricating Spherical Plain Bearings Research Articles

Influence of housing hole accuracy on the starting torque of self-lubricating spherical plain bearings

The accuracy of the housing hole mainly refers to the dimensional tolerance, ovality and surface roughness of the hole, which directly affects the swing flexibility and service life of the self-lubricating spherical plain bearings (SSPBs) after the bearing is fitted in the housing hole. The dimension tolerance between the bearing and the housing has been ordained in the aviation standard. However, it is found that the ovality and surface roughness of the housing hole also have a great influence on the interference fit state, thus affecting the starting torque. In this paper, the effects of the three indexes of the housing hole accuracy (dimensional tolerance, ovality and surface roughness) on the starting torque were studied. Firstly, a theoretical analysis of the liner deformation inside the bearing was carried out based on the Lame equation. Then, the concept of equivalent interference was proposed considering the interference fit caused by the ovality and surface roughness, and the amount of the equivalent interference was determined. Furthermore, the influences of the three indexes on the starting torque were studied quantitatively with theoretical solution, numerical analysis and experimental verification. The results showed that the proposed concept of the equivalent interference can analyze the influence of the ovality and the surface roughness effectively, and the ovality has a greater effect on the starting torque than the surface roughness. The three precision indexes of the housing hole are reasonably designed to ensure that the starting torque after fitted meets standard requirements.

Erratum to: Finite element simulation and experimental test of the wear behavior for self-lubricating spherical plain bearings

Erratum to: Finite element simulation and experimental test of the wear behavior for self-lubricating spherical plain bearings

Tribological performances of polytetrafluoroethylene/Kevlar fabric liner reinforced by oil-containing microcapsules

The wear depth of polytetrafluoroethylene (PTFE)/Kevlar fabric liner is directly related to the service life of self-lubricating spherical plain bearings. Herein, two kinds of oil-containing microcapsules were applied to improve the wear properties of PTFE/Kevlar fabric liner at various temperatures. The addition of microcapsules successfully reduced the wear and COF simultaneously, which was owing to the synergistic effect of hard shell and liquid lubricants core. The hard shell enhanced the mechanical strength of PTFE, while the released lubricants made up the shortcoming due to the incomplete PTFE lubricating film, and inhibited the abrasive wear of SiO2 debris. The present work suggested that the addition of oil-containing microcapsules was an effective method to enhance the tribological performances of PTFE/Kevlar fabric liner.

A new coated self-lubricating spherical plain bearing with high performance and excellent security

The low service performance and sudden failure characteristics of coated self-lubricating spherical plain bearings (SPBs) limit their engineering application prospects. In the current study, two new self-lubricating coatings types were applied to the bearing contact surfaces. The bearings were compared and investigated using life tests. Furthermore, the wear failure mechanism was investigated. The results show that self-lubricating coatings improved the service performance of the traditional bearings with manganese phosphate coating. Among them, double-sided coated bearings had the best service performance and life. More interestingly, the torque and temperature rise signal curves will show a notable signal downstage before the signal mutation stage. The large-area and complete mixed friction transfer film on the contact surfaces was the main reason for the signal downstage. Moreover, the failure mechanism changed from abrasive wear to adhesive wear and fatigue wear with the increment of load. This study results preliminarily verify the performance improvement of high-performance coatings and provide a theoretical reference for further breaking through the engineering application bottleneck of coated self-lubricating SPBs.

Wear Mechanism of TC4 Titanium Alloy with TiN Coating against Self-Lubricating Fabric

Vapor deposition technology can improve the surface wear resistance of titanium alloys, and prepare lightweight and corrosion-resistant self-lubricating spherical plain bearings made of titanium alloys. However, titanium alloys with hard films can be worn by soft self-lubricating fabrics. This paper focuses on the wear problem of TiN coating on the surfaces of self-lubricating spherical plain bearings based on titanium alloys. Ring-to-plate wear tests were carried out to study the tribological properties of TiN coating on TC4 titanium alloy against self-lubricating fabric under different working conditions (load: 500–2000 N and speed: 100–500 r/min), along with the investigation of the wear mechanism of TiN coating, and the evaluation of applicable working conditions of GE15 type self-lubricating spherical plain bearings through swing tests. The results have revealed that TiN coatings can maintain a certain friction distance without wear. Increasing friction speed and load can make TiN coatings more prone to wear. A thick transfer film can protect the TiN coating from wear. The main wear mechanism is attributed to fatigue wear induced by the repeated formation and peeling of transfer films. The GE15 bearing has achieved a self-lubricating fabric wear of approximately 0.04 mm when the swinging for 500 m (25,000 times) is under a specific condition of 27 kN and 0.2 Hz without damaging the inner ring of the bearing. The bearing is suitable for swing conditions with applied loads below 27 kN. This study provides a fundamental understanding of designing self-lubricating spherical plain bearings made of titanium alloys.

A strategy to improve tribological performances of solvent-based phenol−formaldehyde resin by using oil-containing nanocapsules

Enhancing the tribological properties of immersed resin is a common and effective method to prolong the service life of aero self-lubricating spherical plain bearing. Herein, using hollow mesoporous carbon nanospheres (HMCNs) as containers, the oil-containing carbon nanocapsules (Oil@C), which were obtained by the incipient wetness impregnation method, were applied as reinforced particles to improve the tribological properties of solvent-based phenol−formaldehyde (PF) resin. Specifically speaking, the Oil@C was monodispersed and uniform with the diameter of 190 nm, shell thickness of 12.5 nm and oil containing rate of 70.59 wt%. With the content of Oil@C reach to 5 wt%, the coefficient of friction (COF) of PF composite decreased about 81.9% (from 0.54 to 0.098). Moreover, the service life of the PF composite prolonged greatly, from 460 s to 5 h. According to the analyzation of morphology and transfer film of the friction pairs, it revealed that the released oil played a key role in the realization of self-lubricating property of PF composites, inhibiting the transfer of PF resin and exfoliation of coating. However, there were different degrees of decline occurred on the shear strength of the PF composites, which needs further improvement.

Novel Tribometer for Coated Self-Lubricating Spherical Plain Bearings in a Vacuum

Coated self-lubricating spherical plain bearings (SSPBs) are a fairly key component of the space-swing mechanism. To examine the operation status and tribological properties of coated SSPBs, a tribometer with a temperature control module in a vacuum condition was developed. The tribometer was mainly composed of a fixture system, reciprocating rotational motion system, environment control system, etc. First, the tribometer was verified with the self-made hydrogenated diamond-like carbon (H-DLC) SSPBs. The sensor signals indicated that the tribometer conformed to the design specifications. Then, the influence of friction heat on the tribological properties of H-DLC SSPBs was analyzed. The results showed that friction torque and temperature increased with the overall test time. Although the temperature had reached 48 °C, the frictional heat had little effect on the H-DLC SSPBs’ lifespans. The damage mechanism of H-DLC SSPBs was dominated by abrasive wear and fatigue wear in vacuum conditions.

Evaluation of wear mechanism between TC4 titanium alloys and self-lubricating fabrics

Self-lubricating spherical plain bearings are important product components in the aerospace industry. With technological advances, the limitations of steel self-lubricating spherical plain bearings with high quality but poor corrosion resistance became quite evident. The preparation of self-lubricating spherical plain bearings made of titanium alloys is of great interest. In general, titanium alloys have poor tribological properties as well as unsatisfactory tribological performance when they rub with soft self-lubricating fabrics. To understand the wear mechanism of TC4 titanium alloys, half-ring wear tests were conducted between TC4 titanium alloys and self-lubricating fabrics under different pressures and swing frequencies, namely, 50 and 100 MPa, and 0.2 and 0.4 Hz, respectively. The corresponding results showed that the friction coefficients (COFs) between TC4 titanium alloys and self-lubricating fabrics greatly fluctuated in the range of 0.1–0.25. The main wear form of titanium alloys was a combination of abrasive wear and adhesive wear, and the accumulated debris generated during the friction process was the main cause of surface damage of TC4 titanium alloys. The accumulated debris tended to increase with increasing pressure and swing frequency. The debris scratched and made large grooves in titanium alloy surfaces and aggravated the wear of self-lubricating fabrics, which led to severe fibre breakage. This paper describes a novel wear mechanism of accumulated debris and offers a solid theoretical foundation for the application of titanium alloys in the field of self-lubricating spherical plain bearings.

Influence of an outer ring end offset on the service life and eccentric wear defects of self-lubricating spherical plain bearings

Improving the service life of self-lubricating spherical plain bearings is important due to the wear failure of the self-lubricating layer is considered as the primary reason for their service life decreases. Currently, there remains a lack of research on the influence of the forming accuracy of radial symmetry on bearing service life. This study investigated the relationship between the forming accuracy of self-lubricating spherical plain bearings and bearings’ life, by proposing the concept of the outer ring end offset. Based on the influence of the offset on the contact stress and strain of the bearing, the range of reasonable offset was analyzed. The correctness of the range of offset was verified by bearing friction tests, and the wear mechanism of the bearing eccentric wear phenomena was described. Results show that the forming accuracy of the outer ring is an essential factor that affects bearing service life. The outer ring end offset can be used as a key indicator of forming accuracy. Considering the GE15DE1TK self-lubricating spherical plain bearing as an example, the outer ring end offset with an over-extrusion bearing should be less than 0.05 mm. However, the required offset for an insufficient-extrusion bearing does not easily produce eccentric wear defects even with an offset of 0.08 mm. This paper provides a new idea and indicator for improving the service life of self-lubricating spherical plain bearings.

Preparation of a titanium alloy outer ring self-lubricating spherical plain bearing by a two-step extrusion forming process

Self-lubricating spherical plain bearings are a key component widely used in the aerospace field. The spherical plain bearing made of titanium alloy has a lighter weight and better corrosion resistance. To manufacture the outer ring of a self-lubricating spherical plain bearing with titanium alloy, a two-step extrusion forming process was proposed. Taking a GE15DE1TK spherical plain bearing as an example, the finite element simulation model of the forming process was established, the forming test was carried out, and the outer ring of a self-lubricating spherical plain bearing made by TC4 titanium alloy was successfully prepared. The results show that forming defects such as over extrusion and insufficient extrusion can be improved by optimizing the die structure, the size of the outer ring blank, and the lubrication conditions. The forming parameters for the GE15DE1TK spherical plain bearing are given. The prepared spherical plain bearing can be used directly without other processes to correct no-load rotational breakaway torque.

Preparation and Tribological Properties of Self-Lubricating Epoxy Resins with Oil-Containing Nanocapsules.

The design and development of self-lubricating materials aid in enhancing the tribological performance and prolonging the service life of aero self-lubricating spherical plain bearings. Herein, monodisperse hollow mesoporous carbon nanospheres (HMCNs) with thin shell thickness and uniform particle size were synthesized. Afterward, oil-containing nanocapsules (Oil@HMCNs) were obtained by the impregnation method, which was an innovative approach to prepare nanoscale capsules. The Oil@HMCNs were monodispersed, and the oil content rate could reach 45 wt %. The nanocapsules were applied as additives to achieve the self-lubricating properties of epoxy resin (EP). It was revealed that the coefficient of friction (COF) and the wear rate of EP in dry friction can be decreased from 0.56 to 0.084 and from 50.15 × 10-6 to 0.27 × 10-6 mm3/N/m, respectively. The tribological properties of EP composites in dry friction are better than those of EP with external oil. According to the analysis of friction pairs, the antifriction performance was contributed by the released automatic transmission fluid 6HP (ATF6) from nanocapsules. Meanwhile, the presence of ATF6 effectively could inhibit fatigue wear, and the decrease of compressive strength and surface hardness led to plastic deformation and mechanical polishing. Both resulted in a lower wear rate of EP. In conclusion, this work explored the application possibility of hollow mesoporous materials in lubrication. The prepared EP composites exhibited excellent tribological performances and self-lubricating properties, which were expected to be applied to self-lubricating spherical plain bearings and other mechanical parts.

Tribological performance of hydrogenated diamond-like carbon coating deposited on superelastic 60NiTi alloy for aviation self-lubricating spherical plain bearings

It is imperative to develop a novel matching of metallic substrate and self-lubricating coating for aircraft spherical plain bearing in a wide range of service conditions. As a new type of superelastic material, 60NiTi alloy meets the performance requirements of aerospace bearing materials, but exhibits poor tribological performance, especially under the conditions of dry sliding friction. A Hydrogenated Diamond-Like Carbon (H-DLC) coating was deposited on the 60NiTi alloy to improve its tribological performance. The microstructure and mechanical behavior of the 60NiTi alloy and its H-DLC coating were explored. Results show that improvement of friction and wear performance of the H-DLC coating deposited on the 60NiTi substrate is mainly achieved by graphitization at the friction interface and the transfer film produced on the counterpart ball. The increased friction load leads to intensification of graphitization at the friction interface and formation of continuous and compact transfer film on the surface of the counterpart ball.

Mesoscopic scale simulation and wear investigation of self-lubricating fabric liner based on representative volume element

As the self-lubricating layer of self-lubricating spherical plain bearings, fabric liner shows obvious heterogeneous anisotropic characteristics, so it is a technical difficulty to predict its wear properties. In this paper, the continuous wear of self-lubricating fabric liner was simulated based on the mesoscopic scale wear model. The macroscopic wear properties of the fabric liner were characterized by establishing a representative volume element, and subsequently imposing periodic boundary restrictions on periodic surfaces. In order to avoid excessive mesh distortion, voxel grids meshing method was used, and then continuous wear of the heterogeneous material was realized by adjusting node coordinates and combining nodes. Detailed comparison between simulation prediction results and wear test data of fabric liner was made. The good correlation of the results confirmed that the mesoscopic scale wear model could be used in accurately predict the tribological performance of fabric composite.

Research on the mathematical modelling of the starting torque of self-lubricating spherical plain bearings

The starting torque of self-lubricating spherical plain bearings (SSPBs) is a key parameter for evaluating the performance of bearings. Therefore, the starting torque of SSPBs should be controlled to within an allowable range. In this paper, the starting torque generation mechanism is analyzed, and the critical load for the separation of the liner from the outer spherical surface of the inner ring is determined. A mathematical model of the starting torque is established; the experimental and theoretical results of the starting torque are compared and analyzed, and then the accuracy of the mathematical model is evaluated by the deterministic coefficient R2. The research reveals that a critical load exists for the starting torque. Below the critical load, the starting torque is dependent on the outer spherical radius of the inner ring, bearing wrap angle, and liner parameters such as the compressive elastic modulus, friction coefficient, and precompression of the liner; however, the starting torque is independent of the radial load. Above the critical load, the starting torque is also dependent on the radial load. The research results provide a theoretical basis for the design and application of bearings.

Research on Multi-Stage Composite Loading Process Control Method for Roller Staking of Self-Lubricating Spherical Plain Bearings

The staking quality of Self-lubricating Spherical Plain Bearings (SSPBs) directly affects the safety of aircraft and the service life of bearings. Reliable loading process control methods and precise process parameter indexes will come into the creation of efficacious staking quality. Therefore, this paper aims to analyze the mechanical state of the roller staking process and give a load control method and corresponding parameter indexes for the high-quality roller staking process. First, based on the analysis of quality inspection requirements, five states of the deformation degree of the flanging lip of the V groove during the roller staking process were proposed, and their relationship with the requirements was studied. Then, the mechanical states corresponding to the five deformation states of the flanging lip deformation were obtained by numerical simulation, and the feeding displacement was determined. Meanwhile, a Multi-Stage Composite Loading (MSCL) process control method was first proposed to control the material damage of the flanging lip, i.e., the rotate speed of the roller tool was constant during the roller staking process, and the displacement–time control was adopted first; when the staking load reaches a staking value, the force–time control was used to make the staking quality meet the requirements. Finally, the staking quality of the MSCL method was verified though the test. The research shows that the feeding displacement needs to be added to the requirements, and the recommended value is 0.5–0.6 times of the V groove depth. A good surface quality and non-material-damage of the flanging lip is more likely to be obtained by the MSCL process control method. The research reveals the formation mechanism of process deformation, and gives more precise process control indexes. At the same time, it provides a theoretical reference for more reliable technical standards.

Research on damage and failure behaviour of coated self-lubricating spherical plain bearings based on detection of friction torque and temperature rise

In this study, the wear failure behaviour of coated self-lubricating spherical plain bearings under different working conditions in the light-load and low-frequency range was studied using a swing wear life tester. The commercial GE17ES bearing was the main research object, and the wear state of the bearing was monitored in real time using torque and temperature rise signals. Furthermore, a scanning electron microscope and an energy dispersive X-ray spectroscope were adopted to analyse the wear failure mechanism of bearings. The results indicated that the friction torque and temperature rise signal changes were mainly divided into three stages, and the characteristics of similar changes could be maintained under different working conditions. Furthermore, at the end of the bearing life, the torque and temperature rise had abrupt changes almost at the same time, which indicated the occurrence of bearing failure. In general, the wear failure mechanisms of bearings included abrasive wear and adhesive wear. Moreover, fatigue wear and oxidation wear were under some conditions. The torque and temperature rise comprehensively reflected the service state of coated self-lubricating spherical plain bearings, which could be used as a physical criterion for determining bearing failure. These results had a strong prospect of engineering application, and were expected to provide a reference for determining the failure behaviour of multi-type coated self-lubricating spherical plain bearings in the future, and provided theoretical support for establishing relevant test standards.

Remaining Useful Life Prediction of High-Frequency Swing Self-Lubricating Liner

The remaining useful life (RUL) prediction of self-lubricating spherical plain bearings is essential for replacement decision-making and the reliability of high-end equipment. The high-frequency swing self-lubricating liner (HSLL) is the key component of self-lubricating spherical plain bearings under high-frequency oscillation conditions. In this study, a RUL prediction method was proposed based on the Wiener process and grey system theory. First, the predictive processing of the wear depth was carried out using the grey model GM(1,1) to reduce the randomness and enhance the inherent regularity of the life test data. A degradation process model was established and the RUL was predicted online with the model parameter estimates based on the Bayesian updating strategy. Finally, examples were provided to elaborate the RUL prediction of the HSLL. The results show that the prediction accuracy of the proposed RUL prediction model is higher than that of the simple Wiener process during the entire residual life cycle of the HSLL. Based on the original wear data, the prediction accuracy of the RUL exhibited a strong dependence on prior samples and was relatively low owing to the larger deviation of the wear rate between the test sample and prior samples.

Effect of ambient temperature on the formation mechanism of PTFE liner transfer film of spherical plain bearings

The relationship between the formation of the transfer film and the tribological properties of the self-lubricating spherical plain bearing was studied at the ambient temperature of 25–145 °C. The results show that the wear, friction coefficient and friction temperature increase of the spherical plain bearings all decreased first and then increased with the increase of the ambient temperature. The general trend is that the bearing has the most excellent anti-friction and wear resistance at 55–85 °C. An increase in ambient temperature will accelerate the formation speed of PTFE transfer film and shorten the running-in period of the bearing, but the ambient temperature above 85 °C will shorten the duration of the PTFE transfer film, thus accelerating the bearing into the degradation period. The transfer film coverage of the liner after wear was characterized, and it was found that the transfer film coverage was the largest when the ambient temperature was 85 °C. The wear form of the bearing is mainly abrasive wear and adhesive wear, and the aramid fiber is more prone to adhesive wear. The anti-friction effect of the bearing is determined by the PTFE transfer film. Elevated ambient temperature can promote the formation of PTFE transfer film and enhance the antifriction effect, but if the ambient temperature is too high, the wear resistance of the PTFE transfer film will be reduced, thus reducing the friction reduction effect.

Role of transfer film formation on the tribological properties of polymeric composite materials and spherical plain bearing at low temperatures

Polymeric composites show promising self-lubricating properties for low temperature applications, yet the lubrication mechanism still remains unresolved. Here, we report the friction and wear behaviors and mechanisms of a hybrid composite based on polyetheretherketone (PEEK) and polytetrafluoroethylene (PTFE) at temperatures ranging from 20 °C to −100 °C in vacuum with relatively large load of 10 N. The friction coefficient maintains lower than 0.1 over this wide temperature range with a minimum value at −70 °C due to the combined effects of transfer film formation and thermally activated sliding. As for self-lubricating spherical plain bearings lubricated by Nomex/PTFE fabric, we show direct evidence of transfer film formation on the outer surface of the inner ring after tribotest at −160 °C in nitrogen gas.

Optimal Design of the Process for Self-lubricating Spherical Plain Bearings Based on Finite Element Analysis

As a key technology for forming of self-lubricating spherical plain bearing, extrusion forming process affects general performances of self-lubricating spherical plain bearing. In order to solve the problems of insufficient force of the extruder and material accumulation of outer ring blank in the extrusion process of wide series and large bearing, a scheme of machining extrusion groove on outer ring blank is put forward, and taken numerical simulation by explicit dynamic finite element code LS-DYNA. Effective plastic plain and effective stress and bearing osculation after springback are investigated as well as applied extrusion force. The experimental results show that the bearing osculation can meet the requirement after machining extrusion groove on outer ring blank, meanwhile, a smaller extrusion force is needed in the extrusion forming process.