Grease Lubrication Research Articles

Tribotechnical Properties of Cu–Al Antifriction Gas Thermal Coatings in Liquid and Grease Lubricants

Tribotechnical Properties of Cu–Al Antifriction Gas Thermal Coatings in Liquid and Grease Lubricants

Effect of magnetic field on the lubricating performance of nano magnetorheological grease at different temperatures

PurposeThe adverse effects of temperature on the lubricating properties of nano magnetorheological grease are reduced by applying of a magnetic field.Design/methodology/approachNano magnetorheological grease was prepared via a thermal water bath with stirring. The lubricating properties of the grease were investigated at different temperatures. Then the lubricity of the prepared nano magnetorheological grease was investigated under the effect of thermomagnetic coupling.FindingsAs the temperature rises, the coefficient of friction of grease lubrication gradually increases, surface wear gradually increases and lubrication performance gradually decreases. Compared with grease, magnetorheological grease has a decreased coefficient of friction and enhanced lubrication effect under the action of a magnetic field at different temperatures.Originality/valueA lubrication method using a magnetic field to reduce the effect of temperature is established, thereby providing new ideas for lubrication design under a wide range of temperature conditions.

Oil-separation properties of aviation grease under thermal aging

During work or long-term storage, grease is susceptible to thermal oxygen or mechanical aging, which affects the performance of grease or even leads to the destruction of the structural surface of grease lubricated products. As the main mechanism of grease lubrication, grease partitioning can explore the aging law of grease and effectively identify whether the grease fails. In this paper, 7014 aerospace grease was selected as the research object and accelerated test was conducted by high temperature thermal oxygen. The combination of grease separation degree and mass change rate was selected for comprehensive characterization of grease separation performance. The results show that the grease partitioning degree has a significant decreasing trend; the quality change has also decreased and the degradation trajectory is similar to the partitioning degree; it is inferred that the main reason for the decrease of grease partitioning performance may be the loss of base oil and the change of thickener fiber structure.

Grease-lubricated triboelectric instantaneous angular speed sensor integrated with signal processing circuit for bearing fault diagnosis

The triboelectric sensor can be integrated with the rolling bearing to diagnose bearing faults by monitoring the periodic fluctuation of instantaneous angular speed, yet continuous dry friction between materials will reduce its signal-to-noise ratio (SNR). Moreover, the signal processing modules used by previous triboelectric sensors need additional wires to transmit the signal, and thus affect the reliability of the system. In this manuscript, a wireless grease-lubricated triboelectric instantaneous angular speed sensor (GL-TEIASS) integrated with the signal processing circuit for bearing fault diagnosis is proposed based on rotary freestanding triboelectric nanogenerators (TENGs). First, electrical insulating grease is presented as an ideal candidate lubricant used in the GL-TEIASS. Next, by establishing the charge density-dependent electromechanical model of grease-lubricated TENG, optimal structural parameters for maximum signal-to-noise ratio and angular resolution as well as minimum friction torque of the GL-TEIASS can be obtained. Moreover, the kurtosis-guided local polynomial fitting method is employed to extract the signal fluctuation caused by bearing faults. The results show that, compared to dry friction, the mass loss of the polymer under grease lubrication decreases by 91.7 % and the electrical output increases by 4.0 times. The durability of the GL-TEIASS in fault diagnosis is improved by at least 2.8 times compared with that of the sensor operating under dry friction. The error between the angular speed measured by the GL-TEIASS and a commercial sensor is less than 0.731 %, and the SNR of the output signal from in-situ monitoring by the GL-TEIASS can reach 39.1 dB, which is 4.9 times higher than that of vibration signal commonly used in fault diagnosis. The designed GL-TEIASS has good application prospects in self-sensing and self-diagnosis of operation state for smart bearings.

Investigation on friction characteristics of imitation clamshell textured surfaces under MXenes-enhanced grease lubrication

To enhance the performance and longevity of bearings by addressing the issue of insufficient lubrication, this research paper introduces a multi-drive lubrication system. Inspired by the micro-texture of clamshells, bionic textured surfaces (BTSs) were designed and fabricated. Calcium complex grease (CCG), aluminum complex grease (ACG), lithium complex grease (LCG), and these three greases containing 2[Formula: see text]wt.% accordion-like MXenes Ti3C2 were prepared, respectively. This study commits to evaluating the tribological properties of BTSs with different densities under different grease lubrication. The results showed that BTSs ([Formula: see text] = 38%) had the best anti-friction and anti-wear under the MXenes lithium complex grease lubrication, the average friction coefficient (0.113) and wear rate reached the smallest ([Formula: see text][Formula: see text]mm3[Formula: see text]N[Formula: see text][Formula: see text]m[Formula: see text]), and only slight wear occurred. This was mainly attributed to the synergistic and complementary lubricating effects of BTSs and MXenes-enhanced grease.

Contact fatigue performance and failure mechanisms of Fe-based small-module gears fabricated using powder metallurgy technique

Gears fabricated using powder metallurgy technique (PMT) have the potential to replace traditional wrought gears due to their very high forming efficiency and relatively low cost. However, their practical applications are restricted due to the lack of understanding of their fatigue properties and failure mechanisms. This work investigates the contact fatigue performance and corresponding failure mechanism of a typical Fe-based small-module gears fabricated by PMT. The fatigue tests were conducted under grease lubrication condition, using a power-opened small-module gear testing rig. The experimental results show that the current PMT fabricated small-module gears exhibit contact fatigue properties comparable to those of traditional wrought steel gears. It has been found that the subsurface cracks induced from pores are the dominant cause of spalling of PMT fabricated small-module gears during rolling contact. The cracks tend to propagate between adjacent pores, playing a vital role in the overall fatigue life. Thus, this study suggests that PMT can be a successful method for fabricating Fe-based small-module gears with excellent contact fatigue property if formation of pores during fabrication is well inhibited.

The tribological performance analysis of palm olein-based grease lubricants containing copper nanoparticle additive

Mineral oil-based lubricants have been widely used for their beneficial properties. However, their environmental impact and toxicity have raised concerns, resulting in a search for sustainable and environmentally friendly alternatives. This study investigates the tribological behavior of palm olein-based grease with copper nanoparticles additive concentrations of 0.25 and 0.50 wt%. The friction and wear tests were conducted using a four-ball tribometer to evaluate the performance of the formulated grease and compare it with commercial mineral grease. The results showed that the palm olein-based grease with a 0.25 wt% copper nanoparticle additive concentration had excellent lubricant performance, with low friction and wear prevention characteristics. This concentration of Cu nanoparticles led to a 35.05% reduction in average friction coefficient (COF), a 32.56% reduction in wear scar diameter (WSD), and a 48.02% reduction in wear volume compared to pure palm olein-based grease.

Comparison of Different Standard Test Methods for Evaluating Greases for Rolling Bearings under Vibration Load or at Small Oscillation Angles

Rolling bearings operated at small oscillation angles or exposed to vibrations during standstill show typical damage after only a short period of operation. This can be false brinelling damage, so-called standstill marks or classic fretting damage (fretting corrosion, tribo-oxidation). It is important to differentiate here according to the amplitude-ratio x/2b, which indicates the ratio between the rolling element Motion (x) and the Hertzian contact half-axis (b). Depending on this ratio, suitable laboratory test methods must be used to test the lubricating grease practically for the particular application. For this purpose, the Fafnir wear test, according to the standard of the American Society for Testing and Materials ASTM D4170, is also listed in the current high-performance multi-use specification of the National Lubricating Grease Institute (NLGI) as a release test for lubricating greases. In Europe, the SNR-FEB2 test is frequently used, which is also required to release greases in the blade bearings of wind turbines, among other things. In the case of standstill marks due to very small oscillation angles or vibrations, the Mannheim Tribology Competence Center (KTM) has developed a special test now established in the industry. The oscillating angles vary in these three different standard tests in the range from ±6° in the Fafnir test to ±3° in the SNR-FEB2 test to ±0.5° in the KTM standstill marking test; the x-to-2b ratios range from 5.5 (Fafnir) to 3.4 (SNR) to 0.5 (KTM). This paper will explain the scientific basis for these special operating and test conditions and compare test results of specially prepared model greases in these three standard rolling bearing tests, two test variations and a classical fretting test under oscillating sliding friction (ASTM D7594). The paper’s main objective is to show that the suitability of grease for such an application depends strongly on the prevailing operating conditions. Different tests in this field are, therefore, not interchangeable. Good results in one test do not automatically mean good results in a similar test at first glance. Therefore, selecting the right test for the application is important.

Comparison of the Tribological Behaviour of Various Graphene Nano-Coatings as a Solid Lubricant for Copper

Among the amazing properties of graphene, superlubricity is one of the most promising properties. This property can be used in industrial field components to reduce friction without using liquid lubricants, and therefore, improve machines’ efficiency and reliability with low environmental impact thanks to the elimination of oil or grease lubricants. In this paper, copper alloy samples for electrical purposes were coated with graphene by four different deposition processes. The investigated synthesis processes are direct grown graphene on bulk Cu, transferred graphene, and self-assembled graphene from graphene flakes. Ball-on-disk tests were performed to evaluate the tribological performance of samples. The aim was to compare the effect on the tribological performance given by different types of coatings, taking also into consideration industrial scalability. Interestingly, not all graphene nano-coatings being compared proved effective in reducing friction and wear in gross sliding conditions. The results show that the cost-effective self-assembled graphene is the longer-lasting nano-coating among those investigated in this work, and can reduce both friction and wear. Tests revealed that graphene coatings can be applied as a solid lubricant, reducing friction up to 78%, and reducing the average wear volume up to 40%.

Innovative Formulation and Characterisation of Grease Made from Waste Engine Oil

Lubricating grease is usually produced from mineral oil, making the relationship between grease and mineral oil unavoidable. Formulation of grease from waste oil can reduce the dependency of the grease industry on mineral oil as well as help to reduce the waste generation of used oil. This study aims to produce fumed silica (FS) grease from waste engine oil (WEO) and analyse the properties of the formulated grease. The method started with treating WEO to remove any contaminants in the used oil. After that, the greases are produced using a weight percentage ratio before being examined for consistency, oil separation, oil bleeding, FTIR (Fourier transform infrared spectroscopy) analysis, and corrosiveness. In terms of uniformity, oil separation, and oil bleeding, WEO percentage content had a substantial impact on the findings. The FTIR demonstrated that synthetic greases had the same spectra when evaluated between 500 cm<sup>-1</sup> and 4000 cm<sup>-1</sup>. The grease's corrosiveness is low, as determined by class 1 corrosiveness toward the copper strip. However, the grease properties differ when consistency, oil bleeding and oil separation test is done. Higher oil content in grease produced high oil bleeding and separation but low consistency. As a conclusion of the results, fumed silica grease with oil percentages of 83 and 82 have the most grease-like features, showing that the grease fits the traits' requirements. Based on the investigation's findings, it was established that WEO may be used as a base oil in grease formulation and that the grease's properties are satisfactory.

Hydrothermally synthesized smectite as a thickener with favorable rheological and tribological properties over a wide temperature range

Lubricating grease is essential for many applications. Although smectite grease is an important type of lubricating grease, its properties can be inconsistent due to variations in the properties of natural smectite. In this work, pure smectite was synthesized from an H2O/NH4F/Mg(CH3COO)2/Al(NO3)3/tetraethoxysilane/NaOH system by a one-step hydrothermal method. Compared with natural smectite, synthetic smectite has a larger cation exchange capacity (CEC), smaller particle size, and fluffier morphology. The pure smectite grease showed higher energy storage modulus and better thixotropy than natural smectite grease at the same thickener content. Synthetic smectite grease showed more stable tribological behavior, maintaining better anti-wear and anti-friction properties at different test temperatures. These improvements can be mainly ascribed to its fixed elemental composition, larger CEC, good rheological properties, and absence of impurities. Due to the convenient synthetic method and superior rheological and tribological performances, synthetic smectite as a grease thickener has promising potential for use in demanding advanced precision instruments, as well as pharmaceuticals, food, and other industrial fields.

A novel layered double hydroxides oleogel lubricant: Inspired by conventional greases

Grease lubricants serve as a simple, effective and convenient source of lubrication for saving energy. However, the most applicable lithium greases meet contradictory pressures of high performance and low pricing demands because of the substantial development of the global electric vehicles industry and the ubiquity of lithium batteries. In order to alleviate the dependence on lithium resources and develop resource-widespread and economical lubricating grease, herein, a class of thickener and oleogel system based on organic acid intercalated layered double hydroxides (LDHs) inspired by traditional grease has been reported. LDHs materials can be regarded as a preassembled “salt-like” compound, comparable to the self-assembled traditional metal soap salt thickener. LDHs oleogels exhibit viscoelastic properties and tribological performance similar to those of lithium greases. This study presents ideas for future development through layered materials regarding as "salt-like" compounds as a replacement for traditional grease thickeners.

The Friction of Radially Loaded Hybrid Spindle Bearings under High Speeds

Friction losses are an important parameter for evaluating the operational behaviour of high-speed rolling bearings. Specifically, in machine tool applications, the bearings are subjected to high radial loads and high speeds, which lead to increased forces in the rolling contact and, as a result, increased bearing friction. In this high-speed application, hybrid spindle bearings, typically made of ceramic balls and steel raceways, show better frictional behaviour compared to full steel-made bearings. Therefore, precise knowledge of the friction characteristics of high-speed hybrid bearings can improve friction models and generalise them to spindle bearings with different types, geometries, and operating conditions. In this article, a new straightforward and cost-efficient method for measuring the frictional torque in spindle bearings is presented. A rigidly arranged 7008 hybrid spindle bearing pair was tested up to rotational speeds of 24,000 rpm and high radial loads of 3 kN. The effects of oil–air and grease lubrication are discussed in characteristic diagrams of the tested bearings. Then, based on the test results, a friction calculation model is presented and validated for the outer race control and minimised power dissipation regarding the influence of radial forces.

Roughness Test of Aluminum A5052 before and after Forming by SPIF Process

Single point incremental forming process, which is also known as SPIF, that forms from one direction. SPIF process was used for milling an aluminum metallic sheet. In this paper, an aluminum (A5052) sheet was formed in a conical shape by the SPIF process. The A5052 sheet was deformed by the computer numerical control (CNC) technology in the T60 series milling machine. The conical shape was deformed by the spiral toolpath in the T60 series milling machine. At the surface of the A5052, lubricant grease was continuously applied manually for reducing the friction generated due to milling. Also, for decreasing the tool rotational force friction on the A5052 sheet there was the use of lubricant. Moreover, 0.1mm step-down is used for the tool path because it takes more time for forming a metallic sheet, however, it deformed the sheet smoothly. The 0.5mm step-down g-code was extracted from fusion 360. Then the metallic sheet A5052 surface was compared before and after deformation. For testing a metallic sheet A5052 the formability parameter was analyzed in the experiment part. The experiment was performed to find the optimal smooth surface from the CNC milling machine. The initial roughness value and the microscopic image were presented in the investigational part. Likewise, the comparison of points for the highest and lowest rough point was further discussed and compared throughout the experiment.

Facile formulation of graphene oxide modified commercial polyurea grease and its enhanced tribological capability under extremely high contact stress

Grease lubricants have been widely applied to the rubbing components to increase the operation stability and reliability. In this study, graphene oxide modified polyurea grease was prepared by formulating different amounts of exfoliated expandable graphite (EG) into commercial polyurea grease with combined mechanical agitation and grinding processes, tribological characteristics of graphene oxide modified polyurea grease were subsequently evaluated under extremely higher contacting stress. Among all the formulated polyurea grease specimens, polyurea grease with 2.0 wt.% graphene oxide displays better capabilities of minimizing friction and wear. In particular, the wear loss reduces considerably by 80% meanwhile the averaged friction coefficient decreases to 0.10, as referenced with existing polyurea grease. The improved tribological behavior is proposed to be associated with the in‐situ produced graphene oxide solids that enhances extremely lower shearing ability and promotes the fatigue resistance of grease, as well as its desired chemical compatibility with resulting iron sulfide tribofilm. Particularly, graphene oxide works as the dispersed precipitate within the dense iron sulfide/oxide layer induced from ZDDP that covers on the steel substrate.

Tribological evaluation of PAO 100 oil‐based lithium greases with chemically functionalized nanoadditives

AbstractThe lubricating greases employed in this study were formulated with polyalphaolefin (PAO) 100 as the base oil and 12‐lithium hydroxystearate as the thickener. The sol–gel method was used to produce the LaF3 nanoadditives. The oleic acid‐treated LaF3 (i.e., OA‐LaF3) and carboxylic acid‐treated MWCNTs (i.e., COOH‐MWCNTs) were blended in the formulated grease samples and used as nanoadditives. The detailed microstructural and chemical properties of chemically treated nanoadditives were investigated using HR‐TEM, XRD, FTIR and XPS analyses. Using a four‐ball tester, the effect of different concentrations of nanoadditives on the tribo‐performance of greases was investigated. Tribo‐test results revealed the role of nanoadditives in PAO‐based grease lubrication performance. COOH‐MWCNTs and OA‐LaF3 nanoadditives outperformed PAO grease in terms of physicochemical and tribo‐performance for steel‐steel tribo‐pair. The improved tribo‐performance is attributed to the formation of tribo‐film on the contact interfaces via nanoadditives. The development of tribo‐film was confirmed by XPS analysis of worn surfaces.

Triboactive coatings for wear and friction reduction in chain drives

Using hard triboactive (Cr,Al,X)N coatings (X = Mo, Cu) on chain pins is promising to increase the energy efficiency and sustainability of chain drives. (Cr,Al,X)N coatings, when interacting with lubricants, can build a tribofilm in the contact zone which has wear- and friction-reducing properties. These tribofilms can also be transferred to the surface of the chain bush, protecting it from wear. In this study, these coatings were tribologically investigated via pin-on-disc tribometer and a chain-joint tribometer under grease lubrication. The results showed that the (Cr,Al,Mo)N coating achieved the lowest total wear volume and lowest friction coefficient. The tribofilms showed a comparable composition on chain pins and bushes after the tests indicating a transfer of the tribofilm.

Tribological properties of hexagonal boron nitride nanoparticles as a lubricating grease additive

AbstractThe friction and anti‐wear behaviours of hexagonal boron nitride (h‐BN) nanoparticles as a lithium lubricating grease additive are being investigated under sliding conditions. The grease with 3 wt% 60 nm h‐BN particles and the grease with 1 wt% 500 nm h‐BN particles lead to 22.34% and 20.18% reductions in the wear scar diameter, respectively, although the friction coefficients slightly decrease. The boric acid H3BO3 with layer‐crystal structure produces during the friction process, and the synergistic effect of h‐BN nanoparticles and H3BO3 makes friction reduction and wear‐resistant enhancement. Furthermore, the 60 nm h‐BN particles filled in the asperity valleys of the friction surface make a rolling effect and establish a protective film, while the 500 nm h‐BN particles shield the asperity contact between friction pairs and make a polishing effect as well.

Influence of Free Water Droplets on Film Formation Under Point Contact Grease Lubrication

Abstract The influence of the grease-water two-phase emulsion on the bearing tracks on the film formation has been studied by carrying out optical interferometry experiments on a ball disk test rig to observe the dynamic film-forming behavior of lubricating grease under steady-state and micro-oscillation conditions. It is found that free water causes a brief rise in the grease film under steady-state conditions, specifically due to the grease-water phase connecting the oil reservoirs on both sides. The experiment determines that the state of the phase before entering the elastohydrodynamic lubrication (EHL) contact is water-in-oil. For the reciprocating motion and oscillation, the effects of the amount of free water and the length of the motion stroke are discussed in detail. It is found that for reciprocating movements with long stroke lengths, the effect of free water is summarized to enhance the fluidity of the fibrous mass of the grease thickener in the first few cycles of the movement. For shorter stroke lengths, the effect of free water is a scouring effect on the contact zone grease after 100 cycles. This paper provides new insights into the effects that cause water pollution to the point of contact lubrication during the transformation from reciprocating motion to fretting.

Thermal-elastohydrodynamic lubrication study of axial-modified greased cycloid drive in rotate vector reducer

A grease lubrication model of the cycloid drive in the rotate vector (RV) reducer is established to study the axial modification effect of the cycloid drive on its thermal-elastohydrodynamic lubrication (TEHL) behavior. Here, the elastic deformation and stress are rapidly calculated using the fast Fourier transform (FFT) technique. Meanwhile, the film thickness experiment is used to verify the above model. It is found that the modification effect on the TEHL performance is obvious and positive in most meshing processes of the cycloid drive. Compared with other common profiles, the Lundberg profile is an optimal modification to improve the end TEHL behavior and relieve the edge stress concentration due to the better adaptability for the meshing process of the cycloid drive.