Experimental measuring procedure for the friction torque in rolling bearings

The reduction of the power loss generated in mechanical transmissions and the use of low friction biodegradable lubricants has been attracting considerable attention in recent times. Therefore, it is necessary to develop methods to test and evaluate the performance of such lubricants and compare them with conventional ones. In this sense, a Four‐Ball Machine was modified allowing the test of rolling bearings. A 51107 thrust ball bearing was used to test two different greases and the corresponding base oils. Friction torque and operating temperatures were continuously monitored to quantify the power loss and the heat evacuation for each lubricant tested. Copyright © 2010 John Wiley & Sons, Ltd.     

现代自动变速箱用的润滑油

用于轿车和商业车辆的ＺＦ自动变速箱要求高性能的润滑油，这种润滑油只好由变速箱厂和润滑油生产者紧密合作才能开发出来。ＺＦ各种润滑油的技术条件中必须说明各润滑油的特性，为经济性和环保的原因，必须对轿车用的ＺＦ多速比自动变速箱提供全程润滑法，滑差可调扭矩变矩器已成为所有新型变速箱的标准部件。由于湿式起动离合器和电子可控换档的发展结果对润滑油产生了一些新的要求。     

Temperature—friction characteristics of cylinder lubrication in large,slow, cross‐head marine diesel engines under boundary lubrication conditions

In large, slow, cross‐head marine diesel engines research has increasingly shown that the lubrication regime between piston rings and cylinder liner at top dead centre is of the boundary lubrication type due to the high gas pressure, low sliding speed, and high temperature. This means that the tribological properties of piston ring, cylinder liner, and cylinder lubricant in these types of engine under boundary lubrication conditions should be considered simultaneously when friction and wear between the piston ring and cylinder liner are studied. Until now there has been no standard method to evaluate boundary lubrication performance. There are a few traditional methods used in lubricant research, but their results are not correlated with service conditions. It is important to find a suitable method to evaluate the boundary lubrication performance of lubricants at the laboratory testing stage or before the engine testing stage. The important parameters, such as sliding speed, normal load, materials of the contacting pairs, and lubricant, need all to be controlled. In this paper a systematic experimental procedure, the ‘five times heating and cooling test’, is introduced to assess lubricant properties under boundary lubrication conditions. Most of the parameters mentioned above are controlled. The model contact, of pin‐on‐plate form, is made from the actual piston and liner materials used in a large‐bore, slow, cross‐head marine diesel engine. The temperature characteristics of different blends of lubricants are investigated under boundary lubrication conditions using a pin‐on‐plate reciprocating test rig. These blends of lubricants have the same additives but different base fluids; they nevertheless fulfil the physical and chemical requirements of a real marine diesel engine. The test temperature range is from room temperature to the working temperature of the top piston ring. The experiments show that there are different temperature—friction characteristics for lubricants with different bases and the same additive package and there are also different temperature—friction characteristics during heating up and cooling down for each blend. Single‐base lubricants have more promising temperature—friction characteristics than those of a blend of a high‐viscosity base and a low‐viscosity base at high temperature.     

Development of an experimental rig with hydrostatic bearings for testing the EHD traction force of high-speed lubricants

A test rig for the EHD traction force of lubricants is essential equipment for the study of the non-Newtonian properties of lubricants operating at high pressure. In this paper is described the development of test equipment with hydrostatic bearings, on the basis of analysis of EHD traction and other test rigs. Hydrostatic bearings allow the rig to pick up the signals of the EHD traction force more precisely. Thus, good test results for an aviation lubricant have been obtained.     

Mapping Shear Stress in Elastohydrodynamic Contacts

A new method has been, devised for investigating the theological properties of lubricant films in two-dimensional EHD contacts. A lubricated, sliding contact is produced between a sapphire flat and a steel ball. Thermal infrared emission microscopy is then employed to obtain 2-D maps of the variation of temperature rise due to friction across the contact. These maps are then used in conjunction with moving heal source theory to produce maps of energy dissipation and thus shear strength, of the lubricant film across the contact.

A series of mixtures of two lubricants, one giving high traction and one with low traction, have been studied using this technique to investigate the influence of lubricant, blending on shear stress and traction.     

Friction behavior of lubricated zinc phosphate coatings

The aim of the present work is to elucidate the influence of lubricants on the friction behavior of zinc phosphated coatings and provide an explanation for the results in terms of physical–chemical interactions between lubricant and phosphate. The friction behavior was studied through a sliding wear test, with a conventional ball-on-disc configuration. Discs, made of AISI 1006 low carbon steel, uncoated and coated with zinc phosphate, were tested against bearing steel balls. A stearate sodium soap, paraffinic oil and both soap and oil were used as lubricants. The sodium stearate soap was found to have the best seizure resistance. The nature of the interfacial forces between the lubricant and surface has an important role in determining the friction behavior.     

Parametric elastohydrodynamic study of a linear contact applied to SiC lubricated with a low‐viscosity fluid

Ceramic materials are often used in tribological applications because of their good mechanical properties. Silicon carbide (SiC) has a low density, high Young's modulus, high corrosion resistance, and very low friction in water. It is a very good candidate for replacing waterproof oil‐lubricated bearings in water pumps and for applications when water is used as a lubricant. In such applications the SiC dissolves very slowly in water to form silicilic acid. After running in, journal and bearing surfaces look like mirrors with very low roughness and very small friction coefficient. This paper reports on a parametric study to model the cylinder—plane configuration, which is then utilised in the specific case of SiC in water. The study was carried out in order to reduce the number of parameters and to ensure good precision of the results. The numerical model is applied to an SiC contact with a very low‐viscosity lubricant (water) using elastohydrodynamic (EHD) conditions. Multigrid techniques for the Reynolds equation and multilevel, multi‐integration methods for the elasticity equations are used to solve the EHD problem. These results are given in non‐dimensional form using the Moes parameters M and L for several velocities and loads. Comparison is made between the two‐ and three‐dimensional cases in order to assess the effects for the cylinder ends.     

Viscosity and working efficiency analysis of soybean oil based bio-lubricants

This article presents significant data about viscosity and working efficiency analysis for developing the soybean oil based bio-lubricants. A suitable viscosity or viscosity index (VI) plays a very important role in a lubricant, which can avoid collision and rubbing between components of mechanical devices in work as well as optimize working efficiency of a machine. In general, low friction between devices can increase working efficiency of a machine, but low viscosity of a lubricant will easily cause collision and rubbing between components of mechanical devices in work. A too viscous lubricant also requires a large amount of energy to move, but a too thin lubricant will easily cause rubbed devices and increased friction. To replace the mineral oils and syntholubes, the soybean oil is recently become one of the most actively studied oils due to its eco-friendly organic property and lower cost. This work used mixtures of the original soybean oil, the epoxidized soybean oil, and the hydrogenated soybean oil as the base oils. Applications are focused on developing engine bio-lubricants. The results show that the epoxidized soybean oil has extremely large viscosity in comparison with the engine lubricants as well as the original soybean oil, whereas the hydrogenated soybean oil is clearly opposite. This viscosity analysis offers good informations to fit viscosity of the engine lubricants by mixing the three soybean oils as base oils.     

The Effect of Lubricant Viscosity and Composition on Engine Friction and Bearing Wear

This paper investigates the effect of lubricant composition on engine friction and connecting-rod bearing wear. Special attention has been given to polymer-thickened (VI improved) oils since these oils are characterized by shear-dependent viscosity and a simultaneous occurrence of viscous and elastic properties. The variables investigated in this study included lubricant viscosity, polymer type, and concentration.

Two sets of engine studies were conducted, one to determine engine friction, the other to measure connecting-rod bearing wear, using irradiated bearings. For Newtonian fluids, the engine friction and wear response can be predicted from classical lubrication theory—that is, (a) friction decreases with increaing viscosity until a viscosity is reached where friction is a minimum; beyond this viscosity, further increases in viscosity result in increased friction. (b) Bearing wear decreases with increasing viscosity, but as a step function, not linearly, and the transition viscosity (of the step) corresponds to the viscosity which gives a minimum engine friction.

The addition of polymeric VI improvers (non-Newtonian fluids) to mineral oil base stocks reduces engine friction and lowers bearing wear—the amount of friction and wear reduction depending on the polymer type and concentration. This paper demonstrates that polymer-thickened oils actually give better bearing wear performance than their comparable mineral oil counterparts despite the fact that they have a lower apparent viscosity at high rates of shear. In addition, it appears that temporary viscosity loss is not the sale cause of the reduced engine friction of polymer-thickened oils.     

Predicting friction reliably and accurately in journal bearings—A systematic validation of simulation results with experimental measurements

It is the aim of this work to predict friction in journal bearings reliably and accurately under realistic dynamic working conditions. To this purpose elastohydrodynamic (EHD) calculations using an extensive oil-model and including an approach to the conformal roughnesses of the bearing surfaces are carried out for transient loads typical for current utility vehicles (40 MPa) as well as for considerably higher specific loads (70 MPa) and for different lubricants (SAE10, SAE20, SAE30 and SAE40) to account for a large span of working conditions ranging from full film lubrication to mixed lubrication with metal-metal contact.The results obtained from this simulation model are compared to measurements performed on a journal bearing test rig. We find that the results of the presented approach agree very closely with the experimental values. The presented approach allows consequently to investigate the effectiveness of changes in bearing geometry, bearing materials, bearing surface roughness, lubricant viscosity and engine operating conditions to reduce friction in journal bearings.     

Evaluation of Hot Pressed Silicon Nitride as a Rolling Bearing Material

A series of tests has been conducted to evaluate the suitability of silicon nitride as a bearing material for rolling contact applications. The ability of silicon nitride to be lubricated by some conventional lubricants was found to be satisfactory. This was determined by wettability studies, lubricant film thickness and traction coefficient measurements on the optical EHD rig and friction coefficient measurements by the pin-on-disk method. The abrasive wear coefficient, measured on a lopping machine using 600 grit SiC abrasive, was found to be high compared to other ceramics. It was also dependent on the composition of the silicon nitride. Comparative rolling contact fatigue tests on steel and silicon nitride flat washers were conducted using steel rollers and balls. A high wear rate leading to grooving in the rolling track on silicon nitride was observed. The spalling resistance of silicon nitride was found to be higher than that of steel under the test conditions used. Surface interactions in the Si₃N₄-M50 steel contacts, detrimental to the life of the steel rolling elements, were recognized. Attempts were made to reduce the severity of these interactions and prolong the life of bearings containing ceramic elements.     

Synthetic Lubricants in Hydrodynamic Journal Bearings: Experimental Results

Synthetic lubricants and additives have seen many major improvements in recent years. However, very little is known about the performance peculiarities of these new lubricants in actual machines. To fill this gap, a new full-scale hydro-dynamic journal bearing test rig has been constructed to evaluate the behavior of conventional and new bearing designs, synthetic lubricants and variations in operating parameters. This test rig’s bearing has diameter 180 mm with measuring capabilities including continuous film thickness and film pressure as well as temperature. The new machine was used to compare a number of synthetic lubricants to mineral based lubricants, finding that performance of the synthetic lubricants was superior to their mineral based counterparts of much higher viscosity grade. These tests showed that high viscosity index (VI) synthetic lubricants had higher viscosity in the region of maximum pressure and lower viscosity elsewhere in the bearing than similar mineral based lubricants. This reduction in viscosity in low pressure zones was found to produce a measurable reduction in friction and power loss in the bearing system. This paper provides comparative performance results of several formulations of current and future turbine oils from measurements of losses, oil film thickness, and temperature under a range of operating parameters. Lubricants tested include ISO VG68 and VG32 mineral based turbine oils (VG68 and VG32), ISO VG32 synthetic ester based oil (SE32), two ISO VG22 synthetic ester based oils (SE22 and SV22), and ISO VG15 synthetic ester based oil (SE15). It was found that SE32 and VG68 provided similar performance at lower speeds while SV22 provided similar performance to VG68 at the highest speed. Likewise, SE22 and SV22 provided similar performance to VG32 at low speeds while SE15 provided similar performance to VG32 at medium to higher speeds. Generally, the results demonstrate the potential for replacing mineral based lubricants with high performance synthetic lubricants of significantly lower viscosity grade without sacrificing bearing safety.     

Rolling Contact Testing of Vapor Phase Lubricants—Part II: System Performance Evaluation

The relative effects of several vapor lubrication parameters on bearing performance were examined using a ball-on-rod tester. Lubricants included in the evaluation were a tertiary-butylphenyl phosphate (TBPP), a 2 cSt polyalphaolefin blended with 15 percent TBPP (PAO+), the TBPP blended with 33 percent tributyl phosphate (TBPP+), a cyclophosphazine (X-1P), a polyphenylether (5P4E), and a perfluoroalkylether (Z). Parameters included in the study were bearing temperature, vapor concentration, and vapor temperature. Additionally, a solid lubricant coating was included to improve the bearing performance under cold-start conditions. The lubricants containing phosphorus demonstrated the best high temperature performance. The TBPP lubricant failed shortly after test at 650°C., while the X-1P lubricant performed satisfactorily over an eight-hour period at 650°C. The TBPP+ lubricant demonstrated the widest temperature range capability, with 600°C operation and a projected pump-ability point of–-45°C. Lubricant concentration was the most significant system parameter affecting bearing friction and wear.     

Ionic liquids with ammonium cations as lubricants or additives

A new class of more effective lubricants could lead to huge energy savings. Limited recent literature has suggested potential for using room-temperature ionic liquids as lubricants, however, only a few out of millions (or more) of species possible have been evaluated. In this study, a series of new protic alkylammonium ionic liquids were synthesized by neutralization and metathesis reactions, and have demonstrated promising lubricating properties as neat lubricants or lubricant additives, particularly for use with difficult-to-lubricate metals like aluminum. More than a 30% friction reduction was observed with ammonium-based ionic liquids compared to conventional hydrocarbon oils in reciprocating sliding tests of 52100 bearing steel on aluminum alloy 6061-T6511. The inherent polarity of ionic liquids is believed to provide strong adsorption to contact surfaces and can form a boundary lubricating film leading to friction and wear reductions. Other advantages of ionic liquids include (1) negligible volatility, (2) high thermal stability, (3) non-flammability, and (4) better intrinsic properties that may eliminate the need for more complex and expensive additive packages. With very flexible molecular structures, this new class of lubricants, particularly ammonium-based ionic liquids, can be tailored to fit a variety of applications.     

Comparison of friction and lubrication of different hip prostheses

It is well documented that an important cause of osteolysis and subsequent loosening of replacement hip joints is polyethylene wear debris. To avoid this, interest has been renewed in metal-on-metal and ceramic-on-ceramic prostheses. Various workers have assessed the lubrication modes of different joints by measuring the friction at the bearing surfaces, using different lubricants. Measurements of friction factors of a series of hip prostheses were undertaken using carboxymethyl cellulose (CMC) fluids, silicone fluids, synovial fluid and different concentrations of bovine serum as the lubricant. The experimental results were compared with theoretical predictions of film thicknesses and lubrication modes. A strong correlation was observed between experiment and theory when employing CMC fluids or silicone fluids as the lubricant. Mixed lubrication was found to occur in the metal-on-metal (CoCrMo/CoCrMo) joints with all lubricants at a viscosity within the physiological range. This was also the case for the metal-on-plastic (CoCrMo/ultra-high molecular weight polyethylene) joints. The ceramic-on-ceramic (Al2O3/Al2O3) joints, however, exhibited full fluid film lubrication with the synthetic lubricants but mixed lubrication with the biological lubricants. Employing a biological fluid as the lubricant affected the friction to varying degrees when compared with the synthetic lubricants. In the case of the ceramic-on-ceramic joints it acted to increase the friction factor tenfold; however, for the metal-on-metal joints, biological fluids gave slightly lower friction than the synthetic lubricants did. This suggests that, when measuring friction and wear of artificial joints, a standard lubricant should be used.     

Influence of micropolar lubricants on the performance of slot-entry hybrid journal bearing

A theoretical study concerning the slot-entry hybrid journal bearing lubricated with micropolar lubricants is presented. The modified Reynolds equation for micropolar lubricant is solved using finite element method along with equation of lubricant flow through slot-entry restrictors as a constraint together with appropriate boundary conditions. It has been observed that a slot-entry hybrid journal bearing operating with micropolar lubricant shows an increase in the value of minimum fluid film thickness and a reduction in the value of coefficient of friction as compared to a corresponding similar slot-entry hybrid journal bearing operating with Newtonian lubricant.     

Influence of micropolar lubricants on the performance of slot-entry hybrid journal bearing

A theoretical study concerning the slot-entry hybrid journal bearing lubricated with micropolar lubricants is presented. The modified Reynolds equation for micropolar lubricant is solved using finite element method along with equation of lubricant flow through slot-entry restrictors as a constraint together with appropriate boundary conditions. It has been observed that a slot-entry hybrid journal bearing operating with micropolar lubricant shows an increase in the value of minimum fluid film thickness and a reduction in the value of coefficient of friction as compared to a corresponding similar slot-entry hybrid journal bearing operating with Newtonian lubricant.     

Lubrication safety in high-speed ball-bearings

In a 7206 C high-speed, mist-lubricated ball-bearing, both the film thickness and lubricant parameter λ in ball—race tribo-systems have been analytically investigated in terms of starvation and thermal effects, and good correlations with experimental results were obtained for 5,000 to 35,000 rpm rotational speeds. Including a lubricant parameter in ball—race tribosystems as a safety criterion for ball-bearings, friction losses, temperature, and limit speeds have been correlated with this criterion for a 7206 C ball-bearing. Friction losses in ball-outer race tribosystems have been computed for both EHD and mixed lubrication regimes. The influence of the bricant parameter on friction losses has been shown both theoretically and experimentally. A new speed limit concept including the lubricant parameter λ, has been developed by the authors.     

Performance Characteristics of Perfluoroalkylpolyether Synthetic Lubricants

An extensive test program was conducted to determine the performance characteristics (friction, wear, temperature) of a linear high temperature perfluoroalkylpolyether (PFPE) and two branched PFPE lubricants under normal and severe operating conditions. These include temperatures up to 200° C and Hertzian pressures as high as 2.8 GPa. A four-ball wear tester was used to measure wear scar and a two-disk machine was used to measure friction. The linear perfluoroalkylpolyether lubricant demonstrated low friction and wear at elevated temperatures under the test conditions considered. A bearing testing apparatus was used to measure friction and temperature of tapered roller bearings under various operating conditions for a branched PFPE and the results are reported.     

The Friction and Wear of Glass

A combination if electron photo micrographs and sensitive friction measurements indicate that the dry friction if glass may be due to surface melting under frictional heat. Stick slip phenomena are examined and the characteristic “tear drop” scratch is seen to be composed if a widening track if fused cracks ending in a “circular spot” if fused glass. Low temperatures or lubrication prevent this. For short durations at low loads, temperatures, and sliding speeds, most lubricants gave the same friction and wear results. When conditions become more severe or time more extended, various classes if lubricant excellence can be found. Wear measurements following “run in” indicate that with proper lubricants and operating conditions glass bearing surfaces can be operated at high loads and high temperatures.