Effect Of Friction Modifiers Research Articles

Thermal–Mechanical Coupling Analysis of Wheel–Rail Sliding Friction under Two-Point Contact Conditions

The generation of wheel–rail-sliding frictional heat is often accompanied by transverse displacement of a wheel. To study the thermal problem of wheel–rail sliding friction at two-point contact, this paper uses an LM tread wheel and a 60 kg·m−1 rail as examples. A thermal–mechanical-coupled finite-element model of equal proportion wheel–rail sliding is established. A direct-coupling method is used to analyze the thermal–mechanical coupling of the wheel–rail interface under sliding contact. This model considers the temperature-dependent material properties and boundary conditions, such as thermal convection and thermal dissipation, in the process of nonstationary frictional-heat conduction. Firstly, the effects of different sliding speeds, axle loads, and contact modes on the temperature and stress fields of the contact area are analyzed. Then, the lubrication and cooling effects of friction modifiers on the rail top and rail gauge angle are compared. The results show that, at a sliding speed of 2 m/s and an axle load of 30 t under a sliding condition of 200 mm, on the top and side of the rail, the temperatures at the contact patch centers are 813 °C and 547.7 °C, respectively. Under different operating conditions, the rail-side temperature is 55–75% of that of the temperature at the rail top, and the rail-side contact and friction stress values are 76–96% of those at the rail top. This indicates that frictional thermal damage on the rail side cannot be ignored. With a lower sliding speed, the thermal response of the two contact patches is closer. The impact of axle load on the frictional temperature and stress on the rail side is more critical than the sliding speed. The optimal lubrication choice is overall lubrication, which can decrease the rail top temperature by 47.2% and frictional stress by 56.2%, as well as decreasing the rail side temperature by 70.3% and frictional stress by 77.4%.

Wear and damage behaviours of wheel and rail materials: Effects of friction modifier and environmental temperature

Low temperature conditions can aggravate the wear and fatigue damage of the wheel and the rail materials. Friction modifier (FM) is used to control the adhesion and alleviate the wear and damage. However, the wear and damage behaviours of wheel and rail with the application of FM under low temperature have not been revealed. Therefore, the present study aims to investigate the effect of FM on the wear and the rolling contact fatigue (RCF) damage of the wheel and the rail under low temperature conditions. Tests were conducted on a rolling-sliding wear testing apparatus with the samples enclosed in a temperature-controlled chamber. The results indicated that the effect of FM on the wear and the RCF damage of the wheel and the rail was influenced by the temperature significantly. Compared with the dry condition, the wear rate of the wheel decreased obviously at all temperature conditions when using FM. Meanwhile, the RCF damage of the wheel was relatively slight. Besides, there was no significant difference among the RCF damage of the wheel at different temperatures both under the dry and the FM conditions. At room temperature, the application of FM could alleviate the RCF damage of the rail. Under the low temperature condition, the performance of FM would fail, which would result in a severe oil wedge effect on the rail specimen. Therefore, the application of FM could induce a higher wear rate and severe RCF damages on rail material at the low temperature condition. The results of the present study could provide guidance for the development and design of FMs used in cold regions.

Tribological evaluation of passenger car engine oil: Effect of friction modifiers

The tribo-performance of passenger car engine oil with three friction modifiers (organic and inorganic) has been experimentally evaluated using benchtop tribometers. Friction coefficient traces and wear and contact potential (qualitative measurement of lubricating film formation) were reported at varying contact pressures, speeds, and temperatures. The in situ film-forming characteristics of friction modifier in engine oil were also investigated and reported using SLIM (Spacer layer imaging) in a Mini traction tribometer. Overall, inorganic friction modifiers (MoDTC) yielded better tribological performances than organic friction modifiers. Typically, thicker protective tribo-film (180 nm) formed at the worn surface by MoDTC than organic friction modifiers (80–120 nm). Scanning electron microscopy (SEM), Raman spectroscopy and Atomic force microscopy (AFM) have been utilised to characterise the worn surfaces and understand the mechanism behind the protective tribo-film formation.

An experimental study on the effects of friction modifiers on wheel–rail dynamic interactions with various angles of attack

By modifying friction to the desired level, the application of friction modifiers (FMs) has been considered as a promising emerging tool in the railway engineering for increasing braking/traction force in poor adhesion conditions and mitigating wheel/rail interface deterioration, energy consumption, vibration and noise. Understanding the effectiveness of FMs in wheel–rail dynamic interactions is crucial to their proper applications in practice, which has, however, not been well explained. This study experimentally investigates the effects of two types of top-of-rail FM, i.e. FM-A and FM-B, and their application dosages on wheel–rail dynamic interactions with a range of angles of attack (AoAs) using an innovative well-controlled V-track test rig. The tested FMs have been used to provide intermediate friction for wear and noise reduction. The effectiveness of the FMs is assessed in terms of the wheel–rail adhesion characteristics and friction rolling induced axle box acceleration (ABA). This study provides the following new insights into the study of FM: the applications of the tested FMs can both reduce the wheel–rail adhesion level and change the negative friction characteristic to positive; stick–slip can be generated in the V-Track and eliminated by FM-A but intensified by FM-B, depending on the dosage of the FMs applied; the negative friction characteristic is not a must for stick–slip; the increase in ABA with AoA is insignificant until stick–slip occurs and the ABA can thus be influenced by the applications of FM.

Numerical prediction on the effect of friction modifiers on adhesion behaviours in the wheel-rail starved EHL contact

Previous third body studies for wheel/rail are mainly concentrated on water or oil condition. However, friction modifiers (FMs) represent a new approach to managing adhesion coefficient in wheel/rail interface. In this paper, the position of the FM-air meniscus was given as input parameter, the thickness of FM can be solved easily with the multi-grid method. Non-Newtonian fluid model and micro-EHL analysis combined to obtain the relation between the adhesion coefficient and the quantity of FM. The results show that the quantity of FM has a significant effect on the initial adhesion coefficient within a certain range. Besides, the initial adhesion coefficient decreases gradually until it reach stable value with the increase of the quantity of FM.

The effect of friction modifiers on tribocorrosion and tribocorrosion-fatigue of austenitic stainless steel

Hydraulic fluids are vastly operated in marine applications however existing mineral-based fluids are not an alternative in these applications due to both security reasons during operation (fire hazard) and the effect for the environment. Water-glycol based fluids shown promising characteristics with low flammability and high cooling capabilities. In the present study, tribocorrosion and multi-degradation of austenitic stainless steel (ASS) exposed to marine environment under lubrication of water-glycol based hydraulic fluids has been investigated. The effect of dimethylaminoethanol and palmitic acid as corrosion inhibitor and friction modifier on wear loss and effect of salt contamination has been studied. The role of the friction modifier in tribocorrosion and multi-degradation has been crucial to control wear and the electrochemical response of the ASS.

Effect of Friction Modifier on Rolling Contact Fatigue and Wear of Wheel and Rail Materials

ABSTRACTIn this study, the effects of a friction modifier (FM) on wear and rolling contact fatigue were investigated by twin-disc testing under various conditions. It was found that the contact fatigue damage occurred on the rail specimen under the FM condition and the water condition. The test under FM condition was performed under three different spraying times: 0.2, 0.5, and 1.0 s. The progress of contact fatigue damage severely increases at a spraying time of 0.2 s. Contact fatigue damage under the FM condition occurred faster than that under the water condition. Results show that the friction modifier induced a mixed condition that exists at the same time as the wet condition and the dry condition. Cracks were initiated by the traction force due to the friction of the dry condition and the crack growth rate became faster due to the fluid entrapment mechanism under the wet condition. Therefore, the amount of FM should be evaluted to avoid severe contact fatigue damage.

Laboratory investigation of ability of oil-based friction modifiers to control adhesion at wheel-rail interface

In the last few years, top-of-rail friction modifiers have been designed and used in many railway systems all over the world. These adhesion enhancers are applied by either off-board or on-board system in order to achieve the intermediate level of friction and positive adhesion curve. Previous scientific effort was mainly focused on the effects of water-based friction modifier on adhesion, rolling contact fatigue, railway noise and corrugation formation. The objective of this study is to investigate the abilities of oil-based friction modifiers to control adhesion and reduce wear at wheel rail interface. For this purpose, two commercial oil-based friction modifiers were particularly utilised. A ball-on-disc tribometer was employed to investigate their traction and braking performance for various slip ratios under dry conditions. Furthermore, the effect of friction modifier amount has been studied. At the end of the performed tests, wear rate, surface damage and changes of surface topography were determined and compared to dry and oil-contaminated contact. The results indicate that oil-based friction modifiers are able to control adhesion in wheel-rail contact but it is strongly dependent on the applied amount of friction modifiers. Regarding to the friction behaviour and wear, the content of metal particles seems to be the crucial parameter.

Innovative technique of measuring follower rotation in real production engine using Gradiometer sensor and the effect of friction modifier

Purpose – The purpose of this paper is to understand the effect of engine operating conditions and lubricant friction modifier on direct acting tappet rotation. In this research work, novel method of measuring engine tappet rotation speed has been developed. The technique is so novel. It allows the measurement on real production engine with no modification to the engine tappet bore. Also, In this paper, the effect of engine operating conditions and the effectiveness of friction modifier on tappet rotation is reported. Design/methodology/approach – For the very first time, for the purpose of measuring follower rotation in a real production engine, a 4 × 6 mm2 electronic chip called Gradiometer is mounted outside the tappet housing, allowing the monitoring of tappet rotation speed without the need to machine a hole in the tappet bore. This novel technique is adopted on Mercedes Benz OM464 engine to study the effect of engine conditions and lubricant chemistry on tappet performance. Findings – The main outcome of this research work is the development of novel method of measuring tappet rotation. Also, during the experiments, it was revealed that although friction modifiers help in reducing friction at the cam/tappet interface, they can also adversely affect the tappet rotation speed. Originality/value – The novel technique developed in the research work is one of the most cost effective and simple to use. Researches can adopt the technique to study the tribological performance of direct acting tappet on real production engine. Researches acknowledge the effectiveness of friction modifiers in valve train but its effect on rotation which plays a key role in the component durability has not been the focus of most of the researches mainly due to lack of effective techniques.

The effect of friction modifiers on wheel/rail isolation at low axle loads

Signalling block occupancy is triggered by the wheelset of a rail vehicle ‘shunting’ the track circuit. The main cause of loss of shunting is the many materials that can be present on the rail head. These include iron oxides, leaves, ballast dust, oil, as well as products deliberately applied to the rail/wheel such as grease, friction modifiers, lubricants and sand. These may lead to an indication that a signal block is unoccupied when a train is actually present. Friction modifiers (FMs) are being increasingly used for reduction of noise, lateral forces, wear, rolling contact fatigue, etc. The Kelsan® high positive friction (HPF) solid stick FM is applied directly to the wheel tread creating a thin film. Although no effects have been observed during field operation, the intention in this work was to build on previous studies and evaluate conductance across a wider range of contact conditions as studies have shown that loss of shunt is more common with light axle loads where films could have a greater effect. The testing was performed at a contact pressure of 470 MPa (equating to a 4.9 tonne axle load). An HPF stick was spring loaded against the rotating wheel disc to generate a FM film at the contact. Tests were run to measure the impedance across the discs using a modified TI21 track circuit. Static testing was also performed using discs with a pre-generated HPF film. Analysis of the results showed that the application of HPF friction modifier had no significant effect on the measured level of impedance. The highest impedance levels were recorded under pure rolling (0% slip) where FMs were not being applied. In cases where the application of FM increased impedance, the recorded impedance values were still lower than the highest values measured in the absence of a FM. This is of key relevance as several light rail operators coast their trains.

The effect of friction modifiers on wheel/rail isolation

Abstract Train detection, for signalling purposes, is often by means of track circuits. Signalling block occupancy is triggered by the wheelset of the train ‘shorting out’ the track circuit, i.e. the wheels and axle act as a shunt. Contamination on the track such as ballast dust, rust, oil, or leaves as well as substances designed to improve train operation such as friction modifiers or sand may cause the contact between the wheelsets and the track to be compromised, inhibiting train identification. In previous work a twin disc approach has been used to study the effect of sand (used to improve adhesion) and leaves on wheel/rail isolation. Friction modifiers are of significant current interest in wheel/rail research. Introducing a new material into the tread/top of rail interface can raise questions about the impact on signalling systems. Although no significant effects have been observed in practical operation on a range of railway systems, the intention in this work was to evaluate conductance between wheel and rail in a more controlled and systematic fashion using the previously established methodology. Using the twin disc technique, friction modifier, in the form of a solid stick, was applied using a spring loaded device to the rotating wheel disc to generate a visible film. Tests were run to measure contact impedance at typical loads and slips. Static tests were also carried out using discs pre-conditioned with a friction modifier film. The electrical circuit used was a modified simplified simulation of audio frequency track circuit. No significant difference was observed in the measured impedance for dry conditions with no friction modifier, versus tests where friction modifier was applied, regardless of percentage slip or input voltage. The analysis suggests that the introduction of friction modifier into the existing wheel/rail interfacial film does not result in increased impedance with all other factors being equal.

Effect of Friction Modifiers and Antiwear Additives on the Tribological Performance of a Hydrogenated DLC Coating

There has been a lot of attention on the effect of lubricant additives on the friction at carbon coated surfaces. But only few papers have addressed the effect of additives on the durability of some diamondlike carbon DLC coatings. This paper presents a systematic study assessing the additive/additive and additive/surface interactions, and their influences on the durability of a low hydrogen-containing (15 at. % hydrogen) metal-free DLC coating (a-C:15H). In this study, lubricating oils containing a zinc dithiophosphate (ZDDP) antiwear additive and/or organomolybdenum friction modifiers (moly dimer and moly trimer) were used. Tribological tests were carried out in a pin-on-plate tribometer under boundary lubrication conditions. To understand the effect of additives, tribofilms formed on the wear tracks were analyzed using surface sensitive analytical techniques such as atomic force microscope, scanning electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy. Results showed that the ZDDP formed a zinc phosphate containing an ultrathin antiwear tribofilm, which offered excellent durability/wear protection to the a-C:15H coating. However, the antiwear performance of this additive was compromised when it was used with moly dimer or moly trimer. Surface analysis revealed that unlike steel surfaces, MoS2 formed on the DLC surfaces had negligible influence on friction, while the low friction DLC wear debris had strong influence on friction. Abrasive wear was found to be the dominating wear mechanism in the cases when additives showed poor wear protection on the a-C:15H coating.

Modelling the effects of friction modifiers on rail corrugation in cornering

A proposed solution to the problem of rail corrugation on curves of urban lines is the use of friction modifiers. Recent experimental research indicates this approach can be effective. This paper makes use of two theoretical models to study the effects of friction modifiers on corrugation in cornering. The first one is a nonlinear time-domain model that predicts corrugation growth from a series of simulated passes of a bogie. The second one is a frequency domain model of wheel–rail interaction, linearised about a steady cornering state, estimated from the nonlinear model. Both models predict reduction of growth of corrugation due to the use of friction modifiers. Friction modifiers can alter the steady cornering state to one with somewhat lower creeps and much lower tractions. Major reductions in corrugation growth rate have been shown to occur if a wheel can be prevented from reaching a state of full sliding when subject to oscillating creeps caused by existing rail corrugation.

Field studies of the effect of friction modifiers on short pitch corrugation generation in curves

This paper describes field evaluation on the effect of top of rail friction control on short pitch corrugation growth on different mass transit railways in Europe and Japan. Measurements were made during baseline conditions to evaluate the rate of growth of corrugations. Friction modifier (FM) was then applied using wayside application equipment, in some cases after grinding to re-establish an uncorrugated state. It was observed that FM application reduced the rate of corrugation growth by a factor of 6–11 times. In most cases there were little or no new corrugations when FM was applied on sites that formed corrugations rapidly under baseline conditions. Modelling showed that the test sites had a range of contact conditions, indicating that the effect of FM on corrugations is insensitive to changes in profile.

Top-of-rail friction control for curve noise mitigation and corrugation rate reduction

Top-of-rail friction modifiers are designed to control frictional characteristics at the wheel–rail interface that influence noise, corrugations, and lateral forces. Key characteristics of the resulting thin film are: (1) controlled intermediate coefficient of friction and (2) positive friction, to reduce roll–slip oscillations related to both curve squeal noise and short pitch corrugation development. Practical results are presented on the effect of friction modifiers on curve squeal and flanging noise generation at a range of European mass transit sites. Reductions in A-weighted noise ranging from 6.3 to 22.8 dB were recorded, with an average reduction across the different test sites of 12 dB. Both top-of-rail squeal and flanging components of noise were reduced. The effect of friction modifier on short pitch corrugation development was evaluated over several years at two curves at Metro Bilbao. Under conditions where corrugations grew from 0.1 to 0.5 mm in 18 months under baseline conditions, with application of friction modifier essentially no corrugation growth occurred in an equivalent time period.

Improvement of bogie curving performance by using friction modifier to rail/wheel interface: Verification by full-scale rolling stand test

In very sharp curved tracks where the efficient difference of rolling radius between inside/outside wheels is hard to obtain by the bogie, various ill-suited phenomena, such as rail corrugation, squeal noise, derailment and etc., are occurring. The purpose of this study is to avoid these phenomena. The effect of friction modifier on the improvement of bogie curving performance has been systematically tested. Stand tests of curving performance were carried out using full-scale test stand and bogie. The test results show that friction modifier can reduce the lateral force of front-outside wheel and longitudinal forces of rear wheels, which produce un-desired phenomena in sharp curves.

Effects of basestock and additive chemistry on traction testing

Over the past few years there have been numerous papers and articles published using the Mini-Traction Machine (MTM) test rig for transport lubricant applications. This rig has gained popularity by allowing the measurement of both high and low entrainment speeds in a single instrument. In this study, the MTM clearly demonstrates the effects of friction modifier on high and low polarity basestocks. It can be observed in the slide–roll ratio curves the friction modifier glycerol monooleate (GMO) is most effective in naphthenic mineral oil and least effective in polyol ester. Additionally, we have used the MTM rig to differentiate the traction of various organic friction modifiers, as well as, investigated the behavior of a variety of overbased sulfonate detergents. Depending on the nature of the friction modifier the traction can be increased or decreased under various conditions of temperature and slide–roll ratio. Conversely, the addition of overbased sulfonate detergents increases the traction at high speeds and decreases the traction at low speeds, resulting in an overall flattening of the traction curve.

Effect of friction modifiers on antifriction properties of motor oil

A molybdenum-containing additive, AF-1, and two ashless additives, AF-2 and AF-3, were used as friction modifiers. The addition of the modifiers lowered the coefficient of friction. Test results on M-6z/10G/sub 1/ oil with various concentrations of the AF-1 additive are presented. The introduction of the modifier should reduce the mechanical losses and fuel consumption and increase the engine speed. The additives were examined for aftereffect; the AF-1 additive gave a prolonged aftereffect and had a high level of antifriction properties, depending on its concentration.