The variation of lateral forces as a function of alignment for a cylindrical wheel rolling on a rail

Those lateral forces that arise between a steel wheel and a rail as a consequence of misalignment are analysed. The magnitude of these forces is calculated for unconed wheels as a function of the misalignment angle and the friction coefficient. The results show that by following an FEM standard (an alignment better than 1:2500) lateral forces should be less than μP/4 where μ is the interfacial friction coefficient and P is the normal load.     

Tribological behavior of reciprocating friction drive system under lubricated contact

The dynamic friction and wear behaviors are investigated in reciprocating friction drive system using a 0.45% carbon steel pair. The effects of various operating parameters on the traction force, stick and slip time, and friction modes are examined under the lubricated contacts. Moreover, the critical operating conditions in classifying three friction modes are also established. Results show that the fluid friction induced by the shearing of lubricant dominates the variation of traction force and produces the positive slope γ at the first period of slip in the traction force–relative sliding velocity curve. The γ value decreases at higher driver speed during stick-slip motion due to the thicker fluid film and shear thinning effect. The γ value increases due to the asperity interactions as the friction region is transferred from stick-slip to sticking with normal load from 196 to 980 N. Furthermore, it is also found that the static friction force is independent of stick time for the tangential loading rate ranged from 1.12 to 16.8 s⁻¹. The transition region produces the severest wear under the different driver speeds, but the wear is insensitive to the friction regions and the severe wear only occurs at higher normal load due to the action of Hertzian contact.     

Externally pressurized conical step bearing with visco-elastic lubricant

A solution for an externally pressurized conical step bearing with visco-elastic lubricant is obtained by using a regular perturbation technique. The effects of the parameters S (elastic number), H = h₁/h₂ (the ratio of film thickness) and sinα (α, the semi-vertical angle) on the pressure, the load capacity and the ratio of flow flux have been studied and are presented graphically. The effect of elasticity on improving bearing performance is very small.     

Tribological characterization of environmentally adapted ester based fluids

Fundamental properties of six synthetic ester base fluids, suitable for the formulation of environmentally adapted lubricants, have been investigated. High pressure viscosity data for the test fluids were obtained through experimental measurements with a high pressure Couette rheometer. The temperature, pressure and viscosity data η(p, T) were parameterized against the Roelands pressure–viscosity equation. Thermal conductivity and specific heat capacity data were obtained using a transient hotwire method, and the EHD friction coefficient, γ, was obtained experimentally as well. The results from these measurements are reported, and the correlation between thermal properties, molecular structure, and the fluid rheology parameters, of the test fluids are discussed.     

Tilting effects on the performance of hydrostatic and hybrid gas thrust bearings

A hydrostatic gas thrust bearing is analysed to determine the effect of tilting on its performance characteristics. The governing Reynolds Equation is solved simultaneously with the mass flow rate equation by numerical methods to determine the inlet pressure and the pressure distribution throughout the pad. It is shown that the load, friction, and lubricant mass flow rate are strong functions of the bearing number λ, restrictor coefficient λ_o, supply pressure and tilt parameter     

Rotation effects on hybrid air journal bearings

Rotation effects of hybrid air journal bearings with multi-arrays of orifice feedings are investigated numerically. Porous air bearings are also solved for comparison. The results show that bearing load capacity W increases faster with eccentricity ratio than with rotation speed, i.e. bearing number Be. There are optimum orifice diameters, i.e. optimum feeding parameters λ_o, which give maximum load capacity W for orifice feeding; but for porous feeding, load capacity W increases with feeding parameters λ_p. It was found that the load capacity increases with feeding arrays of orifices and five rows of orifice feedings can approximate the operations of porous bearings very well. It was also found that load capacity W does not increase further when air supply pressure exceeds 5 atm because there is a critical pressure ratio through orifice (e.g. (P_o/P_s)<0.53).     

Static Performance of Finite Hydrodynamic Journal Bearings Lubricated by Magnetic Fluids with Couple Stresses

Based upon the Stokes micro-continuum theory, the problem of lubrication of finite hydrodynamic journal bearing lubricated by magnetic fluids with couple stresses is investigated. By taking into account the couple stresses due to the microstructure additives and the magnetic effects due to the magnetization of the magnetic fluid, modified Reynolds equation is obtained. The effects of couple stresses are studied by defining the couple stress parameter L that can be considered as a measure of the chain length of the additive molecule. The magnetic effects of the magnetic fluid are investigated by the magnetic coefficient γ. Using the finite-difference technique and for different values of couple stress parameter and magnetic coefficient, the Reynolds equation is solved, and pressure distributions are obtained. The bearing static characteristics namely load carrying capacity, attitude angle, friction coefficient, and side leakage flow are determined. The results indicate that the influence of couple stresses and magnetic effects on the bearing characteristics are significantly apparent. It is concluded that fluids with couple stresses are better than Newtonian fluids. The improvement of the bearing characteristics is enhanced if the magnetic effects are present.     

Boundary lubrication additives for aluminium: A journey from nano to macrotribology

The tribology of two potential additive molecules for aluminium has been studied in the self assembled monolayer state at low loads (10⁻⁹–10⁻² N) and in dispersion state in n-hexadecane at high load (60 N). The two additive molecules are 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane (FOTS), which has a rigid rod-like helical structure and octadecyltrichlorosilane (OTS), which has a zigzag backbone configuration. The adhesion, friction and wear performance have been investigated. Molecular conformational order and stiffness were found to play major roles in frictional dissipation and wear resistance, recorded in a nanotribometer and a pin-on-disc machine, while for very low load measurements (LFM), adhesion was found to have a significant role on friction along with molecular entropy.     

Tribopolymerization of n-butyl acrylate on the steel–steel rubbing surface

Antiwear property of n-butyl acrylate (BA) in hexadecane for steel–steel friction elements was studied. Scanning electronic microscopy (SEM) images of worn scars of balls, and width measurements of worn tracks of disks indicate that BA has good antiwear property.Tribopolymerization (polymerization initiated by the rubbing surface) tests of BA used as lubricant instead of an additive were conducted, to classify antiwear mechanisms. Infrared spectroscopy (IR) of washing solution and thermogravimetry (TG) traces of wear debris confirmed that tribopolymers were generated on steel–steel interface in situ. Also considerable wear products were precipitated from vacuum-condensed worn fluids with methanol as the non-solvent, characterized by IR, gel permeation chromatography (GPC), nuclear magnetic resonance (NMR) spectroscopy, showing that the precipitates were poly(n-butyl acrylate) (PBA) with very high molecular weight.To study tribopolymerization mechanisms of BA, effects of sliding velocities and adding 1 wt% n-butyl alcohol to BA fluids on the mass of tribopolymers were investigated, respectively. Expectedly, the mass of tribopolymers dramatically enhanced with the sliding velocity increasing, showing that the tribopolymers were generated just due to friction processes. Additionally, tribopolymerization of styrene (easy to thermopolymerize) was studied. Unexpectedly—but not unreasonably—no substantial worn products were precipitated from vacuum-condensed worn fluids. Based on these experimental results, an exoelectron-radical-tribopolymerization mode, consistent with some Kajdas’ tribochemistry theories, for BA was proposed.     

Lubrication of a roller on a plane in combined rolling, sliding and normal motion with additives

The micropolar fluid theory-a possible non-Newtonian model for fluids with additives in which the average molecular size may be comparable to a characteristic material length scale—is applied to the problem of lubrication of a lightly loaded roller on a plane in combined sliding, rolling and normal motion. Three new parameters N, L and q are defined. Various bearing characteristics are presented graphically. It is shown that these three dimensionless parameters are of great significance in discussing the flow behavior of a lubricant with additives. The results of Dowson et al.[2] can be obtained as a particular case of the present study. The overall conclusion of the study, in conformity with the experimental results, is an increase in effective viscosity, resulting in an increase in the load carrying capacity and a decrease in coefficient of friction when additives are used.     

Vlasov torsion of non-linearly elastic beams of thin-walled open cross-section

A beam lamina of thin-walled open cross-section is considered. The sectional forces are a bimoment (warping moment) B and a Vlasov torsional moment M_w. A formula for the sectorial co-ordinate containing three unknowns and being suitable for numerical calculations is given. The material is assumed to be non-linearly elastic with the constitutive equation = K|σ|ⁿ sgn σ in uniaxial tension. The relation M_w = B′ is shown to hold also for n ≠ 1. The location of the centre of twist as a function of n is determined for a monosymmetric I cross-section. The torsion-bending analogue for beams is found to be valid also for n ≠ 1.     

Evaluation of a novel UHMWPE bearing for applications in precision slideways

This paper presents a novel slideway bearing design comprised of a thin-film (0.1–0.2 mm) ultra-high molecular weight polyethylene (UHMWPE) bound to a rigid hemispherical substrate. Two prototype bearing designs were fabricated and tested to characterize the coefficient of friction (dynamic and static) and wear of the polymer. In addition, similar bearings were incorporated into a kinematically constrained rectilinear carriage to determine the repeatability of motion during multiple traverses. The first bearing had a radius of curvature on the order of 2.38 mm incorporating an UHMWPE film thickness between 0.1 and 0.2 mm. The friction coefficient was measured to be 0.16 at a normal load of 11.5 N while changing to 0.19 as the normal load is decreased to 2.2 N, at a surface speed of 4.2 mm s⁻¹. This bearing failed after a traverse of approximately 700 m at a load of 11.5 N. A similar evaluation procedure was carried out on a bearing of radius 6.35 mm resulting in a friction coefficient of 0.13 at a normal load of 27.8 N while changing to 0.19 as the load is decreased to 2.2 N, and the bearing endured a traverse of over 2.2 km at a load of approximately 28 N (in both air and vacuum conditions) with a surface speed of 4.2 mm s⁻¹. The second bearing prototype was further subjected to a repeatability test. In this set-up, a carriage incorporating five bearings was traversed in a nominally linear path while vertical deviations for multiple traverses were measured by a custom built displacement sensor. Deviations from a linear path were observed to repeat to within a few nanometers about nominal variations of less than 10 nm for a traverse distance of 10 mm. This system and other subsystems used to characterize the friction coefficient and noise of the polymer bearing are presented.     

Sliding friction and wear performance of Ti6Al4V in the presence of surface-capped copper nanoclusters lubricant

The friction and wear properties of Ti₆Al₄V sliding against AISI52100 steel ball under different lubricative media of surface-capped copper nanoclusters lubricant—Cu nanoparticles capped with O,O′-di-n-octyldithiophosphate (Cu-DTP), rapeseed oil and rapeseed oil containing 1 wt% Cu-DTP was evaluated using an Optimol SRV oscillating friction and wear tester. The wear mechanism was examined using scanning electron microscopy (SEM) and X-ray photoelectron spectrosmeter (XPS). Results indicate that Cu-DTP can act as the best lubricant for Ti₆Al₄V as compared with rapeseed oil and rapeseed oil containing 1 wt% Cu-DTP. The applied load and sliding frequency obviously affected the friction and wear behavior of Ti₆Al₄V under Cu-DTP lubricating. The frictional experiment of the Ti₆Al₄V sliding against AISI52100 cannot continue under the lubricating condition of rapeseed oil or rapeseed oil containing 1 wt% Cu-DTP when the applied load are over 100 N. Surprisingly, the frictional experiment of Ti₆Al₄V sliding against AISI52100 steel can continue at the applied load of 450 N under Cu-DTP lubricating. The tribochemical reaction film containing S and P is responsible for the good wear resistance and friction reduction of Ti₆Al₄V under Cu-DTP at the low applied load. However, a conjunct effect of Cu nanoparticle deposited film and tribochemical reaction film containing S and P contributes to the good tribological properties of Ti₆Al₄V under Cu-DTP at the high-applied load.     

Rotor dynamic instability analysis on hybrid air journal bearings

Hybrid air journal bearings with multi-array of 1, 2, 3, 4, or 5-row orifice feedings are analyzed for the problem of rotor dynamic instability. The bearing stiffness and damping coefficients are calculated numerically to determine threshold rotor mass under various operating conditions. The hybrid porous air journal bearings are also analyzed for comparison to investigate the similarities in dynamic characteristics between the multi-array of orifice feeding bearings and the porous bearings. The results show that the porous bearing is more stable than the orifice feeding bearing at lower rotation speeds (Λ<0.1) or at higher rotation speeds (Λ>1) with lower feeding parameters (λ_P<10⁻⁸). The 5-row orifice feeding bearing is more stable than the porous bearing at moderate speeds (0.3<Λ<0.6) with lower feeding parameters (λ₀<10⁻⁴).     

Tribological studies of 1-alkyl-2,5-dithiohydrazodicarbonamides and their Mo–S complexes as EP and multifunctional additives

This communication reports a comparative study of the anti-wear, extreme pressure and friction reducing properties of 0.5% (w/v) admixtures of paraffin oil with 1-alkyl-2,5- dithiohydrazodicarbonamide ligands and their Mo–S complexes, i.e. μ-hydroxo-μ-(1-alkyl-2,5-dithiohydrazodicarbonamido)-bis-[dioxohydroxo molybdenum VI] using 12.7 mm diameter steel bearing balls in a four-ball test. In particular, the additive μ-hydroxo-μ-(1-n-butyl-2,5-dithiohydrazodicarbonamido)-bis-[dioxohydroxo molybdenum VI] exhibits an increase in the load carrying capacity of the lubricant and a decrease in the values of wear and coefficient of friction. The surface analysis of the wear-scar matrix of the used ball specimen has been investigated by SEM and AES techniques.     

Performance characteristics for the hydrostatic thrust bearing of a saw blade

A semi-analytical solution is presented to obtain the pressure distribution, the load capacity, the frictional moment and the lubricant flow rate for a laminar flow, hydrostatic bearing with its axis offset from thee axis of the saw blade. The general equations are developed and solved for a variable film thickness due to plate vibrations. It was found that the load, friction and lubricant flow rate are strong functions of film thickness variations , bearing number λ and the bearing offset from the rotation axis     

Linear stability of short journal bearings with consideration of flow rheology and surface roughness

The combined effects of surface roughness and flow rheology on the linear stability of a rigid rotor supported on short-length journal bearings are analyzed. The modified Reynolds equation and the rotor motion equation are linearized about the equilibrium position and the fluid film is modeled as spring and damping elements. From the characteristic equation, the instability threshold is then obtained with various surface roughness parameters (standard derivation, σ, and Peklenik number, γ), flow rheology (power-law index, n) and eccentricity ratios (). The results show that the size of the stability regions of different roughness patterns has the following characteristics: longitudinal (γ>1)>isotropic (γ=1)>smooth>transverse (γ<1). The stability of the bearing system deteriorates with decreasing power-law index. Moreover, there are crossing points in the vicinity of =0.45 among the curves of dimensionless speed parameter ( ) associated with various power-law indices and surface roughness parameters.     

Friction and wear behaviour of Thordon XL and LgSn80 in sliding against plasma-sprayed Cr2O3 coatings

This paper studies the friction and wear behaviour of two important bearing materials, Thordon XL and LgSn80, in dry and lubricated sliding vs. plasma-sprayed Cr₂O₃ coatings. As a reference, AISI 1043 steel is also studied under the same conditions. SEM, EDS and surface topography were employed to study the wear mechanisms. The results indicate that the Thordon XL/Cr₂O₃ coating pair gives the lowest dry friction coefficient (0.16) under a normal load of 45.3 N (pressure 0.453 MPa) at a velocity of 1 m/s. The dry friction coefficient of Thordon XL/Cr₂O₃ coating increases to 0.38 under a normal load of 88.5 N (pressure 0.885 MPa). The dry friction coefficients of the LgSn80/Cr₂O₃ coating are in the range of 0.31–0.46. Secondly, both dry wear rate under low normal load (45.3 N) and lubricated wear rate under a load of 680 N for Thordon XL are lower than those of LgSn80 in sliding against plasma-sprayed Cr₂O₃ coatings at a speed of 1 m/s. However, under a normal load of 88.5 N the dry wear rate of Thordon XL is much higher than that of LgSn80. Thirdly, a high viscosity lubricant (SAE 140) leads to lower wear for Thordon XL and LgSn80 than a low viscosity lubricant (SAE 30). Finally, the dominating wear mechanism for Thordon XL is shear fracture when against the plasma-sprayed Cr₂O₃ ceramic coating. For LgSn80 against plasma-sprayed Cr₂O₃ ceramic coating, abrasive wear is the governing failure mechanism.     

Friction and adhesion in boundary lubrication measured by microtribometers

The measuring and modelling of friction are critically important for the motion control in nanopositioning, particularly when bearings are employed to cover the wide working distances. Since the positioning system usually operates at very low speed to achieve fine positioning, the boundary lubrication is the dominant regime. A detailed characterization of the friction of boundary lubrication formed by Poly–α–Olefin (PAO) with and without surfactant and a suspension of MoS₂ in base oil has been performed in reciprocating sliding tests by steel/steel point contacts, and correlated with adhesion measurements by silicon/silicon point contacts. A microtribometer based on laser interferometers and glass springs, which can resolve 100 nN force in a speed range of 1–1000 μm/s was employed to detect the minute changes in forces. We find that a simple linear function instead of a logarithmic function is possible to describe the relationship between the friction force and operating speed for all the lubricants tested, though the gradients are quite different and under the influence of normal load. Comparing to PAO+surfactant and MoS₂ suspension, PAO shows a much higher load-dependent coefficient of friction. This result is further confirmed by the repulsion force measurements, which shows a higher increase of contact pressure with the increase of normal load for PAO.     

The sectorial co-ordinate of non-linearly elastic beams of three thin-walled open cross-sections

A beam lamina of thin-walled open cross-section is considered. The sectional forces are a bimoment B and a Vlasov torsional moment M_w. The material is assumed to be non-linearly elastic with the constitutive equation = K|σ|ⁿ sgn σ in uniaxial tension. A slight redefinition of the generalized sectorial moment of inertia I⁽ⁿ⁾_ω is given. The sectorial co-ordinate as a function of n is determined for three commonly used cross-sections.