Temperature—The Key to Lubricant Capacity

Failure of a nonreactive mineral oil can be predicted by Blok's formula for determining the maximum temperature between two bodies in rolling and sliding contact. Evaluation of many lubricants on a geared roller lest machine revealed that the lubricant failure for any particular lubricant-material combination occurs at a constant, critical contact temperature over wide ranges of load, sliding velocity, surface velocity, specimen temperature, film thickness, and viscosity grade. Coefficient of friction can be predicted by a parameter involving the unit load, inlet viscosity, sum velocity, and sliding velocity. The load capacity of a lubricant varies inversely with specimen temperature for a constant set of lest conditions. Electrical resistance measurements across the contact zone aided in identifying the lubricant failure point and in revealing the action of two deposit-forming additives.     

The correlation between surface temperature and the number of metallic contacts in the concentrated contact

Results of measurements of local changes in electrical resistance in a concentrated contact and contact surface temperatures determined with the aid of a thin-layer transducer are presented in this paper.Good agreement between the oil film thickness corresponding to the measured inlet temperature and the thickness predicted by the thermal reduction factor given by Murch and Wilson has been found. It has also been found that there exists a strong correlation between the number of individual metallic contacts (asperities) in the contact zone and the surface temperature in the inlet zone as well as the maximum surface temperature in the contact. This dependence may be described by a power function for which the exponent depends only on rolling velocity; however, the coefficient is also a function of sliding velocity.     

Failure criteria in thin film lubrication: Investigation of the different stages of film failure

The mechanisms of failure of sliding lubricated concentrated steel contacts have been studied using a newly developed tribometer which allows fast separation of the sliding surfaces at the different stages of film failure. The appearance of the corresponding contact topography was investigated by means of optical interference microscopy. The results indicate a gradual transition from full to partial elastohydrodynamic (EHD) lubrication as a function of increasing load and increasing bulk oil temperature until complete failure of the EHD film occurs at critical triplets of normal load, sliding velocity and bulk oil temperature.     

Experimental investigation of temperature rise in elliptical EHL contacts

A toroidal traction drive continuously variable transmission (CVT) transmits power by the shearing action of lubricant film under heavy loads at contact points on the CVT rollers. In other words, rolling and sliding motions produce traction force. Furthermore, due to the geometry of the toroidal CVT, spin motion is produced at the contact points. These contact points, which are elliptical in shape, are where shear stress of the lubricant generates frictional heat. Temperature rise at the contact point has never been measured under the conditions of high rolling speed (velocity) and minute amounts of sliding (slippage), nor has the influence of spin motion on temperature rise been examined thus far. The authors et al. measured temperature distribution at contact points under conditions of high rolling speed and minute amounts of sliding, such as what is found in a toroidal CVT, using an FZG twin-disk test machine and thin-film sensors. The influence of spin motion on temperature rise was also experimentally investigated.     

Measuring contact pressure distributions under elastohydrodynamic point contacts

Circular point contact EHD pressure distributions were investigated under various operating conditions using a thin film piezo-resistive pressure transducer. A series of controlled experiments, in which the active element of the transducer is positioned at various contact cross-sections, was carried out. The results are analysed and presented in the form of isobars. The pressure distribution closely describes the Hertzian pressure profile with a secondary pressure peak appearing prior to and near the exit constriction.     

Non-newtonian thermal analysis of an EHD contact lubricated with MIL-L-23699 oil

A thermal and non-Newtonian fluid model under elastohydrodynamic lubrication conditions is proposed, integrating some particularities, such as the separation between hydrodynamic and dissipative phenomena inside the contact. The concept of apparent viscosity is used to introduce the non-Newtonian behaviour of the lubricant and the thermal behaviour of the contact into the Reynolds equation, acting as a link element between the hydrodynamic and dissipative components of the EHD film, independently of the rheological and thermal models considered. The apparent viscosity enables the application of the rheological model better adapted to each lubricant, without appealing to special formulations of the EHD problem.The Newton–Raphson technique is used to obtain the lubricant film geometry and the pressure distribution inside the EHD contact. The shear stresses developed in the fluid film are evaluated assuming the non-linear Maxwell rheological model. The surfaces and lubricant temperature distributions are determined using the simplified Houpert's method, applied to the inlet contact zone, and the thermal method proposed by Tevaarwerk is applied in the high pressure contact zone.The non-Newtonian thermal EHD model is applied to the analysis of a contact lubricated with MIL-L-23699 oil. Significant results are obtained for the centre and minimum film thickness, for the inlet shear heating and film thickness reduction factor (φ_T), for the temperature rise of the lubricant and of the surfaces and for the friction coefficient inside the contact, considering wide ranges of the operating conditions (maximum Hertzian pressure, inlet oil temperature, rolling speed and slide-to-roll ratio).Finally, the numerical traction curves determined are compared with the corresponding experimental results, showing very good correlation.     

A study of sliding between rollers and races in a roller bearing with a numerical model for mechanical event simulations

A numerical model of a roller bearing is presented in this paper. These simulations provide us with the spatial and time distributions of stress and strain values, as well as all the nodal displacements at every time step. The model was developed with the finite element method (FEM) for mechanical event simulations (MES) with the commercial code Algor^TM. The model has been validated by verifying that the contact force distributions correspond to those predicted by the analytical model of Harris-Jones.As an application, a study of sliding between the rollers and the races has been carried out. For each roller, a rolling zone can be defined in which local sliding (computed between two consecutive time steps) is negligible. According to the simulations, we conclude that the rolling zone is practically the same for all the rollers in the same simulation; that this rolling zone is smaller than the corresponding load zone, and that rolling and load zones are angularly centered with respect to each other.     

The investigation of oil film thickness in heavily-loaded contact

An experimental study of the shape and thickness of the oil film during rolling in a thrust ball bearing has been carried out by the interference method.The experimental results showed good agreement with theory. Oil film thickness was affected mainly by the rolling velocity, viscosity of oil and maximum Hertzian stress. The groove radius had no effect on the film thickness. With increase of rolling velocity the film thickness increases and then reduces sharply owing to temperature rise and the non-Newtonian properties of the lubricant. A qualitative similarity was derived from the experimentally observed dimensionless shapes of the film and of the dimensionless theoretical shapes of the oil film for the lubricant in the non-Newtonian state. The flat “squashed” contact area diminished and disappeared with rise in velocity, which agreed with theoretical predictions.Good agreement was found between the theoretical and the experimental values of the oil film thickness and the friction coefficients for a ball sliding on a plane. Values of relaxation time for oil agree with values observed by the vibration method.The interference method is proposed to estimate the relation of the relaxation time for lubricants to the pressure and temperature up to maximum Hertzian pressures of 14,000 kg/cm². Experimental studies by the interference method and the solution of the non-isothermal hydrodynamic contact problem for liquids both in the Newtonian and non-Newtonian state provide a method of calculation of the friction coefficient.     

Computational fluid dynamics and full elasticity model for sliding line thermal elastohydrodynamic contacts

Classically, the EHD problem is solved using the Reynolds assumptions to model the fluid behaviour, and the Boussinesq elastic deformation equation to model the solid response, both being coupled with the load balance equation. The development of an alternative approach is presented here in order to solve at once the Navier-Stokes equations (mass conservation and momentum equilibrium), the full elasticity and energy equations for the line EHD problem in a fluid-structure interaction approach.The Finite Element Method is used to solve the mathematical formulation in a fully coupled way, inspired from Habchi et al. (2008) [1]. After linearisation with the Newton procedure, all the physical quantities (pressure, velocity field, deformations and temperature) are solved together in a unique system. An important benefit of this approach is the possibility to implement in a simple manner the non-Newtonian and thermal effects; in fact all the quantities can vary through the film thickness. The extension to non-Newtonian rheology and the pressure and temperature dependencies for the viscosity and density are taken into account in a direct way to allow an acceptable prediction of the friction coefficient. Gradients across the film thickness and temperature fields in both the fluid and the two solids are naturally computed and analysed. As a case study, we focus first on the pure sliding cylinder-on-plane contact. It is shown that thermal effects due to friction in the central zone of the contact play a role in heating the lubricant at the inlet zone, via heat conduction in the solids. By increasing the Slide-to-Roll Ratio (SRR), the occurrence of dimples and the subsequent effects in different parts of the contact under zero entrainment velocity conditions are then studied.     

Influence of operating conditions on friction and temperature characteristics of a wet clutch engagement

To optimise the gear change in automatic transmissions, more knowledge is needed of the engagement behaviour of wet clutches. A factorial design investigation of the engagement of a wet clutch has been carried out. The friction and temperature characteristics have been studied. The experiments were carried out in an apparatus that can vary sliding velocity, drive torque, inertia, force rate, and lubricant flow. The results show how these parameters affect the response data: engagement time, developed energy, temperature rise, maximum torque, maximum power, static friction, dynamic friction, and initial friction. There are interaction effects between some of the input parameters, but they are relatively small. The friction coefficient varies over time but is independent of the input parameters, except for dynamic friction, which decreases with increasing sliding velocity, drive torque, and inertia. The temperature rise was found to be proportional to developed energy and both these were most affected by sliding velocity. The drive torque and force rate have the greatest effect on the engagement time.     

Effect of Base Oil Structure on Elastohydrodynamic Friction

The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding–rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide–roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.     

Thin film lubrication of sliding point contacts of AISI 52100 steel

The mechanism of thin film lubrication of sliding point contacts of AISI 52100 steel has been studied as a function of load, sliding speed, composition and temperature of the lubricant.Below certain critical combinations of Hertzian pressure, speed and temperature the surfaces are kept apart by an elastohydrodynamic lubricant film. The load carrying capacity of this film depends primarily on the effective viscosity of the lubricant in the contact region which decreases with bulk oil temperature and with increasing sliding speed, because of friction induced thermal effects. After breakdown of the EHD film, boundary lubrication may still prevent severe adhesive wear. The transition from the boundary lubricated regime towards the regime of severe adhesive wear is a function of load (normal force), speed and bulk oil temperature and possibly depends on the conjunction temperature. Irrespective of the initial lubrication condition, oxidation of the steel surfaces leads to the (re)establishment of low friction, mild wear conditions.     

Thin lubricating films behaviour at very high contact pressure

Thin film colorimetric interferometry has been used to examine the behaviour of thin elastohydrodynamic (EHD) lubricant films under very high contact pressures of the order of 0.5–3 GPa. It has been shown that at moderate pressures, the variation of film thickness with speed follows the Hamrock and Dowson prediction down to one nanometer. As the load is increased, however, thin films behave differently from the prediction of the conventional EHD theory. For a certain lubricant and operational conditions, there is a critical rolling speed below which a reduction of film thickness is observed. This behaviour is very similar to that previously predicted computationally by Zhu.     

Synthesis of cam-follower systems with rolling contactSynthese der kurvengetriebe mit rollberührung

The present work deals with the synthesis of cam follower systems with rolling contact during the rise period of a dwell-rise-dwell return cam. It has been found that the total lift of the follower can not be achieved solely by rolling contact, therefore sliding is permitted for a fraction of the lift. On the basis of minimum work-loss, the respective percentages of contribution by pure rolling contact and rolling cum sliding action have been worked out.     

Traction and wear of an elastomer in combined rolling and sliding

Under combined rolling and sliding materials can experience millions of cycles as well as complex loading and slip conditions, which can dramatically affect their friction and wear behaviour. It was shown that for a carbon black‐filled natural rubber compound in combined rolling and sliding contact with a smooth alumina coated disk, the traction coefficient, as a function of slip percent, was dependent upon the normal load and independent of rolling velocity. The wear rate of this material pair was found to be independent of slip percentage as well as rolling velocity but dependent upon sliding distance. The wear rate was found to be approximately the same for all tested cases (K ~ 1 × 10⁻⁴ mm³·Nm⁻¹). The worn profiles of the ball specimens showed that this wear occurred preferentially on the left side (inner radius) of the contacting area. Copyright © 2015 John Wiley & Sons, Ltd.     

Instantaneous Coefficients of Gear Tooth Friction

In a gear contact as simulated on a roller test machine, the instantaneous coefficient of friction follows the concept of transition from boundary to hydrodynamic lubrication. The coefficient has been found to increase with increasing load and to decrease with increasing sum velocity, sliding velocity, and oil viscosity as each of these quantities is varied individually. The viscosity was determined by the temperature of the oil entering contact and the viscosity-temperature characteristics of the lubricant. The results have been combined in a formula which closely represents the data. When this formula is used in gear scoring calculations, the same type of U-shaped load-speed curve is obtained as has been found on several gear test rigs.     

Numerical and Experimental Studies on Tribological Behaviors of Cu-Based Friction Pairs from Hydrodynamic to Boundary Lubrication

When studying the tribological behaviors of a Cu-based friction pair in different lubrication regimes, calculation of the real contact area of asperity contacts is crucial but difficult. In this work, a mixed lubrication model in plane contacts is developed, and pin-on-disc tests are carried out. The real contact area ratio, load sharing ratio, and friction coefficient are investigated. Effects of sliding velocity, temperature, and pressure are considered. The results show that when the maximum contact area ratio is about 14.6%, the load sharing ratio of asperity contacts is about 95%. The friction coefficient obviously increases from less than 0.04 to about 0.15 as the regime changes from hydrodynamic to boundary lubrication. Asperities have a significant influence on the local lubrication of a Cu-based friction pair, and the action of hydrodynamic pressure cannot be ignored.     

外摆线型单螺杆泵螺杆-衬套副的运动特性分析

通过分析外摆线型单螺杆泵螺杆-衬套副的相对运动,发现螺杆-衬套副的固定接触点出现在衬套骨线上,衬套可采用金属材料,螺杆可在金属基体表面黏接橡胶外套;用瞬心法推导出齿凸接触点和齿凹接触点处相对滑动速度的计算公式,并用MATLAB生成速度曲线,用SolidWorks建立螺杆泵实体模型,用SolidWorks Motion进行运动仿真,对比速度仿真值与速度公式曲线可知,拟合度很高,最大偏差率仅为-0.101%;将速度曲线与啮合状态进行对应,发现齿凹接触点处相对滑动速度的最大值发生在螺杆齿凸中点与衬套齿凹中点相接触处,齿凸接触点处相对滑动速度最大值发生在螺杆齿凸中点与衬套齿凸中点相接触处。     

碳化硼增强铝基复合材料的摩擦磨损性能

为了比较两种含量不同的碳化硼颗粒增强铝基复合材料的摩擦学性能,将其加工成销试样,在多功能摩擦磨损试验机上分别与钢盘试样进行对比摩擦磨损试验,重点研究了接触载荷和相对滑动速度对两种复合材料摩擦磨损性能的影响.结果表明:碳化硼增强铝基复合材料的磨损量随载荷与相对滑动速度的增大而增大,而摩擦因数随载荷与相对滑动速度的增大而减小,较高碳化硼含量的复合材料的耐磨性能比较低含量的复合材料好.     

Multilevel solution for thermal elastohydrodynamic lubrication of rolling/sliding circular contacts

A fast multigrid approach is presented for the analysis of thermal elastohydrodynamic lubrication (EHL) under rolling/sliding circular contacts at high loads and high slip ratios with low computing time on a personal computer. This fast solver combines directiteration, multigrid, Newton-Raphson, Gauss-Seidel iteration, and multilevel multi-integration methods into one working environment that can reduce the computational complexity from O(n³ to O(nlnn) for the thermal EHL problem under rolling/sliding circular contacts. Since the couped Reynolds and energy equations are simultaneously solved by the Newton-Raphson scheme, the iteration for the convergence solution is less than those of the classical approach. Results show that thermal effects on the pressure profile and film thickness are significant for a wide range of loads, speeds and slip ratios. The maximum midfilm and surface temperature rise in the Hertzian contact region increases with increasing slip ratio, dimensionless speed, and load. The minimum film thickness decreases with increasing load and slip ratio, and decreasing dimensionless speed.